Non-recombining sex chromosomes are expected to undergo evolutionary decay,
ending up genetically degenerated, as has happened in birds and mammals. Why are
then sex chromosomes so often homomorphic in cold-blooded vertebrates? One
possible explanation is a high rate of turnover events, replacing master
sex-determining genes by new ones on other chromosomes. An alternative is that
X-Y similarity is maintained by occasional recombination events, occurring in
sex-reversed XY females. Based on mitochondrial and nuclear gene sequences, we
estimated the divergence times between European tree frogs (Hyla
arborea, H. intermedia, and H.
molleri) to the upper Miocene, about 5.4–7.1 million years
ago. Sibship analyses of microsatellite polymorphisms revealed that all three
species have the same pair of sex chromosomes, with complete absence of X-Y
recombination in males. Despite this, sequences of sex-linked loci show no
divergence between the X and Y chromosomes. In the phylogeny, the X and Y
alleles cluster according to species, not in groups of gametologs. We conclude
that sex-chromosome homomorphy in these tree frogs does not result from a recent
turnover but is maintained over evolutionary timescales by occasional X-Y
recombination. Seemingly young sex chromosomes may thus carry old-established
sex-determining genes, a result at odds with the view that sex chromosomes
necessarily decay until they are replaced. This raises intriguing perspectives
regarding the evolutionary dynamics of sexually antagonistic genes and the
mechanisms that control X-Y recombination.
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