Editing Sex (section)

== Sex-determination systems ==
{{Main|Sex-determination system}}
[[File:Evolsex-dia2a.svg|thumb|left|Sex helps the spread of advantageous traits through recombination. The diagrams compare the evolution of [[allele frequency]] in a sexual population (top) and an asexual population (bottom). The vertical axis shows frequency and the horizontal axis shows time. The alleles a/A and b/B occur at random. The advantageous alleles A and B, arising independently, can be rapidly combined by sexual reproduction into the most advantageous combination AB. Asexual reproduction takes longer to achieve this combination because it can only produce AB if A arises in an individual which already has B or vice versa.]]

The biological cause of an organism developing into one sex or the other is called ''sex determination''.  The cause may be genetic, environmental, [[haplodiploidy]], or multiple factors.<ref name="Bachtrog-2014" />  Within animals and other organisms that have genetic sex-determination systems, the determining factor may be the presence of a [[sex chromosome]]. In plants that are sexually dimorphic, such as ''Ginkgo biloba'',<ref name="Judd-2002"/>{{rp|203}} the liverwort ''[[Marchantia polymorpha]]'' or the dioecious species in the flowering plant genus ''[[Silene]]'', sex may also be determined by sex chromosomes.<ref name="Tanurdzic-2004">{{cite journal | vauthors = Tanurdzic M, Banks JA | title = Sex-determining mechanisms in land plants | journal = The Plant Cell | volume = 16 | issue = Suppl | pages = S61–S71 | date = 2004 | pmid = 15084718 | pmc = 2643385 | doi = 10.1105/tpc.016667 | bibcode = 2004PlanC..16S..61T }}</ref> Non-genetic systems may use environmental cues, such as the [[Temperature-dependent sex determination|temperature]] during early development in [[crocodile]]s, to determine the sex of the offspring.<ref name="Warner-2008">{{cite journal | vauthors = Warner DA, Shine R | title = The adaptive significance of temperature-dependent sex determination in a reptile | journal = Nature | volume = 451 | issue = 7178 | pages = 566–568 | date = January 2008 | pmid = 18204437 | doi = 10.1038/nature06519 | bibcode = 2008Natur.451..566W | s2cid = 967516 }}</ref>

[[Sex-determination system|Sex determination]] is often distinct from [[Sexual differentiation|sex differentiation]]. Sex determination is the designation for the development stage towards either male or female while sex differentiation is the pathway towards the development of the [[phenotype]].<ref>{{cite book| vauthors = Beukeboom LW, Perrin N |url={{GBurl|id=7yrnAwAAQBAJ|q=the+evolution+of+sex+determination}}|title=The Evolution of Sex Determination |date=2014|publisher=Oxford University Press|isbn=978-0-19-965714-8|pages=16|language=en}}</ref>

=== Genetic ===
==== XY sex determination ====
[[File:Drosophila XY sex-determination.svg|thumb|The common fruit fly has an [[XY sex-determination system]], as do humans and most mammals.]]
Humans and most other [[mammal]]s have an [[XY sex-determination system]]: the [[Y chromosome]] carries factors responsible for triggering male development, making XY sex determination mostly based on the presence or absence of the [[Y chromosome]]. It is the male [[gamete]] that determines the sex of the offspring.<ref name="Wallis-2008">{{cite journal|vauthors=Wallis MC, Waters PD, Graves JA|date=October 2008|title=Sex determination in mammals – before and after the evolution of SRY|journal=Cellular and Molecular Life Sciences|volume=65|issue=20|pages=3182–95|doi=10.1007/s00018-008-8109-z|pmid=18581056|s2cid=31675679|pmc=11131626}}</ref> In this system XX mammals typically are female and XY typically are male.<ref name="Bachtrog-2014" /> However, individuals with [[Klinefelter syndrome|XXY]] or [[XYY syndrome|XYY]] are males, while individuals with [[Turner syndrome|X]] and [[Triple X syndrome|XXX]] are females.<ref name="Hake-2008" /> Unusually, the [[platypus]], a [[monotreme]] mammal, has ten sex chromosomes; females have ten X chromosomes, and males have five X chromosomes and five Y chromosomes. Platypus egg cells all have five X chromosomes, whereas sperm cells can either have five X chromosomes or five Y chromosomes.<ref>{{cite book | vauthors = Pierce BA |title=Genetics: a conceptual approach |date=2012 |publisher=W.H. Freeman |isbn=978-1-4292-3250-0 |edition=4th |location=New York |pages=73–74 |oclc=703739906}}</ref>

