Sex‐specific survival to maturity and the evolution of environmental sex determination

Four decades ago, it was proposed that environmental sex determination (ESD) evolves when individual fitness depends on the environment in a sex‐specific fashion—a form of condition‐dependent sex allocation. Many biological processes have been hypothesized to drive this sex asymmetry, yet a general explanation for the evolution of sex‐determining mechanisms remains elusive. Here, we develop a mathematical model for a novel hypothesis of the evolution of ESD, and provide a first empirical test using data across turtles. ESD is favored when the sex‐determining environment affects annual survival rates equivalently in males and females, and males and females mature at different ages. We compare this hypothesis to alternative hypotheses, and demonstrate how it captures a crucially different process. This maturation process arises naturally from common life histories and applies more broadly to condition‐dependent sex allocation. Therefore, it has widespread implications for animal taxa. Across turtle species, ESD is associated with greater sex differences in the age at maturity compared to species without ESD, as predicted by our hypothesis. However, the effect is not statistically significant and will require expanded empirical investigation. Given variation among taxa in sex‐specific age at maturity, our survival‐to‐maturity hypothesis may capture common selective forces on sex‐determining mechanisms.     

Inbreeding effects on gene‐specific DNA methylation among tissues of Chinook salmon

Inbreeding depression is the loss of fitness resulting from the mating of genetically related individuals. Traditionally, the study of inbreeding depression focused on genetic effects, although recent research has identified DNA methylation as also having a role in inbreeding effects. Since inbreeding depression and DNA methylation change with age and environmental stress, DNA methylation is a likely candidate for the regulation of genes associated with inbreeding depression. Here, we use a targeted, multigene approach to assess methylation at 22 growth‐, metabolic‐, immune‐ and stress‐related genes. We developed PCR‐based DNA methylation assays to test the effects of intense inbreeding on intragenic gene‐specific methylation in inbred and outbred Chinook salmon. Inbred fish had altered methylation at three genes, CK‐1, GTIIBS and hsp70, suggesting that methylation changes associated with inbreeding depression are targeted to specific genes and are not whole‐genome effects. While we did not find a significant inbreeding by age interaction, we found that DNA methylation generally increases with age, although methylation decreased with age in five genes, CK‐1, IFN‐?, HNRNPL, hsc71 and FSHb, potentially due to environmental context and sexual maturation. As expected, we found methylation patterns differed among tissue types, highlighting the need for careful selection of target tissue for methylation studies. This study provides insight into the role of epigenetic effects on ageing, environmental response and tissue function in Chinook salmon and shows that methylation is a targeted and regulated cellular process. We provide the first evidence of epigenetically based inbreeding depression in vertebrates.     

Microsatellite variation reveals weak genetic structure and retention of genetic variability in threatened Chinook salmon (<Emphasis Type="Italic">Oncorhynchus tshawytscha</Emphasis>) within a Snake River watershed

Pacific salmon (Oncorhynchus spp.) have been central to the development of management concepts associated with evolutionarily significant units (ESUs), yet there are still relatively few studies of genetic diversity within threatened and endangered ESUs for salmon or other species. We analyzed genetic variation at 10 microsatellite loci to evaluate spatial population structure and genetic variability in indigenous Chinook salmon (Oncorhynchus tshawytscha) across a large wilderness basin within a Snake River ESU. Despite dramatic 20th century declines in abundance, these populations retained robust levels of genetic variability. No significant genetic bottlenecks were found, although the bottleneck metric (M ratio) was significantly correlated with average population size and variability. Weak but significant genetic structure existed among tributaries despite evidence of high levels of gene flow, with the strongest genetic differentiation mirroring the physical segregation of fish from two sub-basins. Despite the more recent colonization of one sub-basin and differences between sub-basins in the natural level of fragmentation, gene diversity and genetic differentiation were similar between sub-basins. Various factors, such as the (unknown) genetic contribution of precocial males, genetic compensation, lack of hatchery influence, and high levels of current gene flow may have contributed to the persistence of genetic variability in this system in spite of historical declines. This unique study of indigenous Chinook salmon underscores the importance of maintaining natural populations in interconnected and complex habitats to minimize losses of genetic diversity within ESUs.     

