Multiple contact zones and karyotypic evolution in a neotropical frog species complex

Previous studies of DNA sequence and karyotypic data have revealed high genetic diversity in the Physalaemus cuvieri - Physalaemus ephippifer species complex-a group of small leptodactylid frogs in South America. To date, seven major genetic lineages have been recognized in this group, with species delimitation tests supporting four to seven of them as valid species. Among these, only P. ephippifer shows heteromorphic sex chromosomes, but the implications of cytogenetic divergence for the evolution of this group are unknown. We analyzed karyotypic, mitochondrial DNA, and 3RAD genomic data to characterize a putative contact zone between P. ephippifer and P. cuvieri Lineage 1, finding evidence for admixture and karyotypic evolution. We also describe preliminary evidence for admixture between two other members of this species complex-Lineage 1 and Lineage 3 of P. cuvieri. Our study sheds new light on evolutionary relationships in the P. cuvieri - P. ephippifer species complex, suggesting an important role of karyotypic divergence in its evolutionary history and underscoring the importance of hybridization as a mechanism of sex chromosome evolution in amphibians.

Population genomics and sexual signals support reproductive character displacement in Uperoleia (Anura: Myobatrachidae) in a contact zone

When closely related species come into contact via range expansion, both may experience reduced fitness as a result of the interaction. Selection is expected to favour traits that minimize costly interspecies reproductive interactions (such as mismating) via a phenomenon called reproductive character displacement (RCD). Research on RCD frequently assumes secondary contact between species, but the geographical history of species interactions is often unknown. Population genomic data permit tests of geographical hypotheses about species origins and secondary contact through range expansion. We used population genomic data from single nucleotide polymorphisms (SNPs), mitochondrial sequence data, advertisement call data and morphological data to investigate a species complex of toadlets (Uperoleia borealis, U. crassa, U. inundata) from northern Australia. Although the three species of frogs were morphologically indistinguishable in our analysis, we determined that U. crassa and U. inundata form a single species (synonymized here) based on an absence of genomic divergence. SNP data identified the phylogeographical origin of U. crassa as the Top End, with subsequent westward invasion into the range of U. borealis in the Kimberley. We identified six F1 hybrids, all of which had the U. borealis mitochondrial haplotype, suggesting unidirectional hybridization. Consistent with the RCD hypothesis, U. borealis and U. crassa sexual signals differ more in sympatry than in allopatry. Hybrid males have intermediate calls, which probably reduces attractiveness to females. Integrating population genomic data, mitochondrial sequencing, morphology and behavioural approaches provides an unusually detailed collection of evidence for reproductive character displacement following range expansion and secondary contact.