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Programmed elimination of DNA during development yields somatic cell nuclei with dramatically different DNA sequence and content relative to germline nuclei, profoundly influencing genome architecture and stability. Whole-genome sequencing has significantly expanded the list of taxa known to exhibit this trait and has revealed the identity of excised genes and transposable elements (TEs) in certain taxa. Here, we compare the diverse mechanisms employed by ciliates, nematodes, copepods, and lamprey to downsize their genomes during development and propose tests of hypotheses about the evolution and/or maintenance of this trait. We explore possible functional roles that programmed DNA elimination (PDE) could play in genomic defense (especially against TEs), regulation of development, sex determination, co-option, and modulating nucleotypic effects, which together argue for a place in the mainstream investigation of genome evolution.

A global decline in seagrass populations has led to renewed calls for their conservation as important providers of biogenic and foraging habitat, shoreline stabilisation, and carbon storage. Eelgrass (Zostera marina) occupies the largest geographic range among seagrass species spanning a commensurately broad spectrum of environmental conditions. However, relatively little is known about their fine-scale genetic structure and broad-scale genomic signatures of environmental adaptation, and in Canada, eelgrass is considered a single phylogroup despite occurring across three oceans. We used a pooled whole-genome re-sequencing approach to characterise population structure, gene flow, and adaptation of 23 eelgrass populations ranging from the Northeast United States, to Atlantic, subarctic, and Pacific Canada. We identified over 500,000 SNPs, which when mapped to a chromosome-level genome assembly revealed six broad clades of eelgrass across the study area, with pairwise FST ranging from 0 among neighbouring populations to 0.54 between Pacific and Atlantic coasts. Genetic diversity was highest in the Pacific and lowest in the Arctic, consistent with colonisation of the Arctic and Atlantic oceans from the Pacific. Using redundancy analyses and two climate change projection scenarios, we found that subarctic populations are more vulnerable to climate change through genomic offset predictions. Conservation planning in Canada should ensure that representative populations from each identified clade are included within a national network so that latent genetic diversity is protected, and gene flow is maintained. Northern populations, in particular, may require stronger protective measures given their susceptibility to change climate.

Adaptation to ocean climate is increasingly recognized as an important driver of diversity in marine species despite the lack of physical barriers to dispersal and the presence of pelagic stages in many taxa. A robust understanding of the genomic and ecological processes involved in structuring populations is lacking for most marine species, often hindering management and conservation action. Cunner (Tautogolabrus adspersus), is a temperate reef fish that displays both pelagic early life history stages and strong site-associated homing as adults; the species is also presently of interest for use as a cleaner fish in salmonid aquaculture in Atlantic Canada. Here we produce a chromosome-level genome assembly for cunner and characterize spatial population structure throughout Atlantic Canada using whole genome resequencing. The genome assembly spanned 0.72 Gbp and resolved 24 chromosomes; whole genome resequencing of 803 individuals from 20 locations spanning from Newfoundland to New Jersey identified approximately 11 million genetic variants. Principal component analysis revealed four distinct regional groups in Atlantic Canada, including three near the range edge in Newfoundland. PairwiseFSTand selection scans revealed consistent signals of differentiation and selection at discrete genomic regions including adjacent peaks on chromosome 10 recurring across multiple pairwise comparisons (i.e.,FST0.5-0.75). Redundancy analysis suggested significant association of environmental variables related to benthic temperature and oxygen range with genomic structure, again highlighting the previously identified region on chromosome 10. Our results suggest that climate associated adaptation in this temperate reef fish drives regional diversity despite high early life history dispersal potential.