Sex‐specific plasticity and genotype × sex interactions for age and size of maturity in the sheepshead swordtail,Xiphophorus birchmanni

Responses to sexually antagonistic selection are thought to be constrained by the shared genetic architecture of homologous male and female traits. Accordingly, adaptive sexual dimorphism depends on mechanisms such as genotype‐by‐sex interaction (G×S) and sex‐specific plasticity to alleviate this constraint. We tested these mechanisms in a population of Xiphophorus birchmanni (sheepshead swordtail), where the intensity of male competition is expected to mediate intersexual conflict over age and size at maturity. Combining quantitative genetics with density manipulations and analysis of sex ratio variation, we confirm that maturation traits are dimorphic and heritable, but also subject to large G×S. Although cross‐sex genetic correlations are close to zero, suggesting sex‐linked genes with important effects on growth and maturation are likely segregating in this population, we found less evidence of sex‐specific adaptive plasticity. At high density, there was a weak trend towards later and smaller maturation in both sexes. Effects of sex ratio were stronger and putatively adaptive in males but not in females. Males delay maturation in the presence of mature rivals, resulting in larger adult size with subsequent benefit to competitive ability. However, females also delay maturation in male‐biased groups, incurring a loss of reproductive lifespan without apparent benefit. Thus, in highly competitive environments, female fitness may be limited by the lack of sex‐specific plasticity. More generally, assuming that selection does act antagonistically on male and female maturation traits in the wild, our results demonstrate that genetic architecture of homologous traits can ease a major constraint on the evolution of adaptive dimorphism.     

Genomic signatures of fine‐scale local selection in Atlantic salmon suggest involvement of sexual maturation,energy homeostasis and immune defence‐related genes

Elucidating the genetic basis of adaptation to the local environment can improve our understanding of how the diversity of life has evolved. In this study, we used a dense SNP array to identify candidate loci potentially underlying fine‐scale local adaptation within a large Atlantic salmon (Salmo salar) population. By combining outlier, gene–environment association and haplotype homozygosity analyses, we identified multiple regions of the genome with strong evidence for diversifying selection. Several of these candidate regions had previously been identified in other studies, demonstrating that the same loci could be adaptively important in Atlantic salmon at subdrainage, regional and continental scales. Notably, we identified signals consistent with local selection around genes associated with variation in sexual maturation, energy homeostasis and immune defence. These included the large‐effect age‐at‐maturity gene vgll3, the known obesity gene mc4r, and major histocompatibility complex II. Most strikingly, we confirmed a genomic region on Ssa09 that was extremely differentiated among subpopulations and that is also a candidate for local selection over the global range of Atlantic salmon. This region colocalized with a haplotype strongly associated with spawning ecotype in sockeye salmon (Oncorhynchus nerka), with circumstantial evidence that the same gene (six6) may be the selective target in both cases. The phenotypic effect of this region in Atlantic salmon remains cryptic, although allelic variation is related to upstream catchment area and covaries with timing of the return spawning migration. Our results further inform management of Atlantic salmon and open multiple avenues for future research.     

