Characterization of natural variation in North American Atlantic Salmon populations (Salmonidae: Salmo salar) at a locus with a major effect on sea age

Age at maturity is a key life-history trait of most organisms. In anadromous salmonid fishes such as Atlantic Salmon (Salmo salar), age at sexual maturity is associated with sea age, the number of years spent at sea before the spawning migration. For the first time, we investigated the presence of two nonsynonymous vgll3 polymorphisms in North American Atlantic Salmon populations that relate to sea age in European salmon and quantified the natural variation at these and two additional candidate SNPs from two other genes. A targeted resequencing assay was developed and 1,505 returning adult individuals of size-inferred sea age and sex from four populations were genotyped. Across three of four populations sampled in Quebec, Canada, the late-maturing component (MSW) of the population of a given sex exhibited higher proportions of SNP genotypes 54Thr(vgll3) and 323Lys(vgll3) compared to early-maturing fish (1SW), for example, 85% versus 53% of females from Trinite River carried 323Lys(vgll3) (n(MSW)=205 vs. n(1SW)=30; p<.001). However, the association between vgll3 polymorphism and sea age was more pronounced in females than in males in the rivers we studied. Logistic regression analysis of vgll3 SNP genotypes revealed increased probabilities of exhibiting higher sea age for 54Thr(vgll3) and 323Lys(vgll3) genotypes compared to alternative genotypes, depending on population and sex. Moreover, individuals carrying the heterozygous vgll3 SNP genotypes were more likely (>66%) to be female. In summary, two nonsynonymous vgll3 polymorphisms were confirmed in North American populations of Atlantic Salmon and our results suggest that variation at those loci correlates with sea age and sex. Our results also suggest that this correlation varies among populations. Future work would benefit from a more balanced sampling and from adding data on juvenile riverine life stages to contrast our data.