The negative genetic impacts of gene flow from domestic to wild populations can be dependent on the degree of domestication and exacerbated by the magnitude of pre-existing genetic differences between wild populations and the domestication source. Recent evidence of European ancestry within North American aquaculture Atlantic salmon (Salmo salar) has elevated the potential impact of escaped farmed salmon on often at-risk wild North American salmon populations. Here we compare the ability of single nucleotide polymorphism (SNP) and microsatellite (SSR) marker panels of different sizes (7-SSR, 100-SSR, and 220K-SNP) to detect introgression of European genetic information into North American wild and aquaculture populations. Linear regression comparing admixture predictions for a set of individuals common to the three data sets showed that the 100-SSR panel and 7-SSR panels replicated the full 220K-SNP-based admixture estimates with low accuracy (r2of 0.64 and 0.49 respectively). Additional tests explored the effects of individual sample size and marker number, which revealed that ~300 randomly selected SNPs could replicate the 220K-SNP admixture predictions with greater than 95% fidelity. We designed a custom SNP panel (301-SNP) for European admixture detection in future monitoring work and then developed and tested a Python package, SalmonEuAdmix (https://github.com/CNuge/SalmonEuAdmix), that uses a deep neural network to makede novoestimates of individuals’ European admixture proportion without the need to conduct complete admixture analysis utilizing baseline samples. The results demonstrate the mobilization of targeted SNP panels and machine learning in support of at-risk species conservation and management.

Characterizing the nature of genetic differentiation among individuals and populations and its distribution across the genome is increasingly important to inform both conservation and management of exploited species. Atlantic Halibut (Hippoglossus hippoglossus) is an ecologically and commercially important fish species, yet knowledge of population structure and genomic diversity in this species remains lacking. Here, we use restriction-site associated DNA sequencing and a chromosome-level genome assembly to identify over 86 000 single nucleotide polymorphisms mapped to 24 chromosome-sized scaffolds, genotyped in 734 individuals across the Northwest Atlantic. We describe subtle but significant genome-wide regional structuring between the Gulf of St. Lawrence and adjacent Atlantic continental shelf. However, the majority of genetic divergence is associated with a large putative chromosomal rearrangement (5.74 megabases) displaying high differentiation and linkage disequilibrium, but no evidence of geographic variation. Demographic reconstructions suggest periods of expansion coinciding with glacial retreat, and more recent declines in Ne. This work highlights the utility of genomic data to identify multiple sources of genetic structure and genomic diversity in commercially exploited marine species.

Interactions between spatial dynamics and stock structure in marine fishes have largely focused on stocks in decline; stock structure is rarely re-visited for expanding species. Here, the spatial e...

Atlantic salmon (Salmo salar) populations throughout the North Atlantic have declined in recent decades largely due to reduced marine survival, yet our understanding of marine distribution patterns and migratory routes remains limited. Here, we assigned archived individual samples (n = 3891) collected over a half century (1968–2018) throughout the North Atlantic to region of origin using range-wide genetic assignment. In the Northwest Atlantic, the distribution of assignments reinforced the importance of the Labrador Sea as an aggregation area, with 73% of all reporting groups detected. Moreover, individuals from six European reporting groups were identified in the Northwest Atlantic, and detections decreased with decreasing latitude spanning an area from Greenland to southern Newfoundland. In the Northeast Atlantic, six North American reporting groups were detected in samples from around the Faroe Islands. Based on the distribution of samples, estimates of trans-Atlantic migration distance averaged 3861 and 2889 km for North American and European salmon respectively. Our analysis highlights the widespread importance of the Labrador Sea and Faroe Islands to the species marine distribution patterns, and the prevalence of long-distance trans-Atlantic migration. Ultimately, the results suggest that environmental conditions experienced by many Atlantic salmon populations span much of the North Atlantic Ocean.

Despite widespread biodiversity losses, an understanding of how most taxa will respond to future climate change is lacking. Here we integrate genomics and environmental modelling to assess climate change responses in an ecologically and economically important Arctic species. Environmentally associated genomic diversity and machine learning are used to identify highly vulnerable populations of anadromous (migratory) Arctic charr, and we reconstruct estimates of effective population size spanning the twentieth century to identify past climate-associated declines. We uncover past region-wide declines in effective population size that correspond to decreases in temperature and community biomass in the Northwest Atlantic. We find vulnerable populations near the southern range limit, indicating northward shifts and a possible loss of commercially important life-history variation in response to climate change. The genomic approach used here to investigate climate change response identifies past and future declines that impact species persistence, ecosystem stability and food security in the Arctic. Genomics and environmental modelling are integrated to assess past and future changes in Arctic charr populations in response to changing climate. Southern population vulnerability suggests climate change may lead to northward shifts and the loss of important life-history variation.

