Distribution and hybridisation of barred and common grass snakes (Natrix helvetica, N. natrix) in Baden-Württemberg, South-western Germany

The Endangered Sardinian Grass Snake: Distribution Update, Bioclimatic Niche Modelling, Dorsal Pattern Characterisation, and Literature Review

The Sardinian grass snake, Natrix helvetica cetti, is an endangered endemic snake subspecies with a restricted and highly fragmented geographic distribution. Information on its ecology and detailed geographic distribution are scarce and may negatively impact on its conservation status. Therefore, a literature review on its taxonomy, morphology, ecology, and conservation is presented here. Moreover, field records from the authors, citizen science and the existing literature provide an updated geographic distribution highlighting its presence within 13 new and 7 historic 10 × 10 km cells. Bioclimatic niche modelling was then applied to explore patterns of habitat suitability and phenotypic variation within N. h. cetti. The geographic distribution of the species was found to be positively correlated with altitude and precipitation values, whereas temperature showed a negative correlation. Taken together, these outcomes may explain the snake's presence, particularly in eastern Sardinia. In addition, analysis of distribution overlap with the competing viperine snake (N. maura) and the urodeles as possible overlooked trophic resources (Speleomantes spp. and Euproctus platycephalus) showed overlaps of 66% and 79%, respectively. Finally, geographical or bioclimatic correlations did not explain phenotypic variation patterns observed in this highly polymorphic taxon. Perspectives on future research to investigate N. h. cetti's decline and support effective conservation measures are discussed.

Genotyping the phenotypic diversity in Aegean Natrixnatrixmoreotica (Bedriaga, 1882) (Reptilia, Serpentes, Natricidae)

We examined the mitochondrial identity of Aegean Natrixnatrixmoreotica representing different morphotypes, with a focus on new material from Milos and Skyros. We found no correlation between distinct morphotypes and mitochondrial identity. Our results support that grass snake populations are polyphenetic and that southern subspecies, including island populations, show a higher variability than northern ones.

Rare genetic admixture and unidirectional gene flow between Vipera aspis and Vipera berus at their contact zone in western France

Asp vipers (Vipera aspis) and adders (Vipera berus) coexist in the Loire-Atlantique department in France where the two species reach their respective range limits. This contact zone is of special interest since hybridization has been recently discovered there. We carried out extensive sampling to further investigate the hybrid status of morphologically typical individuals and to evaluate the proportion of genetically admixed individuals in this area. Using microsatellite markers, no hybrids with typical morphological traits of either asp vipers or adders were detected. All recently investigated individuals with intermediate morphological traits were shown to be hybrids. A rather low proportion of genetically admixed individuals (1.5-3%) and a very small number of supposed second-generation hybrids suggest reduced fertility of first-generation hybrids or low viability of their progeny. The investigation of mtDNA of newly sampled hybrids support the finding that hybridization only occurs between female V. aspis and male V. berus. Several possible explanations for the unidirectional hybridization are discussed and consequent future studies suggested.

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

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.