Signals of large scale climate drivers,hatchery enhancement,and marine factors in Yukon River Chinook salmon survival revealed with a Bayesian life history model

Understanding how species might respond to climate change involves disentangling the influence of co‐occurring environmental factors on population dynamics, and is especially problematic for migratory species like Pacific salmon that move between ecosystems. To date, debate surrounding the causes of recent declines in Yukon River Chinook salmon (Oncorhynchus tshawytscha) abundance has centered on whether factors in freshwater or marine environments control variation in survival, and how these populations at the northern extremity of the species range will respond to climate change. To estimate the effect of factors in marine and freshwater environments on Chinook salmon survival, we constructed a stage‐structured assessment model that incorporates the best available data, estimates incidental marine bycatch mortality in trawl fisheries, and uses Bayesian model selection methods to quantify support for alternative hypotheses. Models fitted to two index populations of Yukon River Chinook salmon indicate that processes in the nearshore and marine environments are the most important determinants of survival. Specifically, survival declines when ice leaves the Yukon River later in the spring, increases with wintertime temperature in the Bering Sea, and declines with the abundance of globally enhanced salmon species consistent with competition at sea. In addition, we found support for density‐dependent survival limitations in freshwater but not marine portions of the life cycle, increasing average survival with ocean age, and age‐specific selectivity of bycatch mortality in the Bering Sea. This study underscores the utility of flexible estimation models capable of fitting multiple data types and evaluating mortality from both natural and anthropogenic sources in multiple habitats. Overall, these analyses suggest that mortality at sea is the primary driver of population dynamics, yet under warming climate Chinook salmon populations at the northern extent of the species’ range may be expected to fare better than southern populations, but are influenced by foreign salmon production.     

Identification of multiple genetically distinct populations of Chinook salmon (<Emphasis Type="Italic">Oncorhynchus tshawytscha</Emphasis>) in a small coastal watershed

Management and restoration planning for Pacific salmon is often characterized by efforts at broad multi-basin scales. However, finer-scale genetic and phenotypic variability may be present within individual basins and can be overlooked in such efforts, even though it may be a critical component for long-term viability. Here, we investigate Chinook salmon (Oncorhynchus tshawytscha) within the Siletz River, a small coastal watershed in Oregon, USA. Adult Chinook salmon were genotyped using neutral microsatellite markers, single nucleotide polymorphisms and “adaptive” loci, associated with temporal variation in migratory behavior in many salmon populations, to investigate genetic diversity based upon both spatial and temporal variation in migratory and reproductive behavior. Results from all three marker types identified two genetically distinct populations in the basin, corresponding to early returning fish that spawn above a waterfall, a spring-run population, and later returning fish spawning below the waterfall, a fall-run population. This finding is an important consideration for management of the species, as spring-run populations generally only have been recognized in large watersheds, and highlights the need to evaluate population structure of salmon within smaller watersheds, and thereby increase the probability of successful conservation of salmon species.     

Population genetic evidence for sex‐specific dispersal in an inbred social spider

Dispersal in most group‐living species ensures gene flow among groups, but in cooperative social spiders, juvenile dispersal is suppressed and colonies are highly inbred. It has been suggested that such inbred sociality is advantageous in the short term, but likely to lead to extinction or reduced speciation rates in the long run. In this situation, very low levels of dispersal and gene flow among colonies may have unusually important impacts on fitness and persistence of social spiders. We investigated sex‐specific differences in dispersal and gene flow among colonies, as reflected in the genetic structure within colonies and populations of the African social spider Stegodyphus dumicola Pocock, 1898 (Eresidae). We used DNA fingerprinting and mtDNA sequence data along with spatial mapping of colonies to compare male and female patterns of relatedness within and among colonies at three study sites. Samples were collected during and shortly after the mating season to detect sex‐specific dispersal. Distribution of mtDNA haplotypes was consistent with proliferation of social nests by budding and medium‐ to long‐distance dispersal by ballooning females. Analysis of molecular variance and spatial autocorrelation analyses of AFLPs showed high levels of genetic similarity within colonies, and STRUCTURE analyses revealed that the number of source populations contributing to colonies ranged from one to three. We also showed significant evidence of male dispersal among colonies at one site. These results support the hypothesis that in social spiders, genetic cohesion among populations is maintained by long‐distance dispersal of female colony founders. Genetic diversity within colonies is maintained by colony initiation by multiple dispersing females, and adult male dispersal over short distances. Male dispersal may be particularly important in maintaining gene flow among colonies in local populations.     