Cultured Atlantic salmon Salmo salar are of international socioeconomic value, and the process of domestication has resulted in significant behavioural, morphological, and allelic differences from wild populations. Substantial evidence indicates that direct genetic interactions or interbreeding between wild and escaped farmed Atlantic salmon occurs, genetically altering wild salmon and reducing population viability. However, genetic interactions may also occur through ecological mechanisms (e.g. disease, parasites, predation, competition), both in conjunction with and in the absence of interbreeding. Here we examine existing evidence for ecological and non-reproductive genetic interactions between domestic Atlantic salmon and wild populations and the potential use of genetic and genomic tools to resolve these impacts. Our review identified examples of genetic changes resulting from ecological processes, predominately through pathogen or parasite transmission. In addition, many examples were identified where aquaculture activities have either altered the selective landscape experienced by wild populations or resulted in reductions in population abundance, both of which are consistent with the widespread occurrence of indirect genetic changes. We further identify opportunities for genetic or genomic methods to quantify these impacts, though careful experimental design and pre-impact comparisons are often needed to accurately attribute genetic change to aquaculture activities. Our review indicates that ecological and non-reproductive genetic interactions are important, and further study is urgently needed to support an integrated understanding of aquaculture-ecosystem interactions, their implications for ecosystem stability, and the development of potential mitigation and management strategies.

The genetic underpinnings of incipient speciation, including the genomic mechanisms which contribute to morphological and ecological differentiation and reproductive isolation, remain poorly understood. The repeated evolution of consistently, phenotypically distinct morphs of Arctic Charr (Salvelinus alpinus) within the Quaternary period offer an ideal model to study the repeatability of evolution at the genomic level. Sympatric morphs of Arctic Charr are found across this species' circumpolar distribution. However, the specific genetic mechanisms driving this morph differentiation are largely unknown despite the cultural and economic importance of the anadromous morph. We used a newly designed 87k SNP chip to investigate the character and consistency of the genomic differences among sympatric morphs within three recently deglaciated and geographically proximate lakes in Labrador, Canada. We found genetically distinct small and large morph Arctic Charr in all three lakes consistent with resident and anadromous morphs, respectively. A degree of reproductive isolation among sympatric morphs is likely given genome-wide distributions of outlier SNPs and high genome-wide FST s. Across all lakes, outlier SNPs were largely nonoverlapping suggesting a lack of genetic parallelism driving morph differentiation. Alternatively, several genes and paralogous copies of the same gene consistently differentiated morphs across multiple lakes suggesting their importance to the manifestation of morphs. Our results confirm the utility of Arctic Charr as a model for investigating the predictability of evolution and support the importance of both genetic parallelism and nonparallelism to the incipient speciation of Arctic Charr morphs.

The resiliency of populations and species to environmental change is dependent on the maintenance of genetic diversity, and as such, quantifying diversity is central to combating ongoing widespread reductions in biodiversity. With the advent of next‐generation sequencing, several methods now exist for resolving fine‐scale population structure, but the comparative performance of these methods for genetic assignment has rarely been tested. Here, we evaluate the performance of sequenced microsatellites and a single nucleotide polymorphism (SNP) array to resolve fine‐scale population structure in a critically important salmonid in north eastern Canada, Arctic Charr (Salvelinus alpinus). We also assess the utility of sequenced microsatellites for fisheries applications by quantifying the spatial scales of movement and exploitation through genetic assignment of fishery samples to rivers of origin and comparing these results with a 29‐year tagging dataset. Self‐assignment and simulation‐based analyses of 111 genome‐wide microsatellite loci and 500 informative SNPs from 28 populations of Arctic Charr in north‐eastern Canada identified largely river‐specific genetic structure. Despite large differences (~4X) in the number of loci surveyed between panels, mean self‐assignment accuracy was similar with the microsatellite loci and the SNP panel (>90%). Subsequent analysis of 996 fishery‐collected samples using the microsatellite panel revealed that larger rivers contribute greater numbers of individuals to the fishery and that coastal fisheries largely exploit individuals originating from nearby rivers, corroborating results from traditional tagging experiments. Our results demonstrate the efficacy of sequence‐based microsatellite genotyping to advance understanding of fine‐scale population structure and harvest composition in northern and understudied species.

