Molecular and pedigree measures of relatedness provide similar estimates of inbreeding depression in a bottlenecked population

Individual‐based estimates of the degree of inbreeding or parental relatedness from pedigrees provide a critical starting point for studies of inbreeding depression, but in practice wild pedigrees are difficult to obtain. Because inbreeding increases the proportion of genomewide loci that are identical by descent, inbreeding variation within populations has the potential to generate observable correlations between heterozygosity measured using molecular markers and a variety of fitness related traits. Termed heterozygosity‐fitness correlations (HFCs), these correlations have been observed in a wide variety of taxa. The difficulty of obtaining wild pedigree data, however, means that empirical investigations of how pedigree inbreeding influences HFCs are rare. Here, we assess evidence for inbreeding depression in three life‐history traits (hatching and fledging success and juvenile survival) in an isolated population of Stewart Island robins using both pedigree‐ and molecular‐derived measures of relatedness. We found results from the two measures were highly correlated and supported evidence for significant but weak inbreeding depression. However, standardized effect sizes for inbreeding depression based on the pedigree‐based kin coefficients (k) were greater and had smaller standard errors than those based on molecular genetic measures of relatedness (RI), particularly for hatching and fledging success. Nevertheless, the results presented here support the use of molecular‐based measures of relatedness in bottlenecked populations when information regarding inbreeding depression is desired but pedigree data on relatedness are unavailable.     

Heterozygosity–fitness correlations in a declining seabird population

Loss of genetic diversity is thought to lead to increased risk of extinction in endangered populations due to decreasing fitness of homozygous individuals. Here, we evaluated the presence of inbreeding depression in a long‐lived seabird, the European shag (Phalacrocorax aristotelis), after a severe decline in population size by nearly 70%. During three reproductive seasons, 85 breeders were captured and genotyped at seven microsatellite loci. Nest sites were monitored during the breeding season to estimate reproductive success as the number of chicks surviving to full‐size‐grown per nest. Captured birds were tagged with a ring with an individual code, and resighting data were collected during 7‐year period. We found a strong effect of multilocus heterozygosity on female reproductive performance, and a significant, although weaker, effect on breeder survival. However, our matrix population model suggests that this relatively small effect of genetic diversity on breeder survival may have a profound effect on fitness. This highlights the importance of integrating life history consequences in HFC studies. Importantly, heterozygosity was correlated across loci, suggesting that genomewide effects, rather than single loci, are responsible for the observed HFCs. Overall, the HFCs are a worrying symptom of genetic erosion in this declining population. Many long‐lived species are prone to extinction, and future studies should evaluate the magnitude of fitness impact of genetic deterioration on key population parameters, such as survival of breeders.     

Population demography and heterozygosity–fitness correlations in natural guppy populations: An examination using sexually selected fitness traits

Heterozygosity–fitness correlations (HFCs) have been examined in a wide diversity of contexts, and the results are often used to infer the role of inbreeding in natural populations. Although population demography, reflected in population‐level genetic parameters such as allelic diversity or identity disequilibrium, is expected to play a role in the emergence and detectability of HFCs, direct comparisons of variation in HFCs across many populations of the same species, with different genetic histories, are rare. Here, we examined the relationship between individual microsatellite heterozygosity and a range of sexually selected traits in 660 male guppies from 22 natural populations in Trinidad. Similar to previous studies, observed HFCs were weak overall. However, variation in HFCs among populations was high for some traits (although these variances were not statistically different from zero). Population‐level genetic parameters, specifically genetic diversity levels (number of alleles, observed/expected heterozygosity) and measures of identity disequilibrium (g2 and heterozygosity–heterozygosity correlations), were not associated with variation in population‐level HFCs. This latter result indicates that these metrics do not necessarily provide a reliable predictor of HFC effect sizes across populations. Importantly, diversity and identity disequilibrium statistics were not correlated, providing empirical evidence that these metrics capture different essential characteristics of populations. A complex genetic architecture likely underpins multiple fitness traits, including those associated with male fitness, which may have reduced our ability to detect HFCs in guppy populations. Further advances in this field would benefit from additional research to determine the demographic contexts in which HFCs are most likely to occur.     

