Editing Sex-determination system

{{Short description|Biological system that determines the development of an organism's sex}}
{{About|the internal mechanisms that determine what sex an organism develops into|how people identify the sex of a human or other animal|Determination of sex}}
{{Use dmy dates|date=September 2020}}
[[File:Types of sex determination.png|thumb|Some chromosomal sex determination systems in animals|315x315px|alt=]]
A '''sex-determination system''' is a [[biology|biological]] system that determines the development of [[sexual characteristics]] in an [[organism]].<ref>{{Cite encyclopedia |title=Sex Determination in Humans | encyclopedia = The Embryo Project Encyclopedia | first = Risa Aria | last = Schnebly | date = 2021 | url=https://embryo.asu.edu/pages/sex-determination-humans |access-date=2022-07-06 }}</ref> Most [[organism]]s that create their [[offspring]] using [[sexual reproduction]] have two common sexes, [[Male|males]] and [[Female|females]], and in other species, there are [[hermaphrodite]]s, organisms that can function reproductively as either female or male, or both.<ref>{{cite encyclopedia | vauthors = Rosenfield KA | chapter = Hermaphrodite|date=2018 |encyclopedia=Encyclopedia of Animal Cognition and Behavior|pages=1–2| veditors = Vonk J, Shackelford T |place=Cham|publisher=Springer International Publishing|doi=10.1007/978-3-319-47829-6_329-1|isbn=978-3-319-47829-6 }}</ref>

There are also some species in which only one sex is present, temporarily or permanently. This can be due to [[parthenogenesis]], the act of a female reproducing without [[fertilization]]. In some plants or algae the [[gametophyte]] stage may reproduce itself, thus producing more individuals of the same sex as the parent.

In some species, sex determination is genetic: males and females have different [[allele]]s or even different [[gene]]s that specify their sexual [[Comparative anatomy|morphology]]. In animals this is often accompanied by [[chromosome|chromosomal]] differences, generally through combinations of [[XY sex-determination system|XY]], [[ZW sex-determination system|ZW]], [[X0 sex-determination system|XO]], [[Z0 sex-determination system|ZO]] chromosomes, or [[haplodiploidy]]. The sexual differentiation is generally triggered by a main gene (a "sex locus"), with a multitude of other genes following in a [[domino effect]].

In other cases, the sex of a fetus is determined by [[ecosystem|environmental]] variables (such as [[Temperature-dependent sex determination|temperature]]). The details of some sex-determination systems are not yet fully understood.

Some species such as various plants and fish do not have a fixed sex and instead go through life cycles and [[Sequential hermaphroditism|change sex]] based on genetic cues during corresponding life stages of their type. This could be due to environmental factors such as seasons and temperature. In some [[Gonochorism|gonochoric]] species, a few individuals may have [[Disorders of sex development|conditions]] that cause a mix of different [[Sexual characteristics|sex characteristics]].<ref>{{Cite book | vauthors = Minelli A, Fusco G |author-link1=Alessandro Minelli (biologist)|title=The Biology of Reproduction|url=https://books.google.com/books?id=AKGsDwAAQBAJ&q=Intersex%20|url-status=live|archive-url=https://web.archive.org/web/20201011041143/https://www.google.com/books/edition/The_Biology_of_Reproduction/AKGsDwAAQBAJ?hl=en&gbpv=1&bsq=Intersex+|archive-date=11 October 2020|access-date=11 October 2020|publisher=Cambridge University Press|pages=116–117|date=2019|isbn=978-1108499859}}</ref>

== Discovery ==
{{Expand section|date=June 2021}}
Sex determination was discovered in the [[mealworm]] by the American geneticist [[Nettie Stevens]] in 1903.<ref>{{Cite web|url=https://www.nature.com/scitable/topicpage/nettie-stevens-a-discoverer-of-sex-chromosomes-6580266|title=Nettie Stevens: A Discoverer of Sex Chromosomes {{!}} Learn Science at Scitable|website=www.nature.com|access-date=2018-06-07|archive-date=7 April 2019|archive-url=https://web.archive.org/web/20190407152710/https://www.nature.com/scitable/topicpage/nettie-stevens-a-discoverer-of-sex-chromosomes-6580266|url-status=live}}</ref><ref>{{cite journal | vauthors = Ogilvie MB, Choquette CJ | title = Nettie Maria Stevens (1861–1912): her life and contributions to cytogenetics | journal = Proceedings of the American Philosophical Society | volume = 125 | issue = 4 | pages = 292–311 | date = August 1981 | pmid = 11620765 | jstor = 986332 }}</ref><ref>{{Cite web|url=https://embryo.asu.edu/pages/nettie-maria-stevens-1861-1912|title=Nettie Maria Stevens (1861–1912) {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|access-date=2018-06-07|archive-date=8 April 2019|archive-url=https://web.archive.org/web/20190408010235/https://embryo.asu.edu/pages/nettie-maria-stevens-1861-1912|url-status=live}}</ref>

In 1694, J.R. Camerarius, conducted early experiments on pollination and reported the existence of male and female characteristics in plants(Maize).

In  1866, [[Gregor Mendel]] published on inheritance of genetic traits. This is known as [[Mendelian inheritance]] and it eventually established the modern understanding of inheritance from two [[gametes]].

In 1902, C.E. McClung identified sex chromosomes in bugs.

In 1917, C.E. Allen, discovered sex determination mechanisms in plants.

In 1922, C.B. Bridges, put forth the Genic Balance Theory of sex determination.

In 1928, Fritz Baltzer first described environmental sex determination.<ref>{{Cite journal |last=Baltzer |first=Fritz |date=1928 |title=Über metagame Geschlechtsbestimmung und ihre Beziehung zu einigen Problemen der Entwicklungsmechanik und Vererbung (Auf Grund von Versuchen an Bonellia) |journal=Verhandlungen der Deutschen Zoologischen Gesellschaft 32: 273-325 |language=de |volume=32 |pages=273-325}}</ref>

==Chromosomal systems==
Among animals, the most common chromosomal sex determination systems are XY, XO, ZW, ZO, but with numerous exceptions.

According to the Tree of Sex database<ref>{{Cite journal |date=2014-06-24 |title=Tree of Sex: A database of sexual systems |journal=Scientific Data |volume=1 |pages=140015 |doi=10.1038/sdata.2014.15 |issn=2052-4463 |pmc=4322564 |pmid=25977773 |last1=Ashman |first1=Tia-Lynn |last2=Bachtrog |first2=Doris |last3=Blackmon |first3=Heath |last4=Goldberg |first4=Emma E. |last5=Hahn |first5=Matthew W. |last6=Kirkpatrick |first6=Mark |last7=Kitano |first7=Jun |last8=Mank |first8=Judith E. |last9=Mayrose |first9=Itay |last10=Ming |first10=Ray |last11=Otto |first11=Sarah P. |last12=Peichel |first12=Catherine L. |last13=Pennell |first13=Matthew W. |last14=Perrin |first14=Nicolas |last15=Ross |first15=Laura |last16=Valenzuela |first16=Nicole |last17=Vamosi |first17=Jana C. }}</ref> (as of 2023), the known sex determination systems are:<ref>{{Cite journal |last1=Hayashi |first1=Shun |last2=Abe |first2=Takuya |last3=Igawa |first3=Takeshi |last4=Katsura |first4=Yukako |last5=Kazama |first5=Yusuke |last6=Nozawa |first6=Masafumi |date=2024-07-31 |title=Sex chromosome cycle as a mechanism of stable sex determination |url=https://academic.oup.com/jb/article/176/2/81/7709515 |journal=The Journal of Biochemistry |volume=176 |issue=2 |pages=81–95 |doi=10.1093/jb/mvae045 |issn=0021-924X |pmc=11289310 |pmid=38982631}}</ref>
{| class="wikitable"
|+ Sex determination systems in vertebrates, insects and angiosperms
! Taxonomic group !! XY !! XO !! ZW !! ZO !! Other<sup>1</sup> !! XO/XY ratio !! ZO/ZW ratio
|-
| Vertebrates || 722 || 15 || 480 || 3 || 254 || 0.02 || 0.01 
|-
| Insects || 4415 || 1857 || 37 || 25 || 156 || 0.42 || 0.68
|-
| Angiosperms || 23 || 0 || 1 || 0 || 19 || 0.00 || 0.00
|-
| Total || 5160 || 1872 || 518 || 28 || 429 || 0.36 || 0.05
|}

<sup>1. complex sex chromosomes, homomorphic sex chromosomes, or others</sup>

=== XX/XY sex chromosomes ===
[[File:Drosophila XY sex-determination.svg|thumb|Drosophila sex-chromosomes]]
[[File:Human male karyotpe high resolution - XY chromosome cropped.JPG|thumb|Human male XY chromosomes after [[G-banding]]]]
{{main|XY sex-determination system}}

The '''XX/XY sex-determination system''' is the most familiar, as it is found in humans. The XX/XY system is found in most other [[mammal]]s, as well as some insects. In this system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes ([[autosome]]s), and are sometimes called [[allosome]]s. In some species, such as humans, organisms remain sex indifferent for a time during development ([[embryogenesis]]); in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.<ref name="Hake-2008"/>

==== Y-centered sex determination ====

Some species (including humans) have a gene [[SRY]] on the Y chromosome that determines [[male]]ness. Members of SRY-reliant species can have uncommon XY chromosomal combinations such as [[Klinefelter syndrome|XXY]] and still live.<ref name="Hake-2008">{{cite journal |vauthors=Hake L |title=Genetic Mechanisms of Sex Determination |journal=Nature Education |year=2008 |volume=1 |issue=1 |url=http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314 |access-date=8 December 2011 |archive-date=19 August 2017 |archive-url=https://web.archive.org/web/20170819121941/http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314 |url-status=live }}</ref>
Human sex is determined by the presence or absence of a Y chromosome with a functional SRY gene. Once the SRY gene is activated, cells create [[testosterone]] and [[anti-müllerian hormone]] which typically ensures the development of a single, male reproductive system.<ref name="Hake-2008"/> In typical XX embryos, cells secrete [[estrogen]], which drives the body toward the female pathway.