Teleosts exhibit extensive diversity of sex determination (SD) systems and mechanisms, providing the opportunity to study the evolution of SD and sex chromosomes. Here we sequenced the genome of the common lumpfish (Cyclopterus lumpus Linnaeus), a species of increasing importance to aquaculture, and identified the SD region and master SD locus using a 70 K single nucleotide polymorphism array and tissue-specific expression data. The chromosome-level assembly identified 25 diploid chromosomes with a total size of 572.89 Mb, a scaffold N50 of 23.86 Mb and genome annotation-predicted 21,480 protein-coding genes. Genome-wide association analysis located a highly sex-associated region on chromosome 13, suggesting that anti-Müllerian hormone (AMH) is the putative SD factor. Linkage disequilibrium and heterozygosity across chromosome 13 support a proto-XX/XY system, with an absence of widespread chromosome divergence between sexes. We identified three copies of AMH in the lumpfish primary and alternate haplotype assemblies localized in the SD region. Comparison to sequences from other teleosts suggested a monophyletic relationship and conservation within the Cottioidei. One AMH copy showed similarity to AMH/AMHY in a related species and was also the only copy with expression in testis tissue, suggesting this copy may be the functional copy of AMH in lumpfish. The two other copies arranged in tandem inverted duplication were highly similar, suggesting a recent duplication event. This study provides a resource for the study of early sex chromosome evolution and novel genomic resources that benefits lumpfish conservation management and aquaculture.

Pleistocene glaciations drove repeated range contractions and expansions shaping contemporary intraspecific diversity. Atlantic salmon (Salmo salar) in the western and eastern Atlantic diverged >600,000 years before present, with the two lineages isolated in different southern refugia during glacial maxima, driving trans-Atlantic genomic and karyotypic divergence. Here, we investigate the genomic consequences of glacial isolation and trans-Atlantic secondary contact using 108,870 single nucleotide polymorphisms genotyped in 80 North American and European populations. Throughout North America, we identified extensive interindividual variation and discrete linkage blocks within and between chromosomes with known trans-Atlantic differences in rearrangements: Ssa01/Ssa23 translocation and Ssa08/Ssa29 fusion. Spatial genetic analyses suggest independence of rearrangements, with Ssa01/Ssa23 showing high European introgression (>50%) in northern populations indicative of post-glacial trans-Atlantic secondary contact, contrasting with low European ancestry genome-wide (3%). Ssa08/Ssa29 showed greater intrapopulation diversity, suggesting a derived chromosome fusion polymorphism that evolved within North America. Evidence of potential selection on both genomic regions suggests that the adaptive role of rearrangements warrants further investigation in Atlantic salmon. Our study highlights how Pleistocene glaciations can influence large-scale intraspecific variation in genomic architecture of northern species.

The post-glacial colonization of Gander Lake in Newfoundland, Canada, by Arctic Charr (Salvelinus alpinus) provides the opportunity to study the genomic basis of adaptation to extreme deep-water environments. Colonization of deep-water (>50 m) habitats often requires extensive adaptation to cope with novel environmental challenges from high hydrostatic pressure, low temperature, and low light, but the genomic mechanisms underlying evolution in these environments are rarely known. Here, we compare genomic divergence between a deep-water morph adapted to depths of up to 288 m and a larger, piscivorous pelagic morph occupying shallower depths. Using both a SNP array and resequencing of whole nuclear and mitochondrial genomes, we find clear genetic divergence (FST = 0.11-0.15) between deep and shallow water morphs, despite an absence of morph divergence across the mitochondrial genome. Outlier analyses identified many diverged genomic regions containing genes enriched for processes such as gene expression and DNA repair, cardiac function, and membrane transport. Detection of putative copy number variants (CNVs) uncovered 385 genes with CNVs distinct to piscivorous morphs, and 275 genes with CNVs distinct to deep-water morphs, enriched for processes associated with synapse assembly. Demographic analyses identified evidence for recent and local morph divergence, and ongoing reductions in diversity consistent with postglacial colonization. Together, these results show that Arctic Charr morph divergence has occurred through genome-wide differentiation and elevated divergence of genes underlying multiple cellular and physiological processes, providing insight into the genomic basis of adaptation in a deep-water habitat following postglacial recolonization.

