Abstract
Recent studies found major conflicts between traditional taxonomy and genetic differentiation of grass snakes and identified previously unknown secondary contact zones. Until now, little is known about gene flow across these contact zones. Using two mitochondrial markers and 13 microsatellite loci, we examined two contact zones. One, largely corresponding to the Rhine region, involves the western subspecies Natrix natrix helvetica and the eastern subspecies N. n. natrix, whereas in the other, more easterly, contact zone two lineages meet that are currently identified with N. n. natrix and N. n. persa. This second contact zone runs across Central Europe to the southern Balkans. Our analyses reveal that the western contact zone is narrow, with parapatrically distributed mitochondrial lineages and limited, largely unidirectional nuclear gene flow. In contrast, the eastern contact zone is very wide, with massive nuclear admixture and broadly overlapping mitochondrial lineages. In combination with additional lines of evidence (morphology, phylogeny, divergence times), we conclude that these differences reflect different stages in the speciation process and that Natrix helvetica should be regarded as a distinct species. We suggest a nomenclatural framework for presently recognized grass snake taxa and highlight the need for reconciling the conflicts between genetics and taxonomy.
Highlights
Even though species delimitation became a Renaissance issue in zoology, with new approaches being developed for assessing species boundaries[1,2,3,4,5], the validity of many approaches largely depends on the underlying species concept
In contrast to the situation in the Rhine region, the haplotypes of these two eastern lineages occur in wide sympatry and represent lineages that are placed in phylogenetic analyses in the same major clade, while the mitochondrial lineage of N. n. helvetica belongs to another one of the three major clades (Supplementary Fig. S1)
This second contact zone runs across Central Europe to the southern Balkans and concerns two genetic lineages (‘red’ and ‘yellow lineages’), which occur within the distribution ranges of N. n. natrix and N. n. persa
Summary
Even though species delimitation became a Renaissance issue in zoology, with new approaches being developed for assessing species boundaries[1,2,3,4,5], the validity of many approaches largely depends on the underlying species concept. One of these major clades matches with the Ibero-Maghrebian taxon astreptophora, which had traditionally been recognized as a subspecies of N. natrix. Helvetica) and concordant morphological and genetic evidence, Ibero-Maghrebian grass snakes have recently been split off as the distinct species N. astreptophora[20]. In agreement with earlier morphological investigations[27], the recent phylogeographic assessment of grass snakes[26] revealed a contact zone of two deeply divergent mitochondrial lineages for the Rhine region, with an unexpectedly clear-cut parapatric distribution pattern. Persa in the Balkan Peninsula comprise a previously unknown, more easterly located, contact zone of two distinct lineages, which conflict with morphological taxon delimitation and occur across the distribution ranges of N. n. The previous sampling[26] was increased fourfold and mitochondrial DNA data (1,983 bp) were combined with evidence from 13 highly polymorphic nuclear microsatellite loci
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