Transforming ecology and conservation biology through genome editing.

As the conservation challenges increase, new approaches are needed to help combat losses in biodiversity and slow or reverse the decline of threatened species. Genome-editing technology is changing the face of modern biology, facilitating applications that were unimaginable only a decade ago. The technology has the potential to make significant contributions to the fields of evolutionary biology, ecology, and conservation, yet the fear of unintended consequences from designer ecosystems containing engineered organisms has stifled innovation. To overcome this gap in the understanding of what genome editing is and what its capabilities are, more research is needed to translate genome-editing discoveries into tools for ecological research. Emerging and future genome-editing technologies include new clustered regularly interspaced short palindromic repeats (CRISPR) targeted sequencing and nucleic acid detection approaches as well as species genetic barcoding and somatic genome-editing technologies. These genome-editing tools have the potential to transform the environmental sciences by providing new noninvasive methods for monitoring threatened species or for enhancing critical adaptive traits. A pioneering effort by the conservation community is required to apply these technologies to real-world conservation problems.

Transformación de la Ecología y la Biología de la Conservación por medio de la Edición Genómica Resumen Conforme aumentan los retos de conservación, se necesitan nuevas estrategias para ayudar a combatir las pérdidas de biodiversidad y para disminuir o revertir la declinación de especies. La tecnología de edición genómica está cambiando el rostro de la biología moderna, facilitando aplicaciones que eran inimaginables hace una década. Esta tecnología tiene el potencial de contribuir significativamente en los campos de la biología evolutiva, la ecología y la conservación, aun así, el miedo a las consecuencias accidentales de los ecosistemas planeados que contienen organismos diseñados ha sofocado a la innovación. Para sobreponerse a este vacío en el entendimiento de lo que es la edición genómica y cuáles son sus capacidades se requiere de mayor investigación para traducir los descubrimientos de la edición genómica a herramientas para la investigación ecológica. Las tecnologías de edición genómica emergentes y futuras incluyen nuevas estrategias CRISPR enfocadas en la secuenciación y detección de ácidos nucleicos, así como tecnologías de definición del código de barras genético de las especies y de edición somática de genes. Estas herramientas de edición genómica tienen el potencial para transformar las ciencias ambientales al proporcionar nuevos métodos no invasivos para el monitoreo de especies amenazadas o para mejorar las características adaptativas más importantes. Se requiere de un esfuerzo vanguardista por parte de la comunidad conservadora para aplicar esta tecnología a los problemas de conservación en el mundo real.

Retention of a chromosomal inversion from an anadromous ancestor provides the genetic basis for alternative freshwater ecotypes in rainbow trout.

Migratory behaviour patterns in animals are controlled by a complex genetic architecture. Rainbow trout (Oncorhynchus mykiss) is a salmonid fish that spawns in streams but exhibits three primary life history pathways: stream-resident (fluvial), lake-migrant (adfluvial) and ocean-migrant (anadromous). Previous studies examining fluvial and anadromous O. mykiss have identified several genes associated with life history divergence including the presence of an inversion complex within chromosome 5 (Omy05) that appears to maintain a suite of linked genes controlling migratory behaviour. However, adfluvial trout are migratory without being anadromous, and the genetic basis for this life history has not been investigated from evolutionary perspectives. We sampled wild, native nonanadromous rainbow trout occupying connected stream and lake habitats in a southwest Alaskan watershed to determine whether these fish exhibit genetic divergence between fluvial and adfluvial ecotypes, and whether that divergence parallels that documented in fluvial and anadromous O. mykiss. Data from restriction site-associated DNA (RAD) sequencing revealed an association between frequencies of both the Omy05 inversion complex and other single nucleotide polymorphisms (SNPs) with habitat type (stream or lake), supporting the genetic divergence of fluvial and adfluvial individuals in sympatry. The presence of a genetic basis for migration into lakes, analogous to that documented for anadromy, indicates that the adfluvial ecotype must be recognized separately from the fluvial form of O. mykiss even though neither is anadromous. These results highlight the genetic architecture underlying migration and the importance of chromosomal inversions in promoting and sustaining intraspecific diversity.

Parallel signatures of selection at genomic islands of divergence and the major histocompatibility complex in ecotypes of sockeye salmon across Alaska.

