Editing Sex-determination system (section)

== 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>