Spatiotemporal patterns and habitat associations of smallmouth bass (Micropterus dolomieu) invading salmon‐rearing habitat

1. Smallmouth bass (Micropterus dolomieu) have been widely introduced to fresh waters throughout the world to promote recreational fishing opportunities. In the Pacific Northwest (U.S.A.), upstream range expansions of predatory bass, especially into subyearling salmon‐rearing grounds, are of increasing conservation concern, yet have received little scientific inquiry. Understanding the habitat characteristics that influence bass distribution and the timing and extent of bass and salmon overlap will facilitate the development of management strategies that mitigate potential ecological impacts of bass. 2. We employed a spatially continuous sampling design to determine the extent of bass and subyearling Chinook salmon (Oncorhynchus tshawytscha) sympatry in the North Fork John Day River (NFJDR), a free‐flowing river system in the Columbia River Basin that contains an upstream expanding population of non‐native bass. Extensive (i.e. 53 km) surveys were conducted over 2 years and during an early and late summer period of each year, because these seasons provide a strong contrast in the river’s water temperature and flow condition. Classification and regression trees were applied to determine the primary habitat correlates of bass abundance at reach and channel‐unit scales. 3. Our study revealed that bass seasonally occupy up to 22% of the length of the mainstem NFJDR where subyearling Chinook salmon occur, and the primary period of sympatry between these species was in the early summer and not during peak water temperatures in late summer. Where these species co‐occurred, bass occupied 60–76% of channel units used by subyearling Chinook salmon in the early summer and 28–46% of the channel units they occupied in the late summer. Because these rearing salmon were well below the gape limitation of bass, this overlap could result in either direct predation or sublethal effects of bass on subyearling Chinook salmon. The upstream extent of bass increased 10–23 km (2009 and 2010, respectively) as stream temperatures seasonally warmed, but subyearling Chinook salmon were also found farther upstream during this time. 4. Our multiscale analysis suggests that bass were selecting habitat based on antecedent thermal history at a broad scale, and if satisfactory temperature conditions were met, mesoscale habitat features (i.e. channel‐unit type and depth) played an additional role in determining bass abundance. The upstream extent of bass in the late summer corresponded to a high‐gradient geomorphic discontinuity in the NFJDR, which probably hindered further upstream movements of bass. The habitat determinants and upstream extent of bass were largely consistent across years, despite marked differences in the magnitude and timing of spring peak flows prior to bass spawning. 5. The overriding influence of water temperature on smallmouth bass distribution suggests that managers may be able limit future upstream range expansions of bass into salmon‐rearing habitat by concentrating on restoration activities that mitigate climate‐ or land‐use‐related stream warming. These management activities could be prioritised to capitalise on survival bottlenecks in the life history of bass and spatially focused on landscape knick points such as high‐gradient discontinuities to discourage further upstream movements of bass.     

Genetic signals of artificial and natural dispersal linked to colonization of South America by non‐native Chinook salmon (Oncorhynchus tshawytscha)

Genetics data have provided unprecedented insights into evolutionary aspects of colonization by non‐native populations. Yet, our understanding of how artificial (human‐mediated) and natural dispersal pathways of non‐native individuals influence genetic metrics, evolution of genetic structure, and admixture remains elusive. We capitalize on the widespread colonization of Chinook salmon Oncorhynchus tshawytscha in South America, mediated by both dispersal pathways, to address these issues using data from a panel of polymorphic SNPs. First, genetic diversity and the number of effective breeders (N_b) were higher among artificial than natural populations. Contemporary gene flow was common between adjacent artificial and natural and adjacent natural populations, but uncommon between geographically distant populations. Second, genetic structure revealed four distinct clusters throughout the Chinook salmon distributional range with varying levels of genetic connectivity. Isolation by distance resulted from weak differentiation between adjacent artificial and natural and between natural populations, with strong differentiation between distant Pacific Ocean and Atlantic Ocean populations, which experienced strong genetic drift. Third, genetic mixture analyses revealed the presence of at least six donor geographic regions from North America, some of which likely hybridized as a result of multiple introductions. Relative propagule pressure or the proportion of Chinook salmon propagules introduced from various geographic regions according to government records significantly influenced genetic mixtures for two of three artificial populations. Our findings support a model of colonization in which high‐diversity artificial populations established first; some of these populations exhibited significant admixture resulting from propagule pressure. Low‐diversity natural populations were likely subsequently founded from a reduced number of individuals.     

