Skin morphology and humoral non-specific defence parameters of mucus and plasma in rainbow trout,coho and Atlantic salmon

Susceptibility to different diseases among related species, such as coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhyncus mykiss) and Atlantic salmon (Salmo salar), is variable. The prominence of these species in aquaculture warrants investigation into sources of this variability to assist future disease management. To develop a better understanding of the basis for species variability, several important non-specific humoral parameters were examined in juvenile fish of these three economically important species. Mucous protease, alkaline phosphatase and lysozyme, as well as plasma lysozyme activities and histological parameters (epidermal thickness and mucous cell density, and size) were characterized and compared for three salmonids: rainbow trout, Atlantic salmon and coho salmon. Rainbow trout had a thicker epidermis and significantly more mucous cells per cross-sectional area than the other two species. Rainbow trout also had significantly higher mucous protease activity than Atlantic salmon and significantly higher lysozyme (plasma and mucus) activities than coho and Atlantic salmon, in seawater. Atlantic salmon, on the other hand, had the lowest activities of mucous lysozyme and proteases, the thinnest epidermal layer and the sparsest distribution of mucous cells, compared with the two other salmonids in seawater. Only coho salmon had sacciform cells. Atlantic and coho salmon had higher mucous lysozyme activities in freshwater as compared to seawater. There was no significant difference between mucous lysozyme activities in any of the three species reared in freshwater; however, rainbow trout still had a significantly higher plasma lysozyme activity compared with the other two species. All three species exhibited significantly lower mucous alkaline phosphatase and protease activities in freshwater than in seawater. Our results demonstrate that there are significant histological and biochemical differences between the skin and mucus of these three salmonid species, which may change as a result of differing environments. Variation in these innate immune factors is likely to have differing influences on each species response to disease processes.     

Maternal immune factors and the evolution of secondary sexual characters

Secondary sexual characters have been hypothesized to revealthe ability of males to resist debilitating parasites. Althoughsuch reliable signaling of parasite resistance may be maintainedby parasite–host coevolution, maternal effects potentiallyprovide a previously neglected factor that could affect thelevel of genetic variation in resistance to parasites. Thatcould be the case because maternal effects have an entirelyenvironmental basis, or because they can maintain considerableamounts of genetic variation through epistatic effects, evenin the presence of strong directional selection. Maternal effectshave been shown to occur as maternal allocation of immune factorsto offspring, and such allocation may depend on the mating prospectsof sons, causing mothers to differentially allocate maternaleffects to eggs in species subject to intense sexual selection.Here we show that a maternal effect through innate antibacterialimmune defense, lysozyme, which is transferred from the motherto the egg in birds, is positively associated with the evolutionof secondary sexual characters. Previous studies have shownthat females differentially allocate lysozyme to their eggswhen mated to attractive males, and elevated levels of lysozymeare associated with reduced hatching failure and superior healthamong neonates and adults. In this study, comparative analysesof lysozyme from eggs of 85 species of birds showed a strongpositive relationship between brightness of male plumage andegg lysozyme, even when controlling for potentially confoundingvariables. These findings suggest that maternal immune factorsmay play a role in the evolution of secondary sexual characters.     

Quantitative genetics of growth and development time in the burying beetle Nicrophorus pustulatus in the presence and absence of post-hatching parental care

Despite a growing interest in the evolutionary aspects of maternal effects, few studies have examined the genetic consequences of maternal effects associated with parental care. To begin to provide data on nonlaboratory or nondomestic animals, we compared the effect of presence and absence of parental care on phenotype expression of larval mass and development time at different life-history stages in the burying beetle Nicrophorus pustulatus. This beetle has facultative care; parents can feed their larvae through regurgitation of digested carrion or offspring can feed by themselves from previously prepared carrion. To investigate larval responses to these two levels of care, including estimates of additive genetic effects, maternal effects, and genotype-by-environment interactions, we used a half-sibling split-family breeding experiment-raising half of the offspring of a family in the presence of their mother and the other half without their mother present. Larvae reared with their mother present were on average heavier and developed faster, although some of the differences in development decreased or were eliminated by the adult stage. These results suggest that presence or absence of post-hatching maternal care plays an important role in phenotype expression early in life, whereas later the phenotype of the offspring is determined mainly by the genotype and/or unshared environmental effects. Our study also permitted us to examine the differences in genetic effects between the two care environments. Heritabilities, maternal/common environment effect, and most genetic correlations did not differ between the care treatments. Genetic analyses revealed substantial additive genetic effects for development time but small effects for measures of body mass. Maternal plus common environment effects were high for measures of mass but low for development time, suggesting that indirect genetic effects of maternal and/or common environment are less important for the evolution of development time than for mass. Estimates of genetic correlations revealed a trade-off between the duration of the two development stages after the offspring left the carrion. There was also a negative genetic correlation between the time spent on carrion and the mass at 72 h, when mothers usually stop feeding. The analysis of genotype-by-environment interactions indicates substantial variation among maternal families in response to care. Presence or absence of parental care may therefore contribute to the additive genetic variance through its interaction with the maternal component of the additive genetic variance. The presence of this interaction further suggests that parents may vary in care strategies, with some parents dispersing after preparation of the carrion and some parents staying with the larvae. This interaction may help maintain genetic variation in growth, development time, and parental care behavior. Additional work is needed, however, to quantify indirect genetic effects and genetic variation in parental care behavior itself.     

