Fine-scale genetic structure and diversity in native and introduced Arctic charr populations

Genetic diversity is a central indicator for assessing the conservation status, since it influences evolutionary processes and demographic dynamics of populations. In fragmented habitats, introduced populations are often genetically impoverished due to founder effects and isolation, potentially leading to reduced population viability. Here, we investigated the genetic structure, diversity and demographic history of native and introduced populations of Arctic charr ( Salvelinus alpinus ) inhabiting peri-Alpine and Alpine lakes using SNP and microsatellite (STR) markers. Contrary to expectations of genetic erosion from founder effects, most introduced populations displayed moderate levels of genetic diversity, although effective population sizes were variable and often lower than those of native populations. Alpine populations form a heterogeneous genetic mosaic structured into several genetic clusters, reflecting distinct introduction histories and isolation. Historical demographic trajectories indicated a general decline in effective population size, consistent with genetic drift in small and isolated populations. Estimates were significantly correlated between SNPs and STRs, confirming that both marker types provide reliable estimates of population genetic structure. These findings suggest that established introduced populations may contribute to maintaining genetic diversity in threatened species at their thermal margin. However, this should not be interpreted as support for introductions, given the risks of hybridization, introgression and loss of local adaptation. Management strategies need to consider populations individually, as translocations may disrupt genetic structure but could also support the conservation of declining native populations.