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 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 vs. autosomal markers across both zones. This absence of Large X-effects i...