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

Based on 1031 samples of grass snakes from Central Europe, we examine the recently reported occurrence southern subspecies barred snake (Natrix helvetica sicula) in Bavaria, Germany. Using 13 microsatellite loci and mtDNA coding for cytb gene partial ND4 plus adjacent tRNAs, show that N. h. sicula is restricted to a few river valleys (Inn, Isar, Loisach) southernmost Bavaria Tyrol, Austria. At widening into pre-Alpine plains, hybridizes locally with common natrix) bimodal hybrid zone. Our study provides evidence Europe was colonized by Natrix over two distinct immigration routes. In addition previously known western route nominotypical subspecies, leading colonization Rhine region, crossed Alps, most likely using Brenner Pass and/or Reschen Pass. underlines Alps are not an impermeable biogeographic barrier, as often assumed. North combination geographic setting (occurrence sheltered Alpine valleys) population-density-dependent blocking immigrants resident species (N. natrix), acting concert intrinsic genetic factors, prevented formation geographically more extended Unlike helvetica, natrix hybridize north broadly, accordance their better compatibility. Many populations European have been “genetically swamped” vulgaris immigrating Balkans. This led complete replacement n. some regions, where today only persists.