Dynamics of introgressive hybridization assessed by SNP population genomics of coding genes in stocked brook charr (Salvelinus fontinalis)

Salmonid fishes rank among species being most severely affected by introgressive hybridization as a result of a long tradition of stocking with hatchery-reared conspecifics. The objectives of this study were (i) to evaluate the genetic consequences of stocking and resulting introgression rates on the genetic integrity of natural populations of brook charr, (ii) to identify genomic regions potentially associated with adaptation to natural and artificial rearing environments, and (iii) to test the null hypothesis that introgression from domesticated brook charr into wild populations is homogeneous among loci. A total of 336 individuals were sampled from nine lakes, which were stocked at different intensities with domestic fish. Individuals were genotyped at 280 SNPs located in transcribed regions and developed by means of next-generation sequencing. As previously reported with microsatellites, we observed a positive relationship between stocking intensity and genetic diversity among stocking groups, and a decrease in population differentiation. Individual admixture proportions also increased with stocking intensity. Moreover, genomic cline analysis revealed 27 SNPs, seven of which were also identified as outliers in a genome scan, which showed an introgression rate either more restricted or enhanced relative to neutral expectations. This indicated that selection, mainly for growth-related biological processes, has favored or hampered the introgression of genomic blocks into the introgressed wild populations. Overall, this study highlights the usefulness of investigating the impact of stocking on the dynamics of introgression of potentially adaptive genetic variation to better understand the consequences of such practice on the genomic integrity of wild populations.