Masu salmon species complex relationships and sex chromosomes revealed from analyses of the masu salmon (Oncorhynchus masou masou) genome assembly

Masu salmon (Oncorhynchus masou) are the only Pacific salmon endemic to Asia. Some researchers prefer to categorize these salmon into 4 subspecies (masu-Oncorhynchus masou masou, amago-Oncorhynchus masou ishikawae, Biwa-Oncorhynchus masou subsp., and Formosan-Oncorhynchus masou formosanus), while others prefer individual species designations. Even though the masu salmon fishery is thousands of years old, classification of the diversity within the masu salmon species complex remains elusive. In this study, a genetic map and reference genome assembly were generated for 1 species/subspecies (masu) to provide resources for understanding the species complex. In O. m. masou, the sex chromosome was determined to be chromosome 7. Resequenced genomes from 2 other putative subspecies (amago and Biwa) provided evidence that they do not share the same sex chromosome. Principal component and admixture analyses clustered the amago and Biwa salmon close together. This supported previous findings of a close relationship between amago and Biwa salmon and a more distant relationship to masu salmon for both. Additional analyses of the masu salmon species complex will benefit from using the new reference genome assembly.

Limited evidence for extensive genetic differentiation between X and Y chromosomes in Hybognathus amarus (Cypriniformes: Leuciscidae)

Sex determination systems and genetic sex differentiation across fishes are highly diverse but are unknown for most Cypriniformes, including Rio Grande silvery minnow (Hybognathus amarus). In this study, we aimed to detect and validate sex-linked markers to infer sex determination system and to demonstrate the utility of combining several methods for sex-linked marker detection in nonmodel organisms. To identify potential sex-linked markers, Nextera-tagmented reductively amplified DNA (nextRAD) libraries were generated from 66 females, 64 males, and 60 larvae of unknown sex. These data were combined with female and male de novo genomes from Nanopore long-read sequences. We identified five potential unique male nextRAD-tags and one potential unique male contig, suggesting an XY sex determination system. We also identified two single-nucleotide polymorphisms (SNPs) in the same contig with values of FST, allele frequencies, and heterozygosity conforming with expectations of an XY system. Through PCR we validated the marker containing the sex-linked SNPs and a single nextRAD-tag sex-associated marker but it was not male specific. Instead, more copies of this locus in the male genome were suggested by enhanced amplification in males. Results are consistent with an XY system with low differentiation between sex-determining regions. Further research is needed to confirm the level of differentiation between the sex chromosomes. Nonetheless, this study highlighted the power of combining reduced representation and whole-genome sequencing for identifying sex-linked markers, especially when reduced representation sequencing does not include extensive variation between sexes, either because such variation is not present or not captured.

Identification and Genomic Localization of Autosomal sdY Locus in a Population of Atlantic Salmon (Salmo salar)

The determination of sex in salmonid fishes is controlled by genetic mechanisms, with males being the heterogametic sex. The master sex-determining gene, the sexually dimorphic gene on the Y chromosome (sdY), is a conserved gene across various salmonid species. Nevertheless, variations in the genomic location of sdY have been observed both within and between species. Furthermore, different studies have reported discordances in the association between the sdY and the phenotypic gender. While some males seem to lack this locus, there have been reports of females carrying sdY. Although the exact reasons behind this discordance remain under investigation, some recent studies have proposed the existence of an autosomal, non-functional copy of sdY as a potential cause. In this study, we confirmed the presence of this autosomal sdY in the SalmoBreed strain of Atlantic salmon using a genotyping platform through a novel approach that allows for high-throughput screening of a large number of individuals. We further characterized the segregation profile of this locus across families and found the ratio of genetically assigned female-to-male progeny to be in accordance with the expected profile of a single autosomal sdY locus. Additionally, our mapping efforts localized this locus to chromosome 3 and suggested a putative copy on chromosome 6.