In Y-centered sex determination, the SRY gene is the main gene in determining male characteristics, but multiple genes are required to develop testes. In XY mice, lack of the gene [[DAX1]] on the X chromosome results in sterility, but in humans it causes [[adrenal hypoplasia congenita]].<ref name="Goodfellow-1999">{{cite journal | vauthors = Goodfellow PN, Camerino G | title = DAX-1, an 'antitestis' gene | journal = Cellular and Molecular Life Sciences | volume = 55 | issue = 6–7 | pages = 857–863 | date = June 1999 | pmid = 10412368 | doi = 10.1007/PL00013201 | s2cid = 19764423 | pmc = 11147076 }}</ref> However, when an extra DAX1 gene is placed on the X chromosome, the result is a female, despite the existence of SRY, since it overrides the effects of SRY.<ref name="Chandra-1999">{{cite journal |author=Chandra, H. S. |title=Another way of looking at the enigma of sex determination in Ellobius lutescens |journal=Current Science |date=25 April 1999 |page=1072 |volume=76 |issue=8}}</ref> Even when there are normal sex chromosomes in XX females, duplication or expression of [[SOX9]] causes testes to develop.<ref name="Cox-2011">{{cite journal | vauthors = Cox JJ, Willatt L, Homfray T, Woods CG | title = A SOX9 duplication and familial 46,XX developmental testicular disorder | journal = The New England Journal of Medicine | volume = 364 | issue = 1 | pages = 91–93 | date = January 2011 | pmid = 21208124 | doi = 10.1056/NEJMc1010311 | doi-access = free }}</ref><ref name="Huang-1999">{{cite journal | vauthors = Huang B, Wang S, Ning Y, Lamb AN, Bartley J | title = Autosomal XX sex reversal caused by duplication of SOX9 | journal = American Journal of Medical Genetics | volume = 87 | issue = 4 | pages = 349–353 | date = December 1999 | pmid = 10588843 | doi = 10.1002/(SICI)1096-8628(19991203)87:4<349::AID-AJMG13>3.0.CO;2-N }}</ref> Gradual [[sex reversal]] in developed mice can also occur when the gene [[FOXL2]] is removed from females.<ref name="Uhlenhaut-2009">{{cite journal | vauthors = Uhlenhaut NH, Jakob S, Anlag K, Eisenberger T, Sekido R, Kress J, Treier AC, Klugmann C, Klasen C, Holter NI, Riethmacher D, Schütz G, Cooney AJ, Lovell-Badge R, Treier M | display-authors = 6 | title = Somatic sex reprogramming of adult ovaries to testes by FOXL2 ablation | journal = Cell | volume = 139 | issue = 6 | pages = 1130–1142 | date = December 2009 | pmid = 20005806 | doi = 10.1016/j.cell.2009.11.021 | doi-access = free }}</ref> Even though the gene [[DMRT1]] is used by birds as their sex locus, species who have XY chromosomes also rely upon DMRT1, contained on chromosome 9, for sexual differentiation at some point in their formation.<ref name="Hake-2008"/>

==== X-centered sex determination ====

Some species, such as [[Drosophila melanogaster|fruit flies]], use the presence of two X chromosomes to determine [[female]]ness.<ref name="Penalva-2003">{{cite journal | vauthors = Penalva LO, Sánchez L | title = RNA binding protein sex-lethal (Sxl) and control of Drosophila sex determination and dosage compensation | journal = Microbiology and Molecular Biology Reviews | volume = 67 | issue = 3 | pages = 343–59, table of contents | date = September 2003 | pmid = 12966139 | pmc = 193869 | doi = 10.1128/MMBR.67.3.343-359.2003 }}</ref> Species that use the number of Xs to determine sex are nonviable with an extra X chromosome.

==== Other variants of XX/XY sex determination ====

Some fish have variants of the [[XY sex-determination system]], as well as the regular system. For example, while having an XY format, ''[[Xiphophorus nezahualcoyotl]]'' and ''X. milleri'' also have a second Y chromosome, known as Y', that creates XY' females and YY' males.<ref name="Schartl-2004a">{{cite journal | vauthors = Schartl M | title = A comparative view on sex determination in medaka | journal = Mechanisms of Development | volume = 121 | issue = 7–8 | pages = 639–645 | date = July 2004 | pmid = 15210173 | doi = 10.1016/j.mod.2004.03.001 | s2cid = 17401686 | doi-access = free }}</ref>

At least one [[monotreme]], the [[platypus#Evolution|platypus]], presents a particular sex determination scheme that in some ways resembles that of the [[ZW sex-determination system|ZW sex chromosomes]] of birds and lacks the SRY gene. The [[platypus]] has sex chromosomes <math>X_1, X_2, X_3, X_4, X_5, Y_1, Y_2, Y_3, Y_4, Y_5</math>. The males have <math>X_1Y_1/X_2Y_2/X_3Y_3/X_4Y_4/X_5Y_5</math>, while females have <math>X_1X_1/X_2X_2/X_3X_3/X_4X_4/X_5X_5</math>. During meiosis, 5 of X form one chain, and 5 of Y form another chain. Thus, they behave effectively as a typical XY chromosomal system, except each of X and Y is broken into 5 parts, with the effect that [[Homologous recombination|recombinations]] occur very frequently at 4 particular points.<ref>{{Cite journal |last1=Gruetzner |first1=Frank |last2=Ashley |first2=Terry |last3=Rowell |first3=David M. |last4=Marshall Graves |first4=Jennifer A. |date=2006-04-01 |title=How did the platypus get its sex chromosome chain? A comparison of meiotic multiples and sex chromosomes in plants and animals |url=https://doi.org/10.1007/s00412-005-0034-4 |journal=Chromosoma |volume=115 |issue=2 |pages=75–88 |doi=10.1007/s00412-005-0034-4 |pmid=16344965 |issn=1432-0886}}</ref> One of the X chromosomes is homologous to the human X chromosome, and another is homologous to the bird Z chromosome.<ref>{{Cite journal |last1=Grützner |first1=Frank |last2=Rens |first2=Willem |last3=Tsend-Ayush |first3=Enkhjargal |last4=El-Mogharbel |first4=Nisrine |last5=O'Brien |first5=Patricia C. M. |last6=Jones |first6=Russell C. |last7=Ferguson-Smith |first7=Malcolm A. |last8=Marshall Graves |first8=Jennifer A. |date=December 2004 |title=In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes |url=https://www.nature.com/articles/nature03021 |journal=Nature |volume=432 |issue=7019 |pages=913–917 |doi=10.1038/nature03021 |pmid=15502814 |bibcode=2004Natur.432..913G |issn=1476-4687}}</ref>

Although it is an XY system, the platypus' sex chromosomes share no homologues with [[eutherian]] sex chromosomes.<ref name="Warren-2008">{{cite journal | vauthors = Warren WC, Hillier LW, Marshall Graves JA, Birney E, Ponting CP, Grützner F, Belov K, Miller W, Clarke L, Chinwalla AT, Yang SP, Heger A, Locke DP, Miethke P, Waters PD, Veyrunes F, Fulton L, Fulton B, Graves T, Wallis J, Puente XS, López-Otín C, Ordóñez GR, Eichler EE, Chen L, Cheng Z, Deakin JE, Alsop A, Thompson K, Kirby P, Papenfuss AT, Wakefield MJ, Olender T, Lancet D, Huttley GA, Smit AF, Pask A, Temple-Smith P, Batzer MA, Walker JA, Konkel MK, Harris RS, Whittington CM, Wong ES, Gemmell NJ, Buschiazzo E, Vargas Jentzsch IM, Merkel A, Schmitz J, Zemann A, Churakov G, Kriegs JO, Brosius J, Murchison EP, Sachidanandam R, Smith C, Hannon GJ, Tsend-Ayush E, McMillan D, Attenborough R, Rens W, Ferguson-Smith M, Lefèvre CM, Sharp JA, Nicholas KR, Ray DA, Kube M, Reinhardt R, Pringle TH, Taylor J, Jones RC, Nixon B, Dacheux JL, Niwa H, Sekita Y, Huang X, Stark A, Kheradpour P, Kellis M, Flicek P, Chen Y, Webber C, Hardison R, Nelson J, Hallsworth-Pepin K, Delehaunty K, Markovic C, Minx P, Feng Y, Kremitzki C, Mitreva M, Glasscock J, Wylie T, Wohldmann P, Thiru P, Nhan MN, Pohl CS, Smith SM, Hou S, Nefedov M, de Jong PJ, Renfree MB, Mardis ER, Wilson RK | display-authors = 6 | title = Genome analysis of the platypus reveals unique signatures of evolution | journal = Nature | volume = 453 | issue = 7192 | pages = 175–183 | date = May 2008 | pmid = 18464734 | pmc = 2803040 | doi = 10.1038/nature06936 | bibcode = 2008Natur.453..175W }}</ref> Instead, homologues with eutherian sex chromosomes lie on the platypus chromosome 6, which means that the eutherian sex chromosomes were [[autosomes]] at the time that the monotremes diverged from the therian mammals (marsupials and eutherian mammals). However, homologues to the avian [[DMRT1]] gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene for the platypus is the same one that is involved in bird sex-determination. More research must be conducted in order to determine the exact sex determining gene of the platypus.<ref name="Gruetzner-2006">{{cite journal | vauthors = Gruetzner F, Ashley T, Rowell DM, Marshall Graves JA | title = How did the platypus get its sex chromosome chain? A comparison of meiotic multiples and sex chromosomes in plants and animals | journal = Chromosoma | volume = 115 | issue = 2 | pages = 75–88 | date = April 2006 | pmid = 16344965 | doi = 10.1007/s00412-005-0034-4 | name-list-style = amp | s2cid = 23603889 }}</ref>

[[File:Critique of the Theory of Evolution Fig 060.svg |thumb|Heredity of sex chromosomes in XO sex determination]]