Heterozygosity–fitness correlations in a wild mammal population: accounting for parental and environmental effects

HFCs (heterozygosity–fitness correlations) measure the direct relationship between an individual's genetic diversity and fitness. The effects of parental heterozygosity and the environment on HFCs are currently under‐researched. We investigated these in a high‐density U.K. population of European badgers (Meles meles), using a multimodel capture–mark–recapture framework and 35 microsatellite loci. We detected interannual variation in first‐year, but not adult, survival probability. Adult females had higher annual survival probabilities than adult males. Cubs with more heterozygous fathers had higher first‐year survival, but only in wetter summers; there was no relationship with individual or maternal heterozygosity. Moist soil conditions enhance badger food supply (earthworms), improving survival. In dryer years, higher indiscriminate mortality rates appear to mask differential heterozygosity‐related survival effects. This paternal interaction was significant in the most supported model; however, the model‐averaged estimate had a relative importance of 0.50 and overlapped zero slightly. First‐year survival probabilities were not correlated with the inbreeding coefficient (f); however, small sample sizes limited the power to detect inbreeding depression. Correlations between individual heterozygosity and inbreeding were weak, in line with published meta‐analyses showing that HFCs tend to be weak. We found support for general rather than local heterozygosity effects on first‐year survival probability, and g2 indicated that our markers had power to detect inbreeding. We emphasize the importance of assessing how environmental stressors can influence the magnitude and direction of HFCs and of considering how parental genetic diversity can affect fitness‐related traits, which could play an important role in the evolution of mate choice.     

Recent admixture generates heterozygosity–fitness correlations during the range expansion of an invading species

Admixture, the mixing of historically isolated gene pools, can have immediate consequences for the genetic architecture of fitness traits. Admixture may be especially important for newly colonized populations, such as during range expansion and species invasions, by generating heterozygosity that can boost fitness through heterosis. Despite widespread evidence for admixture during species invasions, few studies have examined the demographic history leading to admixture, how admixture affects the heterozygosity and fitness of invasive genotypes, and whether such fitness effects are maintained through time. We address these questions using the invasive plant Silene vulgaris, which shows evidence of admixture in both its native Europe and in North America where it has invaded. Using multilocus genotype data in conjunction with approximate Bayesian computation analysis of demographic history, we showed that admixture during the invasion of North America was independent from and much younger than admixture in the native range of Europe. We tested for fitness consequences of admixture in each range and detected a significant positive heterozygosity–fitness correlation (HFC) in North America; in contrast, no HFC was present in Europe. The lack of HFC in Europe may reflect the longer time since admixture in the native range, dissipating associations between heterozygosity at markers and fitness loci. Our results support a key short‐term role for admixture during the early stages of invasion by generating HFCs that carry populations past the threat of extinction from inbreeding and demographic stochasticity.     

Heterozygosity at a single locus explains a large proportion of variation in two fitness‐related traits in great tits: a general or a local effect?

In natural populations, mating between relatives can have important fitness consequences due to the negative effects of reduced heterozygosity. Parental level of inbreeding or heterozygosity has been also found to influence the performance of offspring, via direct and indirect parental effects that are independent of the progeny own level of genetic diversity. In this study, we first analysed the effects of parental heterozygosity and relatedness (i.e. an estimate of offspring genetic diversity) on four traits related to offspring viability in great tits (Parus major) using 15 microsatellite markers. Second, we tested whether significant heterozygosity–fitness correlations (HFCs) were due to ‘local’ (i.e. linkage to genes influencing fitness) and/or ‘general’ (genome‐wide heterozygosity) effects. We found a significant negative relationship between parental genetic relatedness and hatching success, and maternal heterozygosity was positively associated with offspring body size. The characteristics of the studied populations (recent admixture, polygynous matings) together with the fact that we found evidence for identity disequilibrium across our set of neutral markers suggest that HFCs may have resulted from genome‐wide inbreeding depression. However, one locus (Ase18) had disproportionately large effects on the observed HFCs: heterozygosity at this locus had significant positive effects on hatching success and offspring size. It suggests that this marker may lie near to a functional locus under selection (i.e. a local effect) or, alternatively, heterozygosity at this locus might be correlated to heterozygosity across the genome due to the extensive ID found in our populations (i.e. a general effect). Collectively, our results lend support to both the general and local effect hypotheses and reinforce the view that HFCs lie on a continuum from inbreeding depression to those strictly due to linkage between marker loci and genes under selection.     