Chromosomal rearrangements (e.g., inversions, fusions, and translocations) have long been associated with environmental variation in wild populations. New genomic tools provide the opportunity to examine the role of these structural variants in shaping adaptive differences within and among wild populations of non-model organisms. In Atlantic Salmon (Salmo salar), variations in chromosomal rearrangements exist across the species natural range, yet the role and importance of these structural variants in maintaining adaptive differences among wild populations remains poorly understood. We genotyped Atlantic Salmon (n = 1429) from 26 populations within a highly genetically structured region of southern Newfoundland, Canada with a 220K SNP array. Multivariate analysis, across two independent years, consistently identified variation in a structural variant (translocation between chromosomes Ssa01 and Ssa23), previously associated with evidence of trans-Atlantic secondary contact, as the dominant factor influencing population structure in the region. Redundancy analysis suggested that variation in the Ssa01/Ssa23 chromosomal translocation is strongly correlated with temperature. Our analyses suggest environmentally mediated selection acting on standing genetic variation in genomic architecture introduced through secondary contact may underpin fine-scale local adaptation in Placentia Bay, Newfoundland, Canada, a large and deep embayment, highlighting the importance of chromosomal structural variation as a driver of contemporary adaptive divergence.

Changes in the genetic mechanisms that control sexual determination have occurred independently across the tree of life, and with exceptional frequency in teleost fishes. To investigate the genomic changes underlying the evolution of sexual determination, we sequenced a chromosome-level genome, multi-tissue transcriptomes, and reduced representation population data for the Atlantic halibut (Hippoglossus hippoglossus), which has an XY/XX sex determination mechanism and has recently diverged (0.9-3.8 MYA) from the Pacific halibut (Hippoglossus stenolepis), which has a ZZ/ZW system. We used frequency and coverage-based population approaches to identify a putative sex-determining factor, GSDF. We characterized regions with elevated heterozygosity and linkage disequilibrium indicating suppression of recombination across a nascent sex chromosome. We detected testis-specific expression of GSDF, the sequence of which is highly conserved across flatfishes. Based on evidence from genome-wide association, coverage, linkage disequilibrium, testis and brain transcriptomes, and sequence conservation with other flatfishes, we propose a mechanism for the recent evolution of an XY sex-determination mechanism in Atlantic halibut. Changes to the ancestral sex-determining gene DMRT1 in regulating the downstream gene GSDF likely coincided with GSDF, or a proximal regulatory element of it, becoming the primary sex-determining factor. Our results suggest changes to a small number of elements can have drastic repercussions for the genomic substrate available to sex-specific evolutionary forces, providing insight into how certain elements repeatedly evolve to control sex across taxa. Our chromosome-level assembly, multi-tissue transcriptomes, and population genomic data provide a valuable resource and understanding of the evolution of sexual systems in fishes.