=== XX/X0 sex chromosomes ===
{{Main |X0 sex-determination system}}

In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The ''0'' denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order [[Orthoptera]] and in cockroaches (order [[cockroach|Blattodea]]). A small number of mammals also lack a Y chromosome. These include the Amami spiny rat (''[[Tokudaia osimensis]]'') and the Tokunoshima spiny rat (''[[Tokudaia tokunoshimensis]]'') and ''Sorex araneus'', a [[shrew]] species. Transcaucasian mole voles (''[[Ellobius lutescens]]'') also have a form of XO determination, in which both sexes lack a second sex chromosome.<ref name="Chandra-1999"/> The mechanism of sex determination is not yet understood.<ref name="Kuroiwa-2011">{{cite journal | vauthors = Kuroiwa A, Handa S, Nishiyama C, Chiba E, Yamada F, Abe S, Matsuda Y | title = Additional copies of CBX2 in the genomes of males of mammals lacking SRY, the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis) | journal = Chromosome Research | volume = 19 | issue = 5 | pages = 635–644 | date = July 2011 | pmid = 21656076 | doi = 10.1007/s10577-011-9223-6 | s2cid = 23311263 }}</ref>

The [[nematode]] ''[[Caenorhabditis elegans|C. elegans]]'' is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.<ref name="Majerus-2003">{{Cite book | vauthors = Majerus ME | title = Sex wars: genes, bacteria, and biased sex ratios | publisher = Princeton University Press | pages = 250 | url = https://books.google.com/books?id=vDHOYPQ2mmYC&q=zo,+zww,+zzww+lepidoptera | isbn = 978-0-691-00981-0 | access-date = 4 November 2011 | year = 2003 }}
</ref> Its main sex gene is XOL, which encodes [[XOL-1 Switch protein N-terminal domain|XOL-1]] and also controls the expression of the genes TRA-2 and HER-1. These genes reduce male gene activation and increase it, respectively.<ref name="Kuwabara-1992">{{cite journal | vauthors = Kuwabara PE, Okkema PG, Kimble J | title = tra-2 encodes a membrane protein and may mediate cell communication in the Caenorhabditis elegans sex determination pathway | journal = Molecular Biology of the Cell | volume = 3 | issue = 4 | pages = 461–473 | date = April 1992 | pmid = 1498366 | pmc = 275596 | doi = 10.1091/mbc.3.4.461 }}</ref>

=== ZW/ZZ sex chromosomes ===
{{Main |ZW sex-determination system}}

The '''ZW sex-determination system''' is found in birds, some reptiles, and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of [[chromosomes]] (ZW), and males have two of the same kind of chromosomes (ZZ). In the chicken, this was found to be dependent on the expression of DMRT1.<ref name="Smith-2009">{{cite journal | vauthors = Smith CA, Roeszler KN, Ohnesorg T, Cummins DM, Farlie PG, Doran TJ, Sinclair AH | title = The avian Z-linked gene DMRT1 is required for male sex determination in the chicken | journal = Nature | volume = 461 | issue = 7261 | pages = 267–271 | date = September 2009 | pmid = 19710650 | doi = 10.1038/nature08298 | s2cid = 4413389 | bibcode = 2009Natur.461..267S }}</ref> In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY.<ref name="Hake-2008"/> However, not all species depend upon the W for their sex. For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW.<ref name="Majerus-2003"/> Also, while mammals deactivate one of their extra X chromosomes when female, it appears that in the case of [[Lepidoptera]], the males produce double the normal amount of enzymes, due to having two Z's.<ref name="Majerus-2003"/> Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex.<ref name="Majerus-2003"/> However, reportedly, the silkworm ''Bombyx mori'' uses a single female-specific [[Piwi-interacting RNA|piRNA]] as the primary determiner of sex.<ref>{{cite journal | vauthors = Kiuchi T, Koga H, Kawamoto M, Shoji K, Sakai H, Arai Y, Ishihara G, Kawaoka S, Sugano S, Shimada T, Suzuki Y, Suzuki MG, Katsuma S | display-authors = 6 | title = A single female-specific piRNA is the primary determiner of sex in the silkworm | journal = Nature | volume = 509 | issue = 7502 | pages = 633–636 | date = May 2014 | pmid = 24828047 | doi = 10.1038/nature13315 | s2cid = 205238635 | bibcode = 2014Natur.509..633K }}</ref> Despite the similarities between the ZW and XY systems, these sex chromosomes evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9.<ref name="Stiglec-2007">{{cite journal | vauthors = Stiglec R, Ezaz T, Graves JA | title = A new look at the evolution of avian sex chromosomes | journal = Cytogenetic and Genome Research | volume = 117 | issue = 1–4 | pages = 103–109 | year = 2007 | pmid = 17675850 | doi = 10.1159/000103170 | s2cid = 12932564 }}</ref> The chicken's Z chromosome also seems to be related to the X chromosome of the platypus.<ref name="Grützner-2004">{{cite journal | vauthors = Grützner F, Rens W, Tsend-Ayush E, El-Mogharbel N, O'Brien PC, Jones RC, Ferguson-Smith MA, Marshall Graves JA | display-authors = 6 | title = In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes | journal = Nature | volume = 432 | issue = 7019 | pages = 913–917 | date = December 2004 | pmid = 15502814 | doi = 10.1038/nature03021 | name-list-style = amp | s2cid = 4379897 | bibcode = 2004Natur.432..913G }}</ref> When a ZW species, such as the [[Komodo dragon]], reproduces [[Parthenogenesis|parthenogenetically]], usually only males are produced. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term.<ref name="BBCNews-2006">{{cite news |work=BBC News |url=http://news.bbc.co.uk/2/hi/science/nature/6196225.stm |title=Virgin births for giant lizards |access-date=13 March 2008 |date=20 December 2006 |archive-date=4 November 2014 |archive-url=https://web.archive.org/web/20141104002358/http://news.bbc.co.uk/2/hi/science/nature/6196225.stm |url-status=live }}</ref>

In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.<ref>{{cite web|title=Evolution of the Y Chromosome|url=http://www.learner.org/channel/courses/biology/textbook/gender/gender_4.html|url-status=dead|archive-url=https://web.archive.org/web/20041104181945/http://www.learner.org/channel/courses/biology/textbook/gender/gender_4.html|archive-date=November 4, 2004|access-date=1 April 2008|website=Annenberg Media}}</ref>{{Better source needed|date=May 2021}}

=== ZZ/Z0 sex chromosomes ===
{{Main |Z0 sex-determination system}}

The '''ZZ/Z0 sex-determination system''' is found in some moths. In these insects there is one sex chromosome, Z. Males have two Z chromosomes, whereas females have one Z. Males are ZZ, while females are Z0.<ref name="Traut-2007">{{cite journal | vauthors = Traut W, Sahara K, Marec F | title = Sex chromosomes and sex determination in Lepidoptera | journal = Sexual Development | volume = 1 | issue = 6 | pages = 332–346 | date = 2007 | pmid = 18391545 | doi = 10.1159/000111765 | s2cid = 6885122 }}</ref><ref>{{cite web|url=http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314|title=Genetic Mechanisms of Sex Determination - Learn Science at Scitable|website=www.nature.com|access-date=8 December 2011|archive-date=19 August 2017|archive-url=https://web.archive.org/web/20170819121941/http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314|url-status=live}}</ref><ref>{{cite book|url=https://books.google.com/books?id=5w8FgSGuH34C&q=ZO+sex-determination+system+moth&pg=PA461|title=Handbuch Der Zoologie / Handbook of Zoology|publisher=Walter de Gruyter|via=Google Books|isbn=9783110162103|year=1925}}</ref>

===UV sex chromosomes===
In some [[bryophyte]] and some [[algae]] species, the [[gametophyte]] stage of the life cycle, rather than being hermaphrodite, occurs as separate male or female individuals that produce male and female gametes respectively. When meiosis occurs in the [[sporophyte]] generation of the life cycle, the sex chromosomes known as U and V assort in spores that carry either the U chromosome and give rise to female gametophytes, or the V chromosome and give rise to male gametophytes.<ref name="Bachtrog-2011">{{cite journal | vauthors = Bachtrog D, Kirkpatrick M, Mank JE, McDaniel SF, Pires JC, Rice W, Valenzuela N | title = Are all sex chromosomes created equal? | journal = Trends in Genetics | volume = 27 | issue = 9 | pages = 350–357 | date = September 2011 | pmid = 21962970 | doi = 10.1016/j.tig.2011.05.005 }}</ref><ref>{{cite journal |author1=Renner, S. S. |author2=Heinrichs, J. |author3=Sousa, A. |year=2017 |title= The sex chromosomes of bryophytes: Recent insights, open questions, and reinvestigations of Frullania dilatata and Plagiochila asplenioides. |journal=Journal of Systematics and Evolution |volume=55 |issue=4 |pages=333–339 |doi=10.1111/jse.12266|doi-access=free }}</ref>

=== Mating types ===
{{Main|Mating type}}
The [[mating type]] in [[microorganism]]s is analogous to sex in multi-cellular organisms, and is sometimes described using those terms, though they are not necessarily correlated with physical body structures. Some species have more than two mating types. ''[[Tetrahymena]],'' a type of [[ciliate]], has 7 mating types

Mating types are extensively studied in fungi. Among fungi, mating type is determined by chromosomal regions called [[Mating-type locus|mating-type loci]]. Furthermore, it is not as simple as "two different mating types can mate", but rather, a matter of combinatorics. As a simple example, most ''[[Basidiomycota|basidiomycete]]'' have a "tetrapolar [[heterothallism]]" mating system: there are two loci, and mating between two individuals is possible if the alleles on ''both'' loci are different. For example, if there are 3 alleles per locus, then there would be 9 mating types, each of which can mate with 4 other mating types.<ref>{{Cite journal |last1=Idnurm |first1=Alexander |last2=Hood |first2=Michael E. |last3=Johannesson |first3=Hanna |last4=Giraud |first4=Tatiana |date=2015-12-01 |title=Contrasted patterns in mating-type chromosomes in fungi: Hotspots versus coldspots of recombination |journal=Fungal Biology Reviews |series=Special Issue: Fungal sex and mushrooms – A credit to Lorna Casselton |volume=29 |issue=3 |pages=220–229 |doi=10.1016/j.fbr.2015.06.001 |pmid=26688691 |bibcode=2015FunBR..29..220I |issn=1749-4613|pmc=4680991 }}</ref> By multiplicative combination, it generates a vast number of mating types. For example, ''[[Schizophyllum commune]],'' a type of fungus, has <math>9 \times 32 \times 9 \times 9 = 23328</math> mating types.[[File:Haplodiploid-sex-determination-system3.png |thumb|Haplodiploid sex chromosomes]]

