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The genetic mechanisms determining sex in teleost fishes are highly variable and the master sex determining gene has only been identified in few species. Here we characterize a male-specific region of 9 kb on linkage group 11 in Atlantic cod ( Gadus morhua ) harboring a single gene named zkY for zinc knuckle on the Y chromosome. Diagnostic PCR test of phenotypically sexed males and females confirm the sex-specific nature of the Y-sequence. We identified twelve highly similar autosomal gene copies of zkY , of which eight code for proteins containing the zinc knuckle motif. 3D modeling suggests that the amino acid changes observed in six copies might influence the putative RNA-binding specificity. Cod zkY and the autosomal proteins zk1 and zk2 possess an identical zinc knuckle structure, but only the Y-specific gene zkY was expressed at high levels in the developing larvae before the onset of sex differentiation. Collectively these data suggest zkY as a candidate master masculinization gene in Atlantic cod. PCR amplification of Y-sequences in Arctic cod ( Arctogadus glacialis ) and Greenland cod ( Gadus macrocephalus ogac ) suggests that the male-specific region emerged in codfishes more than 7.5 million years ago.

Background Molecular quantitative trait loci (molQTL) mapping, particularly for gene expression and chromatin accessibility, provides crucial insights into the regulatory and functional potential of genetic variation. While significant progress has been made in humans and model organisms, aquatic genomics remains underexplored due to the large sample sizes typically required for statistical power. Results In this work, we enhance the scalability, reproducibility, and accessibility of the well-established RASQUAL method, which efficiently detects molQTLs in small datasets, by leveraging the Nextflow workflow framework. This adaptation, named nf-RASQUAL , supports fully automated QTL mapping and incorporates a robust, comprehensive multiple-testing correction process. We apply the pipeline to a comprehensive multi-omics dataset from 12 Atlantic salmon, identifying numerous significant expression and chromatin accessibility QTLs across multiple tissues. Our analysis reveals that a large proportion of lead variants for these loci reside in non-coding regions, with caQTL lead SNPs more likely to disrupt transcription factor motifs. Additionally, the enriched colocalization of eQTL and caQTL lead SNPs in brain, liver, and gonad tissues suggests shared regulatory mechanisms. Conclusions These findings highlight the scalability and utility of nf-RASQUAL for advancing genetic regulation research in aquaculture, facilitating molQTL studies in less-explored species, and improving our understanding of molecular phenotypes shaped by genetic diversity.

Background The functional divergence of duplicate genes (ohnologues) retained from whole genome duplication (WGD) is thought to promote evolutionary diversification. However, species radiation and phenotypic diversification are often temporally separated from WGD. Salmonid fish, whose ancestor underwent WGD by autotetraploidization ~95 million years ago, fit such a ‘time-lag’ model of post-WGD radiation, which occurred alongside a major delay in the rediploidization process. Here we propose a model, ‘lineage-specific ohnologue resolution’ (LORe), to address the consequences of delayed rediploidization. Under LORe, speciation precedes rediploidization, allowing independent ohnologue divergence in sister lineages sharing an ancestral WGD event. Results Using cross-species sequence capture, phylogenomics and genome-wide analyses of ohnologue expression divergence, we demonstrate the major impact of LORe on salmonid evolution. One-quarter of each salmonid genome, harbouring at least 4550 ohnologues, has evolved under LORe, with rediploidization and functional divergence occurring on multiple independent occasions >50 million years post-WGD. We demonstrate the existence and regulatory divergence of many LORe ohnologues with functions in lineage-specific physiological adaptations that potentially facilitated salmonid species radiation. We show that LORe ohnologues are enriched for different functions than ‘older’ ohnologues that began diverging in the salmonid ancestor. Conclusions LORe has unappreciated significance as a nested component of post-WGD divergence that impacts the functional properties of genes, whilst providing ohnologues available solely for lineage-specific adaptation. Under LORe, which is predicted following many WGD events, the functional outcomes of WGD need not appear ‘explosively’, but can arise gradually over tens of millions of years, promoting lineage-specific diversification regimes under prevailing ecological pressures.

