Conservation genetics of the Adriatic grayling: recent and historical exotic imprints on native Adriatic Thymallus thymallus populations

Patterns of population structure and possible genetic introgression have not exhaustively been investigated so far for the 'Adriatic grayling', Thymallus thymallus, despite its conservation critical risk status owing to stocking-induced secondary contact. We analysed 683 grayling from 30 Adriatic sites and from Danubian and Atlantic reference samples, merging mtDNA sequence and microsatellite genotypic data and applying model-based clustering as well as Approximate Bayesian Computation (ABC). Remnant Adriatic populations displayed different frequencies of exotic genetic signatures, spanning from marginal genetic introgression to the total collapse of native genepools. Genetic introgression involved multiple exotic source populations of Danubian and Atlantic origin and evidenced the negative impact that few decades of stocking provoked on the original genetic architecture of Adriatic grayling. Within the Adige River system, a contact zone of Western Adriatic and Eastern Danubian grayling populations was highlighted, with ABC analyses suggesting a historical anthropogenic origin of Eastern Adige populations, most likely founded by medieval stocking. Based on genetic results, we propose a catalogue of management measures, including the immediate legal prohibition of stocking exotic grayling strains and the use of marker-assisted genetic selection in supportive- and captive-breeding programs. Finally, we point to substantial river-specific population substructure within the ‘Adriatic Grayling’ Evolutionary Significant Unit, worth to be considered in future restoration programs.