Inbreeding, energy use and condition

Inbreeding is associated with shorter early-life telomere length in a wild passerine

Inbreeding can have negative effects on survival and reproduction, which may be of conservation concern in small and isolated populations. However, the physiological mechanisms underlying inbreeding depression are not well-known. The length of telomeres, the DNA sequences protecting chromosome ends, has been associated with health or fitness in several species. We investigated effects of inbreeding on early-life telomere length in two small island populations of wild house sparrows (Passer domesticus) known to be affected by inbreeding depression. Using genomic measures of inbreeding we found that inbred nestling house sparrows (n = 371) have significantly shorter telomeres. Using pedigree-based estimates of inbreeding we found a tendency for inbred nestling house sparrows to have shorter telomeres (n = 1195). This negative effect of inbreeding on telomere length may have been complemented by a heterosis effect resulting in longer telomeres in individuals that were less inbred than the population average. Furthermore, we found some evidence of stronger effects of inbreeding on telomere length in males than females. Thus, telomere length may reveal subtle costs of inbreeding in the wild and demonstrate a route by which inbreeding negatively impacts the physiological state of an organism already at early life-history stages.

Repeatability and heritability of resting metabolic rate in a long-lived amphibian

Resting metabolic rate (RMR), i.e. spent energy necessary to maintain basic life functions, is a basic component of energy budget in ectotherms. The evolution of RMR through natural selection rests on the premise of its non-zero repeatability and heritability, i.e. consistent variation within individual lifetimes and resemblance between parents and their offspring, respectively. Joint estimates of RMR repeatability and heritability are missing in ectotherms, however, which precludes estimations of the evolutionary potential of this trait. We examined RMR repeatability and heritability in a long-lived ectotherm, the alpine newt (Ichthyosaura alpestris). Individual RMR was repeatable over both six-month (0.28 ± 0.09 [SE]) and five-year (0.16 ± 0.07) periods. While there was no resemblance between parent and offspring RMR (0.21 ± 0.34), the trait showed similarity among offspring within families (broad-sense heritability; 0.25 ± 0.09). Similar repeatability and broad-sense heritability values in parental and offspring generations, respectively, and non-conclusive narrow-sense heritability suggest the contribution of non-additive genetic factors to total phenotypic variance in this trait. We conclude that RMR evolutionary trajectories are shaped by other processes than natural selection in this long-lived ectotherm.

Evaluating responses to temperature during pre-metamorphosis and carry-over effects at post-metamorphosis in the wood tiger moth (Arctia plantaginis)

Insect metamorphosis is one of the most recognized processes delimiting transitions between phenotypes. It has been traditionally postulated as an adaptive process decoupling traits between life stages, allowing evolutionary independence of pre- and post-metamorphic phenotypes. However, the degree of autonomy between these life stages varies depending on the species and has not been studied in detail over multiple traits simultaneously. Here, we reared full-sib larvae of the warningly coloured wood tiger moth (Arctia plantaginis) in different temperatures and examined their responses for phenotypic (melanization change, number of moults), gene expression (RNA-seq and qPCR of candidate genes for melanization and flight performance) and life-histories traits (pupal weight, and larval and pupal ages). In the emerging adults, we examined their phenotypes (melanization and size) and compared them at three condition proxies: heat absorption (ability to engage flight), flight metabolism (ability to sustain flight) and overall flight performance. We found that some larval responses, as evidenced by gene expression and change in melanization, did not have an effect on the adult (i.e. size and wing melanization), whereas other adult traits such as heat absorption, body melanization and flight performance were found to be impacted by rearing temperature. Adults reared at high temperature showed higher resting metabolic rate, lower body melanization, faster heating rate, lower body temperature at take-off and inferior flight performance than cold-reared adults. Thus, our results did not unambiguously support the environment-matching hypothesis. Our results illustrate the importance of assessing multiple traits across life stages as these may only be partly decoupled by metamorphosis. This article is part of the theme issue 'The evolution of complete metamorphosis'.