Background Understanding genetic architecture is essential for determining how traits will change in response to evolutionary processes such as selection, genetic drift and/or gene flow. In Atlantic salmon, age at maturity is an important life history trait that affects factors such as survival, reproductive success, and growth. Furthermore, age at maturity can seriously impact aquaculture production. Therefore, characterizing the genetic architecture that underlies variation in age at maturity is of key interest. Results Here, we refine our understanding of the genetic architecture for age at maturity of male Atlantic salmon using a genome-wide association study of 11,166 males from a single aquaculture strain, using imputed genotypes at 512,397 single nucleotide polymorphisms (SNPs). All individuals were genotyped with a 50K SNP array and imputed to higher density using parents genotyped with a 930K SNP array and pedigree information. We found significant association signals on 28 of 29 chromosomes ( P -values: 8.7 × 10 −133 –9.8 × 10 −8 ), including two very strong signals spanning the six6 and vgll3 gene regions on chromosomes 9 and 25, respectively. Furthermore, we identified 116 independent signals that tagged 120 candidate genes with varying effect sizes. Five of the candidate genes found here were previously associated with age at maturity in other vertebrates, including humans. Discussion These results reveal a mixed architecture of large-effect loci and a polygenic component that consists of multiple smaller-effect loci, suggesting a more complex genetic architecture of Atlantic salmon age at maturity than previously thought. This more complex architecture will have implications for selection on this key trait in aquaculture and for management of wild salmon populations.

Infectious pancreatic necrosis virus (IPNV) is the cause of one of the most prevalent diseases in farmed Atlantic salmon (Salmo salar). A quantitative trait locus (QTL) has been found to be responsible for most of the genetic variation in resistance to the virus. Here we describe how a linkage disequilibrium-based test for deducing the QTL allele was developed, and how it was used to produce IPN-resistant salmon, leading to a 75% decrease in the number of IPN outbreaks in the salmon farming industry. Furthermore, we describe how whole-genome sequencing of individuals with deduced QTL genotypes was used to map the QTL down to a region containing an epithelial cadherin (cdh1) gene. In a coimmunoprecipitation assay, the Cdh1 protein was found to bind to IPNV virions, strongly indicating that the protein is part of the machinery used by the virus for internalization. Immunofluorescence revealed that the virus colocalizes with IPNV in the endosomes of homozygous susceptible individuals but not in the endosomes of homozygous resistant individuals. A putative causal single nucleotide polymorphism was found within the full-length cdh1 gene, in phase with the QTL in all observed haplotypes except one; the absence of a single, all-explaining DNA polymorphism indicates that an additional causative polymorphism may contribute to the observed QTL genotype patterns. Cdh1 has earlier been shown to be necessary for the internalization of certain bacteria and fungi, but this is the first time the protein is implicated in internalization of a virus.

The long-term evolutionary impacts of whole-genome duplication (WGD) are strongly influenced by the ensuing rediploidization process. Following autopolyploidization, rediploidization involves a transition from tetraploid to diploid meiotic pairing, allowing duplicated genes (ohnologs) to diverge genetically and functionally. Our understanding of autopolyploid rediploidization has been informed by a WGD event ancestral to salmonid fishes, where large genomic regions are characterized by temporally delayed rediploidization, allowing lineage-specific ohnolog sequence divergence in the major salmonid clades. Here, we investigate the long-term outcomes of autopolyploid rediploidization at genomewide resolution, exploiting a recent “explosion” of salmonid genome assemblies, including a new genome sequence for the huchen (Hucho hucho). We developed a genome alignment approach to capture duplicated regions across multiple species, allowing us to create 121,864 phylogenetic trees describing genome-wide ohnolog divergence across salmonid evolution. Using molecular clock analysis, we show that 61% of the ancestral salmonid genome experienced an initial “wave” of rediploidization in the late Cretaceous (85–106 Ma). This was followed by a period of relative genomic stasis lasting 17–39 My, where much of the genome remained tetraploid. A second rediploidization wave began in the early Eocene and proceeded alongside species diversification, generating predictable patterns of lineage-specific ohnolog divergence, scaling in complexity with the number of speciation events. Using gene set enrichment, gene expression, and codon-based selection analyses, we provide insights into potential functional outcomes of delayed rediploidization. This study enhances our understanding of delayed autopolyploid rediploidization and has broad implications for future studies of WGD events

Background The Atlantic salmon genome is in the process of returning to a diploid state after undergoing a whole genome duplication (WGD) event between 25 and100 million years ago. Existing data on the proportion of paralogous sequence variants (PSVs), multisite variants (MSVs) and other types of complex sequence variation suggest that the rediplodization phase is far from over. The aims of this study were to construct a high density linkage map for Atlantic salmon, to characterize the extent of rediploidization and to improve our understanding of genetic differences between sexes in this species. Results A linkage map for Atlantic salmon comprising 29 chromosomes and 5650 single nucleotide polymorphisms (SNPs) was constructed using genotyping data from 3297 fish belonging to 143 families. Of these, 2696 SNPs were generated from ESTs or other gene associated sequences. Homeologous chromosomal regions were identified through the mapping of duplicated SNPs and through the investigation of syntenic relationships between Atlantic salmon and the reference genome sequence of the threespine stickleback ( Gasterosteus aculeatus ). The sex-specific linkage maps spanned a total of 2402.3 cM in females and 1746.2 cM in males, highlighting a difference in sex specific recombination rate (1.38:1) which is much lower than previously reported in Atlantic salmon. The sexes, however, displayed striking differences in the distribution of recombination sites within linkage groups, with males showing recombination strongly localized to telomeres. Conclusion The map presented here represents a valuable resource for addressing important questions of interest to evolution (the process of re-diploidization), aquaculture and salmonid life history biology and not least as a resource to aid the assembly of the forthcoming Atlantic salmon reference genome sequence.