A comparative study on innate immune parameters in the epidermal mucus of various fish species

Fish epidermal mucus and its components provide the first line of defense against pathogens. Little is known about the role of epidermal mucus enzymes in the innate immune system of fish species such as Arctic char (Salvelinus alpinus), brook trout (S. fontinalis), koi carp(Cyprinus carpio), striped bass (Morone saxatilis), haddock, (Melanogrammus aeglefinus), Atlantic cod (Gadus morhua) and hagfish (Myxine glutinosa). The epidermal mucus samples from these fish were analysed for the specific activities of various hydrolytic enzymes including lysozyme, alkaline phosphatase, cathepsin B and proteases and the enzyme levels were compared among the fish species. Of all the species hagfish mucus showed a high activity for lysozyme and proteases and koi carp mucus had the highest levels of alkaline phosphatase and cathepsin B. A wide variation in enzyme activities was observed among the seven species and also between species of same family such as Arctic char and brook trout (salmonidae), haddock and cod (gadidae). Only lysozyme levels showed a marked variation with salinity where seawater fish showed approximately two times higher lysozyme activity than freshwater-reared fish species. Characterization of proteases with specific inhibitors showed Arctic char, brook trout, haddock and cod having higher levels of serine over metalloproteases whereas koi carp and striped bass had higher levels of metalloproteases over serine proteases. In contrast, hagfish had almost equal proportion of both serine and metalloproteases. This study demonstrates variation in the level of hydrolytic enzymes in the epidermal mucus of fish. These results provide preliminary information for a better understanding of the role of epidermal mucus and its components in the fish innate immune system.     

Coho salmon Oncorhynchus kisutch strain differences in disease resistance and non-specific immunity, following immersion challenges with Vibrio anguillarum.

Two strains of freshwater-reared coho salmon Oncorhynchus kisutch were compared for differences in the activity of selected non-specific immune factors before and after lethal and non-lethal immersion challenges with the marine bacterial pathogen Vibrio anguillarum (Vang). Two disease challenge experiments were performed. The first experimental challenge resulted in no mortality; however, significant strain and challenge treatment effects were detected at Day 16 post-challenge. Strain differences in plasma lysozyme activity were found in pre-challenge samples. The second challenge experiment compared the same strains of coho salmon following immersion challenges in different doses of Vang. The fish were sampled at Days 0, 2, 7, and 18 post-challenge and mortality, plasma lysozyme, and anterior kidney phagocyte respiratory burst activity were compared. There were significant strain differences in mortality in the high dose group. The more disease-resistant strain was found to have higher levels of plasma lysozyme and anterior kidney phagocyte respiratory burst activity. These strain differences were detected at various times in the lethal (high dose) and non-lethal challenge groups. There was a clear relationship between the enhanced survival of the more disease-resistant strain and a more sustained, elevated non-specific immune response following the experimental disease challenges. The results of this study suggest that the basis for strain differences in innate disease resistance is related to the ability of the fish to respond quickly to the initial infection and to maintain the response until the infection is quelled.     

