Genetic population structure of the grass snake (Natrix natrix) in human-altered landscapes in Switzerland

Both the conversion of natural habitats to farmland and efforts at increasing the yield of existing crops contribute to a decline in biodiversity. As a consequence of land conversion, specialised species are restricted to remnants of original habitat patches, which are frequently isolated. This may lead to a genetic differentiation of the subpopulations. A lack of gene flow may also result in genetically impoverished subpopulations increasing the risk of local extinction. Highly variable genetic markers, like microsatellites, can be used to investigate genetic differences among subpopulations. The grass snake (Natrix natrix) primarily feeds on amphibians and is therefore associated with wetlands. As a result of pronounced changes in land use, the area actually occupied by the grass snake in Switzerland is restricted to remnants of pristine habitats and the populations are declining in many regions. A few years ago, microsatellite markers were not available for the grass snake. Therefore, six microsatellite markers were developed for the grass snake (N. natrix) and three microsatellite markers of the dice snake (Natrix tessellata) were checked for cross-amplification. These microsatellite loci were used to examine the genetic structure of grass snakes sampled in remnants of pristine habitat embedded in an intensively used agricultural landscape and in a former floodplain in the Swiss lowlands, as well as in a rural valley in the Bernese Alps. The three study areas were 30–100 km apart, but were interconnected by the river Aare. At the local scale, no genetic differentiation was found in either of the N. natrix populations inhabiting the intensively used agricultural area or the rural alpine valley. However, two subpopulations in the former wetland area were genetically differentiated as indicated by a low but significant FST-value. This slight genetic differentiation can be explained by isolation by distance. At the regional scale, significant genetic differentiation between N. natrix populations inhabiting the three study areas was found. The genetic structure was highly related to isolation by distance with 85% of the among-population genetic variance explained only by the geographical distance between subpopulations. The present findings indicate regular gene flow between N. natrix subpopulations. Human activity and habitat alteration do not seem to reduce significantly the movements of grass snakes. These results suggest that conservation actions in landscapes altered by humans should focus on the maintenance of a habitat mosaic with anuran breeding ponds and adequate oviposition sites.
Another aspect of this thesis was to investigate the occurrence and frequency of multiple paternity in the grass snake as a source of genetic diversity. Males can enhance their reproductive success through mating with multiple females. For females, however, one mating is usually sufficient to inseminate all their ova. Females may benefit from multiple mating by producing genetically more diverse offspring, and by having the opportunity to choose sperm of the genetically most compatible male. The frequency of multiple paternity was assessed in 11 clutches of the grass snake using the microsatellite markers. Two and more fathers were found to sire offspring in 27% of the clutches using a very conservative estimate. However, based on a maximum likelihood, multiple paternity occurred in 91% of the clutches with 2–5 contributing males per clutch. This is the first study demonstrating multiple paternity in a European natricine, with a frequency similar to those found in new world natricines.
To sum up, this thesis demonstrated that the genetic variability in grass snake populations is maintained by regular gene flow between subpopulations, and through multiple mating by females resulting in multiple paternity.