Understanding the genetic mechanisms that facilitate adaptive radiation is an important component of evolutionary biology. Here, we genotyped 82 neutral SNPs, seven SNPs in islands of divergence identified in a previous study (island SNPs), and a region of the major histocompatibility complex (MHC) in 32 populations of sockeye salmon to investigate whether conserved genes and genomic regions are involved in adaptive radiation. Populations representing three ecotypes were sampled from seven drainages with differing habitats and colonization histories spanning a range of 2,000 km. We found strong signatures of parallel selection across drainages at the island SNPs and MHC, suggesting that the same loci undergo divergent selection during adaptive radiation. However, patterns of differentiation at most island SNPs and the MHC were not associated with ecotypes, suggesting that these loci are responding differently to a mosaic of selective pressures. Our study provides some of the first evidence that conserved genomic islands may be involved in adaptive divergence of salmon populations. Additionally, our data provide further support for the hypothesis that sockeye salmon inhabiting rivers unconnected to lakes harbour similar genetic diversity across large distances, are likely the ancestral form of the species, and have repeatedly recolonized lake systems as they have become available after glacial recession. Finally, our results highlight the value and importance of validating outlier loci by screening additional populations and regions, a practice that will hopefully become more common in the future.

"Chromosomes and genes, spawned these fateful scenes": Rapid adaptation in an introduced fish.

Humans by their very nature alter the distribution of species. Be it introduction of exotic species, habitat alterations or construction of barriers, anthropogenic changes provide novel experimental systems for the molecular ecologist to study evolutionary change. These events often provide a contradiction. Effective population sizes are generally low, and introduced populations are typically characterized by reduced diversity consistent with theoretical predictions of population bottlenecks and founder effects. However, despite reduced diversity, rapid change sometimes occurs. Identification of genomic regions associated with these rapid adaptive responses to novel selection pressures provides a window into genomic regions important in adaptive diversity, both in the novel and native ranges. These studies also provide an important means to estimate the pace of evolutionary change. In this issue, Willoughby et al. () compared the heterozygosity of steelhead (the anadromous form of rainbow trout Oncorhynchus mykiss) introduced into Lake Michigan in the late 1880s to the putative source population from the ancestral California range. After 25 generations of isolation in Lake Michigan, Willoughby et al. () found consistent genomewide reductions in genetic diversity as estimated by a measure of pooled heterozygosity. Despite this overall reduction in heterozygosity, three chromosomal regions showed signals of rapid adaptation and contained genes associated with osmoregulatory and wound-healing functions.

Screening of duplicated loci reveals hidden divergence patterns in a complex salmonid genome.

A whole-genome duplication (WGD) doubles the entire genomic content of a species and is thought to have catalysed adaptive radiation in some polyploid-origin lineages. However, little is known about general consequences of a WGD because gene duplicates (i.e., paralogs) are commonly filtered in genomic studies; such filtering may remove substantial portions of the genome in data sets from polyploid-origin species. We demonstrate a new method that enables genome-wide scans for signatures of selection at both nonduplicated and duplicated loci by taking locus-specific copy number into account. We apply this method to RAD sequence data from different ecotypes of a polyploid-origin salmonid (Oncorhynchus nerka) and reveal signatures of divergent selection that would have been missed if duplicated loci were filtered. We also find conserved signatures of elevated divergence at pairs of homeologous chromosomes with residual tetrasomic inheritance, suggesting that joint evolution of some nondiverged gene duplicates may affect the adaptive potential of these genes. These findings illustrate that including duplicated loci in genomic analyses enables novel insights into the evolutionary consequences of WGDs and local segmental gene duplications.

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 FST  > 0.14 (average FST 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.

Sorting duplicated loci disentangles complexities of polyploid genomes masked by genotyping by sequencing.

