The genetic architecture of embryonic developmental rate and genetic covariation with age at maturation in rainbow trout Oncorhynchus mykiss

Genomic Selection in Aquaculture: Application, Limitations and Opportunities With Special Reference to Marine Shrimp and Pearl Oysters

Within aquaculture industries, selection based on genomic information (genomic selection) has the profound potential to change genetic improvement programs and production systems. Genomic selection exploits the use of realized genomic relationships among individuals and information from genome-wide markers in close linkage disequilibrium with genes of biological and economic importance. We discuss the technical advances, practical requirements, and commercial applications that have made genomic selection feasible in a range of aquaculture industries, with a particular focus on molluscs (pearl oysters, Pinctada maxima) and marine shrimp (Litopenaeus vannamei and Penaeus monodon). The use of low-cost genome sequencing has enabled cost-effective genotyping on a large scale and is of particular value for species without a reference genome or access to commercial genotyping arrays. We highlight the pitfalls and offer the solutions to the genotyping by sequencing approach and the building of appropriate genetic resources to undertake genomic selection from first-hand experience. We describe the potential to capture large-scale commercial phenotypes based on image analysis and artificial intelligence through machine learning, as inputs for calculation of genomic breeding values. The application of genomic selection over traditional aquatic breeding programs offers significant advantages through being able to accurately predict complex polygenic traits including disease resistance; increasing rates of genetic gain; minimizing inbreeding; and negating potential limiting effects of genotype by environment interactions. Further practical advantages of genomic selection through the use of large-scale communal mating and rearing systems are highlighted, as well as presenting rate-limiting steps that impact on attaining maximum benefits from adopting genomic selection. Genomic selection is now at the tipping point where commercial applications can be readily adopted and offer significant short- and long-term solutions to sustainable and profitable aquaculture industries.

Significant Synteny and Colocalization of Ecologically Relevant Quantitative Trait Loci Within and Across Species of Salmonid Fishes

The organization of functional regions within genomes has important implications for evolutionary potential. Considerable research effort has gone toward identifying the genomic basis of phenotypic traits of interest through quantitative trait loci (QTL) analyses. Less research has assessed the arrangement of QTL in the genome within and across species. To investigate the distribution, extent of colocalization, and the synteny of QTL for ecologically relevant traits, we used a comparative genomic mapping approach within and across a range of salmonid species. We compiled 943 QTL from all available species [lake whitefish (Coregonus clupeaformis), coho salmon (Oncorhynchus kisutch), rainbow trout (O. mykiss), Chinook salmon (O. tshawytscha), Atlantic salmon (Salmo salar), and Arctic charr (Salvelinus alpinus)]. We developed a novel analytical framework for mapping and testing the distribution of these QTL. We found no correlation between QTL density and gene density at the chromosome level but did at the fine-scale. Two chromosomes were significantly enriched for QTL. We found multiple synteny blocks for morphological, life history, and physiological traits across species, but only morphology and physiology had significantly more than expected. Two or three pairs of traits were significantly colocalized in three species (lake whitefish, coho salmon, and rainbow trout). Colocalization and fine-scale synteny suggest genetic linkage between traits within species and a conserved genetic basis across species. However, this pattern was weak overall, with colocalization and synteny being relatively rare. These findings advance our understanding of the role of genomic organization in the renowned ecological and phenotypic variability of salmonid fishes.

