Harald Sægrov

Sex-dependent dominance at a single locus maintains variation in age at maturity in salmon

Males and females share many traits that have a common genetic basis; however, selection on these traits often differs between the sexes, leading to sexual conflict. Under such sexual antagonism, theory predicts the evolution of genetic architectures that resolve this sexual conflict. Yet, despite intense theoretical and empirical interest, the specific loci underlying sexually antagonistic phenotypes have rarely been identified, limiting our understanding of how sexual conflict impacts genome evolution and the maintenance of genetic diversity. Here we identify a large effect locus controlling age at maturity in Atlantic salmon (Salmo salar), an important fitness trait in which selection favours earlier maturation in males than females, and show it is a clear example of sex-dependent dominance that reduces intralocus sexual conflict and maintains adaptive variation in wild populations. Using high-density single nucleotide polymorphism data across 57 wild populations and whole genome re-sequencing, we find that the vestigial-like family member 3 gene (VGLL3) exhibits sex-dependent dominance in salmon, promoting earlier and later maturation in males and females, respectively. VGLL3, an adiposity regulator associated with size and age at maturity in humans, explained 39% of phenotypic variation, an unexpectedly large proportion for what is usually considered a highly polygenic trait. Such large effects are predicted under balancing selection from either sexually antagonistic or spatially varying selection. Our results provide the first empirical example of dominance reversal allowing greater optimization of phenotypes within each sex, contributing to the resolution of sexual conflict in a major and widespread evolutionary trade-off between age and size at maturity. They also provide key empirical evidence for how variation in reproductive strategies can be maintained over large geographical scales. We anticipate these findings will have a substantial impact on population management in a range of harvested species where trends towards earlier maturation have been observed.

Documentation of successful spawning of escaped farmed female Atlantic salmon, Salmo salar, in Norwegian rivers

Abstract Spawning of escaped farmed female Atlantic salmon is reported. Samples of fertilized eggs and alevins from two Norwegian rivers showed that the number of redds spawned by escaped salmon were three out of eight and two out of twelve, respectively. In a third river three out of three samples redds were spawned by farmed females, but all the eggs sampled in this river were unfertilized. This study confirms successful spawning of farmed female escapees and suggests their success is similar, or slightly lower, compared to wild spawners in the same area. Farmed females are able to locate suitable spawning sites. Gene-flow from farmed to wild stocks of Atlantic salmon can thus not be neglected as an influence on the genetic structure of local wild populations. Further, farmed spawners may reduce the spawning success of wild females by digging up their eggs.

Dimensionless numbers and life history variation in Brown Trout

SummaryDimensionless numbers, made up from components of life history as defined by growth, mortality and maturation, may provide fresh insights into life history evolution. Most studies have previously shown that these numbers are more or less constants within taxa. The variation between taxa may clarify the evolution of different life histories. We examine the variation in three dimensionless numbers using data from 29 populations of Brown TroutSalmo trutta from Norway, and find that the dimensionless numbers are not constants for the Brown Trout populations. We find that the relationship betweenK of the von Bertalanffy growth equation and the mortality rate (M) increased with increasing growth rate. Also, relative length at maturity (Lα/Linf) increased with increasing asymptotic length (Linf). We suggest that more such data should be collected from a large number of species and taxonomic groups, to allow a more detailed assessment of why these dimensionless numbers appear to be constants in some taxa and not in others.

Gene flow from domesticated escapes alters the life history of wild Atlantic salmon

Interbreeding between domesticated and wild animals occurs in several species. This gene flow has long been anticipated to induce genetic changes in life-history traits of wild populations, thereby influencing population dynamics and viability. Here, we show that individuals with high levels of introgression (domesticated ancestry) have altered age and size at maturation in 62 wild Atlantic salmon Salmo salar populations, including seven ancestral populations to breeding lines of the domesticated salmon. This study documents widespread changes to life-history traits in wild animal populations following gene flow from selectively bred, domesticated conspecifics. The continued high abundance of escaped, domesticated Atlantic salmon thus threatens wild Atlantic salmon populations by inducing genetic changes in fitness-related traits. Our results represent key evidence and a timely warning concerning the potential ecological impacts of the globally increasing use of domesticated animals.