XY sex determination is found in other organisms, including insects like the [[Drosophila melanogaster|common fruit fly]],<ref name="Kaiser-2010">{{cite journal|vauthors=Kaiser VB, Bachtrog D|year=2010|title=Evolution of sex chromosomes in insects|journal=Annual Review of Genetics|volume=44|pages=91–112|doi=10.1146/annurev-genet-102209-163600|pmc=4105922|pmid=21047257}}</ref> and some plants.<ref name="Dellaporta-1993">{{cite journal | vauthors = Dellaporta SL, Calderon-Urrea A | title = Sex determination in flowering plants | journal = The Plant Cell | volume = 5 | issue = 10 | pages = 1241–1251 | date = October 1993 | pmid = 8281039 | pmc = 160357 | doi = 10.1105/tpc.5.10.1241 | jstor = 3869777 }}</ref> In some cases, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome.<ref name="Hake-2008" /> In the fruit fly individuals with XY are male and individuals with XX are female; however, individuals with XXY or XXX can also be female, and individuals with X can be males.<ref>{{cite book| vauthors = Fusco G, Minelli A |author-link2=Alessandro Minelli (biologist) |url={{GBurl|id=AKGsDwAAQBAJ}}|title=The Biology of Reproduction|year=2019|publisher=Cambridge University Press|isbn=978-1-108-49985-9|pages=306–308 }}</ref>

==== ZW sex determination ====
In birds, which have a [[ZW sex-determination system]], the W chromosome carries factors responsible for female development, and default development is male.<ref>{{cite journal | vauthors = Smith CA, Katz M, Sinclair AH | title = DMRT1 is upregulated in the gonads during female-to-male sex reversal in ZW chicken embryos | journal = Biology of Reproduction | volume = 68 | issue = 2 | pages = 560–570 | date = February 2003 | pmid = 12533420 | doi = 10.1095/biolreprod.102.007294 | doi-access = free }}</ref> In this case, ZZ individuals are male and ZW are female. It is the female [[gamete]] that determines the sex of the offspring. This system is used by birds, some fish, and some [[crustacean]]s.<ref name="Hake-2008" />

The majority of [[Lepidoptera|butterflies and moths]] also have a ZW sex-determination system. Females can have Z, ZZW, and even ZZWW.<ref>{{cite book| vauthors = Majerus ME | author-link = Michael Majerus |url= {{GBurl|id=vDHOYPQ2mmYC|q=ZW+sex+determination}}|title=Sex Wars: Genes, Bacteria, and Biased Sex Ratios|date=2003|publisher=Princeton University Press|isbn=978-0-691-00981-0|page=59|language=en}}</ref>

==== XO sex determination ====
In the [[XO sex-determination system]], males have one X chromosome (XO) while females have two (XX). All other chromosomes in these diploid organisms are paired, but organisms may inherit one or two X chromosomes. This system is found in most [[arachnid]]s, insects such as [[silverfish]] ([[Apterygota]]), [[dragonfly|dragonflies]] ([[Paleoptera]]) and [[grasshopper]]s ([[Exopterygota]]), and some nematodes, crustaceans, and gastropods.<ref name="Bull-1983">{{cite book|title=Evolution of sex determining mechanisms|vauthors=Bull JJ|year=1983|isbn=0-8053-0400-2|page=17|publisher=Benjamin/Cummings Publishing Company, Advanced Book Program |author-link=James J. Bull}}</ref><ref>{{cite journal|vauthors=Thiriot-Quiévreux C|date=2003|title=Advances in chromosomal studies of gastropod molluscs.|journal=Journal of Molluscan Studies|volume=69|issue=3|pages=187–202|doi=10.1093/mollus/69.3.187|doi-access=free}}</ref>

