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Sexual differentiation – the developmental process by which an embryo matures into one or the other phenotypic sex – is consequential from an evolutionary perspective because it is generally a requirement for reproduction in species with separate male and female individuals. In most mammals, sexual differentiation is triggered by the presence or absence of a male-specific gene called Sry that is located on the Y chromosome. This trigger for sexual differentiation has remained almost invariant during dozens of millions of years of mammalian diversification, and its stability is a hallmark of purifying selection and biological importance. Surprisingly, however, the genetic basis of sexual differentiation evolves incredibly rapidly in some groups, including amphibians. For example, Evans et al. 2024 discovered unexpected diversity in the genetic processes that determine the sex in different species of the African clawed frog (genus Xenopus), a significant discovery in what was already one of the most widely studied amphibians. Using genomic approaches, a total of eight different sex chromosomes were identified in just 11 Xenopus species, and many or all of these may contain unique and newly evolved genes that trigger male or female sexual differentiation. Previously, researchers had known of only three different sex chromosomes in these frogs. Prevailing theory had suggested that sex-determining genes should typically arise in regions of the genome with a low rate of genetic recombination – the exchange of genetic material within each parent that creates new mixtures of traits in their offspring. Instead, the newly evolved sex determining genes in Xenopus are almost universally located in genomic regions where genetic recombination is high. These findings raise questions about how and why the genetic basis of very important biological traits – such as sexual differentiation – may evolve so quickly, how new genes and genetic function arise, and how factors such as sexual selection may drive rapid evolution.
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