Cryptic recombination in the ever‐young sex chromosomes of H ylid frogs

Sex chromosomes are expected to evolve suppressed recombination, which leads to degeneration of the Y and heteromorphism between the X and Y . Some sex chromosomes remain homomorphic, however, and the factors that prevent degeneration of the Y in these cases are not well understood. The homomorphic sex chromosomes of the E uropean tree frogs ( H yla spp.) present an interesting paradox. Recombination in males has never been observed in crossing experiments, but molecular data are suggestive of occasional recombination between the X and Y . The hypothesis that these sex chromosomes recombine has not been tested statistically, however, nor has the X‐Y recombination rate been estimated. Here, we use approximate B ayesian computation coupled with coalescent simulations of sex chromosomes to quantify X ‐ Y recombination rate from existent data. We find that microsatellite data from H . arborea , H . intermedia and H . molleri support a recombination rate between X and Y that is significantly different from zero. We estimate that rate to be approximately 10 5 times smaller than that between X chromosomes. Our findings support the notion that very low recombination rate may be sufficient to maintain homomorphism in sex chromosomes.