=== Haplodiploidy ===
{{main|Haplodiploidy}}

[[Haplodiploidy]] is found in insects belonging to [[Hymenoptera]], such as [[ant]]s and [[bee]]s. Sex determination is controlled by the [[zygosity]] of a complementary sex determiner (''csd'') locus. Unfertilized eggs develop into [[haploid]] individuals which have a single, hemizygous copy of the ''csd'' locus and are therefore males. Fertilized eggs develop into [[diploid]] individuals which, due to high variability in the ''csd'' locus, are generally heterozygous females. In rare instances diploid individuals may be homozygous, these develop into sterile males.
The gene acting as a ''csd'' locus has been identified in the [[honeybee]] and several candidate genes have been proposed as a ''csd'' locus for other Hymenopterans.<ref>{{cite journal | vauthors = Beye M, Hasselmann M, Fondrk MK, Page RE, Omholt SW | title = The gene csd is the primary signal for sexual development in the honeybee and encodes an SR-type protein | journal = Cell | volume = 114 | issue = 4 | pages = 419–429 | date = August 2003 | pmid = 12941271 | doi = 10.1016/S0092-8674(03)00606-8 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Privman E, Wurm Y, Keller L | title = Duplication and concerted evolution in a master sex determiner under balancing selection | journal = Proceedings. Biological Sciences | volume = 280 | issue = 1758 | pages = 20122968 | date = May 2013 | pmid = 23466984 | pmc = 3619454 | doi = 10.1098/rspb.2012.2968 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Miyakawa MO, Tsuchida K, Miyakawa H | title = The doublesex gene integrates multi-locus complementary sex determination signals in the Japanese ant, Vollenhovia emeryi | journal = Insect Biochemistry and Molecular Biology | volume = 94 | pages = 42–49 | date = March 2018 | pmid = 29408414 | doi = 10.1016/j.ibmb.2018.01.006 | bibcode = 2018IBMB...94...42M }}</ref>
Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their [[spermatheca]] and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony.<ref name="van Wilgenburg-2006">{{cite journal | vauthors = van Wilgenburg E, Driessen G, Beukeboom LW | title = Single locus complementary sex determination in Hymenoptera: an "unintelligent" design? | journal = Frontiers in Zoology | volume = 3 | issue = 1 | pages = 1 | date = January 2006 | pmid = 16393347 | pmc = 1360072 | doi = 10.1186/1742-9994-3-1 | doi-access = free }}</ref>

=== Polygenic sex determination ===
Polygenic sex determination is when the sex is primarily determined by genes that occur on multiple non-[[homologous chromosome]]s. The environment may have a limited, minor influence on sex determination. Examples include African cichlid fish (''[[Maylandia|Metriaclima]] spp.''), lemmings (''[[Wood lemming|Myopus schisticolor]]''), [[swordtail fish|green swordtail]],<ref name="Schartl-2004a" /> [[Japanese rice fish|medaka]],<ref name="Schartl-2004a" /> etc. In such systems, there is typically a dominance hierarchy, where one system is dominant over another if in conflict. For example, in some species of cichlid fish from Lake Malawi, if an individual has both the XY locus (on one chromosome pair) and the WZ locus (on another chromosome pair), then the W is dominant and the individual has a female phenotype.<ref>{{Cite journal |last1=Moore |first1=Emily C. |last2=Roberts |first2=Reade B. |date=June 2013 |title=Polygenic sex determination |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982213004120 |journal=Current Biology |volume=23 |issue=12 |pages=R510–R512 |doi=10.1016/j.cub.2013.04.004|pmid=23787041 |bibcode=2013CBio...23.R510M }}</ref>

The sex-determination system of [[zebrafish]] is polygenic. Juvenile zebrafishes (0–30 days after hatching) have both ovary-like tissue to testis tissue. They then develop into male or female adults, with the determination based on a complex interaction genes on multiple chromosomes, but not affected by environmental variations.<ref>{{Cite journal |last1=Liew |first1=Woei Chang |last2=Bartfai |first2=Richard |last3=Lim |first3=Zijie |last4=Sreenivasan |first4=Rajini |last5=Siegfried |first5=Kellee R. |last6=Orban |first6=Laszlo |date=2012-04-10 |editor-last=Alsina |editor-first=Berta |title=Polygenic Sex Determination System in Zebrafish |journal=PLOS ONE |volume=7 |issue=4 |pages=e34397 |doi=10.1371/journal.pone.0034397 |doi-access=free |issn=1932-6203 |pmc=3323597 |pmid=22506019|bibcode=2012PLoSO...734397L }}</ref><ref>{{Cite journal |last1=Liew |first1=Woei Chang |last2=Orbán |first2=László |date=March 2014 |title=Zebrafish sex: a complicated affair |journal=Briefings in Functional Genomics |volume=13 |issue=2 |pages=172–187 |doi=10.1093/bfgp/elt041 |issn=2041-2649 |pmc=3954038 |pmid=24148942}}</ref>

===Other chromosomal systems===
In systems with two sex chromosomes, they can be heteromorphic or homomorphic. Homomorphic sex chromosomes are almost identical in size and gene content. The two familiar kinds of sex chromosome pairs (XY and ZW) are heteromorphic. Homomorphic sex chromosomes exist among pufferfish, ratite birds, pythons, and European tree frogs. Some are quite old, meaning that there is some evolutionary force that resists their differentiation.<ref name="Wright-2016">{{Cite journal |last1=Wright |first1=Alison E. |last2=Dean |first2=Rebecca |last3=Zimmer |first3=Fabian |last4=Mank |first4=Judith E. |date=2016-07-04 |title=How to make a sex chromosome |journal=Nature Communications |volume=7 |issue=1 |pages=12087 |doi=10.1038/ncomms12087 |pmid=27373494 |pmc=4932193 |bibcode=2016NatCo...712087W |issn=2041-1723}}</ref> For example, three species of [[European tree frogs]] have homologous, homomorphic sex chromosomes, and this homomorphism was maintained for at least 5.4 million years by occasional recombination.<ref>{{Cite journal |last1=Stöck |first1=Matthias |last2=Horn |first2=Agnès |last3=Grossen |first3=Christine |last4=Lindtke |first4=Dorothea |last5=Sermier |first5=Roberto |last6=Betto-Colliard |first6=Caroline |last7=Dufresnes |first7=Christophe |last8=Bonjour |first8=Emmanuel |last9=Dumas |first9=Zoé |last10=Luquet |first10=Emilien |last11=Maddalena |first11=Tiziano |last12=Sousa |first12=Helena Clavero |last13=Martinez-Solano |first13=Iñigo |last14=Perrin |first14=Nicolas |date=2011-05-17 |editor-last=Rice |editor-first=William R. |title=Ever-Young Sex Chromosomes in European Tree Frogs |journal=PLOS Biology |volume=9 |issue=5 |pages=e1001062 |doi=10.1371/journal.pbio.1001062 |doi-access=free |issn=1545-7885 |pmc=3100596 |pmid=21629756}}</ref>

The ''[[Nematocera]]'', particularly the ''[[Black fly|Simuliids]]'' and [[Chironomidae|''Chironomus'']], have sex determination regions that are labile, meaning that one species may have the sex determination region in one chromosome, but a closely related species might have the same region moved to a different non-[[Sequence homology|homologous chromosome]]. Some species even have the sex determination region different among individuals ''within'' ''the same species'' ([[intraspecific variation]]).<ref>{{Cite journal |last1=Furman |first1=Benjamin L S |last2=Metzger |first2=David C H |last3=Darolti |first3=Iulia |last4=Wright |first4=Alison E |last5=Sandkam |first5=Benjamin A |last6=Almeida |first6=Pedro |last7=Shu |first7=Jacelyn J |last8=Mank |first8=Judith E |date=2020-06-01 |editor-last=Fraser |editor-first=Bonnie |title=Sex Chromosome Evolution: So Many Exceptions to the Rules |url=https://academic.oup.com/gbe/article/12/6/750/5823304 |journal=Genome Biology and Evolution |volume=12 |issue=6 |pages=750–763 |doi=10.1093/gbe/evaa081 |issn=1759-6653 |pmc=7268786 |pmid=32315410}}</ref><ref>{{Cite journal |last1=Martin |first1=Jon |last2=Lee |first2=B. T. O. |date=September 1984 |title=A phylogenetic study of sex determiner location in a group of Australasian Chironomus species (Diptera, Chironomidae) |url=http://link.springer.com/10.1007/BF00292396 |journal=Chromosoma |volume=90 |issue=3 |pages=190–197 |doi=10.1007/BF00292396 |issn=0009-5915}}</ref><ref>{{Cite journal |last1=Martin |first1=Jon |last2=Kuvangkadilok |first2=Chaliow |last3=Peart |first3=Dianne H. |last4=Lee |first4=Barry T. O. |date=June 1980 |title=Multiple sex determining regions in a group of related Chironomus species (Diptera:Chironomidae) |url=https://www.nature.com/articles/hdy198034 |journal=Heredity |volume=44 |issue=3 |pages=367–382 |doi=10.1038/hdy.1980.34 |issn=1365-2540}}</ref> In some species, some populations have homomorphic sex chromosomes while other populations have heteromorphic sex chromosomes.