Aquaculture genomics, genetics and breeding in the United States: current status, challenges, and priorities for future research

Advancing the production efficiency and profitability of aquaculture is dependent upon the ability to utilize a diverse array of genetic resources. The ultimate goals of aquaculture genomics, genetics and breeding research are to enhance aquaculture production efficiency, sustainability, product quality, and profitability in support of the commercial sector and for the benefit of consumers. In order to achieve these goals, it is important to understand the genomic structure and organization of aquaculture species, and their genomic and phenomic variations, as well as the genetic basis of traits and their interrelationships. In addition, it is also important to understand the mechanisms of regulation and evolutionary conservation at the levels of genome, transcriptome, proteome, epigenome, and systems biology. With genomic information and information between the genomes and phenomes, technologies for marker/causal mutation-assisted selection, genome selection, and genome editing can be developed for applications in aquaculture. A set of genomic tools and resources must be made available including reference genome sequences and their annotations (including coding and non-coding regulatory elements), genome-wide polymorphic markers, efficient genotyping platforms, high-density and high-resolution linkage maps, and transcriptome resources including non-coding transcripts. Genomic and genetic control of important performance and production traits, such as disease resistance, feed conversion efficiency, growth rate, processing yield, behaviour, reproductive characteristics, and tolerance to environmental stressors like low dissolved oxygen, high or low water temperature and salinity, must be understood. QTL need to be identified, validated across strains, lines and populations, and their mechanisms of control understood. Causal gene(s) need to be identified. Genetic and epigenetic regulation of important aquaculture traits need to be determined, and technologies for marker-assisted selection, causal gene/mutation-assisted selection, genome selection, and genome editing using CRISPR and other technologies must be developed, demonstrated with applicability, and application to aquaculture industries.Major progress has been made in aquaculture genomics for dozens of fish and shellfish species including the development of genetic linkage maps, physical maps, microarrays, single nucleotide polymorphism (SNP) arrays, transcriptome databases and various stages of genome reference sequences. This paper provides a general review of the current status, challenges and future research needs of aquaculture genomics, genetics, and breeding, with a focus on major aquaculture species in the United States: catfish, rainbow trout, Atlantic salmon, tilapia, striped bass, oysters, and shrimp. While the overall research priorities and the practical goals are similar across various aquaculture species, the current status in each species should dictate the next priority areas within the species. This paper is an output of the USDA Workshop for Aquaculture Genomics, Genetics, and Breeding held in late March 2016 in Auburn, Alabama, with participants from all parts of the United States.

Shifting Thresholds: Rapid Evolution of Migratory Life Histories in Steelhead/Rainbow Trout, Oncorhynchus mykiss

Expression of phenotypic plasticity depends on reaction norms adapted to historic selective regimes; anthropogenic changes in these selection regimes necessitate contemporary evolution or declines in productivity and possibly extinction. Adaptation of conditional strategies following a change in the selection regime requires evolution of either the environmentally influenced cue (e.g., size-at-age) or the state (e.g., size threshold) at which an individual switches between alternative tactics. Using a population of steelhead (Oncorhynchus mykiss) introduced above a barrier waterfall in 1910, we evaluate how the conditional strategy to migrate evolves in response to selection against migration. We created 9 families and 917 offspring from 14 parents collected from the above- and below-barrier populations. After 1 year of common garden-rearing above-barrier offspring were 11% smaller and 32% lighter than below-barrier offspring. Using a novel analytical approach, we estimate that the mean size at which above-barrier fish switch between the resident and migrant tactic is 43% larger than below-barrier fish. As a result, above-barrier fish were 26% less likely to express the migratory tactic. Our results demonstrate how rapid and opposing changes in size-at-age and threshold size contribute to the contemporary evolution of a conditional strategy and indicate that migratory barriers may elicit rapid evolution toward the resident life history on timescales relevant for conservation and management of conditionally migratory species.

