Environmental dependence of inbreeding depression and purging in Drosophila melanogaster

Elimination or reduction of inbreeding depression by natural selection at the contributing loci (purging) has been hypothesized to effectively mitigate the negative effects of inbreeding in small isolated populations. This may, however, only be valid when the environmental conditions are relatively constant. We tested this assumption using Drosophila melanogaster as a model organism. By means of chromosome balancers, chromosomes were sampled from a wild population and their viability was estimated in both homozygous and heterozygous conditions in a favourable environment. Around 50% of the chromosomes were found to carry a lethal or sublethal mutation, which upon inbreeding would cause a considerable amount of inbreeding depression. These detrimentals were artificially purged by selecting only chromosomes that in homozygous condition had a viability comparable to that of the heterozygotes (quasi-normals), thereby removing most deleterious recessive alleles. Next, these quasi-normals were tested both for egg-to-adult viability and for total fitness under different environmental stress conditions: high-temperature stress, DDT stress, ethanol stress, and crowding. Under these altered stressful conditions, particularly for high temperature and DDT, novel recessive deleterious effects were expressed that were not apparent under control conditions. Some of these chromosomes were even found to carry lethal or near-lethal mutations under stress. Compared with heterozygotes, homozygotes showed on average 25% additional reduction in total fitness. Our results show that, except for mutations that affect fitness under all environmental conditions, inbreeding depression may be due to different loci in different environments. Hence purging of deleterious recessive alleles can be effective only for the particular environment in which the purging occurred, because additional load will become expressed under changing environmental conditions. These results not only indicate that inbreeding depression is environment dependent, but also that inbreeding depression may become more severe under changing stressful conditions. These observations have significant consequences for conservation biology.     

Environmental stress increases selection against and dominance of deleterious mutations in inbred families of the Pacific oyster Crassostrea gigas

The deleterious effects of inbreeding are well documented and of major concern in conservation biology. Stressful environments have generally been shown to increase inbreeding depression; however, little is known about the underlying genetic mechanisms of the inbreeding-by-stress interaction and to what extent the fitness of individual deleterious mutations is altered under stress. Using microsatellite marker segregation data and quantitative trait locus (QTL) mapping methods, I performed a genome scan for deleterious mutations affecting viability (viability or vQTL) in two inbred families of the Pacific oyster Crassostrea gigas, reared in a stressful, nutrient-poor diet and a favourable, nutrient-rich diet, which had significant effects on growth and survival. Twice as many vQTL were detected in the stressful diet compared with the favourable diet, resulting primarily from substantially greater mortality of homozygous genotypes. At vQTL, estimates of selection (s) and dominance (h) were greater in the stressful environment (= 0.86 vs. 0.54 and = 0.35 vs. 0.18, in stressful and nonstressful diets, respectively). There was no evidence of interaction between vQTL. Individual vQTL differed across diets in selection only, or in both selection and dominance, and some vQTL were not affected by diet. These results suggest that stress-associated increases in selection against individual deleterious alleles underlie greater inbreeding depression with stress. Furthermore, the finding that inbreeding-by-environment interaction appears, to some extent, to be locus specific, helps to explain previous observations of lineage-specific expression of inbreeding depression and environment-specific purging, which have important implications for conservation and evolutionary biology.     

Inbreeding-environment interactions for fitness: complex relationships between inbreeding depression and temperature stress in a seed-feeding beetle