The genomic basis of reproductive and migratory behaviour in a polymorphic salmonid

Recent ecotypic differentiation provides unique opportunities to investigate the genomic basis and architecture of local adaptation, while offering insights into how species form and persist. Sockeye salmon (Oncorhynchus nerka) exhibit migratory and resident ("kokanee") ecotypes, which are further distinguished into shore-spawning and stream-spawning reproductive ecotypes. Here, we analysed 36 sockeye (stream-spawning) and kokanee (stream- and shore-spawning) genomes from a system where they co-occur and have recent common ancestry (Okanagan Lake/River in British Columbia, Canada) to investigate the genomic basis of reproductive and migratory behaviour. Examination of the genomic landscape of differentiation, differences in allele frequencies and genotype-phenotype associations revealed three main blocks of sequence differentiation on chromosomes 7, 12 and 20, associated with migratory behaviour, spawning location and spawning timing. Structural variants identified in these same areas suggest they could contribute to ecotypic differentiation directly as causal variants or via maintenance of their genomic architecture through recombination suppression mechanisms. Genes in these regions were related to spatial memory and swimming endurance (SYNGAP, TPM3), as well as eye and brain development (including SIX6), potentially associated with differences in migratory behaviour and visual habitats across spawning locations, respectively. Additional genes (GREB1L, ROCK1) identified here have been associated with timing of migration in other salmonids and could explain variation in timing of O. nerka spawning. Together, these results based on the joint analysis of sequence and structural variation represent a significant advance in our understanding of the genomic landscape of ecotypic differentiation at different stages in the speciation continuum.

Sex determination mechanisms and sex control approaches in aquaculture animals

Aquaculture is one of the most efficient modes of animal protein production and plays an important role in global food security. Aquaculture animals exhibit extraordinarily diverse sexual phenotypes and underlying mechanisms, providing an ideal system to perform sex determination research, one of the important areas in life science. Moreover, sex is also one of the most valuable traits because sexual dimorphism in growth, size, and other economic characteristics commonly exist in aquaculture animals. Here, we synthesize current knowledge of sex determination mechanisms, sex chromosome evolution, reproduction strategies, and sexual dimorphism, and also review several approaches for sex control in aquaculture animals, including artificial gynogenesis, application of sex-specific or sex chromosome-linked markers, artificial sex reversal, as well as gene editing. We anticipate that better understanding of sex determination mechanisms and innovation of sex control approaches will facilitate sustainable development of aquaculture.

A nonfunctional copy of the salmonid sex-determining gene (sdY) is responsible for the “apparent” XY females in Chinook salmon, Oncorhynchus tshawytscha

Many salmonids have a male heterogametic (XX/XY) sex determination system, and they are supposed to have a conserved master sex-determining gene (sdY) that interacts at the protein level with Foxl2 leading to the blockage of the synergistic induction of Foxl2 and Nr5a1 of the cyp19a1a promoter. However, this hypothesis of a conserved master sex-determining role of sdY in salmonids is challenged by a few exceptions, one of them being the presence of naturally occurring “apparent” XY Chinook salmon, Oncorhynchus tshawytscha, females. Here, we show that some XY Chinook salmon females have a sdY gene (sdY-N183), with 1 missense mutation leading to a substitution of a conserved isoleucine to an asparagine (I183N). In contrast, Chinook salmon males have both a nonmutated sdY-I183 gene and the missense mutation sdY-N183 gene. The 3-dimensional model of SdY-I183N predicts that the I183N hydrophobic to hydrophilic amino acid change leads to a modification in the SdY β-sandwich structure. Using in vitro cell transfection assays, we found that SdY-I183N, like the wild-type SdY, is preferentially localized in the cytoplasm. However, compared to wild-type SdY, SdY-I183N is more prone to degradation, its nuclear translocation by Foxl2 is reduced, and SdY-I183N is unable to significantly repress the synergistic Foxl2/Nr5a1 induction of the cyp19a1a promoter. Altogether, our results suggest that the sdY-N183 gene of XY Chinook females is nonfunctional and that SdY-I183N is no longer able to promote testicular differentiation by impairing the synthesis of estrogens in the early differentiating gonads of wild Chinook salmon XY females.

