Insights into the Evolutionary History of an Extinct South American Freshwater Snail Based on Historical DNA.

Highly oxygenated freshwater habitats in the High Paraná River (Argentina-Paraguay) were home to highly endemic snails of the genus Aylacostoma, which face extinction owing to the impoundment of the Yacyretá Reservoir in the 1990s. Two species, A. chloroticum and A. brunneum, are currently included in an ongoing ex situ conservation programme, whereas A. guaraniticum and A. stigmaticum 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 A. stigmaticum 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 ex situ programme. We further use this gene to derive a secondary structure model, to examine the specific status of A. stigmaticum and to explore the evolutionary history of these snails. The secondary structure model based on A. stigmaticum 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 A. stigmaticum is recognized here as a distinct evolutionary genetic species. Molecular phylogenetic analyses revealed a sister group relationship between A. chloroticum and A. brunneum, and estimated divergence times suggest that diversification of Aylacostoma in the High Paraná 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

Threatened Neotropical mollusks: analysis of shape differences in three endemic snails from High Paraná River by geometric morphometrics Moluscos neotropicales amenazados: análisis de diferencias de forma en tres caracoles endémicos del río Alto Paraná mediante morfometría geométrica

Variation in shape among a living and 2 extinct aquatic snails of the genus Aylacostoma, using a geometric morphometric method of thin plate splines and multivariate analysis was investigated. The analysis was performed to evaluate the diagnostic capability of this method and to explore shell shape differences, due to the lack of other data, in an attempt to answer why only 1 of the species persisted in the wild. Sixteen landmarks in a bi-dimensional space for 32 shells of type, paratype and reference specimens deposited in museums of Argentina were defined. Analysis was successful in assigning individual specimens to particular species. Statistically significant differences in last whorl, aperture, and spire were found for the first 4 non-uniform components explaining an 85% of local variation observed. Differences could be related to a differential use of habitat and/or to the degree of exposure to water current. More globose shell found in the extinct species could be associated to habitats and substrata with the highest water currents, whereas the more stylized shell in the third species could be related to a preference for more protected habitats, like those where it presently occurs.<br>La variación de forma entre una especie viviente y dos extintas de caracoles acuáticos del género Aylacostoma, fue investigada mediante el método de morfometría geométrica de "thin plate splines" y análisis multivariado. El análisis se realizó para evaluar la capacidad diagnóstica del método y explorar las diferencias de forma de conchilla, debido a la falta de otros datos, en un intento por responder por qué sólo una de las especies persistió en la naturaleza. Dieciséis "landmarks" fueron definidos en un espacio bi-dimensional para 32 conchillas de ejemplares tipo, paratipo y de referencia depositados en museos de Argentina. El análisis fue exitoso en la asignación de los individuos a especies particulares. Se encontraron diferencias significativas en el último anfracto, apertura y espira respecto de los 4 primeros componentes no uniformes, que explicaron el 85% de la variación observada. Estas diferencias podrían vincularse a un uso diferencial del hábitat y/o al grado de exposición a la corriente. La conchilla más globosa en las especies extintas pudo estar asociada a hábitats y sustratos con mayores corrientes, mientras que la conchilla más estilizada en la tercera especie podría estar relacionada con la preferencia por hábitats más protegidos, como los que actualmente habita

Secondary structure model of the third domain of the 12S mt rRNA gene for <i>Aylacostoma stigmaticum</i>.

<p>Mutational changes for <i>A</i>. <i>brunneum</i> and <i>A</i>. <i>chloroticum</i> are indicated by red and blue arrows, respectively (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0169191#pone.0169191.t002" target="_blank">Table 2</a>).</p

Divergence times among <i>Aylacostoma</i> species based on net divergence and Bayesian approaches.

<p>Divergence times among <i>Aylacostoma</i> species based on net divergence and Bayesian approaches.</p

Nucleotide composition of 12S mt rRNA sequences for <i>Aylacostoma</i> species.

<p>Nucleotide composition of 12S mt rRNA sequences for <i>Aylacostoma</i> species.</p

<i>K</i>/θ ratio calculations for <i>Aylacostoma</i> 12S mt rRNA sequences.

<p><i>K</i>/θ ratio calculations for <i>Aylacostoma</i> 12S mt rRNA sequences.</p

Genetic distances between <i>Aylacostoma</i> species.

<p>Uncorrected (below the diagonal) and corrected (K2P; above the diagonal) distances are shown.</p

Collection information and GenBank accession numbers for the samples analysed herein.

<p>Collection information and GenBank accession numbers for the samples analysed herein.</p

Polymorphic positions based on a 234 bp fragment of the 12S mt rRNA gene for <i>Aylacostoma</i> species.

<p>Polymorphic positions based on a 234 bp fragment of the 12S mt rRNA gene for <i>Aylacostoma</i> species.</p

Results of ABGD analysis for the 12S mt rRNA dataset using the K2P model to calculate pairwise distances.

<p>Results of ABGD analysis for the 12S mt rRNA dataset using the K2P model to calculate pairwise distances.</p