Adaptation to environmental stress at different timescales

Environments are changing rapidly, and to cope with these changes, organisms have to adapt. Adaptation can take many shapes and occur at different speeds, depending on the type of response, the trait, the population, and the environmental conditions. The biodiversity crisis that we are currently facing illustrates that numerous species and populations are not capable of adapting with sufficient speed to ongoing environmental changes. Here, we discuss current knowledge on the ability of animals and plants to adapt to environmental stress on different timescales, mainly focusing on thermal stress and ectotherms. We discuss within-generation responses that can be fast and induced within minutes or hours, evolutionary adaptations that are often slow and take several generations, and mechanisms that lay somewhere in between and that include epigenetic transgenerational effects. To understand and predict the impacts of environmental change and stress on biodiversity, we suggest that future studies should (1) have an increased focus on understanding the type and speed of responses to fast environmental changes; (2) focus on the importance of environmental fluctuations and the predictability of environmental conditions on adaptive capabilities, preferably in field studies encompassing several fitness components; and (3) look at ecosystem responses to environmental stress and their resilience when disturbed.

A physiological perspective on fisheries-induced evolution

There is increasing evidence that intense fishing pressure is not only depleting fish stocks but also causing evolutionary changes to fish populations. In particular, body size and fecundity in wild fish populations may be altered in response to the high and often size‐selective mortality exerted by fisheries. While these effects can have serious consequences for the viability of fish populations, there are also a range of traits not directly related to body size which could also affect susceptibility to capture by fishing gears—and therefore fisheries‐induced evolution (FIE)—but which have to date been ignored. For example, overlooked within the context of FIE is the likelihood that variation in physiological traits could make some individuals within species more vulnerable to capture. Specifically, traits related to energy balance (e.g., metabolic rate), swimming performance (e.g., aerobic scope), neuroendocrinology (e.g., stress responsiveness) and sensory physiology (e.g., visual acuity) are especially likely to influence vulnerability to capture through a variety of mechanisms. Selection on these traits could produce major shifts in the physiological traits within populations in response to fishing pressure that are yet to be considered but which could influence population resource requirements, resilience, species’ distributions and responses to environmental change.

The thermodynamics of protein aggregation reactions may underpin the enhanced metabolic efficiency associated with heterosis, some balancing selection, and the evolution of ploidy levels

Identifying the physical basis of heterosis (or "hybrid vigor") has remained elusive despite over a hundred years of research on the subject. The three main theories of heterosis are dominance theory, overdominance theory, and epistasis theory. Kacser and Burns (1981) identified the molecular basis of dominance, which has greatly enhanced our understanding of its importance to heterosis. This paper aims to explain how overdominance, and some features of epistasis, can similarly emerge from the molecular dynamics of proteins. Possessing multiple alleles at a gene locus results in the synthesis of different allozymes at reduced concentrations. This in turn reduces the rate at which each allozyme forms soluble oligomers, which are toxic and must be degraded, because allozymes co-aggregate at low efficiencies. The model developed in this paper can explain how heterozygosity impacts the metabolic efficiency of an organism. It can also explain why the viabilities of some inbred lines seem to decline rapidly at high inbreeding coefficients (F > 0.5), which may provide a physical basis for truncation selection for heterozygosity. Finally, the model has implications for the ploidy level of organisms. It can explain why polyploids are frequently found in environments where severe physical stresses promote the formation of soluble oligomers. The model can also explain why complex organisms, which need to synthesize aggregation-prone proteins that contain intrinsically unstructured regions (IURs) and multiple domains because they facilitate complex protein interaction networks (PINs), tend to be diploid while haploidy tends to be restricted to relatively simple organisms.