The New Zealand frog, ''[[Hochstetter's frog|Leiopelma hochstetteri]]'', uses a [[B chromosome|supernumerary sex chromosome]]. With zero of that chromosome, the frog develops into a male. With one or more, the frog develops into a female. One female had as many as 16 of that chromosome.<ref>{{Cite journal |last=Green |first=David M. |date=1988-09-01 |title=Cytogenetics of the endemic New Zealand frog, Leiopelma hochstetteri: extraordinary supernumerary chromosome variation and a unique sex-chromosome system |url=https://doi.org/10.1007/BF00331795 |journal=Chromosoma |volume=97 |issue=1 |pages=55–70 |doi=10.1007/BF00331795 |issn=1432-0886}}</ref>

Different populations of the [[Japanese wrinkled frog|Japanese frog ''Rana rugosa'']] uses different systems. Two use homomorphic male heterogamety, one uses XX/XY, one uses ZZ/ZW. Remarkably, the X and Z chromosomes are homologous, and the Y and W as well. ''Dmrt1'' is on autosome 1 and not sex-linked. This means that an XX female individual is genetically similar to a ZZ male individual, and an XY male individual is to a ZW female individual. The mechanism behind this is yet unclear, but it is hypothesized that during its recent evolution, the XY-to-ZW transition occurred twice.<ref>{{Cite journal |last1=Uno |first1=Yoshinobu |last2=Nishida |first2=Chizuko |last3=Oshima |first3=Yuki |last4=Yokoyama |first4=Satoshi |last5=Miura |first5=Ikuo |last6=Matsuda |first6=Yoichi |last7=Nakamura |first7=Masahisa |date=June 2008 |title=Comparative chromosome mapping of sex-linked genes and identification of sex chromosomal rearrangements in the Japanese wrinkled frog (Rana rugosa, Ranidae) with ZW and XY sex chromosome systems |url=https://link.springer.com/10.1007/s10577-008-1217-7 |journal=Chromosome Research |volume=16 |issue=4 |pages=637–647 |doi=10.1007/s10577-008-1217-7 |pmid=18484182 |issn=0967-3849}}</ref><ref name="Graves-2008">{{Cite journal |last=Graves |first=Jennifer A. Marshall |date=2008-12-01 |title=Weird Animal Genomes and the Evolution of Vertebrate Sex and Sex Chromosomes |url=http://dx.doi.org/10.1146/annurev.genet.42.110807.091714 |journal=Annual Review of Genetics |volume=42 |issue=1 |pages=565–586 |doi=10.1146/annurev.genet.42.110807.091714 |issn=0066-4197}}</ref>

''[[Clarias gariepinus]]'' uses both XX/XY and ZW/ZZ system within the species, with some populations using homomorphic XX/XY while others using heteromorphic ZW/ZZ. A population in Thailand appears to use both systems simultaneously, possibly because ''C. gariepinus'' were not native to Thailand, and were introduced from different source populations which resulted in a mixture.<ref>{{Cite journal |last1=Nguyen |first1=Dung Ho My |last2=Panthum |first2=Thitipong |last3=Ponjarat |first3=Jatupong |last4=Laopichienpong |first4=Nararat |last5=Kraichak |first5=Ekaphan |last6=Singchat |first6=Worapong |last7=Ahmad |first7=Syed Farhan |last8=Muangmai |first8=Narongrit |last9=Peyachoknagul |first9=Surin |last10=Na-Nakorn |first10=Uthairat |last11=Srikulnath |first11=Kornsorn |date=2021-01-05 |title=An Investigation of ZZ/ZW and XX/XY Sex Determination Systems in North African Catfish (Clarias gariepinus, Burchell, 1822) |journal=Frontiers in Genetics |volume=11 |doi=10.3389/fgene.2020.562856 |doi-access=free |pmid=33584785 |issn=1664-8021|pmc=7874028 }}</ref>

Multiple sex chromosomes like those of platypus also occurs in bony fish.<ref>{{Cite journal |last1=Sember |first1=Alexandr |last2=Nguyen |first2=Petr |last3=Perez |first3=Manolo F. |last4=Altmanová |first4=Marie |last5=Ráb |first5=Petr |last6=Cioffi |first6=Marcelo de Bello |date=2021-09-13 |title=Multiple sex chromosomes in teleost fishes from a cytogenetic perspective: state of the art and future challenges |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=376 |issue=1833 |pages=20200098 |doi=10.1098/rstb.2020.0098 |issn=0962-8436 |pmc=8310710 |pmid=34304595}}</ref> Some moths and butterflies have <math>W_1W_2Z \text {♀}/ZZ \text{♂}</math> or <math>WZ_1Z_2 \text {♀}/Z_1Z_1Z_2Z_2 \text{♂}</math>.<ref>{{Cite journal |last1=Traut |first1=W. |last2=Sahara |first2=K. |last3=Marec |first3=F. |date=2008-01-18 |title=Sex Chromosomes and Sex Determination in Lepidoptera |url=https://doi.org/10.1159/000111765 |journal=Sexual Development |volume=1 |issue=6 |pages=332–346 |doi=10.1159/000111765 |pmid=18391545 |issn=1661-5425}}</ref>

The [[Southern platyfish#Sex|Southern platyfish]] has a complex sex determination system involving 3 sex chromosomes and 4 autosomal alleles.<ref>{{Citation |last=Kallman |first=Klaus D. |title=A New Look at Sex Determination in Poeciliid Fishes |date=1984 |work=Evolutionary Genetics of Fishes |pages=95–171 |editor-last=Turner |editor-first=Bruce J. |url=https://doi.org/10.1007/978-1-4684-4652-4_3 |access-date=2024-08-04 |place=Boston, MA |publisher=Springer US |doi=10.1007/978-1-4684-4652-4_3 |isbn=978-1-4684-4652-4}}</ref><ref name="Schartl-2004b">{{cite journal |vauthors=Schartl M |date=July 2004 |title=A comparative view on sex determination in medaka |journal=Mechanisms of Development |volume=121 |issue=7–8 |pages=639–645 |doi=10.1016/j.mod.2004.03.001 |pmid=15210173 |s2cid=17401686 |doi-access=free}}</ref>

''[[Gastrotheca pseustes]]'' has <math>XY_b \text {♀} / XY_a \text{♂}</math> [[Karyotype|C-banding]] heteromorphism, meaning that both males and females have XY chromosomes, but their Y chromosomes are different on one or more C-bands. ''[[Strabomantis biporcatus|Eleutherodactylus maussi]]'' has a <math>\mathrm{X}_1 \mathrm{X}_1 \mathrm{X}_2 \mathrm{X}_2 \text {♀} / \mathrm{X}_1 \mathrm{X}_2 \mathrm{Y} \text{♂}</math> system.<ref>{{Cite journal |last1=Schmid |first1=M. |last2=Steinlein |first2=C. |last3=Feichtinger |first3=W. |date=March 1992 |title=Chromosome banding in amphibia: XVII. First demonstration of multiple sex chromosomes in amphibians: Eleutherodactylus maussi (Anura, Leptodactylidae) |url=http://link.springer.com/10.1007/BF00346007 |journal=Chromosoma |volume=101 |issue=5–6 |pages=284–292 |doi=10.1007/BF00346007 |pmid=1576881 |issn=0009-5915}}</ref><ref>{{Cite journal |last1=Schmid |first1=M. |last2=Feichtinger |first2=W. |last3=Steinlein |first3=C. |last4=Haaf |first4=T. |last5=Schartl |first5=M. |last6=Visbal García |first6=R. |last7=Manzanilla Pupo |first7=J. |last8=Fernández Badillo |first8=A. |date=2003-08-14 |title=Chromosome banding in Amphibia: XXVI. Coexistence of homomorphic XY sex chromosomes and a derived Y-autosome translocation in Eleutherodactylus maussi (Anura, Leptodactylidae) |url=https://doi.org/10.1159/000071612 |journal=Cytogenetics and Cell Genetics |volume=99 |issue=1–4 |pages=330–343 |doi=10.1159/000071612 |pmid=12900583 |issn=0301-0171}}</ref>

== Evolution ==
See <ref name="Zhu-2024">{{Cite journal |last1=Zhu |first1=Zexian |last2=Younas |first2=Lubna |last3=Zhou |first3=Qi |date=2024-07-18 |title=Evolution and regulation of animal sex chromosomes |url=https://www.nature.com/articles/s41576-024-00757-3 |journal=Nature Reviews Genetics |pages=1–16 |doi=10.1038/s41576-024-00757-3 |pmid=39026082 |issn=1471-0064}}</ref> for a review.

=== Origin of sex chromosomes ===
Sexual chromosome pairs can arise from an autosomal pair that, for various reasons, stopped recombination, allowing for their divergence. The rate at which recombination is suppressed, and therefore the rate of sex chromosome divergence, is very different across [[clade]]s.<ref name="Wright-2016" />

In analogy with [[Stratum|geological strata]], historical events in the evolution of sex chromosomes are called evolutionary strata. The human Y-chromosome has had about 5 strata since the origin of the X and Y chromosomes about 300 Mya from a pair of autosomes. Each stratum was formed when a [[pseudoautosomal region]] (PAR) of the Y chromosome is [[Chromosomal inversion|inverted]], stopping it from recombination with the X chromosome. Over time, each inverted region decays, possibly due to [[Muller's ratchet]].<ref>{{Cite journal |last1=Lahn |first1=Bruce T. |last2=Page |first2=David C. |date=1999-10-29 |title=Four Evolutionary Strata on the Human X Chromosome |url=https://www.science.org/doi/10.1126/science.286.5441.964 |journal=Science |volume=286 |issue=5441 |pages=964–967 |doi=10.1126/science.286.5441.964 |pmid=10542153 |issn=0036-8075}}</ref><ref>{{Cite journal |last1=Lemaitre |first1=Claire |last2=Braga |first2=Marilia D. V. |last3=Gautier |first3=Christian |last4=Sagot |first4=Marie-France |last5=Tannier |first5=Eric |last6=Marais |first6=Gabriel A. B. |date=2009-01-01 |title=Footprints of Inversions at Present and Past Pseudoautosomal Boundaries in Human Sex Chromosomes |journal=Genome Biology and Evolution |volume=1 |pages=56–66 |doi=10.1093/gbe/evp006 |issn=1759-6653 |pmc=2817401 |pmid=20333177}}</ref> Primate Y-chromosome evolution was rapid, with multiple inversions and shifts of the boundary of PAR.<ref>{{Cite journal |last1=Zhou |first1=Yang |last2=Zhan |first2=Xiaoyu |last3=Jin |first3=Jiazheng |last4=Zhou |first4=Long |last5=Bergman |first5=Juraj |last6=Li |first6=Xuemei |last7=Rousselle |first7=Marjolaine Marie C. |last8=Belles |first8=Meritxell Riera |last9=Zhao |first9=Lan |last10=Fang |first10=Miaoquan |last11=Chen |first11=Jiawei |last12=Fang |first12=Qi |last13=Kuderna |first13=Lukas |last14=Marques-Bonet |first14=Tomas |last15=Kitayama |first15=Haruka |date=July 2023 |title=Eighty million years of rapid evolution of the primate Y chromosome |url=https://www.nature.com/articles/s41559-022-01974-x |journal=Nature Ecology & Evolution |volume=7 |issue=7 |pages=1114–1130 |doi=10.1038/s41559-022-01974-x |pmid=37268856 |bibcode=2023NatEE...7.1114Z |issn=2397-334X}}</ref>

