Insights into the evolutionary history of an extinct South American freshwater snail based on historical DNA

Abstract

Highly oxygenated freshwater habitats in the High Parana River (Argentina-Paraguay) were home to highly endemic snails of the genus <i>Aylacostoma</i>, which face extinction owing to the impoundment of the Yacyretá Reservoir in the 1990s. Two species, <i>A. chloroticum</i> and <i>A. brunneum</i>, are currently included in an ongoing <i>ex situ</i> conservation programme, whereas <i>A. guaraniticum</i> and <i>A. stigmaticum</i> are presumed extinct. Consequently, the validity and affinities of the latter two have remained enigmatic. Here, we provide the first molecular data on the extinct <i>A. stigmaticum</i> by means of historical DNA analysis. We describe patterns of molecular evolution based on partial sequences of the mitochondrial 12S ribosomal RNA gene from the extinct species and from those being bred within the <i>ex situ</i> programme. We further use this gene to derive a secondary structure model, to examine the specific status of <i>A. stigmaticum</i> and to explore the evolutionary history of these snails. The secondary structure model based on <i>A. stigmaticum</i> revealed that most polymorphic sites are located in unpaired regions. Our results support the view that the mitochondrial 12S region is an efficient marker for the discrimination of species, and the extinct <i>A. stigmaticum</i> is recognized here as a distinct evolutionary genetic species. Molecular phylogenetic analyses revealed a sister group relationship between <i>A. chloroticum</i> and <i>A. brunneum</i>, and estimated divergence times suggest that diversification of <i>Aylacostoma</i> in the High Parana River might have started in the late Miocene via intra-basin speciation due to a past marine transgression. Finally, our findings reveal that DNA may be obtained from dried specimens at least 80 years after their collection, and confirms the feasibility of extracting historical DNA from museum collections for elucidating evolutionary patterns and processes in gastropods.