It is commonly argued that inbred individuals should be more sensitive to environmental stress than are outbred individuals, presumably because stress increases the expression of deleterious recessive alleles. However, the degree to which inbreeding depression is dependent on environmental conditions is not clear. We use two populations of the seed-feeding beetle, Callosobruchus maculatus, to test the hypotheses that (a) inbreeding depression varies among rearing temperatures, (b) inbreeding depression is greatest at the more stressful rearing temperatures, (c) the degree to which high or low temperature is stressful for larval development varies with inbreeding level, and (d) inbreeding depression is positively correlated between different environments. Inbreeding depression (δ) on larval development varied among temperatures (i.e., there was a significant inbreeding-environment interaction). Positive correlations for degree of inbreeding depression were consistently found between all pairs of temperatures, suggesting that at least some loci affected inbreeding depression across all temperatures examined. Despite variation in inbreeding depression among temperatures, inbreeding depression did not increase consistently with our proxy for developmental stress. However, inbreeding changed which environments are benign versus stressful for beetles; although 20°C was not a stressful rearing temperature for outbred beetles, it became the most stressful environment for inbred larvae. The finding that inbreeding-environment interactions can cause normally benign environments to become stressful for inbred populations has important consequences for many areas of evolutionary genetics, artificial breeding (for conservation or food production), and conservation of natural populations.     

Selection and inbreeding depression: effects of inbreeding rate and inbreeding environment

The magnitude of inbreeding depression in small populations may depend on the effectiveness with which natural selection purges deleterious recessive alleles from populations during inbreeding. The effectiveness of this purging process, however, may be influenced by the rate of inbreeding and the environment in which inbreeding occurs. Although some experimental studies have examined these factors individually, no study has examined their joint effect or potential interaction. In the present study, therefore, we performed an experiment in which 180 lineages of Drosophila melanogaster were inbred at slow and fast inbreeding rates within each of three inbreeding environments (benign, high temperature, and competitive). The fitness of all lineages was then measured in a common benign environment. Although slow inbreeding reduced inbreeding depression in lineages inbred under high temperature stress, a similar reduction was not observed with respect to the benign or competitive treatments. Overall, therefore, the effect of inbreeding rate was nonsignificant. The inbreeding environment, in contrast, had a larger and more consistent effect on inbreeding depression. Under both slow and fast rates of inbreeding, inbreeding depression was significantly reduced in lineages inbred in the presence of a competitor D. melanogaster strain. A similar reduction of inbreeding depression occurred in lineages inbred under high temperature stress at a slow inbreeding rate. Overall, our findings show that inbreeding depression is reduced when inbreeding takes place in a stressful environment, possibly due to more effective purging under such conditions.     

Equivalent inbreeding depression under laboratory and field conditions in a tree-hole-breeding mosquito

Understanding the consequences of inbreeding has important implications for a wide variety of topics in population biology. However, most studies quantifying the effects of inbreeding are performed under artificial farm, greenhouse, laboratory or zoo conditions. Although several authors have argued that the deleterious effects of inbreeding (inbreeding depression) are likely to be more severe under natural field conditions than in artificial experimental environments, these arguments are usually speculative or based on indirect comparisons. We quantified the effects of inbreeding on fitness traits in a tree-hole-breeding mosquito Aedes geniculatus) under near-optimal laboratory conditions and in three natural tree holes. Our index of fitness (Ro) was lower in the field than in the laboratory and declined due to inbreeding in both However, environments, we found no significant interactions between inbreeding depression and environmental conditions. In both the field and laboratory a 10% increase in the inbreeding coefflicient (F) led to a 12-15) decline in fitness (Ro) These results suggest that inbreeding depression will not necessarily be more extreme under natural field conditions than in the laboratory.     

Seasonal stress drives predictable changes in inbreeding depression in field-tested captive populations of Drosophila melanogaster

Recent meta-analyses conducted across a broad range of taxa have demonstrated a strong linear relationship between the change in magnitude of inbreeding depression under stress and stress level, measured as fitness loss in outbred individuals. This suggests that a general underlying response may link stress and inbreeding depression. However, this relationship is based primarily on laboratory data, and it is unknown whether natural environments with multiple stressors and fluctuating stress levels alter how stress affects inbreeding depression. To test whether the same pattern persists in the field, we investigated the effect of seasonal variation on stress level and inbreeding depression in a 3-year field study measuring the productivity of captive populations of inbred and outbred Drosophila melanogaster. We found cold winter temperatures were most stressful and induced the greatest inbreeding depression. Furthermore, these data, collected under natural field conditions, conformed to the same predictive linear relationship seen in Drosophila laboratory studies, with inbreeding depression increasing by 0.17 lethal equivalents for every 10 per cent increase in stress level. Our results suggest that under natural conditions stress level is a primary determinant of the magnitude of inbreeding depression and should be considered when assessing extinction vulnerability in small populations.     