Genome-wide association study (GWAS) for growth rate and age at sexual maturation in Atlantic salmon (Salmo salar)

Early sexual maturation is considered a serious drawback for Atlantic salmon aquaculture as it retards growth, increases production times and affects flesh quality. Although both growth and sexual maturation are thought to be complex processes controlled by several genetic and environmental factors, selection for these traits has been continuously accomplished since the beginning of Atlantic salmon selective breeding programs. In this genome-wide association study (GWAS) we used a 6.5K single-nucleotide polymorphism (SNP) array to genotype ∼480 individuals from the Cermaq Canada broodstock program and search for SNPs associated with growth and age at sexual maturation. Using a mixed model approach we identified markers showing a significant association with growth, grilsing (early sexual maturation) and late sexual maturation. The most significant associations were found for grilsing, with markers located in Ssa10, Ssa02, Ssa13, Ssa25 and Ssa12, and for late maturation with markers located in Ssa28, Ssa01 and Ssa21. A lower level of association was detected with growth on Ssa13. Candidate genes, which were linked to these genetic markers, were identified and some of them show a direct relationship with developmental processes, especially for those in association with sexual maturation. However, the relatively low power to detect genetic markers associated with growth (days to 5 kg) in this GWAS indicates the need to use a higher density SNP array in order to overcome the low levels of linkage disequilibrium observed in Atlantic salmon before the information can be incorporated into a selective breeding program.

Phenotypic and QTL allelic associations among embryonic developmental rate, body size, and precocious maturation in male rainbow trout

We examined associations among embryonic developmental rate (EDR) as measured by hatching time, juvenile body weight (BW) and propensity for precocial sexual maturation (PM) at two years in two sets of diallel crosses of rainbow trout produced in two spawning seasons (September and December) at both the phenotypic and genotypic levels. Dams and sires had highly significant effects on the body weight of their male juvenile progeny on three measurement dates where parental effects remained consistent through time. Dams spawning earlier in the season produced a greater number of mature male progeny (56.7%) than did later spawning females (25.6%). The families from the December lot showed the expected associations among traits in that earlier hatching fish were significantly heavier on all three measurement dates than later hatching fish and were more likely to mature earlier when families were combined. Moreover, earlier maturing fish were significantly heavier on the third measurement date than those that did not mature. In the September lot, mature fish were significantly heavier as juveniles on all three measurement dates than immature fish as predicted but no significant associations were detected between EDR and BW or between PM and EDR. Significant QTL were detected for all three traits but the linkage group location varied depending on the trait and half-sib group analyzed (across dams and sires in each lot). A strong QTL for EDR with genome-wide effects was detected on linkage group RT-8 in all four half-sib analyses. None of the four linkage groups analyzed had QTL for all three traits. However, the phenotypic association between EDR and BW observed in the December lot was supported by the co-localization of QTL to linkage group RT-8 and a positive coupling of allelic effects. RT-8 marker alleles significantly associated with faster EDR were also associated with larger BW and this was observed in numerous families on all three measurement dates. Linkage group RT-24 had weaker QTL for all three traits in the September lot but these were not detected in the same half-sib group simultaneously. At the allelic level, marker alleles for faster EDR were also associated with BW but only at the third measurement date and the progeny of one male. Similarly, RT-30 had weaker QTL for EDR and PM in the December paternal half-sib analysis but no associations were evident at the allelic level. The detection of associations between life history traits and growth at both the phenotypic and genotypic levels has significant implications to aquaculture breeding programs where selection for a desirable trait may lead to unwanted alterations of other traits. Furthermore, the differences between spawning season lots emphasize the complex interaction between environment and genotype on economically important traits and the resulting challenges for aquaculture.

Molecular markers for variation in spawning date in a hatchery population of rainbow trout (Oncorhynchus mykiss)

We examined the distribution of alleles at 63 microsatellite loci distributed across 29 linkage groups in broodstock females from a commercial population of rainbow trout spawning on different dates throughout the season (August to January). A total of 368 females, 184 and 117 females from each of the tail-ends of the spawning distribution and a subsample of 67 females spawning in the middle, were used to detect marker-trait associations. Twenty-one loci in a subset of genomic regions (RT-5, 7, 8, 10, 12, 14, 15, 22, 23, 24, 25, 29, 30, and 31) were significantly associated with variation in spawning date. Many of these markers localize to regions with known spawning date quantitative trait loci based on previous studies. An individual assignment analysis was used to test how well the molecular data could be used to assign individuals to their correct spawning group, and markers were given a ranking reflecting their contribution to the accuracy of assignment. The top 15 ranked markers were successful at assigning the majority of females to the correct spawning group based on genotype with an average accuracy of 76 %. The most likely genes that could contribute to these differences in spawning date are discussed. Together, these data indicate that the loci could be incorporated into a selection index with phenotype data to increase the accuracy of selection for spawning date.