Telomere length reveals cumulative individual and transgenerational inbreeding effects in a passerine bird

Inbreeding results in more homozygous offspring that should suffer reduced fitness, but it can be difficult to quantify these costs for several reasons. First, inbreeding depression may vary with ecological or physiological stress and only be detectable over long time periods. Second, parental homozygosity may indirectly affect offspring fitness, thus confounding analyses that consider offspring homozygosity alone. Finally, measurement of inbreeding coefficients, survival and reproductive success may often be too crude to detect inbreeding costs in wild populations. Telomere length provides a more precise measure of somatic costs, predicts survival in many species and should reflect differences in somatic condition that result from varying ability to cope with environmental stressors. We studied relative telomere length in a wild population of Seychelles warblers (Acrocephalus sechellensis) to assess the lifelong relationship between individual homozygosity, which reflects genome‐wide inbreeding in this species, and telomere length. In juveniles, individual homozygosity was negatively associated with telomere length in poor seasons. In adults, individual homozygosity was consistently negatively related to telomere length, suggesting the accumulation of inbreeding depression during life. Maternal homozygosity also negatively predicted offspring telomere length. Our results show that somatic inbreeding costs are environmentally dependent at certain life stages but may accumulate throughout life.

Starvation resistance and tissue-specific gene expression of stress-related genes in a naturally inbred ant population

Starvation is one of the most common and severe stressors in nature. Not only does it lead to death if not alleviated, it also forces the starved individual to allocate resources only to the most essential processes. This creates energetic trade-offs which can lead to many secondary challenges for the individual. These energetic trade-offs could be exacerbated in inbred individuals, which have been suggested to have a less efficient metabolism. Here, we studied the effect of inbreeding on starvation resistance in a natural population of Formica exsecta ants, with a focus on survival and tissue-specific expression of stress, metabolism and immunity-related genes. Starvation led to large tissue-specific changes in gene expression, but inbreeding had little effect on most of the genes studied. Our results illustrate the importance of studying stress responses in different tissues instead of entire organisms.

The influence of mitonuclear genetic variation on personality in seed beetles

There is a growing awareness of the influence of mitochondrial genetic variation on life-history phenotypes, particularly via epistatic interactions with nuclear genes. Owing to their direct effect on traits such as metabolic and growth rates, mitonuclear interactions may also affect variation in behavioural types or personalities (i.e. behavioural variation that is consistent within individuals, but differs among individuals). However, this possibility is largely unexplored. We used mitonuclear introgression lines, where three mitochondrial genomes were introgressed into three nuclear genetic backgrounds, to disentangle genetic effects on behavioural variation in a seed beetle. We found within-individual consistency in a suite of activity-related behaviours, providing evidence for variation in personality. Composite measures of overall activity of individuals in behavioural assays were influenced by both nuclear genetic variation and by the interaction between nuclear and mitochondrial genomes. More importantly, the degree of expression of behavioural and life-history phenotypes was correlated and mitonuclear genetic variation affected expression of these concerted phenotypes. These results show that mitonuclear genetic variation affects both behavioural and life-history traits, and they provide novel insights into the maintenance of genetic variation in behaviour and personality.

Heritability of flight and resting metabolic rates in the Glanville fritillary butterfly

Dispersal capacity is a key life-history trait especially in species inhabiting fragmented landscapes. Evolutionary models predict that, given sufficient heritable variation, dispersal rate responds to natural selection imposed by habitat loss and fragmentation. Here, we estimate phenotypic variance components and heritability of flight and resting metabolic rates (RMRs) in an ecological model species, the Glanville fritillary butterfly, in which flight metabolic rate (FMR) is known to correlate strongly with dispersal rate. We modelled a two-generation pedigree with the animal model to distinguish additive genetic variance from maternal and common environmental effects. The results show that FMR is significantly heritable, with additive genetic variance accounting for about 40% of total phenotypic variance; thus, FMR has the potential to respond to selection on dispersal capacity. Maternal influences on flight metabolism were negligible. Heritability of flight metabolism was context dependent, as in stressful thermal conditions, environmentally induced variation dominated over additive genetic effects. There was no heritability in RMR, which was instead strongly influenced by maternal effects. This study contributes to a mechanistic understanding of the evolution of dispersal-related traits, a pressing question in view of the challenges posed to many species by changing climate and fragmentation of natural habitats.