Many plants and animals of polyploid origin are currently enjoying a genomics explosion enabled by modern sequencing and genotyping technologies. However, routine filtering of duplicated loci in most studies using genotyping by sequencing introduces an unacceptable, but often overlooked, bias when detecting selection. Retained duplicates from ancient whole-genome duplications (WGDs) may be found throughout genomes, whereas retained duplicates from recent WGDs are concentrated at distal ends of some chromosome arms. Additionally, segmental duplicates can be found at distal ends or nearly anywhere in a genome. Evidence shows that these duplications facilitate adaptation through one of two pathways: neo-functionalization or increased gene expression. Filtering duplicates removes distal ends of some chromosomes, and distal ends are especially known to harbour adaptively important genes. Thus, filtering of duplicated loci impoverishes the interpretation of genomic data as signals from contiguous duplicated genes are ignored. We review existing strategies to genotype and map duplicated loci; we focus in detail on an overlooked strategy of using gynogenetic haploids (1N) as a part of new genotyping by sequencing studies. We provide guidelines on how to use this haploid strategy for studies on polyploid-origin vertebrates including how it can be used to screen duplicated loci in natural populations. We conclude by discussing areas of research that will benefit from better inclusion of polyploid loci; we particularly stress the sometimes overlooked fact that basing genomic studies on dense maps provides value added in the form of locating and annotating outlier loci or colocating outliers into islands of divergence.

Identification and Characterization of Sex-Associated Loci in Sockeye Salmon Using Genotyping-by-Sequencing and Comparison with a Sex-Determining Assay Based on the sdY Gene.

Loci that can be used to screen for sex in salmon can provide important information for study of both wild and cultured populations. Here, we tested for associations between sex and genotypes at thousands of loci available from a genotyping-by-sequencing (GBS) dataset to discover sex-associated loci in sockeye salmon (Oncorhynchus nerka). We discovered 7 sex-associated loci, developed high-throughput assays for 2 loci, and tested the utility of these 2 assays in 8 collections of sockeye salmon sampled throughout North America. We also screened an existing assay based on the master sex-determining gene in salmon (sdY) in these collections. The ability of GBS-derived loci to assign fish to their phenotypic sex varied substantially among collections suggesting that recombination between the loci that we discovered and the sex-determining gene has occurred. Assignment accuracy to phenotypic sex was much higher with the sdY assay but was still less than 100%. Alignment of sequences from GBS-derived loci to draft genomes for 2 salmonids provided strong evidence that many of these loci are found on chromosomes orthologous to the known sex chromosome in sockeye salmon. Our study is the first to describe the approximate location of the sex-determining region in sockeye salmon and indicates that sdY is also the master sex-determining gene in this species. However, discordances between sdY genotypes and phenotypic sex and the variable performance of GBS-derived loci warrant more research.

Identification of Multiple QTL Hotspots in Sockeye Salmon (Oncorhynchus nerka) Using Genotyping-by-Sequencing and a Dense Linkage Map.

Understanding the genetic architecture of phenotypic traits can provide important information about the mechanisms and genomic regions involved in local adaptation and speciation. Here, we used genotyping-by-sequencing and a combination of previously published and newly generated data to construct sex-specific linkage maps for sockeye salmon (Oncorhynchus nerka). We then used the denser female linkage map to conduct quantitative trait locus (QTL) analysis for 4 phenotypic traits in 3 families. The female linkage map consisted of 6322 loci distributed across 29 linkage groups and was 4082 cM long, and the male map contained 2179 loci found on 28 linkage groups and was 2291 cM long. We found 26 QTL: 6 for thermotolerance, 5 for length, 9 for weight, and 6 for condition factor. QTL were distributed nonrandomly across the genome and were often found in hotspots containing multiple QTL for a variety of phenotypic traits. These hotspots may represent adaptively important regions and are excellent candidates for future research. Comparing our results with studies in other salmonids revealed several regions with overlapping QTL for the same phenotypic trait, indicating these regions may be adaptively important across multiple species. Altogether, our study demonstrates the utility of genomic data for investigating the genetic basis of important phenotypic traits. Additionally, the linkage map created here will enable future research on the genetic basis of phenotypic traits in salmon.

Effects of crossovers between homeologs on inheritance and population genomics in polyploid-derived salmonid fishes.

