How often do they do it? An in-depth analysis of the hybrid zone of two grass snake species (Natrix astreptophoraandNatrix helvetica)

Abstract

We examined the contact zone of two parapatric species of grass snake (Natrix astreptophora and Natrix helvetica) in southern France. To this end, we used comprehensive sampling, analysed mtDNA sequences and microsatellite loci, and built Species Distribution Models for current and past climatic conditions. The contact zone had 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.

Transalpine dispersal: Italian barred grass snakes in southernmost Bavaria—This far but no further!

“…natrix survived the Last Glacial Maximum (LGM) there. According to species distribution models for the mid‐Holocene and the LGM presented by Asztalos et al (2020), N . h .…”

“…Recent studies based on mitochondrial and nuclear DNA changed this picture. Due to limited gene flow across geographic contact zones, three full species are now recognized (Asztalos et al, 2020; Kindler et al, 2017; Pokrant et al, 2016; Schultze et al, 2019, 2020; Speybroeck et al, 2020). These three species were previously identified with distinct subspecies (Kabisch, 1999; Mertens & Wermuth, 1960).…”

“…These three species were previously identified with distinct subspecies (Kabisch, 1999; Mertens & Wermuth, 1960). The red‐eyed grass snake Natrix astreptophora (Seoane, 1884) occurs in the northern Maghreb region, the Iberian Peninsula, and southwestern France (Asztalos et al, 2020; Geniez, 2015; Pokrant et al, 2016). The barred grass snake Natrix helvetica (Lacepède, 1789) occurs in France, Great Britain, the Benelux countries, in the Rhine region and southern Bavaria (Germany), Switzerland, and in Italy (Geniez, 2015; Glaw et al, 2019; Kindler et al, 2017; Schultze et al, 2020).…”

“…The barred grass snake Natrix helvetica (Lacepède, 1789) occurs in France, Great Britain, the Benelux countries, in the Rhine region and southern Bavaria (Germany), Switzerland, and in Italy (Geniez, 2015; Glaw et al, 2019; Kindler et al, 2017; Schultze et al, 2020). Between the two species, limited bidirectional gene flow was detected in a narrow, bimodal contact zone in southern France (Asztalos et al, 2020; Pokrant et al, 2016). The third species is the common grass snake, Natrix natrix (Linnaeus, 1758) sensu stricto, which is widely distributed from the Rhine region to Lake Baikal in Central Asia.…”

“…natrix and N . helvetica corresponds to a distinct microsatellite cluster and is characterized by at least one distinct mtDNA lineage (Asztalos et al, 2020; Fritz & Schmidtler, 2020; Kindler et al, 2013, 2017; Schultze et al, 2020). Using the terminology of Kindler et al (2013) and Schultze et al (2020), the mtDNA lineage of N .…”