Multiple sex chromosomes in teleost fishes from a cytogenetic perspective: state of the art and future challenges

Despite decades of cytogenetic and genomic research of dynamic sex chromosome evolution in teleost fishes, multiple sex chromosomes have been largely neglected. In this review, we compiled available data on teleost multiple sex chromosomes, identified major trends in their evolution and suggest further trajectories in their investigation. In a compiled dataset of 440 verified records of fish sex chromosomes, we counted 75 multiple sex chromosome systems with 60 estimated independent origins. We showed that male-heterogametic systems created by Y-autosome fusion predominate and that multiple sex chromosomes are over-represented in the order Perciformes. We documented a striking difference in patterns of differentiation of sex chromosomes between male and female heterogamety and hypothesize that faster W sex chromosome differentiation may constrain sex chromosome turnover in female-heterogametic systems. We also found no significant association between the mechanism of multiple sex chromosome formation and percentage of uni-armed chromosomes in teleost karyotypes. Last but not least, we hypothesized that interaction between fish populations, which differ in their sex chromosomes, can drive the evolution of multiple sex chromosomes in fishes. This underlines the importance of broader inter-population sampling in studies of fish sex chromosomes. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.

Lessons from an unusual vertebrate sex-determining gene

So far, very few sex-determining genes have been identified in vertebrates and most of them, the so-called 'usual suspects', evolved from genes which fulfil essential functions during sexual development and are thus already tightly linked to the process that they now govern. The single exception to this 'usual suspects' rule in vertebrates so far is the conserved salmonid sex-determining gene, sdY (sexually dimorphic on the Y chromosome), that evolved from a gene known to be involved in regulation of the immune response. It is contained in a jumping sex locus that has been transposed or translocated into different ancestral autosomes during the evolution of salmonids. This special feature of sdY, i.e. being inserted in a 'jumping sex locus', could explain how salmonid sex chromosomes remain young and undifferentiated to escape degeneration. Recent knowledge on the mechanism of action of sdY demonstrates that it triggers its sex-determining action by deregulating oestrogen synthesis that is a conserved and crucial pathway for ovarian differentiation in vertebrates. This result suggests that sdY has evolved to cope with a pre-existing sex differentiation regulatory network. Therefore, 'limited options' for the emergence of new master sex-determining genes could be more constrained by their need to tightly interact with a conserved sex differentiation regulatory network rather than by being themselves 'usual suspects', already inside this sex regulatory network. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

The sockeye salmon genome, transcriptome, and analyses identifying population defining regions of the genome

Sockeye salmon (Oncorhynchus nerka) is a commercially and culturally important species to the people that live along the northern Pacific Ocean coast. There are two main sockeye salmon ecotypes—the ocean-going (anadromous) ecotype and the fresh-water ecotype known as kokanee. The goal of this study was to better understand the population structure of sockeye salmon and identify possible genomic differences among populations and between the two ecotypes. In pursuit of this goal, we generated the first reference sockeye salmon genome assembly and an RNA-seq transcriptome data set to better annotate features of the assembly. Resequenced whole-genomes of 140 sockeye salmon and kokanee were analyzed to understand population structure and identify genomic differences between ecotypes. Three distinct geographic and genetic groups were identified from analyses of the resequencing data. Nucleotide variants in an immunoglobulin heavy chain variable gene cluster on chromosome 26 were found to differentiate the northwestern group from the southern and upper Columbia River groups. Several candidate genes were found to be associated with the kokanee ecotype. Many of these genes were related to ammonia tolerance or vision. Finally, the sex chromosomes of this species were better characterized, and an alternative sex-determination mechanism was identified in a subset of upper Columbia River kokanee.

Autosomal sdY Pseudogenes Explain Discordances Between Phenotypic Sex and DNA Marker for Sex Identification in Atlantic Salmon

Despite the key role that sex-determination plays in evolutionary processes, it is still poorly understood in many species. In salmonids, which are among the best studied fishes, the master sex-determining gene sexually dimorphic on the Y-chromosome (sdY) has been identified. However, sdY displays unexplained discordance to the phenotypic sex, with a variable frequency of phenotypic females being reported as genetic males. Multiple sex determining loci in Atlantic salmon have also been reported, possibly as a result of recent transposition events in this species. We hypothesized the existence of an autosomal copy of sdY, causing apparent discordance between phenotypic and genetic sex, that is transmitted in accordance with autosomal inheritance. To test this, we developed a qPCR methodology to detect the total number of sdY copies present in the genome. Based on the observed phenotype/genotype frequencies and linkage analysis among 2,025 offspring from 64 pedigree-controlled families of accurately phenotyped Atlantic salmon, we identified both males and females carrying one or two autosomal copies of sdY in addition to the Y-specific copy present in males. Patterns across families were highly consistent with autosomal inheritance. These autosomal sdY copies appear to have lost the ability to function as a sex determining gene and were only occasionally assigned to the actual sex chromosome in any of the affected families.

