Data from: 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 co-evolves 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.