Trait specific consequences of fast and slow inbreeding: lessons from captive populations of <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

The increased homozygosity due to inbreeding leads to expression of deleterious recessive alleles, which may cause inbreeding depression in small populations. The severity of inbreeding depression has been suggested to depend on the rate of inbreeding, with slower inbreeding being more effective in purging deleterious alleles of smaller effect. The effectiveness of purging is however dependent on various factors such as the effect of the deleterious, recessive alleles, the genetic background of inbreeding depression and the environment in which purging occurs. Investigations have shown inconclusive results as to whether purging efficiently diminish inbreeding depression. Here we used an ecologically relevant inbreeding coefficient (f ≈ 0.25) and generated ten slow and ten fast inbred lines of Drosophila melanogaster by keeping the effective population size constant at respectively 32 and 2 for 19 or 2 generations. These inbred lines were contrasted to non-inbred control lines. We investigated the effect of inbreeding and inbreeding rate in traits associated with fitness including heat, cold and desiccation stress resistance, egg-to-adult viability, development time, productivity, metabolic rate and wet weight under laboratory conditions. The results showed highly trait specific consequences of inbreeding and generally no support for the hypothesis that slow inbreeding is less deleterious than fast inbreeding. Egg-to-adult viability and development time were investigated under both benign and heat stress conditions. Reduced viability and increased developmental time were observed at stressful temperatures and inbreeding depression was on average more severe at stressful compared to benign temperatures.     

Stress and adaptation in conservation genetics

Stress, adaptation and evolution are major concerns in conservation biology. Stresses from pollution, climatic changes, disease etc. may affect population persistence. Further, stress typically occurs when species are placed in captivity. Threatened species are usually managed to conserve their ability to adapt to environmental changes, whilst species in captivity undergo adaptations that are deleterious upon reintroduction into the wild. In model studies using Drosophila melanogaster, we have found that; (a) inbreeding and loss of genetic variation reduced resistance to the stress of disease, (b) extinction rates under inbreeding are elevated by stress, (c) adaptive evolutionary potential in an increasingly stressful environment is reduced in small population, (d) rates of inbreeding are elevated under stressful conditions, (e) genetic adaptation to captivity reduces fitness when populations are reintroduced into the 'wild', and (f) the deleterious effects of adaptation on reintroduction success can be reduced by population fragmentation.     

INBREEDING DEPRESSION INCREASES WITH ENVIRONMENTAL STRESS: AN EXPERIMENTAL STUDY AND META‐ANALYSIS

Inbreeding–environment interactions occur when inbreeding leads to differential fitness loss in different environments. Inbred individuals are often more sensitive to environmental stress than are outbred individuals, presumably because stress increases the expression of deleterious recessive alleles or cellular safeguards against stress are pushed beyond the organism's physiological limits. We examined inbreeding–environment interactions, along two environmental axes (temperature and rearing host) that differ in the amount of developmental stress they impose, in the seed‐feeding beetle Callosobruchus maculatus. We found that inbreeding depression (inbreeding load, L) increased with the stressfulness of the environment, with the magnitude of stress explaining as much as 66% of the variation in inbreeding depression. This relationship between L and developmental stress was not explainable by an increase in phenotypic variation in more stressful environments. To examine the generality of this experimental result, we conducted a meta‐analysis of the available data from published studies looking at stress and inbreeding depression. The meta‐analysis confirmed that the effect of the environment on inbreeding depression scales linearly with the magnitude of stress; a population suffers one additional lethal equivalent, on average, for each 30% reduction in fitness induced by the stressful environment. Studies using less‐stressful environments may lack statistical power to detect the small changes in inbreeding depression. That the magnitude of inbreeding depression scales with the magnitude of the stress applied has numerous repercussions for evolutionary and conservation genetics and may invigorate research aimed at finding the causal mechanism involved in such a relationship.     