Littorina saxatilis is becoming a model system for understanding the genomic basis of ecological speciation. The parallel formation of crab-adapted ecotypes that exhibit partial reproductive isolation from wave-adapted ecotypes has enabled genomic investigation of conspicuous shell traits. Recent genomic studies suggest that chromosomal rearrangements may enable ecotype divergence by reducing gene flow. However, the genomic architecture of traits that are divergent between ecotypes remains poorly understood. Here, we use 11,504 single nucleotide polymorphism (SNP) markers called using the recently released L. saxatilis genome to genotype 462 crab ecotype, wave ecotype and phenotypically intermediate Spanish L. saxatilis individuals with scored phenotypes. We used redundancy analysis to study the genetic architecture of loci associated with shell shape, shape corrected for size, shell size and shell ornamentation, and to compare levels of co-association among different traits. We discovered 341 SNPs associated with shell traits. Loci associated with trait divergence between ecotypes were often located inside putative chromosomal rearrangements recently characterized in Swedish L. saxatilis. In contrast, we found that shell shape corrected for size varied primarily by geographic site rather than by ecotype and showed little association with these putative rearrangements. We conclude that genomic regions of elevated divergence inside putative rearrangements were associated with divergence of L. saxatilis ecotypes along steep environmental axes-consistent with models of adaptation with gene flow-but were not associated with divergence among the three geographical sites. Our findings support predictions from models indicating the importance of genomic regions of reduced recombination allowing co-association of loci during ecological speciation with ongoing gene flow.

As populations diverge many processes can shape genomic patterns of differentiation. Regions of high differentiation can arise due to divergent selection acting on selected loci, genetic hitchhiking of nearby loci, or through repeated selection against deleterious alleles (linked background selection); this divergence may then be further elevated in regions of reduced recombination. Atlantic salmon (Salmo salar) from Europe and North America diverged >600,000 years ago and despite some evidence of secondary contact, the majority of genetic data indicate substantial divergence between lineages. This deep divergence with potential gene flow provides an opportunity to investigate the role of different mechanisms that shape the genomic landscape during early speciation. Here, using 184,295 single nucleotide polymorphisms (SNPs) and 80 populations, we investigate the genomic landscape of differentiation across the Atlantic Ocean with a focus on highly differentiated regions and the processes shaping them. We found evidence of high (mean FST = 0.26) and heterogeneous genomic differentiation between continents. Genomic regions associated with high trans-Atlantic differentiation ranged in size from single loci (SNPs) within important genes to large regions (1-3 Mbp) on four chromosomes (Ssa06, Ssa13, Ssa16 and Ssa19). These regions showed signatures consistent with selection, including high linkage disequilibrium, despite no significant reduction in recombination. Genes and functional enrichment of processes associated with differentiated regions may highlight continental differences in ocean navigation and parasite resistance. Our results provide insight into potential mechanisms underlying differences between continents, and evidence of near-fixed and potentially adaptive trans-Atlantic differences concurrent with a background of high genome-wide differentiation supports subspecies designation in Atlantic salmon.

Genomic architecture and standing variation can play a key role in ecological adaptation and contribute to the predictability of evolution. In Atlantic cod (Gadus morhua), four large chromosomal rearrangements have been associated with ecological gradients and migratory behavior in regional analyses. However, the degree of parallelism, the extent of independent inheritance, and functional distinctiveness of these rearrangements remain poorly understood. Here, we use a 12K single nucleotide polymorphism (SNP) array to demonstrate extensive individual variation in rearrangement genotype within populations across the species range, suggesting that local adaptation to fine‐scale ecological variation is enabled by rearrangements with independent inheritance. Our results demonstrate significant association of rearrangements with migration phenotype and environmental gradients across the species range. Individual rearrangements exhibit functional modularity, but also contain loci showing multiple environmental associations. Clustering in genetic distance trees and reduced differentiation within rearrangements across the species range are consistent with shared variation as a source of contemporary adaptive diversity in Atlantic cod. Conversely, we also find that haplotypes in the LG12 and LG1 rearranged region have diverged across the Atlantic, despite consistent environmental associations. Exchange of these structurally variable genomic regions, as well as local selective pressures, has likely facilitated individual diversity within Atlantic cod stocks. Our results highlight the importance of genomic architecture and standing variation in enabling fine‐scale adaptation in marine species., Understanding parallel evolution in wild populations has long been central to evolutionary biology; genomic evidence has recently indicated an important role for chromosomal structural variation in parallel adaptation. Atlantic cod show repeated evidence of local adaptation across diverse marine habitats, with recent evidence implicating the involvement of a series of chromosomal rearrangements. We explore the distribution and trait and environmental associations of these chromosomal rearrangements within populations range‐wide. We find that across the North Atlantic contemporary adaptation and individual‐based biocomplexity in Atlantic cod is frequently underlain by shared chromosomal rearrangements.