Inbreeding effects on standard metabolic rate investigated at cold, benign and hot temperatures in Drosophila melanogaster

Inbreeding increases homozygosity, which is known to affect the mean and variance of fitness components such as growth, fecundity and mortality rate. Across inbred lines inbreeding depression is typically observed and the variance between lines is increased in inbred compared to outbred lines. It has been suggested that damage incurred from increased homozygosity entails energetic cost associated with cellular repair. However, little is known about the effects of inbreeding on standard metabolic rate. Using stop-flow respirometry we performed repeated measurements of metabolic rate in replicated lines of inbred and outbred Drosophila melanogaster at stressful low, benign and stressful high temperatures. The lowest measurements of metabolic rate in our study are always associated with the low activity period of the diurnal cycle and these measurements therefore serve as good estimates of standard metabolic rate. Due to the potentially added costs of genetic stress in inbred lines we hypothesized that inbred individuals have increased metabolic rate compared to outbred controls and that this is more pronounced at stressful temperatures due to synergistic inbreeding by environment interactions. Contrary to our hypothesis we found no significant difference in metabolic rate between inbred and outbred lines and no interaction between inbreeding and temperature. Inbreeding however effected the variance; the variance in metabolic rate was higher between the inbred lines compared to the outbred control lines with some inbred lines having very high or low standard metabolic rate. Thus genetic drift and not inbreeding per se seem to explain variation in metabolic rate in populations of different size.

Pleiotropic effects of a mitochondrial-nuclear incompatibility depend upon the accelerating effect of temperature in Drosophila

Interactions between mitochondrial and nuclear gene products that underlie eukaryotic energy metabolism can cause the fitness effects of mutations in one genome to be conditional on variation in the other genome. In ectotherms, the effects of these interactions are likely to depend upon the thermal environment, because increasing temperature accelerates molecular rates. We find that temperature strongly modifies the pleiotropic phenotypic effects of an incompatible interaction between a Drosophila melanogaster polymorphism in the nuclear-encoded, mitochondrial tyrosyl-transfer (t)RNA synthetase and a D. simulans polymorphism in the mitochondrially encoded tRNA(Tyr). The incompatible mitochondrial-nuclear genotype extends development time, decreases larval survivorship, and reduces pupation height, indicative of decreased energetic performance. These deleterious effects are ameliorated when larvae develop at 16° and exacerbated at warmer temperatures, leading to complete sterility in both sexes at 28°. The incompatible genotype has a normal metabolic rate at 16° but a significantly elevated rate at 25°, consistent with the hypothesis that inefficient energy metabolism extends development in this genotype at warmer temperatures. Furthermore, the incompatibility decreases metabolic plasticity of larvae developed at 16°, indicating that cooler development temperatures do not completely mitigate the deleterious effects of this genetic interaction. Our results suggest that the epistatic fitness effects of metabolic mutations may generally be conditional on the thermal environment. The expression of epistatic interactions in some environments, but not others, weakens the efficacy of selection in removing deleterious epistatic variants from populations and may promote the accumulation of incompatibilities whose fitness effects will depend upon the environment in which hybrids occur.

Discontinuous gas exchange exhibition is a heritable trait in speckled cockroaches Nauphoeta cinerea

The regulation of insect respiratory gas exchange has long been an area of interest. In particular, the reason why insects from at least five orders exhibit patterns of gas exchange that include regular periods of spiracular closure has been the source of much controversy. Three adaptive hypotheses propose that these discontinuous gas-exchange cycles (DGCs) evolved to either limit water loss across respiratory surfaces, facilitate gas exchange in underground environments or to limit oxidative damage. It is possible that DGCs evolved independently multiple times and for different reasons, but for DGCs to be a plausible target for natural selection, they must be heritable and confer a fitness benefit. In a previous study of cockroaches Nauphoeta cinerea, we demonstrated that DGCs are repeatable and extend survival under food and water restriction. Here, we show for the first time that DGCs are heritable, suggesting that they are a plausible target for natural selection.

