Editing Sex-determination system (section)

=== 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" />