In [[Gryllus|field cricket]]s, for example, insects with a single X chromosome develop as male, while those with two develop as female.<ref>{{cite journal |title=Karyotypes of two American field crickets: Gryllus rubens and Gryllus sp. (Orthoptera: Gryllidae) | vauthors = Yoshimura A |journal=Entomological Science |volume=8 |issue=3 |pages=219–222 |year=2005 |doi=10.1111/j.1479-8298.2005.00118.x| s2cid = 84908090 }}</ref>

In the nematode ''[[Caenorhabditis elegans]]'', most worms are self-fertilizing hermaphrodites with an XX karyotype, but occasional abnormalities in chromosome inheritance can give rise to individuals with only one X chromosome—these XO individuals are fertile males (and half their offspring are male).<ref>{{cite book|title=''C. elegans'' II|vauthors=[[Barbara J. Meyer|Meyer BJ]]|publisher=Cold Spring Harbor Laboratory Press|year=1997|isbn=978-0-87969-532-3|veditors=Riddle DL, Blumenthal T, Meyer BJ, Priess JR|chapter=Sex Determination and X Chromosome Dosage Compensation: Sexual Dimorphism|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK20094/|access-date=17 April 2021|archive-date=6 May 2021|archive-url=https://web.archive.org/web/20210506154914/http://www.ncbi.nlm.nih.gov/books/NBK20094/|url-status=live}}</ref>

==== ZO sex determination ====
In the [[ZO sex-determination system]], males have two Z chromosomes whereas females have one. This system is found in several species of moths.<ref>{{cite book | vauthors = De Prins J, Saithoh K | chapter = Karyology and Sex Determination |chapter-url={{GBurl|id=5w8FgSGuH34C|q=ZO+sex-determination+system+moth|p=461}} | veditors =  Kristensen N |title=Handbuch Der Zoologie / Handbook of Zoology | volume = Arthropoda: Insecta: Lepidoptera, Moths and Butterflies |publisher=Walter de Gruyter|year=2003 | pages = 449–468 | doi = 10.1515/9783110893724.449 |isbn=978-3-11-016210-3|access-date=29 September 2020|via=Google Books}}</ref>

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

For many species, sex is not determined by inherited traits, but instead by environmental factors such as temperature experienced during development or later in life.<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 }}</ref>

In the [[fern]] ''[[Ceratopteris]]'' and other [[Glossary of botanical terms#homospory|homosporous]] fern species, the default sex is hermaphrodite, but individuals which grow in soil that has previously supported hermaphrodites are influenced by the [[pheromone]] [[antheridiogen]] to develop as male.<ref name="Tanurdzic-2004" /> The [[bonelliidae|bonelliidae larvae]] can only develop as males when they encounter a female.<ref name="Bachtrog-2014" />

==== Sequential hermaphroditism ====
[[File:Ocellaris clownfish.JPG|thumb|[[Clownfish]]es are initially male; the largest fish in a group becomes female.]]

Some species can change sex over the course of their lifespan, a phenomenon called [[sequential hermaphroditism]].<ref name="Fusco-2019">{{cite book| vauthors = Fusco G, Minelli A |author-link2=Alessandro Minelli (biologist) |url={{GBurl|id=AKGsDwAAQBAJ|q=sequential+hermaphroditism+in+plants}}|title=The Biology of Reproduction |date=2019|publisher=Cambridge University Press|isbn=978-1-108-49985-9|pages=124|language=en}}</ref> 

[[Teleost|Teleost fishes]] are the only vertebrate [[Lineage (evolution)|lineage]] where sequential hermaphroditism occurs. In [[clownfish]], smaller fish are male, and the dominant and largest fish in a group becomes female; when a dominant female is absent, then her partner changes sex from male to female. In many [[wrasse]]s the opposite is true:  the fish are initially female and become male when they reach a certain size.<ref>{{cite journal|author-link4=Neil Gemmell|vauthors=Todd EV, Liu H, Muncaster S, Gemmell NJ|date=2016|title=Bending Genders: The Biology of Natural Sex Change in Fish|journal=Sexual Development|language=english|volume=10|issue=5–6|pages=223–2241|doi=10.1159/000449297|pmid=27820936|s2cid=41652893|doi-access=free|hdl=10536/DRO/DU:30153787|hdl-access=free}}</ref> 

Sequential hermaphroditism also occurs in plants such as ''[[Arisaema triphyllum]]''.

