Incomplete penetrance of sex determination in frogs, and achiasmatic meiosis in males, allows for interesting possibilities in the evolution of a new sex chromosome.
Rana temporaria shows extraordinary variation in its sex determination system.
Most population have indistinguishable haplotypes in males and females in all chromosomes (XX-undifferentiated race), but many show varying levels of XY haplotype differentiation. In the former, sex determination is non-genetic, as far as we know (random sex determination), while in the latter there is a genetic component to sex determination.
Any time an allele causing sex determination evolves, the population sex ratio would be distorted, and that would select against the new allele. However it is possible for sexually antagonistic variation in combination with the very limited recombination in male meiosis, to retain such an allele.
We are simulating the invasion of non-random sex determining genes, to try to understand more about the sex chromosome evolution dynamics in frogs, and other species where recombination depends on phenotypic sex.