Oxygen uptake of growth hormone transgenic coho salmon during starvation and feeding

Oxygen uptake of growth hormone transgenic coho salmon Oncorhynchus kisutch was measured in individual fish with a closed-system respirometer and was compared with that of similar-sized non-transgenic control coho salmon during starvation and when fed a fixed ration or to satiation. Transgenic and control fish did not differ in their standard oxygen uptake after 4 days of starvation, although control fish had a higher routine oxygen uptake, scope for spontaneous activity and initial acclimation oxygen uptake. During feeding, transgenic fish ate significantly more than control fish, and had an overall oxygen uptake that was 1·7 times greater than control fish. When fish that had eaten the same per cent body mass were compared, transgenic fish had an oxygen uptake that was 1·4 times greater than control fish. Differences in oxygen uptake in growth hormone transgenic coho salmon and non-transgenic fish appear to be due to the effects of feeding, acclimation and activity level, and not to a difference in basal metabolism.     

Evidence for Morphometric Differentiation of Wild and Captively Reared Adult Coho Salmon: A Geometric Analysis

As part of a comprehensive genetic evaluation of reproduction in naturally spawning coho salmon, Oncorhynchus kisutch, we examined morphometric variation in captively reared and wild adults from Hood Canal, Washington (U.S.A.) for evidence of differentiation between these groups. We collected captively reared fish as parr from two stocks and reared to adulthood at a freshwater hatchery, maturing in 1995 and 1996; we sampled closely size-matched wild fish as they returned to a neighboring stream in both years. Multivariate analysis of shape variation by Procrustes coordinates, visualized by thin-plate splines, indicated that the captively reared adults were differentiated from the wild fish by sharply reduced sexual dimorphism as well as smaller heads and less hooked snouts, increased trunk depth, larger caudal peduncles, shorter dorsal fins, larger hindbodies and a reduction in body streamlining. The differences between the captively reared and wild fish were similar to but more pronounced than some differences previously reported between hatchery and wild coho salmon. The magnitude and pattern of differences suggested that at least some of them were environmentally induced. Shape variation showed an allometric relationship with variation in body (measured as centroid) size. Morphometric variation was a poor correlate of most spawning behaviors. Nevertheless, our results suggest that the morphometric consequences of captive rearing for mate selection and reproductive activity of spawning fish may limit its effectiveness as a restorative tool.     

Density of the waterborne parasite Ceratomyxa shasta and its biological effects on salmon

The myxozoan parasite Ceratomyxa shasta is a significant pathogen of juvenile salmonids in the Pacific Northwest of North America and is limiting recovery of Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon populations in the Klamath River. We conducted a 5-year monitoring program that comprised concurrent sentinel fish exposures and water sampling across 212 river kilometers of the Klamath River. We used percent mortality and degree-days to death to measure disease severity in fish. We analyzed water samples using quantitative PCR and Sanger sequencing, to determine total parasite density and relative abundance of C. shasta genotypes, which differ in their pathogenicity to salmonids. We detected the parasite throughout the study zone, but parasite density and genetic composition fluctuated spatially and temporally. Chinook and coho mortality increased with density of their specific parasite genotype, but mortality-density thresholds and time to death differed. A lethality threshold of 40% mortality was reached with 10 spores liter(-1) for Chinook but only 5 spores liter(-1) for coho. Parasite density did not affect degree-days to death for Chinook but was negatively correlated for coho, and there was wider variation among coho individuals. These differences likely reflect the different life histories and genetic heterogeneity of the salmon populations. Direct quantification of the density of host-specific parasite genotypes in water samples offers a management tool for predicting host population-level impacts.     

Heritability,Environmental Effects,and Genetic and Phenotypic Correlations of Oxidative Stress Resistance-Related Enzyme Activities During Early Life Stages in Atlantic Salmon

Oxidative stress (OS) may pose important physiological constraints on individuals, affecting trade-offs between growth and reproduction or ageing and survival. Despite such evolutionary and ecological importance, the results from studies on the magnitude of individual variation in OS resistance and the underlying causes of this variation such as genetic, environmental, and maternal origins, remain inconclusive. Using a high throughput methodology, we investigated the activity levels in three OS resistance-related enzymes (superoxide dismutase, SOD; glutathione reductase, GR; glutathione S-transferase, GST) during the early life stages of 1000 individuals from 50 paternal half-sib families in two populations of Atlantic salmon. Using animal mixed models, we detected the presence of narrow-sense heritability for SOD and GST; that for GST differed between populations due to differences in environmental variance. We found support for the presence of common environmental variation, including maternal effects, for only GR. Using a bivariate animal model, we detected a positive environmental correlation between activity levels of SOD and GST but were unable to detect an additive genetic correlation. Our results complement previous heritability findings for levels of reactive oxygen species or OS resistance by demonstrating the presence of heritability for OS-related enzyme activities. Our findings provide a foundation for future work, such as investigations on the evolutionary importance of variation in enzyme activities. In addition, our findings emphasise the importance of accounting for developmental stage, environmental variance, and kin relationships when investigating the OS-response at the enzyme activity level.     