Sex‐specific graphs: Relating group‐specific topology to demographic and landscape data

Sex‐specific genetic structure is a commonly observed pattern among vertebrate species. Facing differential selective pressures, individuals may adopt sex‐specific life history traits that ultimately shape genetic variation among populations. Although differential dispersal dynamics are commonly detected in the literature, few studies have used genetic structure to investigate sex‐specific functional connectivity. The recent use of graph theoretic approaches in landscape genetics has demonstrated network capacities to describe complex system behaviours where network topology represents genetic interaction among subunits. Here, we partition the overall genetic structure into sex‐specific graphs, revealing different male and female dispersal dynamics of a fisher (Pekania [Martes] pennanti) metapopulation in southern Ontario. Our analyses based on network topologies supported the hypothesis of male‐biased dispersal. Furthermore, we demonstrated that the effect of the landscape, identified at the population level, could be partitioned among sex‐specific strata. We found that female connectivity was negatively correlated with snow depth, whereas connectivity among males was not. Our findings underscore the potential of conducting sex‐specific analysis by identifying landscape elements or configuration that differentially promotes or impedes functional connectivity between sexes, revealing processes that may otherwise remain cryptic. We propose that the sex‐specific graph approach would be applicable to other vagile species where differential sex‐specific processes are expected to occur.     

Characterization of a Y‐specific duplication/insertion of the anti‐Mullerian hormone type II receptor gene based on a chromosome‐scale genome assembly of yellow perch,Perca flavescens

Yellow perch, Perca flavescens, is an ecologically and economically important species native to a large portion of the northern United States and southern Canada and is also a promising candidate species for aquaculture. However, no yellow perch reference genome has been available to facilitate improvements in both fisheries and aquaculture management practices. By combining Oxford Nanopore Technologies long‐reads, 10X Genomics Illumina short linked reads and a chromosome contact map produced with Hi‐C, we generated a high‐continuity chromosome‐scale yellow perch genome assembly of 877.4 Mb. It contains, in agreement with the known diploid chromosome yellow perch count, 24 chromosome‐size scaffolds covering 98.8% of the complete assembly (N50 = 37.4 Mb, L50 = 11). We also provide a first characterization of the yellow perch sex determination locus that contains a male‐specific duplicate of the anti‐Mullerian hormone type II receptor gene (amhr2by) inserted at the proximal end of the Y chromosome (chromosome 9). Using this sex‐specific information, we developed a simple PCR genotyping assay which accurately differentiates XY genetic males (amhr2by⁺) from XX genetic females (amhr2by^?). Our high‐quality genome assembly is an important genomic resource for future studies on yellow perch ecology, toxicology, fisheries and aquaculture research. In addition, characterization of the amhr2by gene as a candidate sex‐determining gene in yellow perch provides a new example of the recurrent implication of the transforming growth factor beta pathway in fish sex determination, and highlights gene duplication as an important genomic mechanism for the emergence of new master sex determination genes.     