Mapping and Expression of Candidate Genes for Development Rate in Rainbow Trout (Oncorhynchus mykiss)

Development rate has important implications for individual fitness and physiology. In salmonid fishes, development rate correlates with many traits later in life, including life-history diversity, growth, and age and size at sexual maturation. In rainbow trout (Oncorhynchus mykiss), a quantitative trait locus for embryonic development rate has been detected on chromosome 5 across populations. However, few candidate genes have been identified within this region. In this study, we use gene mapping, gene expression, and quantitative genetic methods to further identify the genetic basis of embryonic developmental rate in O. mykiss Among the genes located in the region of the major development rate quantitative trait locus (GHR1, Clock1a, Myd118-1, and their paralogs), all were expressed early in embryonic development (fertilization through hatch), but none were differentially expressed between individuals with the fast- or slow-developing alleles for a major embryonic development rate quantitative trait locus. In a follow-up study of migratory and resident rainbow trout from natural populations in Alaska, we found significant additive variation in development rate and, moreover, found associations between development rate and allelic variation in all 3 candidate genes within the quantitative trait locus for embryonic development. The mapping of these genes to this region and associations in multiple populations provide positional candidates for further study of their roles in growth, development, and life-history diversity in this model salmonid.

Rapid parallel evolution of standing variation in a single, complex, genomic region is associated with life history in steelhead/rainbow trout

Rapid adaptation to novel environments may drive changes in genomic regions through natural selection. Such changes may be population-specific or, alternatively, may involve parallel evolution of the same genomic region in multiple populations, if that region contains genes or co-adapted gene complexes affecting the selected trait(s). Both quantitative and population genetic approaches have identified associations between specific genomic regions and the anadromous (steelhead) and resident (rainbow trout) life-history strategies of Oncorhynchus mykiss. Here, we use genotype data from 95 single nucleotide polymorphisms and show that the distribution of variation in a large region of one chromosome, Omy5, is strongly associated with life-history differentiation in multiple above-barrier populations of rainbow trout and their anadromous steelhead ancestors. The associated loci are in strong linkage disequilibrium, suggesting the presence of a chromosomal inversion or other rearrangement limiting recombination. These results provide the first evidence of a common genomic basis for life-history variation in O. mykiss in a geographically diverse set of populations and extend our knowledge of the heritable basis of rapid adaptation of complex traits in novel habitats.

Detection of quantitative trait loci (QTL) related to grilsing and late sexual maturation in Atlantic salmon (Salmo salar)

In Atlantic salmon aquaculture, early sexual maturation represents a major problem for producers. This is especially true for grilse, which mature after one sea winter before reaching a desirable harvest weight, rather than after two sea winters. Salmon maturing as grilse have a much lower market value than later maturing individuals. For this reason, most companies desire fish that grow fast and mature late. Marker-assisted selection has the potential to improve the efficiency of selection against early maturation and for late sexual maturation; however, studies identifying age of sexual maturation-related genetic markers are lacking for Atlantic salmon. Therefore, we used a 6.5K single-nucleotide polymorphism (SNP) array to genotype five families from the Mainstream Canada broodstock program and search for SNPs associated with early (grilsing) or late sexual maturation. There were 529 SNP loci that were variable across all five families, and this was the set that was used for quantitative trait loci (QTL) analysis. GridQTL identified two chromosomes, Ssa10 and Ssa21, containing QTL related to grilsing. In contrast, only one QTL, on Ssa18, was found linked to late maturation in Atlantic salmon. Our previous work on these five families did not identify genome-wide significant growth-related QTL on Ssa10, Ssa21, or Ssa18. Therefore, taken together, these results suggest that both grilsing and late sexual maturation are controlled independently of one another and also from growth-related traits. The identification of genomic regions associated with grilsing or late sexual maturation provide an opportunity to incorporate this information into selective breeding programs that will enhance Atlantic salmon farming.