Age at maturity in Atlantic salmon ( Salmo salar ) is governed to a substantial extent by a locus showing dominance reversal, providing a resolution for sexual conflict in this trait, for which selection favours different ages in the two sexes. Major-effect puberty gene tuned to male and female needs Craig Primmer and colleagues use genome-wide association studies for age at maturity in Atlantic salmon to show that a single gene, VGLL3 , strongly influences the variation in age at maturity, and therefore body size—a key trait in an important fished species. They find that the VGLL3 locus is an example of sex-dependent dominance, promoting earlier and later maturation in males and females, respectively. This mechanism provides a resolution for sexual conflict in this trait, for which selection favours different reproductive ages in the two sexes. Females benefit from being large (5–15 kg), and stay longer at sea feeding by maturing later, while males can have high fitness maturing at smaller sizes (1–3 kg). This discovery will have a substantial impact on population management of Atlantic salmon, where a decrease in the frequency of late maturation has been observed in many populations. 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 1 , 2 . Under such sexual antagonism, theory predicts the evolution of genetic architectures that resolve this sexual conflict 2 , 3 , 4 , 5 . 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 3 , 6 and the maintenance of genetic diversity 6 , 7 . 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 8 , 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 9 , 10 . 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.

Background The development of effective disease management strategies is crucial for the assurance of welfare and sustainability of the aquaculture industries. Pancreas disease (PD) is a major challenge faced by Atlantic salmon aquaculture with viral outbreaks resulting in substantial production losses and raising significant welfare concerns for farmed salmon populations. Previous research has identified several quantitative trait loci (QTL) associated with PD resistance accounting for a substantial additive genetic component. However, pinpointing the underlying causal variation remains challenging, partly due to the location of the QTL within duplicated regions of the Atlantic salmon genome that share high sequence similarity. The present study leverages the latest advancements in Atlantic salmon genomics in order to uncover the genetic landscape underlying PD resistance and identify genomic variation with putative functional impact on disease response. Results Association mapping and haplotype analysis of fish challenged with salmonid alphavirus (SAV3), either through peritoneal injection or infectious cohabitation, confirmed the presence of a major QTL region on chromosome Ssa03. Additionally, another QTL on Ssa07 was detected, linked to infection-specific response. Transcriptomics analysis of the genes overlapping the Ssa03 QTL region revealed significant expression differences among three tandemly duplicated gig1-like genes, whereas allele-specific expression analysis detected several SNPs with putative functional impact on the particular genes. Use of long-read sequencing and construction of disease-associated haplotypes identified more complex variation in the region, offering a detailed exploration of the genetic architecture underlying PD resistance. Finally, integration of the regulatory landscape of Atlantic salmon during response to viral infection improved genomic resolution, providing novel insight into the potential causal variation underlying pancreas disease in Atlantic salmon. Conclusions This study provides a detailed investigation of the genetic architecture underlying PD resistance in farmed Atlantic salmon. Using advanced genomic resources, three copies of the gig1-like gene were identified as likely causal candidates for a major QTL associated with PD resistance. Additionally, genomic variations with potential functional impact on gig1-like expression were uncovered. These findings hold promise for application in developing effective disease management strategies in Atlantic salmon aquaculture.

Teleost fish exhibit a remarkable diversity in the control of sex determination, offering the opportunity to identify novel differentiation mechanisms and their ecological consequences. Here, we perform GWAS using 4715 fish and 46,501 SNP to map sex determination to three separate genomic locations in Atlantic salmon ( Salmo salar ). To characterize each, whole genome sequencing was performed to 30-fold depth of coverage using 20 fish representing each of three identified sex lineages. SNP polymorphism reveals male fish carry a single copy of the male specific region, consistent with an XX/XY or male heterogametric sex system. Haplotype analysis revealed deep divergence between the putatively ancestral locus on chromosome 2, compared with loci on chromosomes 3 and 6. Haplotypes in fish carrying either the chromosome 3 or 6 loci were nearly indistinguishable, indicating a founding event that occurred following the speciation event that defined Salmo salar from other salmonids. These findings highlight the evolutionarily fluid state of sex determination systems in salmonids, and resolve to the sequence level differences in animals with divergent sex lineages.