The effects of inbreeding and heat stress on male sterility in Drosophila melanogaster

Understanding the consequences of inbreeding in combination with stress is important for the persistence of small endangered populations in a changing environment. Inbreeding and stress can influence the population at all stages of the life cycle, and in the last two decades a number of studies have demonstrated inbreeding depression for most life‐cycle components, both in laboratory populations and in the wild. Although male fertility is known to be sensitive to temperature extremes, few studies have focused on this life‐cycle component. We studied the effects of inbreeding on male sterility in benign and stressful environments using Drosophila melanogaster as a model organism. Male sterility was compared in 21 inbred lines and five non‐inbred control lines at 25.0 and 29.0 °C. The effect of inbreeding on sterility was significant only at 29.0 °C. This stress‐induced increase in sterility indicates an interaction between the effects of inbreeding and high‐temperature stress on male sterility. In addition, the stress‐induced temporary and permanent sterility showed significant positive correlation, as did stress‐induced sterility and the decrease in egg‐to‐adult viability. This suggests that the observed stress‐induced decline in fitness could result from conditionally expressed, recessive deleterious alleles affecting both sterility and viability simultaneously. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 432–442.     

Digest: Stress and inbreeding depression in Drosophila melanogaster*

Do stressful conditions exacerbate inbreeding depression? Using Drosophila melanogaster, Schou et al. (2018) examine the mechanisms underlying the interaction between stress and inbreeding depression. The authors found that gene expression in inbred individuals was highly stochastic under benign conditions, but differential gene expression in inbred individuals was reduced compared to controls under stressful conditions.     

Responses of a legume to inbreeding and the intensity of novel and familiar stresses

It is often assumed that the negative effects of inbreeding on fitness (inbreeding depression, ID) are particularly strong under stressful conditions. However, ID may be relatively mild under types of stress that plant populations have experienced for a long time, because environment‐specific deleterious alleles may already have been purged. We examined the performance of open‐ and self‐pollinated progeny of the short‐lived calcareous grassland plant Anthyllis vulneraria under three intensities of each of five types of stress. Drought, nutrient deficiency, and defoliation were chosen as stresses typical for the habitat of origin, while shade and waterlogging were expected to be novel, unfamiliar stresses for A. vulneraria. The stresses reduced plant biomass by up to 91%, and the responses of the plants were mostly in line with the functional equilibrium hypothesis. There was significant ID in biomass (δ = 0.17), leaf chlorophyll content, and the number of root nodules of the legume, but the magnitude of ID was independent of the stress treatments. In particular, there was no significant interaction between inbreeding and the intensity of any stress type, and ID was not higher under novel than under familiar stresses. In addition, phenotypic plasticity in biomass allocation, leaf functional traits and in root nodulation of the legume to the various stress treatments was not influenced by inbreeding. Our findings do not support the common hypothesis of stronger ID under stressful environments, not even if the stresses are novel to the plants.     

Inbreeding and extinction: The effect of environmental stress and lineage

Human activities are simultaneously decreasing the size of wildlife populations (causing inbreeding) and increasing the level of stress that wildlife populations must face. Inbreeding reduces population fitness and increases extinction risk. However, very little information on the impact of stressful environments on extinction risk under inbreeding is available. We evaluated the impact of full sib inbreeding on extinction risk, using Drosophila melanogaster, in a benign and three stressful environments. The three stressful environments involved the addition to the medium of copper sulfate, methanol or alternating copper sulfate and methanol. There were 128 replicate populations for each of the four treatments. Under inbreeding, extinction rates were significantly higher in all three stressful environments compared with the benign environment. The percent extinct at generation eight (F = 0.826) for the four treatments were: 62.5% in the benign environment, 75.8%in the copper sulfate environment, 82.8% in the methanol environment, and 83.6% in the variable stress environment. However, the extinction rate in the variable stress environment did not differ significantly from the constant stress environments. Highly significant differences, among lineages, in extinction risk were detected. The results of this study indicate that wild populations are more vulnerable to inbreeding than indicated by extrapolation from captive environments.     