Effects of variation in resource acquisition during different stages of the life cycle on life‐history traits and trade‐offs in a burying beetle

Individual variation in resource acquisition should have consequences for life‐history traits and trade‐offs between them because such variation determines how many resources can be allocated to different life‐history functions, such as growth, survival and reproduction. Since resource acquisition can vary across an individual's life cycle, the consequences for life‐history traits and trade‐offs may depend on when during the life cycle resources are limited. We tested for differential and/or interactive effects of variation in resource acquisition in the burying beetle Nicrophorus vespilloides. We designed an experiment in which individuals acquired high or low amounts of resources across three stages of the life cycle: larval development, prior to breeding and the onset of breeding in a fully crossed design. Resource acquisition during larval development and prior to breeding affected egg size and offspring survival, respectively. Meanwhile, resource acquisition at the onset of breeding affected size and number of both eggs and offspring. In addition, there were interactive effects between resource acquisition at different stages on egg size and offspring survival. However, only when females acquired few resources at the onset of breeding was there evidence for a trade‐off between offspring size and number. Our results demonstrate that individual variation in resource acquisition during different stages of the life cycle has important consequences for life‐history traits but limited effects on trade‐offs. This suggests that in species that acquire a fixed‐sized resource at the onset of breeding, the size of this resource has larger effects on life‐history trade‐offs than resources acquired at earlier stages.     

Assessment of identity disequilibrium and its relation to empirical heterozygosity fitness correlations: a meta‐analysis

Heterozygosity fitness correlations (HFCs) have frequently been used to detect inbreeding depression, under the assumption that genome‐wide heterozygosity is a good proxy for inbreeding. However, meta‐analyses of the association between fitness measures and individual heterozygosity have shown that often either no correlations are observed or the effect sizes are small. One of the reasons for this may be the absence of variance in inbreeding, a requisite for generating general‐effect HFCs. Recent work has highlighted identity disequilibrium (ID) as a measure that may capture variance in the level of inbreeding within a population; however, no thorough assessment of ID in natural populations has been conducted. In this meta‐analysis, we assess the magnitude of ID (as measured by the g₂ statistic) from 50 previously published HFC studies and its relationship to the observed effect sizes of those studies. We then assess how much power the studies had to detect general‐effect HFCs, and the number of markers that would have been needed to generate a high expected correlation (r² = 0.9) between observed heterozygosity and inbreeding. Across the majority of studies, g₂ values were not significantly different than zero. Despite this, we found that the magnitude of g₂ was associated with the average effect sizes observed in a population, even when point estimates were nonsignificant. These low values of g₂ translated into low expected correlations between heterozygosity and inbreeding and suggest that many more markers than typically used are needed to robustly detect HFCs.     

Effects of inbreeding on fitness‐related traits in a small isolated moose population