Physiological,Behavioral and Maternal Factors That Contribute to Size Variation in Larval Amphibian Populations

Size variance among similarly aged individuals within populations is a pattern common to many organisms that is a result of interactions between intrinsic and extrinsic traits of individuals. While genetic and maternal effects, as well as physiological and behavioral traits have been shown to contribute to size variation in animal populations, teasing apart the influence of such factors on individual growth rates remain a challenge. Furthermore, tracing the effects of these interactions across life stages and in shaping adult phenotypes also requires further exploration. In this study we investigated the relationship between genetics, hatching patterns, behaviors, neuroendocrine stress axis activity and variance in growth and metamorphosis among same-aged larval amphibians. Through parallel experiments we found that in the absence of conspecific interactions, hatch time and to a lesser extent egg clutch identity (i.e. genetics and maternal effects) influenced the propensity for growth and development in individual tadpoles and determined metamorphic traits. Within experimental groups we found that variance in growth rates was associated with size-dependent foraging behaviors and responses to food restriction. We also found an inverse relationship between glucocorticoid (GC) hormone levels and body mass and developmental stage among group-reared tadpoles, which suggests that GC expression plays a role in regulating differing within-population growth trajectories in response to density-dependent conditions. Taken together these findings suggest that factors that influence hatching conditions can have long-term effects on growth and development. These results also raise compelling questions regarding the extent to which maternal and genetic factors influence physiological and behavioral profiles in amphibians.     

Where do all the maternal effects go? Variation in offspring body size through ontogeny in the live-bearing fish Poecilia parae

Maternal effects are an important source of adaptive variation, but little is known about how they vary throughout ontogeny. We estimate the contribution of maternal effects, sire genetic and environmental variation to offspring body size from birth until 1 year of age in the live-bearing fish Poecilia parae. In both the sexes, maternal effects on body size were initially high in juveniles, and then declined to zero at sexual maturity. In sons, this was accompanied by a sharp rise in sire genetic variance, consistent with the expression of Y-linked loci affecting male size. In daughters, all variance components decreased with time, consistent with compensatory growth. There were significant negative among-dam correlations between early body size and the timing of sexual maturity in both sons and daughters. However, there was no relationship between early life maternal effects and adult longevity, suggesting that maternal effects, although important early in life, may not always influence late life-history traits.     

Disease resistance, stress response and effects of triploidy in growth hormone transgenic coho salmon

Diploid and triploid coho salmon Oncorhynchus kisutch transgenic for growth hormone (GH) and control coho salmon were compared for differences in disease resistance and stress response. Resistance to the bacterial pathogen Vibrio anguillarum was not affected in transgenic fish relative to their non‐transgenic counterparts when they were infected at the fry stage, but was lower in transgenic fish when infected near smolting. Vaccination against vibriosis provided equal protection to both transgenic and non‐transgenic fish. Triploid fish showed a lower resistance to vibriosis than their diploid counterparts. Diploid transgenic fish and non‐transgenic fish appeared to show similar physiological and cellular stress responses to a heat shock. These studies provide information useful for both performance and ecological risk assessments of growth‐accelerated coho salmon.     

Have Native Coho Salmon (Oncorhynchus Kisutch) Persisted in the Nooksack and Samish rivers Despite Continuous Hatchery Production Throughout the Past Century?