A long life in the fast lane: positive association between peak metabolic rate and lifespan in a butterfly

High peak metabolic rates may provide a performance advantage, but it may also entail a physiological cost. A long-held assumption is that high mass-specific energy expenditure is associated with short lifespan. To examine the relationship between energy expenditure and lifespan we asked two questions. First, do individuals have a consistent rate of metabolism throughout their life? Second, is metabolic rate correlated with lifespan? We analysed the repeatability of measurements of resting (RMR) and peak flight metabolic rate (MR(peak)) throughout the life of the Glanville fritillary butterfly (Melitaea cinxia). Measurements of MR(peak) showed significant repeatability. Senescence occurred only shortly before death. RMR showed a U-shaped relationship with age and very low repeatability. Intraspecific association between metabolic rates and lifespan was tested under three conditions: in the laboratory, under field conditions and in a laboratory experiment with repeated flight treatments. There was a significant correlation between MR(peak) and lifespan in all three experiments, but the correlation was positive, not negative. RMR was not correlated with lifespan. Both MR(peak) and lifespan may reflect physiological condition and therefore be positively correlated. Individuals with a large resource pool may be able to invest in mechanisms that slow down ageing. Individuals with high metabolic capacity may also possess adaptations against ageing. Molecular polymorphism in the gene phosphoglucose isomerase (Pgi) was significantly associated with both MR(peak) and lifespan, and may have coevolved with defence mechanisms against senescence. Generalisations such as 'live fast, die young' may be too simple to explain the complex processes affecting ageing and lifespan.

High genetic load in an old isolated butterfly population

We investigated inbreeding depression and genetic load in a small (N(e) ∼ 100) population of the Glanville fritillary butterfly (Melitaea cinxia), which has been completely isolated on a small island [Pikku Tytärsaari (PT)] in the Baltic Sea for at least 75 y. As a reference, we studied conspecific populations from the well-studied metapopulation in the Åland Islands (ÅL), 400 km away. A large population in Saaremaa, Estonia, was used as a reference for estimating genetic diversity and N(e). We investigated 58 traits related to behavior, development, morphology, reproductive performance, and metabolism. The PT population exhibited high genetic load (L = 1 - W(PT)/W(ÅL)) in a range of fitness-related traits including adult weight (L = 0.12), flight metabolic rate (L = 0.53), egg viability (L = 0.37), and lifetime production of eggs in an outdoor population cage (L = 0.70). These results imply extensive fixation of deleterious recessive mutations, supported by greatly reduced diversity in microsatellite markers and immediate recovery (heterosis) of egg viability and flight metabolic rate in crosses with other populations. There was no significant inbreeding depression in most traits due to one generation of full-sib mating. Resting metabolic rate was significantly elevated in PT males, which may be related to their short lifespan (L = 0.25). The demographic history and the effective size of the PT population place it in the part of the parameter space in which models predict mutation accumulation. This population exemplifies the increasingly common situation in fragmented landscapes, in which small and completely isolated populations are vulnerable to extinction due to high genetic load.

Gene-expression changes caused by inbreeding protect against inbreeding depression in Drosophila

We present a transcriptomic analysis aimed at investigating whether the changes in gene expression that occur under inbreeding generally reduce or enhance inbreeding depression. Discerning between these two alternatives can be addressed only when both changes in expression due to inbreeding and to inbreeding depression are estimated simultaneously. We used Affymetrix 2.0 arrays to study the changes in gene expression associated with both inbreeding and inbreeding depression for fitness in four sets of inbred sublines of Drosophila melanogaster. We found that for most genes showing changes in expression associated with inbreeding, the least depressed sublines were those showing the largest departures in expression from that of the outbred control. This suggests a pattern consistent with a protective role of expression changes against inbreeding effects, and would reveal a new dimension of the transcriptomics of inbreeding. The variation in depression observed could then be due not only to the genetic damages primarily originating that depression, but also possibly to differences in the ability to carry out the appropriate adjustments in gene expression to cope with the inbreeding. We also found that these expression changes with a putative protective role against inbreeding effects show a clear specificity on RNA synthesis and splicing and energy derivation functions.

