How often do they do it? An in-depth analysis of the hybrid zone of two grass snake species (Natrix astreptophora, N. helvetica). Hybrid zone of Natrix astreptophora and N. helvetica

We examined the contact zone of two parapatric species of grass snakes (Natrix astreptophora and N. helvetica) in southern France. To this end, we used comprehensive sampling, analyzed mtDNA sequences and microsatellite loci, and built Species Distribution Models for current and past climatic conditions. The contact zone established by the mid-Holocene during range expansions from glacial refuges in the Iberian Peninsula (N. astreptophora) and southern or western France (N. helvetica). The contact zone represents a narrow bimodal hybrid zone, with steep genetic transition from one taxon to the other and rare hybridization, supporting species status for N. astreptophora and N. helvetica. Our results suggest that the steepness of the clines is a more robust tool for species delimitation than cline width. In addition, we discovered in western France, beyond the hybrid zone, a remote population of N. helvetica with genetic signatures of hybridization with N. astreptophora, most likely the result of human-mediated long-distance dispersal. For N. helvetica, we identified a southern and a northern population cluster, connected by broad-scale gene flow in a unimodal hybrid zone running across France. This pattern either reflects genetic divergence caused by allopatry in two microrefuges and subsequent secondary contact or introgression of foreign alleles into the southern cluster.