A whole genome duplication occurred in the ancestor of all salmonid fishes some 50-100 million years ago. Early inheritance studies with allozymes indicated that loci in the salmonid genome are inherited disomically in females. However, some pairs of duplicated loci showed patterns of inheritance in males indicating pairing and recombination between homeologous chromosomes. Nearly 20% of loci in the salmonid genome are duplicated and share the same alleles (isoloci), apparently due to homeologous recombination. Half-tetrad analysis revealed that isoloci tend to be telomeric. These results suggested that residual tetrasomic inheritance of isoloci results from homeologous recombination near chromosome ends and that continued disomic inheritance resulted from homologous pairing of centromeric regions. Many current genetic maps of salmonids are based on single nucleotide polymorphisms and microsatellites that are no longer duplicated. Therefore, long sections of chromosomes on these maps are poorly represented, especially telomeric regions. In addition, preferential multivalent pairing of homeologs from the same species in F1 hybrids results in an excess of nonparental gametes (so-called pseudolinkage). We consider how not including duplicated loci has affected our understanding of population and evolutionary genetics of salmonids, and we discuss how incorporating these loci will benefit our understanding of population genomics.

Trade-offs and utility of alternative RADseq methods: reply to Puritz et al.

Puritz et al. provide a review of several RADseq methodological approaches in response to our 'Population Genomic Data Analysis' workshop (Sept 2013) review (Andrews & Luikart 2014). We agree with Puritz et al. on the importance for researchers to thoroughly understand RADseq library preparation and data analysis when choosing an approach for answering their research questions. Some of us are currently using multiple RADseq protocols, and we agree that the different methods may offer advantages in different cases. Our workshop review did not intend to provide a thorough review of RADseq because the workshop covered a broad range of topics within the field of population genomics. Similarly, neither the response of Puritz et al. nor our comments here provide sufficient space to thoroughly review RADseq. Nonetheless, here we address some key points that we find unclear or potentially misleading in their evaluation of techniques.

Signals of heterogeneous selection at an MHC locus in geographically proximate ecotypes of sockeye salmon.

The genes of the major histocompatibility complex (MHC) are an important component of the vertebrate immune system and can provide insights into the role of pathogen-mediated selection in wild populations. Here, we examined variation at the MHC class II peptide-binding region in 27 populations of sockeye salmon (Oncorhynchus nerka), distributed among three distinct spawning ecotypes, from a complex of interconnected rivers and lakes in south-western Alaska. We also obtained genotypes from 90 putatively neutral single nucleotide polymorphisms for each population to compare the relative roles of demography and selection in shaping the observed MHC variation. We found that MHC divergence was generally partitioned by spawning ecotype (lake beaches, rivers and streams) and was 30 times greater than variation at neutral markers. Additionally, we observed substantial differences in modes of selection and diversity among ecotypes, with beach populations displaying higher levels of directional selection and lower MHC diversity than the other two ecotypes. Finally, the level of MHC differentiation in our study system was comparable to that observed over much larger geographic ranges, suggesting that MHC variation does not necessarily increase with increasing spatial scale and may instead be driven by fine-scale differences in pathogen communities or pathogen virulence. The low levels of neutral structure and spatial proximity of populations in our study system indicate that MHC differentiation can be maintained through strong selective pressure even when ample opportunities for gene flow exist.

Temporally isolated lineages of pink salmon reveal unique signatures of selection on distinct pools of standing genetic variation.

A species' genetic diversity bears the marks of evolutionary processes that have occurred throughout its history. However, robust detection of selection in wild populations is difficult and often impeded by lack of replicate tests. Here, we investigate selection in pink salmon (Oncorhynchus gorbuscha) using genome scans coupled with inference from a haploid-assisted linkage map. Pink salmon have a strict 2-year semelparous life history which has resulted in temporally isolated (allochronic) lineages that remain sympatric through sharing of spawning habitats in alternate years. The lineages differ in a range of adaptive traits, suggesting different genetic backgrounds. We used genotyping by sequencing of haploids to generate a high-density linkage map with 7035 loci and screened an existing panel of 8036 loci for signatures of selection. The linkage map enabled identification of novel genomic regions displaying signatures of parallel selection shared between lineages. Furthermore, 24 loci demonstrated divergent selection and differences in genetic diversity between lineages, suggesting that adaptation in the 2 lineages has arisen from different pools of standing genetic variation. Findings have implications for understanding asynchronous population abundances as well as predicting future ecosystem impacts from lineage-specific responses to climate change.

