Introgression Across Hybrid Zones Is Not Mediated by Large X-Effects in Green Toads with Undifferentiated Sex Chromosomes

Divergence between incipient species remains an incompletely understood process. Hybrid zones provide great research potential, reflecting natural organismal genomic interactions and gene evolution in a variety of recombinants over generations. While sex chromosomes are known evolutionary drivers of reproductive isolation, empirical population genetics has mostly examined species with heteromorphic sex chromosomes. We recently reported restricted introgression at sex-linked markers in an amphibian system with homomorphic sex chromosomes (Hyla), consistent with a large X-effect, designating a greater role of sex chromosomes in driving hybrid incompatibilities. Here, using a similar approach, we examined two hybrid zones of Palearctic green toads (Bufo viridis subgroup), involving several lineages that arose at different times and form secondary contacts. We find no evidence for differential introgression of sex-linked versus autosomal markers across both zones. This absence of large X-effects in Bufo indicates that, unlike in Hyla, hybrid incompatibilities may not result from the faster-heterogametic sex and faster-male aspects of Haldane's rule. The recent suppression of XY recombination in Hyla but not in Bufo may have driven greater divergence between Hyla sex chromosomes, causing stronger reproductive isolation. Alternatively, stronger linkage among Hyla's sex-linked markers could restrict introgression. We hypothesize that the degree of sex-specific recombination may condition the importance of homomorphic sex chromosomes in speciation.