Genetic structuring, dispersal and hybridization in European vipers

The asp viper (Vipera aspis) and the adder (Vipera berus) are small venomous snake species. Because both species have large distribution ranges, and as there is insufficient data on their decline, they are considered as least concern on the European red list of threatened species, although they appear to face extinction in numerous regions. Even if the species are well-studied, many aspects of their biology are still unknown. Genetic markers are useful to investigate genetic diversity, population structure and various life-history traits of many species. Such genetic analyses are especially useful to study species with cryptic behaviour whose life-history traits can be hardly investigated in the field directly. Cost- and time-efficient microsatellite markers are still the preferable tool for many population and conservation genetics studies, even if next-generation-sequencing allows to examine whole genomes nowadays. This thesis is composed of five studies focusing on the development of specific markers and their application in assessing genetic diversity, population structure, sex-biased dispersal and hybridization of viper species.
Investigating genetic diversity of endangered species is essential to detect populations which require special consideration in conservation measures. In the Pas-de-Calais department in France, populations of V. berus were studied in the course of an action plan on the functionality of calcareous grasslands what led to the first study of this thesis. Limited genetic diversity of the five investigated populations was revealed. One of the populations showed an especially low level of diversity and seems isolated from other populations. Therefore, it is crucial to thoroughly monitor this population in the future to detect a possible change in the population dynamics due to a lack of genetic heterogeneity.
Microsatellite loci are highly polymorphic, codominant, easily typed, Mendelian inherited and noncoding, therefore they are ideal genetic markers not only for conservation genetics studies. Particularly in threatened species, it is of importance to investigate population structure, genetic diversity and gene flow within and between populations to assess their chance of survival and to take appropriate conservation measures. Since no specific microsatellite markers were available for V. aspis at that time, the second study aimed to develop 12 new polymorphic microsatellite markers for V. aspis, of which eight can also be amplified in V. berus and three in Vipera ammodytes.
V. aspis is morphologically highly variable species not only showing a variety of different colourations but sometimes also deviations from their typical dorsal zigzag pattern with unicoloured individuals (without any pattern) or individuals with a dorsal line only (both referred to as “concolour”). In a specific location in the French Alps, a high density of these particular concolour individuals has been observed. In the third study, I compared the genetic with the morphological variation of individuals from different populations of this area and detected a local selection for the concolour phenotype. A local diversifying selection suggests better adaptation of the concolour individuals to their mainly rocky habitat. I also found evidence for colour-biased dispersal with blotched individuals dispersing further than concolour individuals. Within blotched individuals, females dispersed more than males, but no sex-biased dispersal could be detected in the overall sample.
To understand how V. aspis maintains gene flow between populations or colonises newly created or restored habitats, it is crucial to study dispersal behaviour. Dispersal behaviour often differs between sexes or between locations which is of importance, for example, for the design of monitoring programs. While there is a strong tendency towards female-biased dispersal in birds and male-biased dispersal in mammals, only a few studies demonstrated male-biased dispersal in reptiles. As it was shown in other species, a pattern of sex-biased dispersal can vary among different habitats. Therefore, I conducted the fourth study at two sites in the alpine region of Switzerland and two sites in the lowlands and investigated in each region one population in a fragmented and one population in an unfragmented habitat. We found that in unfragmented habitats, males dispersed further than females while the latter showed the same dispersal propensity in all the study sites irrespective of the habitat.
Even if there are several contact zones between V. aspis and V. berus, hybridization between the two species likely only occurs in two of them. The fifth study aimed to evaluate the hybridization rate between V. aspis and V. berus in the Loire-Atlantique department and to further investigate morphologically intermediate as well as typical individuals of both parental species. I showed that all the investigated individuals with intermediate morphological traits were hybrids, while no hybrids with typical morphological traits of either of the parental species were detected. We also found that the hybridization rate is quite low (1.5–3%) and that mainly first-generation hybrids occur. My results support a unidirectional hybridization with V. aspis as the maternal and V. berus as the paternal parent.
Overall, the findings of my thesis highlight the importance to study numerous aspects of a species’ biology in different locations to be able to apply the most appropriate conservation measures in the respective habitats to save it from extinction.