Deceiving appearances: anthropogenic introgressive hybridization affects phenotypically-selected hatchery broodstock used in supportive breeding programmes of the critically endangered marble trout Salmo marmoratus, Cuvier (Osteichthyes, Salmoniformes, Salmonidae).

Anthropogenic introgressive hybridization is increasingly common in many ecosystems, with inland waters being particularly sensitive to bioinvasions of interfertile non-native taxa. Salmonid native populations are detrimentally affected by such human-induced secondary contacts in several countries promoting sport and commercial salmonid fisheries. In northern Italy, the Critically Endangered marble trout Salmo marmoratus Cuvier, 1829, is a species with exceptional cultural, economic, and conservation value. Supporting breeding programs based on phenotypic selection of wild-caught and captive broodstock is a widespread management practice to supplement local marble populations with both declared conservation and exploitation goals. Using mtDNA and nDNA markers (D-loop; 15 microsatellites) we compare a hatchery-bred sample of marble trout used as broodstock for support breeding with a smaller sample of wild-caught phenotypically selected individuals collected in the Toce River, a large glacial river within the Lake Maggiore basin (northern Italy and southern Switzerland). We measured genetic integrity and introgression levels with non-native Atlantic S. trutta, a non-native species massively introduced in the last two centuries. Consistent levels of introgression were found in these two samples, but asymmetrical introgression with higher mtDNA introgression levels was observed in hatchery-bred individuals. The detected genetic structure of wild-caught and domesticated stocks suggest that in synergy with potential domestication effects, inbreeding and genetic drift, caused by small number of founders and closed reproductive cycle, this practice can have negative effects on the genetic diversity and integrity of the wild population. On the other hand, our preliminary analysis of the Toce River sample suggests that this population may contain genetically pure individuals, thus acting as a key repository of genetic diversity for the long-term conservation of this species. The lack of correspondence between a set of coded coloration traits and genetic traits in both samples highlights the limits of using phenotypic selection during artificial breeding, as also supported by previous studies on this species. Our results strongly suggest to revise current supportive breeding programmes of the marble trout in this system. We finally propose possible future directions for the restoration and sustainable management of the genetic diversity of the Toce River marble trout population.