Discordance for genotypic sex in phenotypic female Atlantic salmon (Salmo salar) is related to a reduced sdY copy number

The master sex determinant in rainbow trout (Oncorhynchus mykiss), sexually dimorphic on the Y chromosome (sdY), is strongly but not perfectly associated with male phenotype in several other species from the family Salmonidae. Currently, the cause and implications of discordance for sdY-predicted genotypic sex and phenotypic sex in these species is unclear. Using an established multiplex PCR test for exons 2 and 3 of sdY, we demonstrated that sdY-predicted genotypic sex was discordant with histologically evidenced phenotypic sex in 4% of 176 Tasmanian Atlantic salmon. All discordant individuals were phenotypic females presenting a male genotype. Using real-time qPCR assays that we developed and validated for exons 2, 3 and 4 of sdY, all genotype-phenotype discordant females were confirmed to possess sdY, albeit at a reduced number of copies when compared to phenotypic males. The real-time qPCR assays also demonstrated reduced levels of sdY in 30% of phenotypic females that the established multiplex PCR-based test indicated to be devoid of sdY. These findings suggest sdY may be reduced in copy number or mosaicked in the genomic DNA of sdY-positive phenotypic female Atlantic salmon and highlight the importance of understanding the effects of reduced sdY copies on the development of phenotypic sex.

The Gene Toolkit Implicated in Functional Sex in Sparidae Hermaphrodites: Inferences From Comparative Transcriptomics

Sex-biased gene expression is the mode through which sex dimorphism arises from a nearly identical genome, especially in organisms without genetic sex determination. Teleost fishes show great variations in the way the sex phenotype forms. Among them, Sparidae, that might be considered as a model family displays a remarkable diversity of reproductive modes. In this study, we sequenced and analyzed the sex-biased transcriptome in gonads and brain (the tissues with the most profound role in sexual development and reproduction) of two sparids with different reproductive modes: the gonochoristic common dentex, Dentex dentex, and the protandrous hermaphrodite gilthead seabream, Sparus aurata. Through comparative analysis with other protogynous and rudimentary protandrous sparid transcriptomes already available, we put forward common male and female-specific genes and pathways that are probably implicated in sex-maintenance in this fish family. Our results contribute to the understanding of the complex processes behind the establishment of the functional sex, especially in hermaphrodite species and set the groundwork for future experiments by providing a gene toolkit that can improve efforts to control phenotypic sex in finfish in the ever-increasingly important field of aquaculture.

An efficient and reliable DNA-based sex identification method for archaeological Pacific salmonid (Oncorhynchus spp.) remains

Pacific salmonid (Oncorhynchus spp.) remains are routinely recovered from archaeological sites in northwestern North America but typically lack sexually dimorphic features, precluding the sex identification of these remains through morphological approaches. Consequently, little is known about the deep history of the sex-selective salmonid fishing strategies practiced by some of the region’s Indigenous peoples. Here, we present a DNA-based method for the sex identification of archaeological Pacific salmonid remains that integrates two PCR assays that each co-amplify fragments of the sexually dimorphic on the Y chromosome (sdY) gene and an internal positive control (Clock1a or D-loop). The first assay co-amplifies a 95 bp fragment of sdY and a 108 bp fragment of the autosomal Clock1a gene, whereas the second assay co-amplifies the same sdY fragment and a 249 bp fragment of the mitochondrial D-loop region. This method’s reliability, sensitivity, and efficiency, were evaluated by applying it to 72 modern Pacific salmonids from five species and 75 archaeological remains from six Pacific salmonids. The sex identities assigned to each of the modern samples were concordant with their known phenotypic sex, highlighting the method’s reliability. Applications of the method to dilutions of modern DNA samples indicate it can correctly identify the sex of samples with as little as ~39 pg of total genomic DNA. The successful sex identification of 70 of the 75 (93%) archaeological samples further demonstrates the method’s sensitivity. The method’s reliance on two co-amplifications that preferentially amplify sdY helps validate the sex identities assigned to samples and reduce erroneous identifications caused by allelic dropout and contamination. Furthermore, by sequencing the D-loop fragment used as a positive control, species-level and sex identifications can be simultaneously assigned to samples. Overall, our results indicate the DNA-based method reported in this study is a sensitive and reliable sex identification method for ancient salmonid remains.

