A few months ago I wrote the following:
I should point out that the mammalian Y chromosome is an anomaly in origin and sex determination. In fact, every single sex determination system and sex chromosome system that I know of differs from all of the others in some manner. It looks like I’m going to have to write an entry on the evolutionary genetics of sex determination in everything other than mammals.
I never did get around to writing that review of sex chromosomes. All is not lost, however, as the most recent issue of Current Biology is devoted to the evolution of sex. There is even an article that reviews sex chromosome evolution in vertebrates. It is from that article that I take this week’s phylogeny:
Figure 1. Vertebrate sex determination systems.
Phylogeny of major vertebrate clades showing the sex determining systems found in members of the respective clade. ‘Mulitple’ indicates involvement of more than one pair of chromosomes in sex determination. Dates of divergence are taken from 42, 82 and 83. TSD: temperature-dependent sex determination.
The XX/XY system is the one with which most people are familiar — males are heterogametic (XY), and females are homogametic (XX). The Y chromosome confers the male phenotype in humans, for example. Monotremes (ie, platypus) have multiple sex chromosomes that form a chain (reviewed here). In other taxa (birds, some other reptiles, some fish, and lepidopterans) males are homogametic (ZZ) and females are heterogametic (ZW). Additionally, many taxa use developmental environment to determine sex, and others can change sex during their life based on environmental cues.
While sexual reproduction is the ancestral state amongst vertebrates (and probably amongst all animals), sex determination systems have evolved multiple times. It is thought that the bird Z and W chromosomes evolved independently from the mammalian X and Y, but data from the platypus genome suggest otherwise. The chain of sex chromosomes in platypus contains the homolog of the avian Z and W on one end and the homolog of the mammalian X and Y at the other. Furthermore, the gene content of the X and Z chromosomes indicates similar selection against potentially deleterious mutations on these two chromosomes. This leads some people to believe that the ancestor of reptiles and mammals had a sex chromosome karyotype like that of monotremes, and one pair of sex chromosomes were retained along the mammalian lineage and a different pair along the lineage leading to birds.
The avian and mammalian W and Y chromosomes are the result of the degradation of the Z and X chromosomes, respectively. An ancestral autosomal pair became specialized as a sex chromosome, with one homolog degrading into the chromosome found in the heterogametic sex. Another well studied system, Drosophila, reveals a different story of sex chromosome evolution. In the case of Drosophila, Y chromosomes are not homologous across the entire taxa. Instead, new Y chromosomes evolve when autosomes fuse with X or Y chromosomes. When this occurs, the homolog of the fused autosome degrades into a new Y chromosome. For an example of such a fusion-degradation event, see here.
T Ezaz, R Stiglec, F Veyrunes, and JAM Graves. 2006. Relationships between Vertebrate ZW and XY Sex Chromosome Systems. Current Biology 16: R736-R743. doi:10.1016/j.cub.2006.08.021
AB Carvalho and AG Clark. 2005. Y chromosome of D. pseudoobscura is not homologous to the ancestral Drosophila Y. Science 307: 108-110. doi:10.1126/science.1101675