“Ich bin ein Berliner” – Genetic identity and admixture of alien grass snakes (Natrix natrix) in Berlin, Germany

Hybridization of native species with introduced closely related taxa is of concern for nature conservation due to the risk of outbreeding depression and genetic swamping. Using mitochondrial DNA sequences and 13 microsatellite loci, we provide first genetic evidence that native grass snake (Natrix natrix natrix) populations in Berlin have been compromised by the introduction of at least one foreign subspecies (N. n. moreotica), originating from the southern Balkans and western Asia Minor. Genetic signatures of a third subspecies (N. n. vulgaris) in the same populations could result either from additional introductions or natural range expansion from the south. Dorsal stripes, absent in N. n. natrix but characteristic of the other two subspecies, have been documented in at least seven independent populations across Berlin, even where the genetic signal detected by the applied markers has been lost through backcrossing with native snakes. It remains unclear whether these hybrid populations will experience reduced fitness in the long term. We therefore recommend close monitoring of these populations and, wherever feasible, containing the dispersal of hybrid snakes.