THE EVOLUTION OF XY RECOMBINATION: SEXUALLY ANTAGONISTIC SELECTION VERSUS DELETERIOUS MUTATION LOAD

Recombination arrest between X and Y chromosomes, driven by sexually antagonistic genes, is expected to induce their progressive differentiation. However, in contrast to birds and mammals (which display the predicted pattern), most cold‐blooded vertebrates have homomorphic sex chromosomes. Two main hypotheses have been proposed to account for this, namely high turnover rates of sex‐determining systems and occasional XY recombination. Using individual‐based simulations, we formalize the evolution of XY recombination (here mediated by sex reversal; the “fountain‐of‐youth” model) under the contrasting forces of sexually antagonistic selection and deleterious mutations. The shift between the domains of elimination and accumulation occurs at much lower selection coefficients for the Y than for the X. In the absence of dosage compensation, mildly deleterious mutations accumulating on the Y depress male fitness, thereby providing incentives for XY recombination. Under our settings, this occurs via “demasculinization” of the Y, allowing recombination in XY (sex‐reversed) females. As we also show, this generates a conflict with the X, which coevolves to oppose sex reversal. The resulting rare events of XY sex reversal are enough to purge the Y from its load of deleterious mutations. Our results support the “fountain of youth” as a plausible mechanism to account for the maintenance of sex‐chromosome homomorphy.