Sex-antagonistic genes, XY recombination, and feminized Y chromosomes

The canonical model of sex-chromosome evolution predicts that sex-antagonistic (SA) genes play an instrumental role in the arrest of XY recombination and ensuing Y-chromosome degeneration. Although this model might account for the highly differentiated sex chromosomes of birds and mammals, it does not fit the situation of many lineages of fish, amphibians or non-avian reptiles, where sex chromosomes are maintained homomorphic through occasional XY recombination and/or high turnover rates. Such situations call for alternative explanatory frameworks. A crucial issue at stake is the effect of XY recombination on the dynamics of SA genes and deleterious mutations. Using individual-based simulations, we show that a complete arrest of XY recombination actually benefits females, not males. Male fitness is maximized at different XY-recombination rates depending on SA selection, but never at zero XY recombination. This should consistently favor some level of XY recombination, which in turn gener...