Coding Gene Single Nucleotide Polymorphism Mapping and Quantitative Trait Loci Detection for Physiological Reproductive Traits in Brook Charr, Salvelinus fontinalis

A linkage map of 40 linkage groups (LGs) was developed for brook charr, Salvelinus fontinalis, using an F(2) interstrain hybrid progeny (n = 171) and 256 coding gene SNP developed specifically for brook charr and validated from a large (>1000) subset of putative SNP, as well as 81 microsatellite markers. To identify quantitative trait loci (QTL) related to reproduction functions, these fish were also phenotyped at six physiological traits, including spermatozoid head diameter, sperm concentration, plasma testosterone, plasma 11-keto-testosterone, egg diameter, and plasma 17β-estradiol. Five significant QTL were detected over four LGs for egg diameter and plasma 17β-estradiol concentration in females, and sperm concentration as well as spermatozoid head diameter in males. In females, two different QTLs located on LG 11 and LG 34 were associated with the egg number, whereas one QTL was associated with plasma 17β-estradiol concentration (LG 8). Their total percent variance explained (PVE) was 26.7% and 27.6%, respectively. In males, two QTL were also detected for the sperm concentration, and their PVE were estimated at 18.58% and 14.95%, respectively. The low QTL number, associated with the high PVE, suggests that the variance in these reproductive physiological traits was either under the control of one major gene or a small number of genes. The QTL associated with sperm concentration, plasma 17β-estradiol, and egg diameter appeared to be under a dominance effect, whereas the two others were under a negative additive effect. These results show that genes underlying the phenotypic variance of these traits are under different modes of action (additive vs. dominance) and may be used to predict an increase or a decrease in their phenotypic values in subsequent generations of selective breeding. Moreover, this newly developed panel of mapped SNP located in coding gene regions will be useful for screening wild populations, especially in the context of investigating the genetic impact of massive stocking of domestic brook charr to support the angling industry throughout eastern North America.

Genetic architecture of body weight, condition factor and age of sexual maturation in Icelandic Arctic charr (Salvelinus alpinus)

The high commercial value from the aquaculture of salmonid fishes has prompted many studies into the genetic architecture of complex traits and the need to identify genomic regions that have repeatable associations with trait variation both within and among species. We searched for quantitative trait loci (QTL) for body weight (BW), condition factor (CF) and age of sexual maturation (MAT) in families of Arctic charr (Salvelinus alpinus) from an Icelandic breeding program. QTL with genome-wide significance were detected for each trait on multiple Arctic charr (AC) linkage groups (BW: AC-4, AC-20; CF: AC-7, AC-20, AC-23, AC-36; MAT: AC-13/34, AC-39). In addition to the genome-wide significant QTL for both BW and CF on AC-20, linkage groups AC-4, AC-7, AC-8, and AC-16 contain QTL for both BW and CF with chromosome-wide significance. These regions had effects (albeit weaker) on MAT with the exception of the region on AC-8. Comparisons with a North American cultured strain of Arctic charr, as well as North American populations of Atlantic salmon (Salmo salar), and rainbow trout (Oncorhynchus mykiss), reveal some conservation in QTL location and structure, particularly with respect to the joint associations of QTL influencing BW and CF. The detection of some differences in genetic architecture between the two aquaculture strains of Arctic charr may be reflective of the differential evolutionary histories experienced by these fishes, and illustrates the importance of including different strains to investigate genetic variation in a species where the intent is to use that variation in selective breeding programs.