Abstract
BackgroundMature male parr (MMP) represent an important alternative life-history strategy in Atlantic salmon populations. Previous studies indicate that the maturation size threshold for male parr varies among wild populations and is influenced by individual growth, environmental conditions, and genetics. More than ten generations of breeding have resulted in domesticated salmon displaying many genetic differences to wild salmon, including greatly increased growth rates. This may have resulted in domesticated fish with the potential to outgrow the size threshold for early maturation, or evolution of the size threshold of the trait itself. To investigate this, we performed a common-garden experiment under farming conditions using 4680 salmon from 39 families representing four wild, two wild-domesticated hybrid, and two domesticated strains.ResultsDomesticated salmon outgrew wild salmon 2–5-fold, and hybrids displayed intermediate growth. Overall, the numbers of MMP varied greatly among families and strains: averaging 4–12% in domesticated, 18–25% in hybrid, and 43–74% in the wild populations. However, when the influence of growth was accounted for, by dividing fish into lower and upper size modes, no difference in the incidence of MMP was detected among domesticated and wild strains in either size mode. In the lower size mode, hybrids displayed significantly lower incidences of mature males than their wild parental strains. No consistent differences in the body size of MMP, connected to domestication, was detected.ConclusionsOur data demonstrate: 1- no evidence for the evolution of the size threshold for MMP in domesticated salmon, 2- the vastly lower incidence of MMP in domesticated strains under aquaculture conditions is primarily due to their genetically increased growth rate causing them to outgrow the size threshold for early maturation, 3- the incidence of MMP is likely to overlap among domesticated and wild salmon in the natural habitat where they typically display overlapping growth, although hybrid offspring may display lower incidences of mature male parr. These results have implications for wild salmon populations that are exposed to introgression from domesticated escapees.
Highlights
Mature male parr (MMP) represent an important alternative life-history strategy in Atlantic salmon populations
The present study demonstrated that while strains of domesticated and hybrid salmon all displayed reduced numbers of MMP compared to several wild strains when reared together under farming conditions, this is primarily a response to selection for increased growth under these conditions, causing them to bypass the size threshold for parr maturation
We found no evidence that the size threshold for MMP has evolved in domesticated salmon, and our results indicate that the propensity to mature as parr is as likely in domesticated salmon as in wild salmon under natural conditions where their growth rates are observed to be similar [26, 33, 34]
Summary
Mature male parr (MMP) represent an important alternative life-history strategy in Atlantic salmon populations. More than ten generations of breeding have resulted in domesticated salmon displaying many genetic differences to wild salmon, including greatly increased growth rates This may have resulted in domesticated fish with the potential to outgrow the size threshold for early maturation, or evolution of the size threshold of the trait itself. Atlantic salmon (Salmo salar L.) display high levels of phenotypic and life history plasticity, both within and among populations and regions [1]. Some of this variation is underpinned by genetic variation, and may reflect adaptations to local environmental conditions [2, 3]. The life-history strategy adopted by male parr is in part influenced by life-history trade-offs between early maturation (MMP) with limited reproductive contribution but increased probability to survive to the reproductive age, and late (adult) maturation and potentially greater reproductive contribution but low probability to survive to the reproductive age [14]
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