Among many species of the [[salamander]]s, the two chromosomes are only distinguished by a [[pericentric inversion]], so that the banding pattern of the X chromosome is the same as that of Y, but with a region near the centromere reversed. (fig 7 <ref name="Solari-1994">{{Cite book |last=Solari |first=Alberto J. |title=Sex chromosomes and sex determination in vertebrates |date=1994 |publisher=CRC Press |isbn=978-0-8493-4571-5 |location=Boca Raton}}</ref>) In some species, the X is pericentrically inverted and the Y is ancestral. In other species it is the opposite. (p.&nbsp;15 <ref name="Solari-1994" />)

The gene content of the X chromosome is almost identical among placental mammals. This is hypothesized to be because the X inactivation means any change would cause serious disruption, thus subjecting it to strong purifying selection. Similarly, birds have highly conserved Z chromosomes.<ref name="Graves-2008" />

=== Neo-sex chromosomes ===
'''Neo-sex chromosomes''' are currently existing sex chromosomes that formed when an autosome pair fused to the previously existing sex chromosome pair. Following this fusion, the autosomal portion undergoes recombination suppression, allowing them to differentiate. Such systems have been observed in insects, reptiles, birds, and mammals. They are useful to the study of the evolution of Y chromosome degeneration and dosage compensation.<ref>{{Cite journal |last1=Zhou |first1=Qi |last2=Wang |first2=Jun |last3=Huang |first3=Ling |last4=Nie |first4=Wenhui |last5=Wang |first5=Jinhuan |last6=Liu |first6=Yan |last7=Zhao |first7=Xiangyi |last8=Yang |first8=Fengtang |last9=Wang |first9=Wen |date=2008-06-14 |title=Neo-sex chromosomes in the black muntjac recapitulate incipient evolution of mammalian sex chromosomes |journal=Genome Biology |volume=9 |issue=6 |pages=R98 |doi=10.1186/gb-2008-9-6-r98 |doi-access=free |issn=1474-760X |pmc=2481430 |pmid=18554412}}</ref><ref>{{Cite journal |last1=Pala |first1=I. |last2=Naurin |first2=S. |last3=Stervander |first3=M. |last4=Hasselquist |first4=D. |last5=Bensch |first5=S. |last6=Hansson |first6=B. |date=March 2012 |title=Evidence of a neo-sex chromosome in birds |journal=Heredity |volume=108 |issue=3 |pages=264–272 |doi=10.1038/hdy.2011.70 |pmid=21897438 |pmc=3282394 |issn=1365-2540}}</ref>

=== Sex-chromosome turnover ===
The '''sex-chromosome turnover''' is an evolutionary phenomenon where sex chromosomes disappear, or becomes autosomal, and autosomal chromosomes become sexual, repeatedly over evolutionary time. Some lineages have extensive turnover, but others don't. Generally, in an XY system, if the Y chromosome is degenerate, mostly different from the X chromosome, and has X [[Sex-chromosome dosage compensation|dosage compensation]], then turnover is unlikely. In particular, this applies to humans.<ref>{{Cite journal |last=Vicoso |first=Beatriz |date=December 2019 |title=Molecular and evolutionary dynamics of animal sex-chromosome turnover |url=https://www.nature.com/articles/s41559-019-1050-8 |journal=Nature Ecology & Evolution |volume=3 |issue=12 |pages=1632–1641 |doi=10.1038/s41559-019-1050-8 |bibcode=2019NatEE...3.1632V |issn=2397-334X}}</ref><ref name="Zhu-2024" /><ref>{{Cite journal |last1=Palmer |first1=Daniela H. |last2=Rogers |first2=Thea F. |last3=Dean |first3=Rebecca |last4=Wright |first4=Alison E. |date=November 2019 |title=How to identify sex chromosomes and their turnover |journal=Molecular Ecology |volume=28 |issue=21 |pages=4709–4724 |doi=10.1111/mec.15245 |issn=0962-1083 |pmc=6900093 |pmid=31538682|bibcode=2019MolEc..28.4709P }}</ref>

The ZW and XY systems can evolve into to each other due to [[sexual conflict]].<ref>{{Cite journal |last1=van Doorn |first1=G Sander |last2=Kirkpatrick |first2=Mark |date=2010-10-01 |title=Transitions Between Male and Female Heterogamety Caused by Sex-Antagonistic Selection |journal=Genetics |volume=186 |issue=2 |pages=629–645 |doi=10.1534/genetics.110.118596 |issn=1943-2631 |pmc=2954476 |pmid=20628036}}</ref>

=== Homomorphism and the fountain of youth ===
It is an evolutionary puzzle why certain sex chromosomes remain homomorphic over millions of years, especially among lineages of fishes, amphibians, and nonavian reptiles. The fountain-of-youth model states that heteromorphy results from recombination suppression, and recombination suppression results from the male phenotype, not the sex chromosomes themselves. Therefore, if some XY sex-reversed females are fertile and adaptive under some circumstances, then the X and Y chromosomes would recombine in these individuals, preventing Y chromosome decay and maintaining long-term homomorphism.<ref>{{Cite journal |last=Perrin |first=Nicolas |date=December 2009 |title=Sex Reversal: A Fountain of Youth for Sex Chromosomes? |url=https://academic.oup.com/evolut/article/63/12/3043/6881025 |journal=Evolution |volume=63 |issue=12 |pages=3043–3049 |doi=10.1111/j.1558-5646.2009.00837.x|pmid=19744117 }}</ref>

[[Sex reversal]] denotes a situation where the phenotypic sex is different from the genotypic sex. While in humans, [[Disorders of sex development|sex reversal]] (such as the [[XX male syndrome]]) are often infertile, sex-reversed individuals of some species are fertile under some conditions. For example, some XY-individuals in population of [[Chinook salmon]] in the [[Columbia River]] became fertile females, producing YY sons. Since Chinook salmons have homomorphic sex chromosomes, such YY sons are healthy. When YY males mate with XX females, all their progeny would be XY male if grown under normal conditions.<ref>{{Cite journal |last1=Nagler |first1=J J |last2=Bouma |first2=J |last3=Thorgaard |first3=G H |last4=Dauble |first4=D D |date=January 2001 |title=High incidence of a male-specific genetic marker in phenotypic female chinook salmon from the Columbia River. |journal=Environmental Health Perspectives |volume=109 |issue=1 |pages=67–69 |doi=10.1289/ehp.0110967 |issn=0091-6765 |pmc=1242053 |pmid=11171527}}</ref>

Support for the hypothesis is found in the [[common frog]], for which XX males and XY males both suppresses sex chromosome recombination, but XX and XY females both recombine at the same rate.<ref>{{Cite journal |last1=Rodrigues |first1=Nicolas |last2=Studer |first2=Tania |last3=Dufresnes |first3=Christophe |last4=Perrin |first4=Nicolas |date=2018-04-01 |title=Sex-Chromosome Recombination in Common Frogs Brings Water to the Fountain-of-Youth |url=https://academic.oup.com/mbe/article/35/4/942/4823133 |journal=Molecular Biology and Evolution |volume=35 |issue=4 |pages=942–948 |doi=10.1093/molbev/msy008 |issn=0737-4038}}</ref>

==Environmental systems==
{{main |Environmental sex determination}}

===Temperature-dependent===
{{Main |Temperature-dependent sex determination}}
[[File:Alligator.jpg |thumb|right |All alligators determine the sex of their offspring by the temperature of the nest.]]

