Evolution in action: allopatry, variable diversity and a stepping-stone model of migration among populations of the freshwater bivalve Triplodon corrugatus from the north-eastern Amazon

Abstract

Triplodon corrugatus is a freshwater bivalve (Hyriidae) endemic to the Amazon, Orinoco and Tocantins basins, and the Piria river. Our understanding of hyriid diversity at, and below, the species level, remains poor. The genetic diversity of T. corrugatus from the Tapajos, Amazon, Tocantins, Irituia and Piria rivers in the north-eastern Brazilian Amazon was investigated. Except for the Irituia, where a single COII–COI haplotype had been fixed, all the other populations had medium to high haplotype diversities, and all populations had low nucleotide diversities. Pairwise fixation indices indicated that all populations were structured, except for comparisons between the Tapajos and Amazon, and the Amazon and Tocantins rivers, which may be explained by a stepping-stone model of migration. AMOVA detected that 81.28% of the variation was among populations. However, STRUCTURE analyses corroborated only the Piria river specimens as comprising a distinct population, which is being maintained by allopatry due to the current isolation between the Piria, and the Amazon and Tocantins basins.