For over a century, Washington State Department of Fish and Wildlife has implemented hatchery programs as a means to boost salmon abundance. Concerns have developed that native populations may be replaced by hatchery strains, decreasing the genetic diversity required to respond to environmental changes. We report a comparison of microsatellite DNA variation in wild-spawning and hatchery-strain coho salmon from the Nooksack and Samish rivers in northern Puget Sound. Significant heterogeneity in genotype frequencies was detected between wild-spawning coho salmon from the upper North Fork (NF) Nooksack River and hatchery-strain coho salmon from the Nooksack River (descendants of primarily Nooksack River broodstock). Little difference in genotype frequencies was detected between wild-spawning coho salmon from the Samish River and hatchery-strain coho salmon from the Nooksack River. The 13-locus suite provided high resolution: in assignment tests over 85% of wild-spawning coho salmon from the upper NF Nooksack River were assigned to source. Wild-spawning coho salmon collected below hatcheries in the Nooksack River and 50% of wild-spawning Samish River coho salmon were assigned to hatchery collections. The genetic divergence of wild-spawning coho salmon in the upper NF Nooksack River is remarkable given the extensive stocking history and proximity of a hatchery. We suggest that these upper river fish are native coho salmon and that wild spawners in the lower Nooksack and Samish River are descendants of hatchery productions. We attribute divergence to earlier run timing in upper NF Nooksack River wild spawners, availability of extensive spawning and rearing habitat upstream of a hatchery in the upper NF Nooksack River, and a longer stocking history in the Samish River.     

Geographical variation in egg mass and egg content in a passerine bird

Reproductive, phenotypic and life-history traits in many animal and plant taxa show geographic variation, indicating spatial variation in selection regimes. Maternal deposition to avian eggs, such as hormones, antibodies and antioxidants, critically affect development of the offspring, with long-lasting effects on the phenotype and fitness. Little is however known about large-scale geographical patterns of variation in maternal deposition to eggs. We studied geographical variation in egg components of a passerine bird, the pied flycatcher (Ficedula hypoleuca), by collecting samples from 16 populations and measuring egg and yolk mass, albumen lysozyme activity, yolk immunoglobulins, yolk androgens and yolk total carotenoids. We found significant variation among populations in most egg components, but ca. 90% of the variation was among individuals within populations. Population however explained 40% of the variation in carotenoid levels. In contrast to our hypothesis, we found geographical trends only in carotenoids, but not in any of the other egg components. Our results thus suggest high within-population variation and leave little scope for local adaptation and genetic differentiation in deposition of different egg components. The role of these maternally-derived resources in evolutionary change should be further investigated.     

Sex-biased genetic component distribution among populations: additive genetic and maternal contributions to phenotypic differences among populations of Chinook salmon

An approach frequently used to demonstrate a genetic basis for population-level phenotypic differences is to employ common garden rearing designs, where observed differences are assumed to be attributable to primarily additive genetic effects. Here, in two common garden experiments, we employed factorial breeding designs between wild and domestic, and among wild populations of Chinook salmon (Oncorhynchus tshawytscha). We measured the contribution of additive (V(A)) and maternal (V(M)) effects to the observed population differences for 17 life history and fitness-related traits. Our results show that, in general, maternal effects contribute more to phenotypic differences among populations than additive genetic effects. These results suggest that maternal effects are important in population phenotypic differentiation and also signify that the inclusion of the maternal source of variation is critical when employing models to test population differences in salmon, such as in local adaptation studies.     

Season- and size-dependent risk taking in juvenile coho salmon: experimental evaluation of asset protection

Using juvenile coho salmon, Oncorhynchus kisutch, we tested predictions arising from dynamic optimization models of foraging under predation risk. Coho juveniles from two size groups raised in the laboratory were individually fed varying food rations. Their willingness to risk predation was measured as the time to resume foraging after presentation of a predator model. Small fish (mean weight 1.5 g) resumed feeding earlier than larger fish (3.5 g) as predicted by dynamic models under summer photoperiod but not under autumn photoperiod. Contrary to predictions, larger fish did not increase risk taking and small fish decreased risk taking between summer and autumn treatments. Food ration significantly influenced time to resume feeding only in small coho. A simple mechanistic model we proposed to explain feeding motivation under risk as a function of body size and prior growth rate was not sufficient to explain observed variation in risk taking. This study suggests that coho salmon use photoperiod and their own body size as cues for long-term, state-dependent adjustments of feeding behaviour. The lower risk taking of larger fish is probably an example of asset protection, whereby larger animals accept less predation risk to protect their greater accumulated fitness value. The decrease of risk taking in small fish in the autumn was possibly caused by a switch of life history trajectory towards delayed smolting. Copyright 1999 The Association for the Study of Animal Behaviour.     

