Mitonuclear and phenotypic discordance in an Atlantic Forest frog hybrid zone

Discordance between mitochondrial and nuclear DNA is common among animals and can be the result of a number of evolutionary processes, including incomplete lineage sorting and introgression. Particularly relevant in contact zones, mitonuclear discordance is expected because the mitochondrial genome is haploid and primarily uniparentally inherited, whereas nuclear loci are evolving at slower rates. In addition, when closely related taxa come together in hybrid zones, the distribution of diagnostic phenotypic characters and their concordance with the mitochondrial or nuclear lineages can also inform on historical and ongoing dynamics within hybrid zones. Overall, genetic and phenotypic discordances provide evidence for evolutionary divergence and processes that maintain boundaries among sister species or lineages. In this study, we characterized patterns of genetic and phenotypic variation in a contact zone between Cycloramphus dubius and Cycloramphus boraceiensis, two sister species of frogs endemic to the Atlantic Coastal Forest of Brazil. We examined genomic-scale nuclear diversification across 19 populations, encompassing the two parental forms and a contact zone between them. We compared the distribution of genomic DNA variability with that of a mitochondrial locus (16S) and two morphological traits (dorsal tubercles and body size). Our results reveal multiple divergent lineages with ongoing admixture. We detected discordance in patterns of introgression across the three data types. Cycloramphus dubius males are significantly larger than C. boraceiensis males, and we posit that competition among males in the hybrid zone, coupled with mate choice by females, may be one mechanism leading to patterns of introgression observed between the species.