Neumann, A., M. Asztalos, U. Fritz & F. Glaw

A spotlight on the hybrid zone of grass snakes (Natrix helvetica sicula and Natrix natrix) in southern Bavaria – the Prien Valley. pp. 17-28 plus Supplementary Material.

Abstract. Based on dense sampling, we examined hybridization between Natrix helvetica sicula and N. natrix in a 24-km long transect in the Prien Valley (southern Bavaria) using phenotypic data (six coloration and pattern traits), mtDNA sequences, and 13 polymorphic microsatellite loci. All three lines of evidence yielded concordant results. Genotypically pure or nearly pure N. h. sicula are restricted to the uppermost part of the valley in the mountains, whereas genotypically pure or nearly pure N. natrix occur downstream in the lowlands. The turnover between the two species takes place in a narrow stretch of only 4 km width in the central Prien Valley, where the flanking mountains spread out and the valley widens to the adjacent northern lowlands. We hypothesize that this extremely narrow hybrid zone is stabilized by different environmental preferences and selection against hybrids, acting in concert with competition and high-density blocking. Our study is the first investigation examining hybridization of the two grass snake species at a small spatial scale. The revealed transition zone is much narrower than previously modelled hybrid zones using cline analyses. This suggests that cline analyses are sensitive to sampling density. Melanistic grass snakes were restricted to the highest studied elevations, suggesting a correlation of elevation and pigmentation. Due to its linear structure, the Prien Valley offers a model area for future studies to examine the effect of environmental and climate change on a currently stable and narrow hybrid zone.

Key words. Squamata, Serpentes, Colubridae, Natricidae, Bavaria, Germany, hybridization.