Speciation without gene-flow in hybridizing deer

Under the ecological speciation model, divergent selection acts on ecological differences between populations, gradually creating barriers to gene flow and ultimately leading to reproductive isolation. Hybridisation is part of this continuum and can both promote and inhibit the speciation process. Here, we used white-tailed (Odocoileus virginianus) and mule deer (O. hemionus) to investigate patterns of speciation in hybridizing sister species. We quantified genome-wide historical introgression and performed genome scans to look for signatures of four different selection scenarios. Despite ample modern evidence of hybridisation, we found negligible patterns of ancestral introgression and no signatures of divergence with gene flow, rather localized patterns of allopatric and balancing selection were detected across the genome. Genes under balancing selection were related to immunity, MHC and sensory perception of smell, the latter of which is consistent with deer biology. The deficiency of historical gene-flow suggests that white-tailed and mule deer were spatially separated during the glaciation cycles of the Pleistocene and genome wide differentiation accrued via genetic drift. Dobzhansky-Muller incompatibilities and selection against hybrids are hypothesised to be acting, and diversity correlations to recombination rates suggests these sister species are far along the speciation continuum.

First assessment of genetic diversity, population structure and historical population dynamics of Myocastor coypus (Rodentia: Echimyidae) in the centre of its native range

Myocastor coypus is a rodent native to South America that is strongly linked to river systems. Past studies on the coypu in its native distribution range were aimed mainly at its ecology and parasitology and genetic studies are very limited. Here, we used sequences of the mitochondrial D-loop control region to study the genetic diversity, population genetic structure and some aspects of historical population dynamics of coypu at the centre of its native range. Our results showed moderate to high levels of genetic diversity and an absence of genetic structure in the study area. Bayesian analysis of population structure (BAPS) showed the existence of two haplogroups distributed in most sampling sites. These results suggest that movement of coypu is independent of the structure of current known river networks and its dispersal patterns are related to flooding events and the presence of lagoons connecting river basins. The demographic expansion patterns in these populations and those of other South American mammals during the Late Pleistocene support the hypothesis that demographic changes in wild populations are related to fluctuations in climate and ecology. The mitochondrial data obtained in this study constitute the first record of these types of sequences in the native range of M. coypus.

Myocastor coypus es un roedor nativo de Sudamérica fuertemente ligado a los sistemas fluviales. En su área de distribución nativa, los estudios llevados a cabo en la especie estuvieron dirigidos principalmente a estudiar su ecología y zoonosis, sin embargo, los estudios genéticos son muy limitados. En el presente trabajo, utilizamos secuencias del d-loop de la Región Control del ADN mitocondrial para estudiar la diversidad genética, la estructura genética de la población y algunos aspectos de la dinámica poblacional histórica del coipo en el centro de su área de distribución. Nuestros resultados develaron niveles moderados y altos de diversidad genética, y ausencia de estructuración genética en el área de estudio. El BAPS exhibió la existencia de dos haplogrupos distribuidos en la mayoría de los sitios de muestreo. Estos resultados sugieren que el movimiento de los coipos sería independiente de la estructura de las redes fluviales actuales conocidas y que sus patrones de dispersión estarían relacionados con eventos de inundación y con la presencia de lagunas que conectan las cuencas fluviales. Los patrones de expansión demográfica observados en esta población de coipos y en otras poblaciones de mamíferos sudamericanos durante el Pleistoceno tardío, apoyan la hipótesis de que las fluctuaciones climáticas y los cambios ecológicos están relacionados con cambios demográficos en las poblaciones silvestres. Los datos mitocondriales obtenidos en este estudio constituyen el primer registro de este tipo de secuencias en el área de distribución nativa de esta especie.

Phylogeography of Otaria flavescens (Carnivora: Pinnipedia): unravelling genetic connectivity at the southernmost limit of its distribution

The Pleistocene glacial period shaped the current genetic structure of numerous species. The last glacial dynamics has been proposed to have split the South American sea lion, Otaria flavescens, into two Evolutionarily Significant Units (ESUs), one on each side of the continent. However, previous studies have not provided genetic information on colonies found along 3000 km of coastline of the southernmost limit of the species distribution, where gene flow could occur. We conducted an exhaustive phylogeographical analysis of O. flavescens using a mtDNA marker, including, for the first time, data from colonies living south of latitude 45° S, in the Argentinian provinces of Santa Cruz and Tierra del Fuego. Our results indicated the presence of five Conservation Units across the distribution range of O. flavescens and suggest that the Patagonian population must have expanded about 150 000 BP. We found evidence for gene flow across the entire species range, supporting a scenario of secondary contact in Tierra del Fuego where representatives of the oldest lineages coexist. The presence of gene flow between oceans leads us to reject the assumption of complete reciprocal monophyly for mtDNA between the presumed ESUs, suggesting that the species constitutes a single Evolutionarily Significant Unit.