Co‐inheritance of sea age at maturity and iteroparity in the Atlantic salmon vgll3 genomic region

Co‐inheritance in life‐history traits may result in unpredictable evolutionary trajectories if not accounted for in life‐history models. Iteroparity (the reproductive strategy of reproducing more than once) in Atlantic salmon (Salmo salar) is a fitness trait with substantial variation within and among populations. In the Teno River in northern Europe, iteroparous individuals constitute an important component of many populations and have experienced a sharp increase in abundance in the last 20 years, partly overlapping with a general decrease in age structure. The physiological basis of iteroparity bears similarities to that of age at first maturity, another life‐history trait with substantial fitness effects in salmon. Sea age at maturity in Atlantic salmon is controlled by a major locus around the vgll3 gene, and we used this opportunity demonstrate that these two traits are co‐inherited around this genome region. The odds ratio of survival until second reproduction was up to 2.4 (1.8–3.5 90% CI) times higher for fish with the early‐maturing vgll3 genotype (EE) compared to fish with the late‐maturing genotype (LL). The L allele was dominant in individuals remaining only one year at sea before maturation, but the dominance was reversed, with the E allele being dominant in individuals maturing after two or more years at sea. Post hoc analysis indicated that iteroparous fish with the EE genotype had accelerated growth prior to first reproduction compared to first‐time spawners, across all age groups, whereas this effect was not detected in fish with the LL genotype. These results broaden the functional link around the vgll3 genome region and help us understand constraints in the evolution of life‐history variation in salmon. Our results further highlight the need to account for genetic correlations between fitness traits when predicting demographic changes in changing environments.     

How relative size and abundance structures the relationship between size and individual growth in an ontogenetically piscivorous fish

While individual growth ultimately reflects the quality and quantity of food resources, intra and interspecific interactions for these resources, as well as individual size, may have dramatic impacts on growth opportunity. Out‐migrating anadromous salmonids make rapid transitions between habitat types resulting in large pulses of individuals into a given location over a short period, which may have significant impact on demand for local resources. We evaluated the spatial and temporal variation in IGF‐1 concentrations (a proxy for growth rate) and the relationship between size and concentration for juvenile Chinook salmon in Puget Sound, WA, USA, as a function of the relative size and abundance of both Chinook salmon and Pacific herring, a species which commonly co‐occurs with salmonids in nearshore marine habitats. The abundance of Chinook salmon and Pacific herring varied substantially among the sub‐basins as function of outmigration timing and spawn timing, respectively, while size varied systematically and consistently for both species. Mean IGF‐1 concentrations were different among sub‐basins, although patterns were not consistent through time. In general, size was positively correlated with IGF‐1 concentration, although the slope of the relationship was considerably higher where Pacific herring were more abundant than Chinook salmon; specifically where smaller individual herring, relative to Chinook salmon, were more abundant. Where Pacific herring were less abundant than Chinook salmon, IGF‐1 concentrations among small and large Chinook salmon were more variable and showed no consistent increase for larger individuals. The noticeable positive effect of relative Pacific herring abundance on the relationship between size and individual growth rates likely represents a shift to predation based on increased IGF‐1 concentrations for individual Chinook salmon that are large enough to incorporate fish into their diet and co‐occur with the highest abundances of Pacific herring.     

Diversity of juvenile Chinook salmon life history pathways

Life history variability includes phenotypic variation in morphology, age, and size at key stage transitions and arises from genotypic, environmental, and genotype-by-environment effects. Life history variation contributes to population abundance, productivity, and resilience, and management units often reflect life history classes. Recent evidence suggests that past Chinook salmon (Oncorhynchus tshawytscha) classifications (e.g., ‘stream’ and ‘ocean’ types) are not distinct evolutionary lineages, do not capture the phenotypic variation present within or among populations, and are poorly aligned with underlying ecological and developmental processes. Here we review recently reported variation in juvenile Chinook salmon life history traits and provide a refined conceptual framework for understanding the causes and consequences of the observed variability. The review reveals a broad continuum of individual juvenile life history pathways, defined primarily by transitions among developmental stages and habitat types used during freshwater rearing and emigration. Life history types emerge from discontinuities in expressed pathways when viewed at the population scale. We synthesize recent research that examines how genetic, conditional, and environmental mechanisms likely influence Chinook salmon life history pathways. We suggest that threshold models hold promise for understanding how genetic and environmental factors influence juvenile salmon life history transitions. Operational life history classifications will likely differ regionally, but should benefit from an expanded lexicon that captures the temporally variable, multi-stage life history pathways that occur in many Chinook salmon populations. An increased mechanistic awareness of life history diversity, and how it affects population fitness and resilience, should improve management, conservation, and restoration of this iconic species.     