Genetic erosion impedes adaptive responses to stressful environments

Biodiversity is increasingly subjected to human-induced changes of the environment. To persist, populations continually have to adapt to these often stressful changes including pollution and climate change. Genetic erosion in small populations, owing to fragmentation of natural habitats, is expected to obstruct such adaptive responses: (i) genetic drift will cause a decrease in the level of adaptive genetic variation, thereby limiting evolutionary responses; (ii) inbreeding and the concomitant inbreeding depression will reduce individual fitness and, consequently, the tolerance of populations to environmental stress. Importantly, inbreeding generally increases the sensitivity of a population to stress, thereby increasing the amount of inbreeding depression. As adaptation to stress is most often accompanied by increased mortality (cost of selection), the increase in the ‘cost of inbreeding’ under stress is expected to severely hamper evolutionary adaptive processes. Inbreeding thus plays a pivotal role in this process and is expected to limit the probability of genetically eroded populations to successfully adapt to stressful environmental conditions. Consequently, the dynamics of small fragmented populations may differ considerably from large nonfragmented populations. The resilience of fragmented populations to changing and deteriorating environments is expected to be greatly decreased. Alleviating inbreeding depression, therefore, is crucial to ensure population persistence.

What causes intraspecific variation in resting metabolic rate and what are its ecological consequences?

Individual differences in the energy cost of self-maintenance (resting metabolic rate, RMR) are substantial and the focus of an emerging research area. These differences may influence fitness because self-maintenance is considered as a life-history component along with growth and reproduction. In this review, we ask why do some individuals have two to three times the 'maintenance costs' of conspecifics, and what are the fitness consequences? Using evidence from a range of species, we demonstrate that diverse factors, such as genotypes, maternal effects, early developmental conditions and personality differences contribute to variation in individual RMR. We review evidence that RMR is linked with fitness, showing correlations with traits such as growth and survival. However, these relationships are modulated by environmental conditions (e.g. food supply), suggesting that the fitness consequences of a given RMR may be context-dependent. Then, using empirical examples, we discuss broad-scale reasons why variation in RMR might persist in natural populations, including the role of both spatial and temporal variation in selection pressures and trans-generational effects. To conclude, we discuss experimental approaches that will enable more rigorous examination of the causes and consequences of individual variation in this key physiological trait.

Analysis of the effects of inbreeding on lifespan and starvation resistance in Drosophila melanogaster

Because of their decreased overall fitness and genetic variability inbred individuals are expected to show reduced survival and lifespan under most environmental conditions as compared with outbred individuals. Whereas evidence for the deleterious effects of inbreeding on lifespan has been previously provided, only a few studies have investigated effects of inbreeding on survival under starved conditions. In the present study we compared the abilities of inbred and outbred adult Drosophila melanogaster to survive under starved and fed conditions. We found that inbreeding reduced lifespan but had no effect on starvation resistance. The results indicate highly trait specific consequences of inbreeding. Possible mechanisms behind the observed results are discussed.

Genetic architecture of metabolic rate: environment specific epistasis between mitochondrial and nuclear genes in an insect

The extent to which mitochondrial DNA (mtDNA) variation is involved in adaptive evolutionary change is currently being reevaluated. In particular, emerging evidence suggests that mtDNA genes coevolve with the nuclear genes with which they interact to form the energy producing enzyme complexes in the mitochondria. This suggests that intergenomic epistasis between mitochondrial and nuclear genes may affect whole-organism metabolic phenotypes. Here, we use crossed combinations of mitochondrial and nuclear lineages of the seed beetle Callosobruchus maculatus and assay metabolic rate under two different temperature regimes. Metabolic rate was affected by an interaction between the mitochondrial and nuclear lineages and the temperature regime. Sequence data suggests that mitochondrial genetic variation has a role in determining the outcome of this interaction. Our genetic dissection of metabolic rate reveals a high level of complexity, encompassing genetic interactions over two genomes, and genotype × genotype × environment interactions. The evolutionary implications of these results are twofold. First, because metabolic rate is at the root of life histories, our results provide insights into the complexity of life-history evolution in general, and thermal adaptation in particular. Second, our results suggest a mechanism that could contribute to the maintenance of nonneutral mtDNA polymorphism.