Many other sex-determination systems exist. In some species of reptiles, including [[alligator]]s, some [[turtle]]s, and the [[tuatara]], sex is determined by the temperature at which the egg is incubated during a temperature-sensitive period. There are no examples of temperature-dependent sex determination (TSD) in birds. [[Megapodes]] had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex.<ref>{{cite journal | vauthors = Göth A, Booth DT | title = Temperature-dependent sex ratio in a bird | journal = Biology Letters | volume = 1 | issue = 1 | pages = 31–33 | date = March 2005 | pmid = 17148121 | pmc = 1629050 | doi = 10.1098/rsbl.2004.0247 }}</ref> For some species with TSD, sex determination is achieved by exposure to hotter temperatures resulting in the offspring being one sex and cooler temperatures resulting in the other. This type of TSD is called ''Pattern&nbsp;I''. For others species using TSD, it is exposure to temperatures on both extremes that results in offspring of one sex, and exposure to moderate temperatures that results in offspring of the opposite sex, called ''Pattern&nbsp;II'' TSD. The specific temperatures required to produce each sex are known as the female-promoting temperature and the male-promoting temperature.<ref name="Torres Maldonado-2002">{{cite journal | vauthors = Torres Maldonado LC, Landa Piedra A, Moreno Mendoza N, Marmolejo Valencia A, Meza Martínez A, Merchant Larios H | title = Expression profiles of Dax1, Dmrt1, and Sox9 during temperature sex determination in gonads of the sea turtle Lepidochelys olivacea | journal = General and Comparative Endocrinology | volume = 129 | issue = 1 | pages = 20–26 | date = October 2002 | pmid = 12409092 | doi = 10.1016/s0016-6480(02)00511-7 }}</ref> When the temperature stays near the threshold during the temperature sensitive period, the [[sex ratio]] is varied between the two sexes.<ref name="Bull-1980">{{cite journal | vauthors = Bull JJ |title=Sex Determination in Reptiles |journal=The Quarterly Review of Biology |date=March 1980 |volume=55 |issue=1 |pages=3–21 |jstor=2826077 |doi=10.1086/411613|s2cid=85177125 }}</ref> Some species' temperature standards are based on when a particular enzyme is created. These species that rely upon temperature for their sex determination do not have the [[SRY gene]], but have other genes such as [[DAX1]], [[DMRT1]], and [[SOX9]] that are expressed or not expressed depending on the temperature.<ref name="Torres Maldonado-2002"/> The sex of some species, such as the [[Nile tilapia]], [[skink|Australian skink lizard]], and [[Agamidae|Australian dragon lizard]], has an initial bias, set by chromosomes, but can later be changed by the temperature of incubation.<ref name="Schartl-2004a"/>

It is unknown how exactly temperature-dependent sex determination evolved.<ref name="Valenzuela-2001">{{cite journal |vauthors=Valenzuela N, Janzen FJ |title=Nest-site philopatry and the evolution of temperature-dependent sex determination |journal=Evolutionary Ecology Research |year=2001 |volume=3 |pages=779–794 |url=http://www.public.iastate.edu/~fjanzen/pdf/01EvolEcolRes.pdf |access-date=7 December 2011 |archive-date=4 May 2013 |archive-url=https://web.archive.org/web/20130504051408/http://www.public.iastate.edu/~fjanzen/pdf/01EvolEcolRes.pdf |url-status=live }}</ref> It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season.<ref name="Valenzuela-2001"/> 
In amniotes, environmental sex determination preceded the genetically determined systems of birds and mammals; it is thought that a temperature-dependent [[amniote]] was the [[common ancestor]] of amniotes with sex chromosomes.<ref>{{cite journal | vauthors = Janzen FJ, Phillips PC | title = Exploring the evolution of environmental sex determination, especially in reptiles | journal = Journal of Evolutionary Biology | volume = 19 | issue = 6 | pages = 1775–1784 | date = November 2006 | pmid = 17040374 | doi = 10.1111/j.1420-9101.2006.01138.x | s2cid = 15485510 }}</ref>

===Other environmental systems===
There are other [[environmental sex determination]] systems including location-dependent determination systems as seen in the marine worm ''[[Bonellia viridis]]'' – larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, [[Bonellia viridis#Bonnelin as a biocide|bonellin]].<ref name="Gilbert-2006">{{cite book | vauthors = Gilbert S |title=Developmental biology |url=https://archive.org/details/developmentalbio00gilb_292 |url-access=limited |date=2006 |publisher=Sinauer Associates, Inc. Publishers |location=Sunderland, Mass. |isbn=9780878932504 |pages=[https://archive.org/details/developmentalbio00gilb_292/page/n569 550]–553 |edition= 8th.}}</ref> Some species, such as some [[snail]]s, practice [[Sequential hermaphroditism|sex change]]: adults start out male, then become female. In tropical [[clownfish]], the dominant individual in a group becomes female while the other ones are male, and bluehead wrasses (''[[Thalassoma bifasciatum]]'') are the reverse. 

[[File:ClownFishCycle.jpg|thumb|Life cycle of clownfish]]

Clownfish live in colonies of several small undifferentiated fish and two large fish (male and female). The male and female are the only sexually mature fish to reproduce. Clownfish are protandrous hermaphrodites, which means after they mature into males, they eventually can transform into females. They develop undifferentiated until they are needed to fill a certain role in their environment, i.e., if they receive the social and environmental cues to do so. <ref>Casas, L., Saborido-Rey, F., Ryu, T., Michell, C., Ravasi, T., & Irigoien, X. (2016). Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis. Scientific Reports, 6(1). https://doi.org/10.1038/srep35461</ref>

Some species, however, have no sex-determination system. Hermaphrodite species include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by [[parthenogenesis]] and are female altogether. There are some reptiles, such as the [[boa constrictor]] and [[Komodo dragon]] that can reproduce both sexually and asexually, depending on whether a mate is available.<ref name="Watts-2006">{{cite journal | vauthors = Watts PC, Buley KR, Sanderson S, Boardman W, Ciofi C, Gibson R | title = Parthenogenesis in Komodo dragons | journal = Nature | volume = 444 | issue = 7122 | pages = 1021–1022 | date = December 2006 | pmid = 17183308 | doi = 10.1038/4441021a | name-list-style = amp | s2cid = 4311088 | bibcode = 2006Natur.444.1021W }}</ref>

== Others ==
There are exceptional sex-determination systems, neither genetic nor environmental.

=== Cytoplasmic sex determination ===
The ''[[Wolbachia]]'' genus of parasitic bacteria lives inside the cytoplasm of its host, and is [[Vertically transmitted infection|vertically transmitted]] from parents to children. They primarily infect arthropods and nematodes. Different ''Wolbachia'' can [[Wolbachia#Method of sexual differentiation in hosts|alter the reproductive abilities]] of its host by a variety of means, including cytoplasmic incompatibility, parthenogenesis, feminization and embryonic male killing.<ref>{{Cite journal |last1=Stevens |first1=Lori |last2=Giordano |first2=Rosanna |last3=Fialho |first3=Roberto F. |date=November 2001 |title=Male-Killing, Nematode Infections, Bacteriophage Infection, and Virulence of Cytoplasmic Bacteria in the Genus Wolbachia |url=https://www.annualreviews.org/doi/10.1146/annurev.ecolsys.32.081501.114132 |journal=Annual Review of Ecology and Systematics |language=en |volume=32 |issue=1 |pages=519–545 |doi=10.1146/annurev.ecolsys.32.081501.114132 |issn=0066-4162}}</ref>

''[[Cytoplasmic male sterility|Mitochondrial male sterility]]'': In many flowering plants, the mitochondria can cause hermaphrodite individuals to be unable to father offspring, effectively turning them into exclusive females. This is a form of [[Mother's curse|mother’s curse]]. It is an evolutionarily adaptive strategy for mitochondria as mitochondria are inherited exclusively from mother to offspring.<ref name="Bachtrog-2014" /> The first published case of mitochondrial male sterility among metazoans was reported in 2022 in the hermaphroditic snail ''[[Physa acuta]]''.<ref>{{Cite journal |last1=David |first1=Patrice |last2=Degletagne |first2=Cyril |last3=Saclier |first3=Nathanaëlle |last4=Jennan |first4=Aurel |last5=Jarne |first5=Philippe |last6=Plénet |first6=Sandrine |last7=Konecny |first7=Lara |last8=François |first8=Clémentine |last9=Guéguen |first9=Laurent |last10=Garcia |first10=Noéline |last11=Lefébure |first11=Tristan |last12=Luquet |first12=Emilien |date=May 2022 |title=Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982222005838 |journal=Current Biology |language=en |volume=32 |issue=10 |pages=2325–2333.e6 |doi=10.1016/j.cub.2022.04.014}}</ref>

=== Paternal genome elimination ===
In some species of insects, springtails and mites, male offspring lose their paternal genome (in whole or in part) during development or in the germline. Males can either be diploid, diploid with missing sex chromosome, functionally haploid or truly haploid, depending on the mechanism of elimination.<ref>{{Cite journal |last=Herbette |first=Marion |last2=Ross |first2=Laura |date=2023 |title=Paternal genome elimination: patterns and mechanisms of drive and silencing |url=https://linkinghub.elsevier.com/retrieve/pii/S0959437X2300045X |journal=Current Opinion in Genetics & Development |language=en |volume=81 |pages=102065 |doi=10.1016/j.gde.2023.102065|doi-access=free }}</ref><ref name="Bachtrog-2014" />

=== Monogeny ===
In some species of [[Hymenoptera]] (ants, bees and wasps), flies and crustaceans, all offspring of a particular individual female are either exclusively male or exclusively female.<ref name="Bachtrog-2014" /> The underlying mechanisms are diverse and include maternally controlled paternal genome elimination and Mendelian inherited maternal sex-determining factors.<ref>{{Cite journal |last=Baird |first=Robert B. |last2=Mongue |first2=Andrew J. |last3=Ross |first3=Laura |date=2023 |title=Why put all your eggs in one basket? Evolutionary perspectives on the origins of monogenic reproduction |url=https://www.nature.com/articles/s41437-023-00632-7 |journal=Heredity |language=en |volume=131 |issue=2 |pages=87–95 |doi=10.1038/s41437-023-00632-7 |issn=0018-067X |pmc=10382564 |pmid=37328587}}</ref>

== Evolution ==

[[File:A region in the pseudoautosomal region of the short arms of the X- and Y-chromosome.jpg |thumb |left |The ends of the XY chromosomes in a human cell in [[metaphase]], highlighted here in green, are all that is left of the original autosomes that can still [[Chromosomal crossover|cross over]] with each other.]]

Sex determination systems may have evolved from [[mating type]], which is a feature of [[microorganism]]s.