Inbreeding can affect fitness‐related traits at different life history stages and may interact with environmental variation to induce even larger effects. We used genetic parentage assignment based on 22 microsatellite loci to determine a 25 year long pedigree for a newly established island population of moose with 20–40 reproducing individuals annually. We used the pedigree to calculate individual inbreeding coefficients and examined for effects of individual inbreeding (f) and heterozygosity on fitness‐related traits. We found negative effects of f on birth date, calf body mass and twinning rate. The relationship between f and calf body mass and twinning rate were found to be separate but weaker after accounting for birth date. We found no support for an inbreeding effect on the age‐specific lifetime reproductive success of females. The influence of f on birth date was related to climatic conditions during the spring prior to birth, indicating that calves with a low f were born earlier after a cold spring than calves with high f. In years with a warm spring, calf f did not affect birth date. The results suggest that severe inbreeding in moose has both indirect effects on fitness through delayed birth and lower juvenile body mass, as well as separate direct effects, as there still was a significant relationship between f and twinning rate after accounting for birth date and body mass as calf. Consequently, severe inbreeding as found in the study population may have consequences for population growth and extinction risk.     

Heterozygosity–fitness correlations in blue tit nestlings (<Emphasis Type="Italic">Cyanistis caeruleus</Emphasis>) under contrasting rearing conditions

Understanding the relation between genetic variation and fitness remains a key question in evolutionary biology. Although heterozygosity has been reported to correlate with many fitness-related traits, the strength of the heterozygosity–fitness correlations (HFCs) is usually weak and it is still difficult to assess the generality of these associations in natural populations. It has been suggested that HFCs may become meaningful only under particular environmental conditions. Moreover, existing evidence suggests that HFCs may also differ between sexes. The aim of this study was to investigate correlations between heterozygosity in neutral markers (microsatellites) and fitness-related traits in a natural population of blue tits (Cyanistes caeruleus). Additionally, we tested whether sex and environmental conditions may influence the magnitude and direction of HFCs. We found a positive relationship between heterozygosity and body mass of 14 days post-hatching nestlings, but only among females. Our results suggest that the correlation between heterozygosity and nestling body mass observed among female offspring could be attributed to within-brood effects. We failed to find any evidence that environmental conditions as simulated by brood size manipulation affect HFCs.     

Reproductive and post‐reproductive life history of wild‐caught Drosophila melanogaster under laboratory conditions

The life history of the fruit fly (Drosophila melanogaster) is well understood, but fitness components are rarely measured by following single individuals over their lifetime, thereby limiting insights into lifetime reproductive success, reproductive senescence and post‐reproductive lifespan. Moreover, most studies have examined long‐established laboratory strains rather than freshly caught individuals and may thus be confounded by adaptation to laboratory culture, inbreeding or mutation accumulation. Here, we have followed the life histories of individual females from three recently caught, non‐laboratory‐adapted wild populations of D. melanogaster. Populations varied in a number of life‐history traits, including ovariole number, fecundity, hatchability and lifespan. To describe individual patterns of age‐specific fecundity, we developed a new model that allowed us to distinguish four phases during a female's life: a phase of reproductive maturation, followed by a period of linear and then exponential decline in fecundity and, finally, a post‐ovipository period. Individual females exhibited clear‐cut fecundity peaks, which contrasts with previous analyses, and post‐peak levels of fecundity declined independently of how long females lived. Notably, females had a pronounced post‐reproductive lifespan, which on average made up 40% of total lifespan. Post‐reproductive lifespan did not differ among populations and was not correlated with reproductive fitness components, supporting the hypothesis that this period is a highly variable, random ‘add‐on’ at the end of reproductive life rather than a correlate of selection on reproductive fitness. Most life‐history traits were positively correlated, a pattern that might be due to genotype by environment interactions when wild flies are brought into a novel laboratory environment but that is unlikely explained by inbreeding or positive mutational covariance caused by mutation accumulation.     