GWAS identifies a single selective sweep for age of maturation in wild and cultivated Atlantic salmon males.

Abstract Background Sea age at sexual maturation displays large plasticity for wild Atlantic salmon males and varies between 1-5 years. This flexibility can also be observed in domesticated salmon. Previous studies have uncovered a genetic predisposition for age at maturity with moderate heritability, thus suggesting a polygenic nature of this trait. The aim with this study was to identify genomic regions and associated SNPs and genes conferring age at maturity in salmon. Results We performed a GWAS using a pool sequencing approach (n=20 per river and trait) of salmon returning as sexually mature either after one sea winter (2009) or after three sea winters (2011) in six rivers in Norway. The study revealed one major selective sweep, which covered 76 significant SNP in a 230 kb region of Chr 25. A SNP assay of other year classes of wild salmon and from cultivated fish supported this finding. The assay in cultivated fish reduced the haplotype conferring the trait to a region which covered 4 SNPs of a 2386 bp region containing the vgll3 gene. 2 of these SNPs caused miss-sense mutations in vgll3. Conclusions This study presents a single selective region in the genome for age at maturation in male Atlantic salmon. The SNPs identified may be used as QTLs to prevent early maturity in aquaculture and in monitoring programs of wild salmon. Interestingly, the identified vgll3 gene has previously been linked to time of puberty in humans, suggesting a conserved mechanism for time of puberty in vertebrates.Wild and domesticated Atlantic salmon males display large variation for sea age at sexual maturation, which varies between 1-5 years. Previous studies have uncovered a genetic predisposition for age at maturity with moderate heritability, thus suggesting a polygenic or complex nature of this trait. The aim of this study was to identify associated genetic loci, genes and ultimately specific sequence variants conferring sea age at maturity in salmon. We performed a GWAS using a pool sequencing approach (20 individuals per river and trait) of salmon returning to rivers as sexually mature either after one sea winter (2009) or three sea winters (2011) in six rivers in Norway. The study revealed one major selective sweep, which covered 76 significant SNP in which 74 were found in a 370 kb region of chromosome 25. Genotyping other smolt year classes of wild salmon and domesticated salmon confirmed this finding. Genotyping domesticated fish narrowed the haplotype region to four SNPs covering 2386 bp, containing the vgll3 gene, including two missense mutations explaining 33-36% phenotypic variation. This study demonstrates a single locus playing a highly significant role in governing sea age at maturation in this species. The SNPs identified may be both used as markers to guide breeding for late maturity in salmon aquaculture and in monitoring programs of wild salmon. Interestingly, a SNP in proximity of the VGLL3 gene in human (Homo sapiens), has previously been linked to age at puberty suggesting a conserved mechanism for timing of puberty in vertebrates. Author summary For most species the factors that contribute to the genetic predisposition for age at maturity are currently unknown. In salmon aquaculture early maturation is negative for the growth, disease resistance and flesh quality. In addition, using populations of salmon selected to mature late may limit the genetic impact of aquaculture escapees, as these late maturing fish are more likely to die before they reach maturity. The aim of this study was to elucidate the genetic predisposition for salmon maturation. We determined the sequences of genomes from Atlantic salmon maturing early and late in six Norwegian rivers. This methodology enabled us to identify a short genomic region involved in determining the age at maturity in Atlantic salmon. This region has also previously been linked to time of puberty in humans – supporting a general mechanism behind age at maturity in vertebrates. The results of this study may be used to breed salmon that are genetically predisposed to mature late which will improve welfare and production in aquaculture industry and aid in the management of escaped farmed salmon.