Abstract
The diversity of reproduction and associated mating patterns in Atlantic salmon (Salmo salar) has long captivated evolutionary biologists. Salmo salar exhibit strategies involving migration, bold mating behaviours and radical morphological and physiological change. One such radical change is the elongation and curvature of the lower jaw in sexually mature males into a hook-like appendage called the kype. The kype is a secondary sexual characteristic used in mating hierarchies and a prime candidate for sexual selection. As one of the core global aquaculture fish species, however, mate choice, and thus sexual selection, has been replaced by industrial artificial fertilization seeking to develop more commercially viable strains. Removal of mate choice provides a unique opportunity to examine the kype over successive generations in the absence of sexual selection. Here we use a large-scale common-garden experiment, incorporating six experimental strains (wild, farmed and wild × farmed hybrids), experiencing one to three sea winters, to assess the impact of age and genetic background. After controlling for allometry, fork length-adjusted kype height (AKH) was significantly reduced in the domesticated strain in comparison to two wild strains. Furthermore, genetic variation at a locus on linkage group SSA1 was associated with kype height, and a locus on linkage group SSA23 was associated with fork length-adjusted kype length (AKL). The reduction in fork length-AKH in domesticated salmon suggests that the kype is of importance in mate choice and that it has decreased due to relaxation of sexual selection. Fork length-AKL showed an increase in domesticated individuals, highlighting that it may not be an important cue in mate choice. These results give us insight into the evolutionary significance of the kype, as well as implications of genetic induced phenotypic change caused by domesticated individuals escaping into the natural environment.
Highlights
Atlantic salmon (Salmo salar) are predominantly anadromous salmonid fish that inhabit coldwater streams on both sides of the northern Atlantic during the freshwater stage of their life cycle
To control for allometry, kype length and kype height were adjusted for fork length, and the residuals generated from an ordinary least-squares linear regression between the two variables were used to produce fork length-adjusted kype length (AKL) and fork lengthAKH
Estimated relative proportion of quantitative trait loci (QTL) variance attributed to KH from the hierarchical generalized linear model (HGLM) was 6.6%. This is the first study to investigate the potential influence of domestication on head morphology in sexually mature adult male Atlantic salmon where sexual selection has been relaxed compared to the wild
Summary
Atlantic salmon (Salmo salar) are predominantly anadromous salmonid fish that inhabit coldwater streams on both sides of the northern Atlantic during the freshwater stage of their life cycle. It undergoes key phenotypic changes manifested in variable morphology [2], physiology [3] and behaviour [4]. Such fundamental biological change is required for survival in both freshwater and marine environments; environments with radically different abiotic and biotic conditions, ranging from differences in salinity and temperature profiles, to changes in competition with conspecifics. The wealth of life-history variation, both among and within populations [5], renders Atlantic salmon well suited for investigating the roles of phenotypic plasticity and heritable genetic change in generating variation for maximized reproductive success [6]. The disparity between mating males and fertile females generates intense male–male competition, fuelled by the increased fertilization success seen in those males who fertilize eggs first [10]
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