How many species of grass snakes are out there? Phylogeography, gene flow across contact zones and taxonomy of Natrix natrix

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The grass snake (Natrix natrix) is the most common and one of the most widely distributed snake species of the Palaearctic region. Recent research on its mitochondrial phylogeography has shown major mismatches between subspecies delineation and in part deeply divergent mitochondrial clades. Further unexpected findings were the clear-cut parapatric distribution of mitochondrial haplotypes of western grass snakes (N. n. helvetica) and eastern grass snakes (currently identified with N. n. natrix) in the Rhine region and the discovery of a contact zone of two newly identified distinct mitochondrial lineages running across the Central European range of eastern grass snakes. Our project analyses the underlying genetic patterns in more detail with the aim of scrutinizing taxonomy and phylogeography of grass snakes. We use large-scale sampling from three contact zones of genetic lineages representing the three most deeply divergent mitochondrial clades of grass snakes plus the newly discovered Central European contact zone. For each contact zone, we examine possible gene flow between distinct lineages using population genetic and phylogenetic approaches based on 13 microsatellite loci and two mitochondrial genes. As a first result, we could show that the Ibero-Maghrebian grass snake represents a distinct species (N. astreptophora), which is reproductively largely isolated from the geographically neighbouring taxon (N. n. helvetica). Our preliminary results for the other two contact zones provide evidence for only very limited hybridization between N. n. helvetica and N. n. natrix, being restricted to a narrow, 10-km-wide strip. By contrast, there is broad admixture between the genetic lineages within N. n. natrix. This provides evidence that the different taxa and genetic lineages of grass snakes represent different stages of the speciation process. Our taxonomic conclusions will be founded on the population genetic and phylogeographic framework to be obtained in our project. Already now it is clear that the taxonomy of the grass snake requires substantial revision. Moreover, using the above mentioned marker systems, we test different biogeographical hypotheses regarding the location of glacial refugia. According to our preliminary results, the glacial refugia of the examined lineages were located in the southern Balkan Peninsula. Ancestral polymorphism clearly cannot explain the observed mitochondrial variation.