The transcriptomic signature of different sexes in two protogynous hermaphrodites: Insights into the molecular network underlying sex phenotype in fish

Sex differentiation is a puzzling problem in fish due to the variety of reproductive systems and the flexibility of their sex determination mechanisms. The Sparidae, a teleost family, reflects this remarkable diversity of sexual mechanisms found in fish. Our aim was to capture the transcriptomic signature of different sexes in two protogynous hermaphrodite sparids, the common pandora Pagellus erythrinus and the red porgy Pagrus pagrus in order to shed light on the molecular network contributing to either the female or the male phenotype in these organisms. Through RNA sequencing, we investigated sex-specific differences in gene expression in both species' brains and gonads. The analysis revealed common male and female specific genes/pathways between these protogynous fish. Whereas limited sex differences found in the brain indicate a sexually plastic tissue, in contrast, the great amount of sex-biased genes observed in gonads reflects the functional divergence of the transformed tissue to either its male or female character. Α common "crew" of well-known molecular players is acting to preserve either sex identity of the gonad in these fish. Lastly, this study lays the ground for a deeper understanding of the complex process of sex differentiation in two species with an evolutionary significant reproductive system.

Sex Chromosome Evolution, Heterochiasmy, and Physiological QTL in the Salmonid Brook Charr Salvelinus fontinalis

Whole-genome duplication (WGD) can have large impacts on genome evolution, and much remains unknown about these impacts. This includes the mechanisms of coping with a duplicated sex determination system and whether this has an impact on increasing the diversity of sex determination mechanisms. Other impacts include sexual conflict, where alleles having different optimums in each sex can result in sequestration of genes into nonrecombining sex chromosomes. Sex chromosome development itself may involve sex-specific recombination rate (i.e., heterochiasmy), which is also poorly understood. The family Salmonidae is a model system for these phenomena, having undergone autotetraploidization and subsequent rediploidization in most of the genome at the base of the lineage. The salmonid master sex determining gene is known, and many species have nonhomologous sex chromosomes, putatively due to transposition of this gene. In this study, we identify the sex chromosome of Brook Charr Salvelinus fontinalis and compare sex chromosome identities across the lineage (eight species and four genera). Although nonhomology is frequent, homologous sex chromosomes and other consistencies are present in distantly related species, indicating probable convergence on specific sex and neo-sex chromosomes. We also characterize strong heterochiasmy with 2.7-fold more crossovers in maternal than paternal haplotypes with paternal crossovers biased to chromosome ends. When considering only rediploidized chromosomes, the overall heterochiasmy trend remains, although with only 1.9-fold more recombination in the female than the male. Y chromosome crossovers are restricted to a single end of the chromosome, and this chromosome contains a large interspecific inversion, although its status between males and females remains unknown. Finally, we identify quantitative trait loci (QTL) for 21 unique growth, reproductive, and stress-related phenotypes to improve knowledge of the genetic architecture of these traits important to aquaculture and evolution.

Genomic islands of divergence linked to ecotypic variation in sockeye salmon

Regions of the genome displaying elevated differentiation (genomic islands of divergence) are thought to play an important role in local adaptation, especially in populations experiencing high gene flow. However, the characteristics of these islands as well as the functional significance of genes located within them remain largely unknown. Here, we used data from thousands of SNPs aligned to a linkage map to investigate genomic islands of divergence in three ecotypes of sockeye salmon (Oncorhynchus nerka) from a single drainage in southwestern Alaska. We found ten islands displaying high differentiation among ecotypes. Conversely, neutral structure observed throughout the rest of the genome was low and not partitioned by ecotype. One island on linkage group So13 was particularly large and contained six SNPs with F_ST  > 0.14 (average F_ST of neutral SNPs = 0.01). Functional annotation revealed that the peak of this island contained a nonsynonymous mutation in a gene involved in growth in other species (TULP4). The islands that we discovered were relatively small (80-402 Kb), loci found in islands did not show reduced levels of diversity, and loci in islands displayed slightly elevated linkage disequilibrium. These attributes suggest that the islands discovered here were likely generated by divergence hitchhiking; however, we cannot rule out the possibility that other mechanisms may have produced them. Our results suggest that islands of divergence serve an important role in local adaptation with gene flow and represent a significant advance towards understanding the genetic basis of ecotypic differentiation.