Outbreeding Depression in Atlantic Salmon Revealed by Hypoxic Stress During Embryonic Development

Multi-generational fitness legacies of natural immigration: theoretical and empirical perspectives and opportunities

Natural dispersal between populations, and resulting immigration, influences population size and genetic variation and is therefore a key process driving reciprocal interactions between ecological and evolutionary dynamics. Here, population dynamic and evolutionary outcomes fundamentally depend not only on the relative fitnesses of natural immigrants and existing residents, but also on the fitness of their various descendants manifested in natural environments. Yet, the fitnesses of different sets of natural immigrants' descendants have rarely been explicitly or rigorously estimated or rationalised in the context of wild spatially structured populations. We therefore still have surprisingly limited capability to understand or predict the ultimate multi-generational impacts of natural immigration on population and evolutionary dynamics. Key theoretical frameworks that predict fitness outcomes of outcrossing between lineages have been developed and widely utilised in the contexts of agriculture and speciation research. These frameworks have also been applied in conservation genetics research to predict positive (widely termed "heterosis") and negative (widely termed "outbreeding depression") outcomes in the context of genetic rescue of highly inbred populations. However, these frameworks have rarely been utilised explicitly to guide analyses of multi-generational legacies of regular natural immigrants in the context of evolutionary ecology, precluding inferences on the basis of, and implications of, sub-population divergence. Accordingly, to facilitate translation of concepts and inspire new empirical efforts, we first review and synthesise key bodies of theory on multi-generational fitness outcomes, developed in the contexts of crosses between inbred lines and between different species. Such theory reveals how diverse fitness outcomes can be generated by common underlying mechanisms, depending on the genetic architecture of fitness, the forms of genotype-phenotype-fitness maps, and the relative roles of adaptive and non-adaptive mechanisms in population differentiation. Interestingly, such theory predicts particularly diverse fitness outcomes of crosses between weakly diverged lineages, constituting the parameter space where spatially structured populations lie. We then conduct a systematic literature review to assess the degree to which multi-generational outcomes of crosses between structured natural populations have actually been quantified. Our review shows a surprising paucity of empirical studies that quantify multi-generational fitness consequences of outcrossing resulting from natural immigration in the wild. Furthermore, studies undertaking experimental crosses among populations have used inconsistent methodologies, precluding quantitative or even qualitative overall conclusions. To initiate new progress, we outline how long-standing and recent methodological developments, including cutting-edge statistical and genomic tools, could be combined with field data sets to quantify the multi-generational fitness outcomes of crosses between residents and immigrants in nature. We thereby highlight key theoretical and empirical gaps that now need to be filled to further our understanding of dispersal-mediated drivers and constraints on eco-evolutionary dynamics arising in structured populations.

Is now the time? Review of genetic rescue as a conservation tool for brook trout

Brook trout populations have been declining throughout their native range in the east coast of the United States. Many populations are now distributed in small, isolated habitat patches where low genetic diversity and high rates of inbreeding reduce contemporary viability and long-term adaptive potential. Although human-assisted gene flow could theoretically improve conservation outcomes through genetic rescue, there is widespread hesitancy to use this tool to support brook trout conservation. Here, we review the major uncertainties that have limited genetic rescue from being considered as a viable conservation tool for isolated brook trout populations and compare the risks of genetic rescue with other management alternatives. Drawing on theoretical and empirical studies, we discuss methods for implementing genetic rescue in brook trout that could yield long-term evolutionary benefits while avoiding negative fitness effects associated with outbreeding depression and the spread of maladapted alleles. We also highlight the potential for future collaborative efforts to accelerate our understanding of genetic rescue as a viable tool for conservation. Ultimately, while we acknowledge that genetic rescue is not without risk, we emphasize the merits that this tool offers for protecting and propagating adaptive potential and improving species' resilience to rapid environmental change.

Population Genomics of Bettongia lesueur: Admixing Increases Genetic Diversity with no Evidence of Outbreeding Depression

Small and isolated populations are subject to the loss of genetic variation as a consequence of inbreeding and genetic drift, which in turn, can affect the fitness and long-term viability of populations. Translocations can be used as an effective conservation tool to combat this loss of genetic diversity through establishing new populations of threatened species, and to increase total population size. Releasing animals from multiple genetically diverged sources is one method to optimize genetic diversity in translocated populations. However, admixture as a conservation tool is rarely utilized due to the risks of outbreeding depression. Using high-resolution genomic markers through double-digest restriction site-associated sequencing (ddRAD-seq) and life history data collected over nine years of monitoring, this study investigates the genetic and fitness consequences of admixing two genetically-distinct subspecies of Bettongia lesueur in a conservation translocation. Using single nucleotide polymorphisms (SNPs) identified from 215 individuals from multiple generations, we found an almost 2-fold increase in genetic diversity in the admixed translocation population compared to the founder populations, and this was maintained over time. Furthermore, hybrid class did not significantly impact on survivorship or the recruitment rate and therefore we found no indication of outbreeding depression. This study demonstrates the beneficial application of mixing multiple source populations in the conservation of threatened species for minimizing inbreeding and enhancing adaptive potential and overall fitness.

