Genetic introgression between wild and stocked salmonids and the prospects for using molecular markers in population rehabilitation: the case of the Adriatic grayling (Thymallus thymallus L. 1785)

In the north Adriatic basin, a morphologically and genetically distinct lineage of grayling is found, designated as the Adriatic grayling. In Slovenia, the Adriatic grayling is restricted to the Soca river system, where it is critically endangered. The most pertinent threat is stocking with non-native, highly divergent Sava (Danubian) drainage stock, and this activity has been going on for more than four decades. The present study was designed to characterise the genetic structure of the Adriatic grayling in Slovenia, with particular emphasis on estimating the degree of introgression with non-indigenous stocked grayling. We analysed polymorphism at 154 microsatellite loci in samples representing grayling from the Adriatic and Danubian drainage stock. A relatively high number (12) of alleles, diagnostic for the Adriatic grayling, were identified. However, a correspondence analysis based on individual multilocus genotypes also revealed that there is no distinctive Adriatic group but rather a dispersed multitude of individuals that cannot be unambiguously distinguished from the more homogenous Danubian population. A Bayesian analysis of individual admixture coefficients confirmed this pattern and revealed extensive introgression between the Adriatic grayling and stocked grayling of Danubian origin. Average individual admixture coefficients showed that only between 50 and 60% of the original gene pools remained, and only few non-introgressed indigenous individuals could be identified. Microsatellite-based individual admixture analysis appear to be an important tool for identifying remaining non-introgressed indigenous individuals that could be used for restoring the original populations.