Comprehensive Analysis of Salamander Hybridization Suggests a Consistent Relationship between Genetic Distance and Reproductive Isolation across Tetrapods

Hybridization between populations along the path to complete reproductive isolation can provide snapshots of speciation in action. Here, we present a comprehensive list of salamander hybrids and estimate genetic distances between the parental hybridizing species using one mitochondrial and one nuclear gene (MT-CYB and RAG1). Salamanders are outliers among tetrapod vertebrates in having low metabolic rates and highly variable sex chromosomes. Both of these features might be expected to impact speciation; mismatches between the mitochondrial and nuclear genomes that encode the proteins for oxidative metabolism, as well as mismatches in heteromorphic sex chromosomes, can lead to reproductive isolation. We compared the genetic distances between hybridizing parental species across four main tetrapod groups that differ in metabolic rates and sex chromosome diversity: salamanders, lizards, mammals, and birds. Our results reveal no significant differences, suggesting that variation in these traits across vertebrates does not translate into predictable patterns of genetic divergence and incompatible loci in hybrids.