Chromosomal sex determination may have evolved early in the history of eukaryotes.<ref name="Lehtonen-2014">{{cite journal | vauthors = Lehtonen J, Parker GA | title = Gamete competition, gamete limitation, and the evolution of the two sexes | journal = Molecular Human Reproduction | volume = 20 | issue = 12 | pages = 1161–1168 | date = December 2014 | pmid = 25323972 | doi = 10.1093/molehr/gau068 | doi-access = free }}</ref> But in plants it has been suggested to have evolved recently.<ref>{{Cite book | vauthors = Clarke R, Merlin M |url=https://books.google.com/books?id=bs4hEAAAQBAJ&q=XY+sex+determination+in+plants&pg=PA359 |title=Cannabis: Evolution and Ethnobotany |date=2016-06-28 |publisher=Univ of California Press |isbn=978-0-520-29248-2 |pages=359 }}</ref>

The accepted hypothesis of XY and ZW sex chromosome evolution in amniotes is that they evolved at the same time, in two different branches.<ref name="Namekawa-2009">{{cite journal | vauthors = Namekawa SH, Lee JT | title = XY and ZW: is meiotic sex chromosome inactivation the rule in evolution? | journal = PLOS Genetics | volume = 5 | issue = 5 | pages = e1000493 | date = May 2009 | pmid = 19461890 | pmc = 2679206 | doi = 10.1371/journal.pgen.1000493 | doi-access = free }}</ref><ref name="Vallender-2006">{{cite journal | vauthors = Vallender EJ, Lahn BT | title = Multiple independent origins of sex chromosomes in amniotes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 48 | pages = 18031–18032 | date = November 2006 | pmid = 17116892 | pmc = 1838700 | doi = 10.1073/pnas.0608879103 | doi-access = free | bibcode = 2006PNAS..10318031V }}</ref>

No genes are shared between the avian ZW and mammal XY chromosomes<ref name="Stiglec-2007"/> and the chicken Z chromosome is similar to the human [[autosomal]] chromosome 9, rather than X or Y. This suggests not that the ZW and XY sex-determination systems share an origin but that the sex chromosomes are derived from autosomal chromosomes of the [[Common descent|common ancestor]] of birds and mammals. In the [[platypus]], a [[monotreme]], the X<sub>1</sub> chromosome shares homology with [[theria]]n mammals, while the X<sub>5</sub> chromosome contains an avian sex-determination gene, further suggesting an evolutionary link.<ref>{{cite journal | vauthors = Veyrunes F, Waters PD, Miethke P, Rens W, McMillan D, Alsop AE, Grützner F, Deakin JE, Whittington CM, Schatzkamer K, Kremitzki CL, Graves T, Ferguson-Smith MA, Warren W, Marshall Graves JA | display-authors = 6 | title = Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes | journal = Genome Research | volume = 18 | issue = 6 | pages = 965–973 | date = June 2008 | pmid = 18463302 | pmc = 2413164 | doi = 10.1101/gr.7101908 }}</ref>

However, there is some evidence to suggest that there could have been transitions between ZW and XY, such as in ''[[Xiphophorus maculatus]]'', which have both ZW and XY systems in the same population, despite the fact that ZW and XY have different gene locations.<ref name="Graves-2000">{{cite journal | vauthors = Marshall Graves JA | title = Human Y chromosome, sex determination, and spermatogenesis- a feminist view | journal = Biology of Reproduction | volume = 63 | issue = 3 | pages = 667–676 | date = September 2000 | pmid = 10952906 | doi = 10.1095/biolreprod63.3.667b | doi-access = free }}</ref><ref name="Ezaz-2006">{{cite journal | vauthors = Ezaz T, Stiglec R, Veyrunes F, Marshall Graves JA | title = Relationships between vertebrate ZW and XY sex chromosome systems | journal = Current Biology | volume = 16 | issue = 17 | pages = R736–R743 | date = September 2006 | pmid = 16950100 | doi = 10.1016/j.cub.2006.08.021 | bibcode = 2006CBio...16.R736E | hdl-access = free | s2cid = 18864471 | hdl = 1885/37887 }}</ref> A recent theoretical model raises the possibility of both transitions between the XY/XX and ZZ/ZW system and environmental sex determination<ref>{{cite journal | vauthors = Quinn AE, Sarre SD, Ezaz T, Marshall Graves JA, Georges A | title = Evolutionary transitions between mechanisms of sex determination in vertebrates | journal = Biology Letters | volume = 7 | issue = 3 | pages = 443–448 | date = June 2011 | pmid = 21212104 | pmc = 3097877 | doi = 10.1098/rsbl.2010.1126 }}</ref> The platypus' genes also back up the possible evolutionary link between XY and ZW, because they have the [[DMRT1]] gene possessed by birds on their X chromosomes.<ref name="Graves-2006" /> Regardless, XY and ZW follow a similar route. All sex chromosomes started out as an original autosome of an original amniote that relied upon temperature to determine the sex of offspring. After the mammals separated, the reptile branch further split into [[Lepidosauria]] and [[Archosauromorpha]]. These two groups both evolved the ZW system separately, as evidenced by the existence of different sex chromosomal locations.<ref name="Vallender-2006" /> In mammals, one of the autosome pair, now Y, mutated its [[SOX3]] gene into the [[SRY]] gene, causing that chromosome to designate sex.<ref name="Vallender-2006" /><ref name="Graves-2006">{{cite journal | vauthors = Graves JA | title = Sex chromosome specialization and degeneration in mammals | journal = Cell | volume = 124 | issue = 5 | pages = 901–914 | date = March 2006 | pmid = 16530039 | doi = 10.1016/j.cell.2006.02.024 | s2cid = 8379688 | doi-access = free }}</ref><ref name="University of Chicago Medical Center-1999">{{cite press release | title=The evolution of the sex chromosomes: Step by step | publisher=University of Chicago Medical Center |date=28 October 1999 |url =http://www.uchospitals.edu/news/1999/19991028-x-vs-y.html |access-date=23 October 2011}}</ref> After this mutation, the SRY-containing chromosome [[Chromosomal inversion|inverted]] and was no longer completely [[Homologous chromosome|homologous]] with its partner. The regions of the [[X chromosome|X]] and [[Y chromosome]]s that are still homologous to one another are known as the [[pseudoautosomal region]].<ref name="Charlesworth-2003">{{cite journal | vauthors = Charlesworth B | title = The organization and evolution of the human Y chromosome | journal = Genome Biology | volume = 4 | issue = 9 | pages = 226 | date = 14 August 2003 | pmid = 12952526 | pmc = 193647 | doi = 10.1186/gb-2003-4-9-226 | doi-access = free }}</ref> Once it inverted, the Y chromosome became unable to remedy deleterious mutations, and thus [[Y chromosome#Degeneration|degenerated]].<ref name="Vallender-2006" /> There is some concern that the Y chromosome will shrink further and stop functioning in ten million years: but the Y chromosome has been strictly conserved after its initial rapid gene loss.<ref name="Graves-2004">{{cite journal | vauthors = Graves JA | title = The degenerate Y chromosome--can conversion save it? | journal = Reproduction, Fertility, and Development | volume = 16 | issue = 5 | pages = 527–534 | date = 22 July 2004 | pmid = 15367368 | doi = 10.1071/RD03096 | s2cid = 23740483 }}</ref><ref name="Hughes-2012">{{cite journal | vauthors = Hughes JF, Skaletsky H, Brown LG, Pyntikova T, Graves T, Fulton RS, Dugan S, Ding Y, Buhay CJ, Kremitzki C, Wang Q, Shen H, Holder M, Villasana D, Nazareth LV, Cree A, Courtney L, Veizer J, Kotkiewicz H, Cho TJ, Koutseva N, Rozen S, Muzny DM, Warren WC, Gibbs RA, Wilson RK, Page DC | display-authors = 6 | title = Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes | journal = Nature | volume = 483 | issue = 7387 | pages = 82–86 | date = February 2012 | pmid = 22367542 | pmc = 3292678 | doi = 10.1038/nature10843 | bibcode = 2012Natur.483...82H }}</ref>

There are some vertebrate species, such as the [[medaka]] fish, that evolved sex chromosomes separately; their Y chromosome never inverted and can still swap genes with the X. These species' sex chromosomes are relatively primitive and unspecialized. Because the Y does not have male-specific genes and can interact with the X, XY and YY females can be formed as well as XX males.<ref name="Schartl-2004a"/> Non-inverted Y chromosomes with long histories are found in [[Python (genus)|python]]s and [[emu]]s, each system being more than 120 million years old, suggesting that inversions are not necessarily an eventuality.<ref name="Bachtrog-2014">{{cite journal | vauthors = Bachtrog D, Mank JE, Peichel CL, Kirkpatrick M, Otto SP, Ashman TL, Hahn MW, Kitano J, Mayrose I, Ming R, Perrin N, Ross L, Valenzuela N, Vamosi JC | display-authors = 6 | title = Sex determination: why so many ways of doing it? | journal = PLOS Biology | volume = 12 | issue = 7 | pages = e1001899 | date = July 2014 | pmid = 24983465 | pmc = 4077654 | doi = 10.1371/journal.pbio.1001899 | doi-access = free }}</ref> XO sex determination can evolve from XY sex determination with about 2 million years.{{clarification needed|reason=What does this sentence mean?|date=April 2022}}<ref>{{Cite book| vauthors = Nei M |url=https://books.google.com/books?id=sJtoAgAAQBAJ&q=sexual+systems+can+evolve&pg=PA168|title=Mutation-Driven Evolution|date=2013-05-02|publisher=OUP Oxford|isbn=978-0-19-163781-0|pages=168}}</ref>

== See also ==
* [[Clarence Erwin McClung]], who discovered the role of chromosomes in sex determination
* [[Testis-determining factor]]
* [[Maternal influence on sex determination]]
* [[Sequential hermaphroditism]]
* [[Sex determination and differentiation (human)]]
* [[Cell autonomous sex identity]]

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* {{cite book |vauthors=Beukeboom L, Perrin N |date=2014 |url=https://books.google.com/books?id=d4cLBAAAQBAJ |title=The Evolution of Sex Determination |publisher=Oxford University Press |isbn=978-0-19-163140-5}}
* {{Cite paper |last=Lahn |first=Bruce T. |date=1999-10-29 |title=Four Evolutionary Strata on the Human X Chromosome |url=https://www.science.org/doi/10.1126/science.286.5441.964 |journal=Science |volume=286 |issue=5441 |pages=964–967 |doi=10.1126/science.286.5441.964 |issn=0036-8075 |pmid=10542153}}
{{refend}}

{{Sex determination and differentiation}}
{{Sex (biology)}}
{{chromo}}
{{Authority control}}

{{DEFAULTSORT:Sex-Determination System}}
[[Category:Sex-determination systems| ]]
[[Category:Epigenetics]]