Abstract
The distribution and hybridisation zone of the two grass snake species occurring in the German state of Baden-Württemberg are described, based on genetic data from maternally inherited mitochondrial DNA (mtDNA, up to 1983 bp) and biparentally inherited microsatellite DNA (13 loci). In agreement with previously published morphological evidence, the barred grass snake (Natrix helvetica) occurs in the Upper Rhine Valley and the Black Forest, while the common grass snake (N. natrix, ‘yellow lineage’) is distributed across the remaining, more eastern parts of Baden-Württemberg. Cline analyses across two transects running through the region of Karlsruhe and the Black Forest indicate that the hybrid zone is similarly narrow here as in the previously characterised stretch near Lake Constance. With respect to nuclear DNA, the Black Forest constitutes no impediment to gene flow in comparison with lowland regions (Karlsruhe, Lake Constance). However, on the eastern slope of the Black Forest, the abrupt replacement of mtDNA ofN. helveticaby that ofN. natrixindicates male-mediated gene flow and that the Black Forest represents a dispersal barrier for female grass snakes.
Highlights
Grass snakes were, until recently, thought to represent a widespread species distributed from North-western Africa and the Iberian Peninsula across much of Western, Central, Northern and Eastern Europe into Central Asia (Kabisch 1999)
Our study genetically confirms the occurrence of two species of grass snake in Baden-Württemberg and defines, for the first time, the exact location of the contact zone of Natrix helvetica and N. natrix, in which the two species hybridise
The contact zone runs from the region of Karlsruhe along the eastern slope of the Black Forest to the Lake Constance region
Summary
Until recently, thought to represent a widespread species distributed from North-western Africa and the Iberian Peninsula across much of Western, Central, Northern and Eastern Europe into Central Asia (Kabisch 1999). Genetic investigations revealed three deeply divergent genealogical lineages without or with very restricted, unidirectional gene flow and prompted the recognition of three distinct species (Kindler et al 2013, 2014, 2017, 2018a, b; Pokrant et al 2016). According to these investigations, the red-eyed grass snake, Natrix astreptophora (Seoane, 1884), occurs in North-western Africa, the Iberian Peninsula and South-western France.
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