Water availability and population origin affect the expression of the tradeoff between reproduction and growth in Plantago coronopus

Investment in reproduction and growth represent a classic tradeoff with implication for life history evolution. The local environment can play a major role in the magnitude and evolutionary consequences of such a tradeoff. Here, we examined the investment in reproductive and vegetative tissue in 40 maternal half‐sib families from four different populations of the herb Plantago coronopus growing in either a dry or wet greenhouse environment. Plants originated from populations with an annual or a perennial life form, with annuals prevailing in drier habitats with greater seasonal variation in both temperature and precipitation. We found that water availability affected the expression of the tradeoff (both phenotypic and genetic) between reproduction and growth, being most accentuated under dry condition. However, populations responded very differently to water treatments. Plants from annual populations showed a similar response to drought condition with little variation among maternal families, suggesting a history of selection favouring genotypes with high allocation to reproduction when water availability is low. Plants from annual populations also expressed the highest level of plasticity. For the perennial populations, one showed a large variation among maternal families in resource allocation and expressed significant negative genetic correlations between reproductive and vegetative biomass under drought. The other perennial population showed less variation in response to treatment and had trait values similar to those of the annuals, although it was significantly less plastic. We stress the importance of considering intraspecific variation in response to environmental change such as drought, as conspecific plants exhibited very different abilities and strategies to respond to high versus low water availability even among geographically close populations.     

Quantitative genetic analysis of larval life history traits in two alpine populations of Rana temporaria

We estimated genetic and maternal variance components of larval life history characters in alpine populations of Rana temporaria (the common frog) using a full-sib/half-sib breeding design. We studied trait plasticity by raising tadpoles at 14 or 20°C in the laboratory. Larval period and metamorphic mass were greater at 14°C. Larval period did not differ between populations, but high elevation metamorphs were larger than low elevation metamorphs. Significant additive variation for larval period was detected in the low altitude population. No significant additive variation was detected for mass at metamorphosis (MM), which instead displayed significant maternal effects. Plasticity in metamorphic mass of froglets was greater in the high altitude population. The plastic response of larval period to temperature did not differ between the populations. Evolution of metamorphic mass is likely constrained by lack of additive genetic variation. In contrast, significant heritability for larval period suggests this trait may evolve in response to environmental change. These results differ from other studies on R. temporaria, suggesting that populations of this broadly distributed species present substantial geographic variation in the genetic architecture and plasticity of tadpole life history traits.     

Molecular cloning and sequencing of coho salmon growth hormone cDNA

A cDNA library was constructed using mRNA isolated from coho salmon pituitaries. By employing rainbow trout growth hormone cDNA as a probe, the coho salmon cDNA was isolated and the complete nucleotide (nt) sequence determined. The coding region contains 630 nt while the 5'- and 3'-untranslated regions are 64 and 489 nt in length, respectively. Comparison of the noncoding regions of coho and chum salmon cDNAs reveal identity at the 5' end but significant variation in the 3' end. Chum salmon and rainbow trout have identical amino acid (aa) sequences, but coho salmon growth hormone has a sequence that differs by 6 of the 188 predicted aa. Since salmonids are tetraploid, this difference may be the result of either divergence of the same growth hormone locus or of variation between different loci. Comparisons of the cDNA restriction maps of these three fish species suggest the former possibility.     

Moose body mass variation revisited: disentangling effects of environmental conditions and genetics

Large-scale geographical variation in phenotypic traits within species is often correlated to local environmental conditions and population density. Such phenotypic variation has recently been shown to also be influenced by genetic structuring of populations. In ungulates, large-scale geographical variation in phenotypic traits, such as body mass, has been related to environmental conditions and population density, but little is known about the genetic influences. Research on the genetic structure of moose suggests two distinct genetic lineages in Norway, structured along a north-south gradient. This corresponds with many environmental gradients, thus genetic structuring provides an additional factor affecting geographical phenotypic variation in Norwegian moose. We investigated if genetic structure explained geographical variation in body mass in Norwegian moose while accounting for environmental conditions, age and sex, and if it captured some of the variance in body mass that previously was attributed to environmental factors. Genetic structuring of moose was the most important variable in explaining the geographic variation in body mass within age and sex classes. Several environmental variables also had strong explanatory power, related to habitat diversity, environmental seasonality and winter harshness. The results suggest that environmental conditions, landscape characteristics, and genetic structure should be evaluated together when explaining large-scale patterns in phenotypic characters or life history traits. However, to better understand the role of genetic and environmental effects on phenotypic traits in moose, an extended individual-based study of variation in fitness-related characters is needed, preferably in an area of convergence between different genetic lineages.