Heterozygosity is unrelated to adult fitness measures in a large, noninbred population of great tits (Parus major)

The extent to which heterozygosity-fitness correlations (HFCs) are expected in wild populations is an important and unresolved question in evolutionary biology, because it relates to our understanding of the genetic architecture of fitness. Here, we report a study of HFCs in a wild, noninbred population of great tits (Parus major), based on a sample comprising 281 individuals typed at 26 markers, resulting in a data set comprising over 5600 genotypes. We regressed pedigree-derived f-score and multilocus genetic diversity against eight life-history traits known to be associated with fitness in this population, including lifetime reproductive success (LRS), as well as several morphological traits under weak selection. We found no evidence for either multilocus or single-locus HFCs for any morphological or fitness trait, and further found no evidence that effect sizes were stronger for those life-history traits more closely associated with reproductive fitness. This result may, in part, be explained by the fact that we found no evidence that our set of 26 markers had any power to infer genome-wide heterozygosity in this population and that marker-derived heterozygosity was uncorrelated with pedigree-derived f-score. Overall, these results emphasize the fact that the often-reported strong HFCs detected in small, inbred populations do not reflect a general phenomenon of increasing individual reproductive fitness with increasing heterozygosity.     

Negative effects of individual heterozygosity on reproductive success in a wild bird population

The evolutionary consequences of individual genetic diversity are frequently studied by assessing heterozygosity–fitness correlations (HFCs). The prevalence of positive and negative HFCs and the predominance of general versus local effects in wild populations are far from understood, partly because comprehensive studies testing for both inbreeding and outbreeding depression are lacking. We studied a genetically diverse population of blue tits in southern Germany using a genome‐wide set of 87 microsatellites to investigate the relationship between proxies of reproductive success and measures of multilocus and single‐locus individual heterozygosity (MLH and SLH). We used complimentary measures of MLH and partitioned markers into functional categories according to their position in the blue tit genome. HFCs based on MLH were consistently negative for functional loci, whereas correlations were rather inconsistent for loci found in nonfunctional areas of the genome. Clutch size was the only reproductive variable showing a general effect. We found evidence for local effects for three measures of reproductive success: arrival date at the breeding site, the probability of breeding at the study site and male reproductive success. For these, we observed consistent, and relatively strong, negative effects at one functional locus. Remarkably, this marker had a similar effect in another blue tit population from Austria (~400 km to the east). We suggest that a genetic local effect on timing of arrival might be responsible for most negative HFCs detected, with carry‐over effects on other reproductive traits. This effect could reflect individual differences in the distance between overwintering areas and breeding sites.     

Post‐fledging survival of altricial birds: ecological determinants and adaptation

For altricial young, fledging is an abrupt step into an unknown environment. Despite increasing numbers of studies addressing the post‐fledging period, our current knowledge of the causes and consequences of post‐fledging survival remains fragmentary. Here, we review the literature on post‐fledging survival of juvenile altricial birds, addressing the following main questions: Is low post‐fledging survival a bottleneck in the altricial reproductive cycle? What is known of proximate and ultimate causal factors such as trophic relations (food and predation), habitat conditions, or abiotic factors acting in the post‐fledging period? We analyzed weekly survival estimates from 123 data series based on studies of 65 species, covering weeks 1–13 post‐fledging. As a general pattern, survival of fledglings was low during the first week post‐fledging (median rate = 0.83), and improved rapidly with time post‐fledging (week 4 median rate = 0.96). For ground‐nesting species, survival immediately after leaving nests was similar to egg‐to‐fledging survival. For species breeding above‐ground, survival during the first week post‐fledging was substantially lower than during both the nestling period and later post‐fledging stages. Thus, the early post‐fledging period is a bottleneck of markedly elevated mortality for most altricial species. Predation was the main proximate cause of mortality. Various factors such as habitat, annual and seasonal variation in the environment, and the physical condition of fledglings have been found to affect post‐fledging survival. Individual survival depended strongly on physical traits such as mass and wing length, which likely influence the ability of fledglings to escape predation. Trophic relationships at various levels are the main ultimate driver of adaptation of traits relevant to survival during the pre‐ and post‐fledging periods. Spatiotemporal dynamics of food resources determine the physical development of juveniles and, in turn, their performance after fledging. However, predators can cause quick and efficient selection for fledgling traits and adult breeding decisions. Parental strategies related to clutch size and timing of breeding, and the age and developmental stage at which young fledge have substantial effects on post‐fledging survival. The intensity and duration of post‐fledging parental investment also influences fledgling survival. Post‐fledging mortality is therefore not a random and inevitable loss. Traits and strategies related to fledging and the post‐fledging stage create large fitness differentials and, therefore, are integral, yet poorly understood, parts of the altricial reproductive strategy.     