Meiotic maps of sockeye salmon derived from massively parallel DNA sequencing.

BACKGROUND: Meiotic maps are a key tool for comparative genomics and association mapping studies. Next-generation sequencing and genotyping by sequencing are speeding the processes of SNP discovery and the development of new genetic tools, including meiotic maps for numerous species. Currently there are limited genetic resources for sockeye salmon, Oncorhynchus nerka. We develop the first dense meiotic map for sockeye salmon using a combination of novel SNPs found in restriction site associated DNA (RAD tags) and SNPs available from existing expressed sequence tag (EST) based assays. RESULTS: We discovered and genotyped putative SNPs in 3,430 RAD tags. We removed paralogous sequence variants leaving 1,672 SNPs; these were combined with 53 EST-based SNP genotypes for linkage mapping. The map contained 29 male and female linkage groups, consistent with the haploid chromosome number expected for sockeye salmon. The female map contains 1,057 loci spanning 4,896 cM, and the male map contains 1,118 loci spanning 4,220 cM. Regions of conservation with rainbow trout and synteny between the RAD based rainbow trout map and the sockeye salmon map were established. CONCLUSIONS: Using RAD sequencing and EST-based SNP assays we successfully generated the first high density linkage map for sockeye salmon.

Single nucleotide polymorphisms unravel hierarchical divergence and signatures of selection among Alaskan sockeye salmon (Oncorhynchus nerka) populations.

BACKGROUND: Disentangling the roles of geography and ecology driving population divergence and distinguishing adaptive from neutral evolution at the molecular level have been common goals among evolutionary and conservation biologists. Using single nucleotide polymorphism (SNP) multilocus genotypes for 31 sockeye salmon (Oncorhynchus nerka) populations from the Kvichak River, Alaska, we assessed the relative roles of geography (discrete boundaries or continuous distance) and ecology (spawning habitat and timing) driving genetic divergence in this species at varying spatial scales within the drainage. We also evaluated two outlier detection methods to characterize candidate SNPs responding to environmental selection, emphasizing which mechanism(s) may maintain the genetic variation of outlier loci. RESULTS: For the entire drainage, Mantel tests suggested a greater role of geographic distance on population divergence than differences in spawn timing when each variable was correlated with pairwise genetic distances. Clustering and hierarchical analyses of molecular variance indicated that the largest genetic differentiation occurred between populations from distinct lakes or subdrainages. Within one population-rich lake, however, Mantel tests suggested a greater role of spawn timing than geographic distance on population divergence when each variable was correlated with pairwise genetic distances. Variable spawn timing among populations was linked to specific spawning habitats as revealed by principal coordinate analyses. We additionally identified two outlier SNPs located in the major histocompatibility complex (MHC) class II that appeared robust to violations of demographic assumptions from an initial pool of eight candidates for selection. CONCLUSIONS: First, our results suggest that geography and ecology have influenced genetic divergence between Alaskan sockeye salmon populations in a hierarchical manner depending on the spatial scale. Second, we found consistent evidence for diversifying selection in two loci located in the MHC class II by means of outlier detection methods; yet, alternative scenarios for the evolution of these loci were also evaluated. Both conclusions argue that historical contingency and contemporary adaptation have likely driven differentiation between Kvichak River sockeye salmon populations, as revealed by a suite of SNPs. Our findings highlight the need for conservation of complex population structure, because it provides resilience in the face of environmental change, both natural and anthropogenic.

Temperature-associated population diversity in salmon confers benefits to mobile consumers.

Habitat heterogeneity can generate intraspecific diversity through local adaptation of populations. While it is becoming increasingly clear that population diversity can increase stability in species abundance, less is known about how population diversity can benefit consumers that can integrate across population diversity in their prey. Here we demonstrate cascading effects of thermal heterogeneity on trout-salmon interactions in streams where rainbow trout rely heavily on the seasonal availability of anadromous salmon eggs. Water temperature in an Alaskan stream varied spatially from 5 degrees C to 17.5 degrees C, and spawning sockeye salmon showed population differentiation associated with this thermal heterogeneity. Individuals that spawned early in cool regions of the 5 km long stream were genetically differentiated from those spawning in warmer regions later in the season. Sockeye salmon spawning generates a pulsed resource subsidy that supports the majority of seasonal growth in stream-dwelling rainbow trout. The spatial and temporal structuring of sockeye salmon spawn timing in our focal stream extended the duration of the pulsed subsidy compared to a thermally homogeneous stream with a single population of salmon. Further, rainbow trout adopted movement strategies that exploited the multiple pulses of egg subsidies in the thermally heterogeneous stream. Fish that moved to track the resource pulse grew at rates about 2.5 times higher than those that remained stationary or trout in the reference stream with a single seasonal pulse of eggs. Our results demonstrate that habitat heterogeneity can have important effects on the population diversity of dominant species, and in turn, influence their value to species that prey upon them. Therefore, habitat homogenization may have farther-reaching ecological effects than previously considered.

