Additive genetic effects of two large effect loci shaping age-at-maturity in Atlantic salmon (Salmo salar)

Fitness-related traits are shaped by both additive and non-additive genetic effects, resulting in a complex and context-dependent genetic architecture. However, the relative contribution of these components remains poorly understood, particularly for major-effect loci underlying life-history traits. In this study, we investigated the additive genetic contributions and potential epistatic interactions between 2 unlinked major-effect loci, vgll3 and six6, that influence age-at-maturity in Atlantic salmon (Salmo salar). By crossing parents heterozygous at both loci to create and rear six large full-sib families in a common-garden environment, we assessed male maturation, and additional fitness related traits, 1-year post fertilization. We found that vgll3 and six6 significantly influenced maturation probability in an additive manner, with the likelihood of early maturation increasing with the number of early-maturing (E) alleles. Double homozygote males with 4 E alleles, vgll3*EE;six6*EE, showed an estimated 60% probability of maturation, compared with only a 20% probability for double late (L) homozygotes, vgll3*LL;six6*LL. Models accounting for the interaction showed no evidence of epistasis between vgll3 and six6, supporting an additive relationship. Additionally, body condition differed by maturation status in a genotype-dependent manner, particularly at vgll3. Studied here in combination for the first time under controlled common garden settings, we show that vgll3 and six6 contribute additively to the regulation of male maturation age in Atlantic salmon. This work, which complements earlier studies conducted separately on these 2 loci, shows the importance of distinguishing additive from non-additive genetic effects when dissecting the genetic architecture of fitness-related traits, as this distinction is fundamental for quantifying heritable variation and more accurately predicting evolutionary responses to selection.