No evidence of inbreeding depression in fast declining herds of migratory caribou

Identifying inbreeding depression early in small and declining populations is essential for management and conservation decisions. Correlations between heterozygosity and fitness (HFCs) provide a way to identify inbreeding depression without prior knowledge of kinship among individuals. In Northern Quebec and Labrador, the size of two herds of migratory caribou (Rivière‐George, RG and Rivière‐aux‐Feuilles, RAF) has declined by one to two orders of magnitude in the last three decades. This raises the question of a possible increase in inbreeding depression originating from, and possibly contributing to, the demographic decline in those populations. Here, we tested for the association of genomic inbreeding indices (estimated with 22,073 SNPs) with body mass and survival in 400 caribou sampled in RG and RAF herds between 1996 and 2016. We found no association of individual heterozygosity or inbreeding coefficient with body mass or annual survival. Furthermore, those genomic inbreeding indices remained stable over the period monitored. These results suggest that the rapid and intense demographic decline of the herds did not cause inbreeding depression in those populations. Although we found no evidence for HFCs, if demographic decline continues, it is possible that such inbreeding depression would be triggered.     

No correlation between multi‐locus heterozygosity and fitness in the common buzzard despite heterozygote advantage for plumage colour

Correlations between heterozygosity and fitness are frequently found but rarely well understood. Fitness can be affected by single loci of large effect which correlate with neutral markers via linkage disequilibrium, or as a result of variation in genome‐wide heterozygosity following inbreeding. We explored these alternatives in the common buzzard, a raptor species in which three colour morphs differ in their lifetime reproductive success. Using 18 polymorphic microsatellite loci, we evaluated potential genetic differences among the morphs which may lead to subpopulation structuring and tested for correlations between three fitness‐related traits and heterozygosity, both genome wide and at each locus separately. Despite their assortative mating pattern, the buzzard morphs were found to be genetically undifferentiated. Multilocus heterozygosity was only found to be correlated with a single fitness‐related trait, infection with the blood parasite, Leucocytozoon buteonis, and this was via interactions with vole abundance and age. One locus also showed a significant relationship with blood parasite infection and ectoparasite infestation. The vicinity of this locus contains two genes, one of which is potentially implicated in the immune system of birds. We conclude that genome‐wide heterozygosity is unlikely to be a major determinant of parasite burden and body condition in the polymorphic common buzzard.     

Lifetime fitness and age‐related female ornament signalling: evidence for survival and fecundity selection in the pied flycatcher

Ornaments displayed by females have often been denied evolutionary interest due to their frequently reduced expression relative to males, habitually attributed to a genetic correlation between the sexes. We estimated annual and lifetime reproductive success of female pied flycatchers (Ficedula hypoleuca) and applied capture–mark–recapture models to analyse annual survival rates in relation to the patterns of expression (absence/presence) of an ornament displayed by all males and a fraction of females. Overall, the likelihood of expressing the ornament increased nonlinearly with female age and was due to within‐individual variation, not to the selective appearance or disappearance of ornament‐related expression of phenotypes in the population. Accordingly, expressing the forehead patch in a given year did not influence survival probability. However, those females expressing the ornament at early ages (1–2 years old) enjoyed survival advantages throughout lifetime. Although ornamented females had higher lifetime fecundity and fledging success, their yearly reproductive performance, in terms of fledging productivity, decreased as they aged so that, late in life, ornamented females reared fewer offspring than nonexpressing females of the same age. In addition, both strategies (expressing vs. not expressing the trait) returned similar fitness payoffs in terms of recruited offspring. Our results support the hypothesis that fecundity and survival selection are involved in the displaying of this ‘male’ ornament by females.     