The pink salmon genome: Uncovering the genomic consequences of a two-year life cycle.

Pink salmon (Oncorhynchus gorbuscha) adults are the smallest of the five Pacific salmon native to the western Pacific Ocean. Pink salmon are also the most abundant of these species and account for a large proportion of the commercial value of the salmon fishery worldwide. A two-year life history of pink salmon generates temporally isolated populations that spawn either in even-years or odd-years. To uncover the influence of this genetic isolation, reference genome assemblies were generated for each year-class and whole genome re-sequencing data was collected from salmon of both year-classes. The salmon were sampled from six Canadian rivers and one Japanese river. At multiple centromeres we identified peaks of Fst between year-classes that were millions of base-pairs long. The largest Fst peak was also associated with a million base-pair chromosomal polymorphism found in the odd-year genome near a centromere. These Fst peaks may be the result of a centromere drive or a combination of reduced recombination and genetic drift, and they could influence speciation. Other regions of the genome influenced by odd-year and even-year temporal isolation and tentatively under selection were mostly associated with genes related to immune function, organ development/maintenance, and behaviour.

Network Analysis of Linkage Disequilibrium Reveals Genome Architecture in Chum Salmon.

Many studies exclude loci that exhibit linkage disequilibrium (LD); however, high LD can signal reduced recombination around genomic features such as chromosome inversions or sex-determining regions. Chromosome inversions and sex-determining regions are often involved in adaptation, allowing for the inheritance of co-adapted gene complexes and for the resolution of sexually antagonistic selection through sex-specific partitioning of genetic variants. Genomic features such as these can escape detection when loci with LD are removed; in addition, failing to account for these features can introduce bias to analyses. We examined patterns of LD using network analysis to identify an overlapping chromosome inversion and sex-determining region in chum salmon. The signal of the inversion was strong enough to show up as false population substructure when the entire dataset was analyzed, while the effect of the sex-determining region on population structure was only obvious after restricting analysis to the sex chromosome. Understanding the extent and geographic distribution of inversions is now a critically important part of genetic analyses of natural populations. Our results highlight the importance of analyzing and understanding patterns of LD in genomic dataset and the perils of excluding or ignoring loci exhibiting LD. Blindly excluding loci in LD would have prevented detection of the sex-determining region and chromosome inversion while failing to understand the genomic features leading to high-LD could have resulted in false interpretations of population structure.

Y-chromosome haplotypes are associated with variation in size and age at maturity in male Chinook salmon.

Variation in size and age at maturity is an important component of life history that is influenced by both environmental and genetic factors. In salmonids, large size confers a direct reproductive advantage through increased fecundity and egg quality in females, while larger males gain a reproductive advantage by monopolizing access to females. In addition, variation in size and age at maturity in males can be associated with different reproductive strategies; younger smaller males may gain reproductive success by sneaking among mating pairs. In both sexes, there is a trade-off between older age and increased reproductive success and increased risk of mortality by delaying reproduction. We identified four Y-chromosome haplogroups that showed regional- and population-specific variation in frequency using RADseq data for 21 populations of Alaska Chinook salmon. We then characterized the range-wide distribution of these haplogroups using GT-seq assays. These haplogroups exhibited associations with size at maturity in multiple populations, suggesting that lack of recombination between X and Y-chromosomes has allowed Y-chromosome haplogroups to capture different alleles that influence size at maturity. Ultimately, conservation of life history diversity in Chinook salmon may require conservation of Y-chromosome haplotype diversity.