==== Temperature-dependent sex determination ====
[[File:Crocoparc Naissance des bébés.jpg|alt=pile of eggs in the sand, with a newly hatched crocodile looking over the top|thumb|Crocodiles do not have [[sex chromosomes]].<ref>{{cite journal | vauthors = González EJ, Martínez-López M, Morales-Garduza MA, García-Morales R, Charruau P, Gallardo-Cruz JA | title = The sex-determination pattern in crocodilians: A systematic review of three decades of research | journal = The Journal of Animal Ecology | volume = 88 | issue = 9 | pages = 1417–1427 | date = September 2019 | pmid = 31286510 | doi = 10.1111/1365-2656.13037 | bibcode = 2019JAnEc..88.1417G }}</ref>  Instead, whether these eggs will produce male or female crocodiles depends on the temperature of the eggs.]]

Many [[reptile]]s, including all [[crocodile]]s and most [[turtles]], have [[temperature-dependent sex determination]]. In these species, the temperature experienced by the embryos during their development determines their sex.<ref name="Bachtrog-2014" /> 

In some turtles, for example, males are produced at lower temperatures than females; but ''[[Alligator snapping turtle|Macroclemys]]'' females are produced at temperatures lower than 22&nbsp;°C or above 28&nbsp;°C, while males are produced in between those temperatures.<ref>{{cite journal|author-link=Scott F. Gilbert|vauthors=Gilbert SF|date=2000|title=Environmental Sex Determination|url=https://www.ncbi.nlm.nih.gov/books/NBK9989/|journal=Developmental Biology. 6th Edition|language=en|access-date=19 May 2021|archive-date=12 June 2021|archive-url=https://web.archive.org/web/20210612123300/https://www.ncbi.nlm.nih.gov/books/NBK9989/|url-status=live}}</ref>

====Haplodiploidy====
Certain insects, such as [[honey bee]]s and [[ant]]s, use a [[Haplodiploidy|haplodiploid sex-determination system]].<ref>{{cite journal|author-link=Brian Charlesworth|vauthors=Charlesworth B|date=August 2003|title=Sex determination in the honeybee|journal=Cell|volume=114|issue=4|pages=397–398|doi=10.1016/S0092-8674(03)00610-X|pmid=12941267|doi-access=free}}</ref> Diploid bees and ants are generally female, and haploid individuals (which develop from unfertilized eggs) are male. This sex-determination system results in highly biased [[sex ratio]]s, as the sex of offspring is determined by fertilization ([[arrhenotoky]] or [[pseudo-arrhenotoky]] resulting in males) rather than the assortment of chromosomes during meiosis.<ref>{{cite journal |vauthors=de la Filia A, Bain S, Ross L |title=Haplodiploidy and the reproductive ecology of Arthropods |journal=Current Opinion in Insect Science |date=June 2015 |volume=9 |pages=36–43 |doi=10.1016/j.cois.2015.04.018 |pmid=32846706 |bibcode=2015COIS....9...36D |url=https://www.pure.ed.ac.uk/ws/files/20137629/1_s2.0_S221457451500084X_main.pdf |hdl=20.500.11820/b540f12f-846d-4a5a-9120-7b2c45615be6 |s2cid=83988416 |hdl-access=free |access-date=25 June 2021 |archive-date=25 June 2021 |archive-url=https://web.archive.org/web/20210625230005/https://www.pure.ed.ac.uk/ws/files/20137629/1_s2.0_S221457451500084X_main.pdf |url-status=live }}</ref>