Parasite‐mediated selection in a natural metapopulation of Daphnia magna

Parasite‐mediated selection varying across time and space in metapopulations is expected to result in host local adaptation and the maintenance of genetic diversity in disease‐related traits. However, nonadaptive processes like migration and extinction‐(re)colonization dynamics might interfere with adaptive evolution. Understanding how adaptive and nonadaptive processes interact to shape genetic variability in life‐history and disease‐related traits can provide important insights into their evolution in subdivided populations. Here we investigate signatures of spatially fluctuating, parasite‐mediated selection in a natural metapopulation of Daphnia magna. Host genotypes from infected and uninfected populations were genotyped at microsatellite markers, and phenotyped for life‐history and disease traits in common garden experiments. Combining phenotypic and genotypic data a Q_ST–F_ST‐like analysis was conducted to test for signatures of parasite mediated selection. We observed high variation within and among populations for phenotypic traits, but neither an indication of host local adaptation nor a cost of resistance. Infected populations have a higher gene diversity (Hs) than uninfected populations and Hs is strongly positively correlated with fitness. These results suggest a strong parasite effect on reducing population level inbreeding. We discuss how stochastic processes related to frequent extinction‐(re)colonization dynamics as well as host and parasite migration impede the evolution of resistance in the infected populations. We suggest that the genetic and phenotypic patterns of variation are a product of dynamic changes in the host gene pool caused by the interaction of colonization bottlenecks, inbreeding, immigration, hybrid vigor, rare host genotype advantage and parasitism. Our study highlights the effect of the parasite in ameliorating the negative fitness consequences caused by the high drift load in this metapopulation.     

The effect of drought stress on heterozygosity–fitness correlations in pedunculate oak (Quercus robur)

Background and Aims

The interaction between forest fragmentation and predicted climate change may pose a serious threat to tree populations. In small and spatially isolated forest fragments, increased homozygosity may directly affect individual tree fitness through the expression of deleterious alleles. Climate change-induced drought stress may exacerbate these detrimental genetic consequences of forest fragmentation, as the fitness response to low levels of individual heterozygosity is generally thought to be stronger under environmental stress than under optimal conditions.

Methods

To test this hypothesis, a greenhouse experiment was performed in which various transpiration and growth traits of 6-month-old seedlings of Quercus robur differing in multilocus heterozygosity (MLH) were recorded for 3 months under a well-watered and a drought stress treatment. Heterozygosity–fitness correlations (HFC) were examined by correlating the recorded traits of individual seedlings to their MLH and by studying their response to drought stress.

Key Results

Weak, but significant, effects of MLH on several fitness traits were obtained, which were stronger for transpiration variables than for the recorded growth traits. High atmospheric stress (measured as vapour pressure deficit) influenced the strength of the HFCs of the transpiration variables, whereas only a limited effect of the irrigation treatment on the HFCs was observed.

Conclusions

Under ongoing climate change, increased atmospheric stress in the future may strengthen the negative fitness responses of trees to low MLH. This indicates the necessity to maximize individual multilocus heterozygosity in forest tree breeding programmes.     

Sex‐specific selection on energy metabolism – selection coefficients for winter survival

Selection for different fitness optima between sexes is supposed to operate on several traits. As fitness‐related traits are often energetically costly, selection should also act directly on the energetics of individuals. However, efforts to examine the relationship between fitness and components of the energy budget are surprisingly scarce. We investigated the effects of basal metabolic rate (BMR, the minimum energy required for basic life functions) and body condition on long‐term survival (8 winter months) with manipulated densities in enclosed populations of bank voles (Myodes glareolus). Here, we show that survival selection on BMR was clearly sex‐specific but density‐independent. Both the linear selection gradient and selection differential for BMR were positive in females, whereas survival did not correlate with male characteristics. Our findings emphasize the relative importance of individual physiology over ecological factors (e.g. intra‐specific competition). Most current models of the origin of endothermy underline the importance of metabolic optima in females, whose physiology evolved to fulfil demands of parental provisioning in mammals. Our novel findings of sex‐specific selection could be related to these life history differences between sexes.