Slow inbred lines of <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> express as much inbreeding depression as fast inbred lines under semi-natural conditions

Selection may reduce the deleterious consequences of inbreeding. This may be due to purging of recessive deleterious alleles or balancing selection favouring heterozygote offspring. Such selection is expected to be more efficient at slower compared to at faster rates of inbreeding. In this study we tested the impact of inbreeding and the rate of inbreeding on fitness related traits (egg productivity, egg-to-adult viability, developmental time and behaviour) under cold and benign semi-natural thermal conditions using Drosophila melanogaster as a model organism. We used non-inbred control and slow and fast inbred lines (both with an expected inbreeding level of 0.25). The results show that contrary to expectations the slow inbred lines do not maintain higher average fitness than the fast inbred lines. Furthermore, we found that stressful environmental conditions increased the level of inbreeding depression but the impact of inbreeding rate on the level of inbreeding depression was not affected by the environmental conditions. The results do not support the hypothesis that inbreeding depression is less severe with slow compared to fast rates of inbreeding and illustrate that although selection may be more efficient with slower rates of inbreeding this does not necessary lead to less inbreeding depression.     

Stresses affect inbreeding depression in complex ways: disentangling stress-specific genetic effects from effects of initial size in plants

The magnitude of inbreeding depression (ID) varies unpredictably among environments. ID often increases in stressful environments suggesting that these expose more deleterious alleles to selection or increase their effects. More simply, ID could increase under conditions that amplify phenotypic variation (CV²), e.g., by accentuating size hierarchies among plants. These mechanisms are difficult to distinguish when stress increases both ID and phenotypic variation. We grew in- and outbred progeny of Mimulus guttatus under six abiotic stress treatments (control, waterlogging, drought, nutrient deficiency, copper addition, and clipping) with and without competition by the grass Poa palustris. ID differed greatly among stress treatments with δ varying from 7% (control) to 61% (waterlogging) but did not consistently increase with stress intensity. Poa competition increased ID under nutrient deficiency but not other stresses. Analyzing effects of initial size on performance of outbred plants suggests that under some conditions (low N, clipping) competition increased ID by amplifying initial size differences. In other cases (e.g., high ID under waterlogging), particular environments amplified the deleterious genetic effects of inbreeding suggesting differential gene expression. Interestingly, conditions that increased the phenotypic variability of inbred progeny regularly increased ID whereas variability among outbred progeny showed no relationship to ID. Our study reconciles the stress- and phenotypic variability hypotheses by demonstrating how specific conditions (rather than stress per se) act to increase ID. Analyzing CV² separately in inbred and outbred progeny while including effects of initial plant size improve our ability to predict how ID and gene expression vary across environments.Subject terms: Genetic variation, Evolutionary ecology, Plant ecology     

Hatching asynchrony aggravates inbreeding depression in a songbird (Serinus canaria): An inbreeding–environment interaction

Understanding how the intensity of inbreeding depression is influenced by stressful environmental conditions is an important area of enquiry in various fields of biology. In birds, environmental stress during early development is often related to hatching asynchrony; differences in age, and thus size, impose a gradient in conditions ranging from benign (first hatched chick) to harsh (last hatched chick). Here, we compared the effect of hatching order on growth rate in inbred (parents are full siblings) and outbred (parents are unrelated) canary chicks (Serinus canaria). We found that inbreeding depression was more severe under more stressful conditions, being most evident in later hatched chicks. Thus, consideration of inbreeding‐environment interactions is of vital importance for our understanding of the biological significance of inbreeding depression and hatching asynchrony. The latter is particularly relevant given that hatching asynchrony is a widespread phenomenon, occurring in many bird species. The exact causes of the observed inbreeding‐environment interaction are as yet unknown, but may be related to a decrease in maternal investment in egg contents with laying position (i.e. prehatching environment), or to performance of the chicks during sibling competition and/or their resilience to food shortage (i.e. posthatching environment).     