Incipient ecological speciation between successional varieties of a dominant tree involves intrinsic postzygotic isolating barriers

Whereas disruptive selection imposed by heterogeneous environments can lead to the evolution of extrinsic isolating barriers between diverging populations, the evolution of intrinsic postzygotic barriers through divergent selection is less certain. Long-lived species such as trees may be especially slow to evolve intrinsic isolating barriers. We examined postpollination reproductive isolating barriers below the species boundary, in an ephemeral hybrid zone between two successional varieties of the landscape-dominant Hawaiian tree, Metrosideros polymorpha, on volcanically active Hawai'i Island. These archipelago-wide sympatric varieties show the weakest neutral genetic divergence of any taxon pair on Hawai'i Island but significant morphological and ecological differentiation consistent with adaptation to new and old lava flows. Cross-fertility between varieties was high and included heterosis of F1 hybrids at the seed germination stage, consistent with a substantial genetic load apparent within varieties through low self-fertility and a lack of self-pollen discrimination. However, a partial, but significant, barrier was observed in the form of reduced female and male fertility of hybrids, especially backcross hybrids, consistent with the accumulation of genetic incompatibilities between varieties. These results suggest that partial intrinsic postzygotic barriers can arise through disruptive selection acting on large, hybridizing populations of a long-lived species.

No evidence for MHC class II-based non-random mating at the gametic haplotype in Atlantic salmon

Genes of the major histocompatibility complex (MHC) are a likely target of mate choice because of their role in inbreeding avoidance and potential benefits for offspring immunocompetence. Evidence for female choice for complementary MHC alleles among competing males exists both for the pre- and the postmating stages. However, it remains unclear whether the latter may involve non-random fusion of gametes depending on gametic haplotypes resulting in transmission ratio distortion or non-random sequence divergence among fused gametes. We tested whether non-random gametic fusion of MHC-II haplotypes occurs in Atlantic salmon Salmo salar. We performed in vitro fertilizations that excluded interindividual sperm competition using a split family design with large clutch sample sizes to test for a possible role of the gametic haplotype in mate choice. We sequenced two MHC-II loci in 50 embryos per clutch to assess allelic frequencies and sequence divergence. We found no evidence for transmission ratio distortion at two linked MHC-II loci, nor for non-random gamete fusion with respect to MHC-II alleles. Our findings suggest that the gametic MHC-II haplotypes play no role in gamete association in Atlantic salmon and that earlier findings of MHC-based mate choice most likely reflect choice among diploid genotypes. We discuss possible explanations for these findings and how they differ from findings in mammals.

Adaptive divergence in embryonic thermal plasticity among Atlantic salmon populations

In the context of global changes, the long-term viability of populations of endangered ectotherms may depend on their adaptive potential and ability to cope with temperature variations. We measured responses of Atlantic salmon embryos from four populations to temperature variations and used a QST -FST approach to study the adaptive divergence among these populations. Embryos were reared under two experimental conditions: a low temperature regime at 4 °C until eyed-stage and 10 °C until the end of embryonic development and a high temperature regime with a constant temperature of 10 °C throughout embryonic development. Significant variations among populations and population × temperature interactions were observed for embryo survival, incubation time and length. QST was higher than FST in all but one comparison suggesting an important effect of divergent selection. QST was also higher under the high-temperature treatment than at low temperature for length and survival due to a higher variance among populations under the stressful warmer treatment. Interestingly, heritability was lower for survival under high temperature in relation to a lower additive genetic variance under that treatment. Overall, these results reveal an adaptive divergence in thermal plasticity in embryonic life stages of Atlantic salmon suggesting that salmon populations may differentially respond to temperature variations induced by climate change. These results also suggest that changes in temperature may alter not only the adaptive potential of natural populations but also the selection regimes among them.

Genetic and phenotypic changes in an Atlantic salmon population supplemented with non-local individuals: a longitudinal study over 21 years

While introductions and supplementations using non-native and potentially domesticated individuals may have dramatic evolutionary effects on wild populations, few studies documented the evolution of genetic diversity and life-history traits in supplemented populations. Here, we investigated year-to-year changes from 1989 to 2009 in genetic admixture at 15 microsatellite loci and in phenotypic traits in an Atlantic salmon (Salmo salar) population stocked during the first decade of this period with two genetically and phenotypically distinct source populations. We detected a pattern of temporally increasing introgressive hybridization between the stocked population and both source populations. The proportion of fish returning to the river after a single winter at sea (versus several ones) was higher in fish assigned to the main source population than in local individuals. Moreover, during the first decade of the study, both single-sea-winter and multi-sea-winter (MSW) fish assigned to the main source population were smaller than local fish. During the second decade of the study, MSW fish defined as hybrids were lighter and smaller than fish from parental populations, suggesting outbreeding depression. Overall, this study suggests that supplementation with non-local individuals may alter not only the genetic diversity of wild populations but also life-history traits of adaptive significance.