Diversity, biogeography, and reproductive evolution in the genus Pipa (Amphibia: Anura: Pipidae)

The genus Pipa is a species-poor clade of Neotropical frogs and one of the most bizarre-looking due to many highly derived anatomical traits related to their fully aquatic lifestyle. With their African relatives, they form the Pipidae family, which has attracted much attention, especially regarding its anatomy, reproductive biology, paleontology and biogeography. However, the actual diversity and phylogenetic relationships within Pipa remain poorly understood, and thus so do their historical biogeography and the evolution of striking features, such as the absence of teeth and endotrophy in some species. Using short mtDNA sequences across the distribution of the genus, we identified 15 main lineages (Operational Taxonomic Units - OTUs). This more than doubles the number of the currently seven valid nominal species. Several closely related OTUs do not share nuDNA alleles, confirming species divergence. Time-calibrated phylogenies obtained from mitogenomes and from 10 nuclear loci provide highly similar topologies but strikingly distinct node ages for Pipa. High dN/dS ratios and the variation of substitution rates across the trees suggest a strong effect of saturation on fast evolving positions of mtDNA, producing a substantially shorter stem branch of Pipa. Focusing on the nuDNA topology, we inferred an early Neogene Amazonian origin of the diversification of Pipa, with an initial split between the Guiana-Brazilian Shields and Western Amazonia, a pattern observed in many other co-distributed groups. All the western species are edentate, suggesting a single loss in the genus. Each of these groups diversified further out of Amazonia, toward the Atlantic Forest and toward trans-Andean forests, respectively. These events are concomitant with paleogeographic changes and match patterns observed in other co-distributed taxonomic groups. The two Amazonian lineages have probably independently acquired endotrophic larval development.