Whole-genome characterization of embryonic stage inbreeding depression in a selfed loblolly pine family

Inbreeding depression is important in the evolution of plant populations and mating systems. Previous studies have suggested that early-acting inbreeding depression in plants is primarily due to lethal alleles and possibly epistatic interactions. Recent advances in molecular markers now make genetic mapping a powerful tool to study the genetic architecture of inbreeding depression. We describe a genome-wide evaluation of embryonic viability loci in a selfed family of loblolly pine (Pinus taeda L.), using data from AFLP markers from an essentially complete genome map. Locus positions and effects were estimated from segregation ratios using a maximum-likelihood interval mapping procedure. We identified 19 loci showing moderately deleterious to lethal embryonic effects. These loci account for >13 lethal equivalents, greater than the average of 8.5 lethal equivalents reported for loblolly pine. Viability alleles show predominantly recessive action, although potential overdominance occurs at 3 loci. We found no evidence for epistasis in the distribution of pairwise marker correlations or in the regression of fitness on the number of markers linked to deleterious alleles. The predominant role of semilethal alleles in embryonic inbreeding depression has implications for the evolution of isolated populations and for genetic conservation and breeding programs in conifers.     

Environmental dependence of inbreeding depression in cultured Coho salmon (Oncorhynchus kisutch): aggressiveness, dominance and intraspecific competition

We evaluated the effects of inbreeding on traits related to territorial dominance and tested whether the magnitude of inbreeding depression (ID) was modified by social environment in Coho salmon. Evaluation of behaviour in paired contests between juvenile salmon with different inbreeding (low, LI=9.5%; medium, MI=29.6%), did not show significant differences between their capacities for establishing territorial dominance (mean aggressiveness score, LI=20.0+/-22; MI=16.7+/-23 or for feeding attempts, LI=18.3+/-12; MI=21.1+/-12). However, fish with low inbreeding (LI) showed almost twice the aggressive pursuit of fish with medium inbreeding (MI), and had a higher specific growth rate (SGR) in culture (SGR(MI)=1.83+/-0.58; SGR(LI)=2.20+/-0.67). Additionally, we found evidence that the magnitude of ID was modified by social environment: (1) Masking: In small groups of fish (N=20), large dominant fish of MI, cultivated with small subordinate fish of LI, showed the same SGR as dominant fish of LI cultivated with small subordinate fish of MI. (2) Magnifying: A significant effect of ID on juvenile survival was detected only in high-density competitive environments. Thus, the number of lethal equivalents was 2.70 at high-density, and only 0.24 in a low-density environment. Our results show that differences in size associated with territorial dominance may mask deleterious effects of inbreeding under certain conditions, and support the concept that intraspecific competition usually magnifies the deleterious effects of inbreeding.     

Ancestral inbreeding reduces the magnitude of inbreeding depression in Drosophila melanogaster

The influence of natural selection on the magnitude of inbreeding depression is an important issue in conservation biology and the study of evolution. It is generally expected that the magnitude of inbreeding depression in small populations will depend upon the average homozygosity of individuals, as measured by the coefficient of inbreeding (F). However, if deleterious recessive alleles are selectively purged from populations during inbreeding, then inbreeding depression may differ among populations in which individuals have the same inbreeding coefficient. In such cases, the magnitude of inbreeding depression will partly depend on the ancestral inbreeding coefficient (fa), which measures the cumulative proportion of loci that have historically been homozygous and therefore exposed to natural selection. We examined the inbreeding depression that occurred in lineages of Drosophila melanogaster maintained under pedigrees that led to the same inbreeding coefficient (F = 0.375) but different levels of ancestral inbreeding (fa = 0.250 or 0.531). Although inbreeding depression varied substantially among individual lineages, we observed a significant 40% decrease in the median level of inbreeding depression in the treatment with higher ancestral inbreeding. Our results demonstrate that high levels of ancestral inbreeding are associated with greater purging effects, which reduces the inbreeding depression that occurs in isolated populations of small size.