Genotype-dependent recovery from acute exposure to heavy metal contamination in the freshwater clam Sphaerium novaezelandiae

The ability to recover from environmental perturbations is essential for the sustainability of ecological systems. Variation in the ability of individual organisms to recover from stressors influences overall resilience at higher levels of biological organisation. Such variation is likely to be genetically based. To investigate this hypothesis we examined the genetic basis of both resistance to and recovery from zinc, a common stormwater contaminant, in the New Zealand freshwater clam Sphaerium novaezelandiae. We undertook a 4-day toxicity test using zinc exposure concentrations ranging from 0.31 to 5.00 mg/L. These concentrations are consistent with levels recorded in urban streams during the first flush of storms. As our response measures we recorded mortality at the end of the 4-day period, as well as reburial rate (time to rebury in sediment) following the 4-day exposure ("exposure") and then again following a 24h period of recovery ("recovery"). Genotypic composition was determined using allozyme electrophoresis, focusing on the enzyme Pgm (phosphoglucomutase). Overall, a significant effect on mortality was observed, with an average value of 78.6% (+/-7.9) at 5.00 mg/L zinc, compared with only 3.8% (+/-3.8) mortality at 0.31 mg/L zinc. An inhibition concentration (IC(50)) of 1.16 mg/L was recorded, when considered regardless of genotypes. There was no significant genotype-specific differences in mortality. There was a significant difference in reburial rates across all genotypes at the end of the exposure period with an average reburial time of 83.0+/-3.6 min at 5.00 mg/L (22.8+/-2.9 min at 0.31 mg/L). There was a near-significant (p=0.058) difference in time taken to rebury when comparing between genotypes at the "exposure" stage for any concentration. Significant differences in reburial rates across all genotypes were also observed following 24h recovery. When individual genotypes were compared at this stage, genotype 33 reburied on average significantly faster (24.0+/-4.5 min) than other genotypes at the highest exposure concentration and was also significantly faster than genotype 44 at 1.25mg/L. Studies investigating the genetic basis to recovery from stressors at an individual level are limited. This study has shown that populations of organisms display genetically-based variation in their ability to recover from zinc exposure in the laboratory and that such variation is linked to a physiological trait (reburial). The potential effects on other life history traits (e.g. feeding), possible physiological trade-offs and the implications for such variation on ecosystem resilience requires further investigation.

Mating tactics determine patterns of condition dependence in a dimorphic horned beetle

The persistence of genetic variability in performance traits such as strength is surprising given the directional selection that such traits experience, which should cause the fixation of the best genetic variants. One possible explanation is 'genic capture' which is usually considered as a candidate mechanism for the maintenance of high genetic variability in sexual signalling traits. This states that if a trait is 'condition dependent', with expression being strongly influenced by the bearer's overall viability, then genetic variability can be maintained via mutation-selection balance. Using a species of dimorphic beetle with males that gain matings either by fighting or by 'sneaking', we tested the prediction of strong condition dependence for strength, walking speed and testes mass. Strength was strongly condition dependent only in those beetles that fight for access to females. Walking speed, with less of an obvious selective advantage, showed no condition dependence, and testes mass was more condition dependent in sneaks, which engage in higher levels of sperm competition. Within a species, therefore, condition dependent expression varies between morphs, and corresponds to the specific selection pressures experienced by that morph. These results support genic capture as a general explanation for the maintenance of genetic variability in traits under directional selection.

Resting metabolic rate can vary with age independently from body mass changes in the Colorado potato beetle, Leptinotarsa decemlineata

Temperature and mass dependency of insect metabolic rates are well known, while less attention has been given to other factors, such as age. Among insect species that experience seasonal variation in environmental conditions, such as in temperate latitudes, age may also have indirect effects on the metabolic rate. We examined the effect of age on the resting metabolic rate of Leptinotarsa decemlineata during 11 days after adult emergence by using flow-through respirometry. Age had a significant mass-independent effect on metabolic rate of beetles. A twofold increase in metabolic rate occurred during the first 2 days of adult life after which metabolic rate decreased with age relatively slowly. Ten day-old adult beetles had a metabolic rate similar to newly emerged beetles. The beetles have to be able to complete their development and prepare for overwintering during the relatively short favourable summer periods. Therefore, the observed pattern in metabolic rate may reflect physiological changes in the pre-diapause beetles adapted to temperate latitudes.