Global losses of biodiversity are occurring at an unprecedented rate, but causes are often unidentified. Genomic data provide an opportunity to isolate drivers of change and even predict future vulnerabilities. Atlantic salmon (Salmo salar) populations have declined range-wide, but factors responsible are poorly understood. Here, we reconstruct changes in effective population size (Ne) in recent decades for 172 range-wide populations using a linkage-based method. Across the North Atlantic, Ne has significantly declined in >60% of populations and declines are consistently temperature-associated. We identify significant polygenic associations with decline, involving genomic regions related to metabolic, developmental, and physiological processes. These regions exhibit changes in presumably adaptive diversity in declining populations consistent with contemporary shifts in body size and phenology. Genomic signatures of widespread population decline and associated risk scores allow direct and potentially predictive links between population fitness and genotype, highlighting the power of genomic resources to assess population vulnerability., The Atlantic salmon has suffered widespread population declines over the last century. Here, Lehnert et al. reconstruct changes in effective population size of 172 populations based on genomic linkage information revealing mostly temperature-associated population declines with over 60% of populations in decline since 1975.

Pleistocene glaciations drove repeated range contractions and expansions shaping contemporary intraspecific diversity. Atlantic salmon (Salmo salar) from the western and eastern Atlantic range diverged >600K YBP, with each clade isolated in independent southern refugia during glacial maxima, driving trans-Atlantic genomic and karyotypic differences. Here, we investigate genomic consequences of glacial isolation and transAtlantic secondary contact using a 220K single nucleotide polymorphism (SNP) array genotyped in 80 North American and European populations. Throughout North America, we identified large inter-individual variation and discrete linkage blocks within and between chromosomes with known rearrangements: Ssa01/Ssa23 translocation and Ssa08/Ssa29 fusion. Spatial genetic analyses suggest independence of rearrangements, with Ssa01/Ssa23 showing high European introgression (>50%) in northern populations indicative of post-glacial trans-Atlantic secondary contact, contrasting low European ancestry genome-wide (3%). Ssa08/Ssa29 showed greater intra-population diversity suggesting a derived chromosome fusion polymorphism within North America. Evidence of selection on both regions suggests adaptive variation associated with karyotypes. Our study highlights how Pleistocene glaciations can drive large-scale intraspecific variation in genomic architecture of northern species.

Seed orchards are the link from tree breeding to reforestation programs and are theoretically expected to function as closed, perfect populations, ensuring gain and diversity are consistently and predictively delivered as improved seed and seedlings. Seed orchard populations often deviate from panmixia due to fertility variation, reproductive asynchrony, and gene flow, leading to reduced seed crops genetic quality. Here, as a part of multiyear monitoring study, we used DNA fingerprinting (simple sequence repeat markers) to assess a Douglas-fir (Pseudotsuga menziesii) seed orchard's seed crop genetic quality (2009 seed crop). The studied seed crop was produced under ambient temperature (i.e. no reproductive phenology manipulation) and pollination was augmented by pollen from within orchard's pollen donors. DNA fingerprinting of the parental population (66 parents) along with 207 gametophyte (1n) – embryo (2n) pairs of random bulk sample of seed allowed parentage (maternal and paternal) assignment and the d...

Gene flow from outside pollen sources reduces the expected genetic gain of seed orchards, and has been quantified by genotyping seed samples from crop years to determine the proportion of seed sired by outside sources. Random bulk seed sampling and microsatellite genotyping allows for fast, unbiased identification of orchard contamination rates. Bulk seed sampling has been employed in multiple studies of seed orchard gene flow, but a method of determining precision of gene flow estimates from this sampling method has yet to be described. In this study, we used jackknife resampling to generate random samples of migrant and within-population genotypes obtained from an empirical sample of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) genotypes of five microsatellite loci. We generated samples of 50, 100, 150, 200, and 250 genotyped individuals from a sample of 400 seeds, with genotypes corresponding to low (6%), moderate (14%), or high (56%) pollen contamination levels. The mean gene flow estimate and c...