How to make a sex chromosome
نویسندگان
چکیده
S ex chromosomes have evolved independently many times throughout the eukaryotes, and represent a remarkable case of genomic convergence, as unrelated sex chromosomes share many properties across distant taxa1–3. Sex chromosomes evolve after recombination is halted between a homologous pair of chromosomes4,5, leading to a cascade of non-adaptive and adaptive processes that produce distinct differences between the X and Y (or Z and W) chromosomes. Owing to detailed studies in Drosophila6–8 and mammals9–11, we have a nuanced understanding of the consequences of arrested recombination1,4,7,8. The non-recombining Y and W chromosomes become highly heterochromatic (see Box 1 for a glossary) and experience profound levels of gene loss even as the X and Z chromosomes remain functional1,12–14. Sex chromosomes have been the focus of intense study and are an important model for understanding the consequences of recombination suppression12,15. It is clear that the loss of recombination triggers a host of evolutionary processes, including Muller’s Ratchet, background selection and genetic hitchhiking, reviewed in ref. 16, that lead to the loss of gene activity and pseudogenization (detailed in Box 2). This work makes very clear the evolutionary consequences of halting recombination between the sex chromosomes. Why recombination is suppressed in the first place is less clear, as the chromosomes that determine sex in many organisms with genetic sex determination never progress to heteromorphic sex chromosomes. For example, a single missense single nucleotide polymorphism in the coding region of the Amhr2 locus appears to control sex in the tiger pufferfish (Takifugu rupripes)17, but recombination is not restricted around this sex-determining gene and there is no evidence of divergence beyond this single nucleotide between the proto-X or proto-Y. Similarly, despite considerable age, the sex chromosomes in many clades (including ratite birds18,19, pythons20 and European tree frogs21) have failed to develop substantial heteromorphism, and remain largely identical. These observations indicate that recombination suppression and sex chromosome divergence are not inevitable consequences of genetic sex determination, leading to three questions at DOI: 10.1038/ncomms12087 OPEN
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