Quantitative genetic and translocation experiments reveal genotype‐by‐environment effects on juvenile life‐history traits in two populations of Chinook salmon (Oncorhynchus tshawytscha)

Understanding the genetic architecture of phenotypic plasticity is required to assess how populations might respond to heterogeneous or changing environments. Although several studies have examined population‐level patterns in environmental heterogeneity and plasticity, few studies have examined individual‐level variation in plasticity. Here, we use the North Carolina II breeding design and translocation experiments between two populations of Chinook salmon to detail the genetic architecture and plasticity of offspring survival and growth. We followed the survival of 50 800 offspring through the larval stage and used parentage analysis to examine survival and growth through freshwater rearing. In one population, we found that additive genetic, nonadditive genetic and maternal effects explained 25%, 34% and 55% of the variance in larvae survival, respectively. In the second population, these effects explained 0%, 24% and 61% of the variance in larvae survival. In contrast, fry survival was regulated primarily by additive genetic effects, which indicates a shift from maternal to genetic effects as development proceeds. Fry growth also showed strong additive genetic effects. Translocations between populations revealed that offspring survival and growth varied between environments, the degree of which differed among families. These results indicate genetic differences among individuals in their degree of plasticity and consequently their ability to respond to environmental variation.     

Multiple molecular approaches yield no evidence for sex‐determining genes in lake sturgeon (Acipenser fulvescens)

Common DNA‐based sexing assays have been widely used for the conservation and management of mammals and birds. However, many fishes do not have genetic sex determination and in those that do, the plasticity of the genes involved means that species‐specific assays are normally required. Such DNA‐sexing markers would be especially valuable in lake sturgeon (Acipenser fulvescens) because of their sexual monomorphism, delayed sexual maturity, and conservation status. We tried to identify genetic differences between male and female lake sturgeon using several different molecular genetic methods, including randomly amplified polymorphic DNA, representational difference analyses, subtractive hybridization, and a candidate gene approach. Ultimately, a number of genes were identified but none was sex‐specific. Although the ultimate mechanism of sex determination is yet unknown, it is possible that sex determination is environmental in lake sturgeon, especially since recent studies have also failed to identify sex determination genes in other sturgeon species.     

Correlated contemporary evolution of life history traits in New Zealand Chinook salmon, Oncorhynchus tshawytscha

Size at age and age at maturity are important life history traits, affecting individual fitness and population demography. In salmon and other organisms, size and growth rate are commonly considered cues for maturation and thus age at maturity may or may not evolve independently of these features. Recent concerns surrounding the potential phenotypic and demographic responses of populations facing anthropogenic disturbances, such as climate change and harvest, place a premium on understanding the evolutionary genetic basis for evolution in size at age and age at maturity. In this study, we present the findings from a set of common-garden rearing experiments that empirically assess the heritable basis of phenotypic divergence in size at age and age at maturity in Chinook salmon (Oncorhynchus tshawytscha) populations introduced to New Zealand. We found consistent evidence of heritable differences among populations in both size at age and age at maturity, often corresponding to patterns observed in the wild. Populations diverged in size and growth profiles, even when accounting for eventual age at maturation. By contrast, most, but not all, cases of divergence in age at maturity were driven by the differences in size or growth rate rather than differences in the threshold relationship linking growth rate and probability of maturation. These findings help us understand how life histories may evolve through trait interactions in populations exposed to natural and anthropogenic disturbances, and how we might best detect such evolution.