Variable outcomes of hybridization between declining and .

Hybridization dynamics between co-occurring species in environments where human-mediated changes take place are important to quantify for furthering our understanding of human impacts on species evolution and for informing management. The allis shad (Linnaeus, 1758) and twaite shad (Lacépède, 1803), two clupeids sister species, have been severely impacted by human activities across Europe. The shrinkage of distribution range along with the decline of the remaining populations' abundance threatens its persistence. The main objective was to evaluate the extent of hybridization and introgression between those interacting species. We developed a set of 77 species-specific SNP loci that allowed a better resolution than morphological traits as they enabled the detection of hybrids up to the third generation. Variable rates of contemporary hybridization and introgression patterns were detected in 12 studied sites across the French Atlantic coast. Mitochondrial markers revealed a cyto-nuclear discordance almost invariably involving individuals with an mitochondrial DNA and provided evidence of historical asymmetric introgression. Overall, contemporary and historical introgression revealed by nuclear and mitochondrial markers strongly suggests that a transfer of genes occurs from toward genome since at least four generations. Moreover, the outcomes of introgression greatly depend on the catchments where local processes are thought to occur. Undoubtedly, interspecific interaction and gene flow should not be overlooked when considering the management of those species.