Heterosis is common and inbreeding depression absent in natural populations of Arabidopsis thaliana

The importance of genetic drift in shaping patterns of adaptive genetic variation in nature is poorly known. Genetic drift should drive partially recessive deleterious mutations to high frequency, and inter‐population crosses may therefore exhibit heterosis (increased fitness relative to intra‐population crosses). Low genetic diversity and greater genetic distance between populations should increase the magnitude of heterosis. Moreover, drift and selection should remove strongly deleterious recessive alleles from individual populations, resulting in reduced inbreeding depression. To estimate heterosis, we crossed 90 independent line pairs of Arabidopsis thaliana from 15 pairs of natural populations sampled across Fennoscandia and crossed an additional 41 line pairs from a subset of four of these populations to estimate inbreeding depression. We measured lifetime fitness of crosses relative to parents in a large outdoor common garden (8,448 plants in total) in central Sweden. To examine the effects of genetic diversity and genetic distance on heterosis, we genotyped parental lines for 869 SNPs. Overall, genetic variation within populations was low (median expected heterozygosity = 0.02), and genetic differentiation was high (median F_ST = 0.82). Crosses between 10 of 15 population pairs exhibited significant heterosis, with magnitudes of heterosis as high as 117%. We found no significant inbreeding depression, suggesting that the observed heterosis is due to fixation of mildly deleterious alleles within populations. Widespread and substantial heterosis indicates an important role for drift in shaping genetic variation, but there was no significant relationship between fitness of crosses relative to parents and genetic diversity or genetic distance between populations.     

Inbreeding depression and drift load in small populations at demographic disequilibrium

Inbreeding depression is a major driver of mating system evolution and has critical implications for population viability. Theoretical and empirical attention has been paid to predicting how inbreeding depression varies with population size. Lower inbreeding depression is predicted in small populations at equilibrium, primarily due to higher inbreeding rates facilitating purging and/or fixation of deleterious alleles (drift load), but predictions at demographic and genetic disequilibrium are less clear. In this study, we experimentally evaluate how lifetime inbreeding depression and drift load, estimated by heterosis, vary with census (N_c) and effective (estimated as genetic diversity, H_e) population size across six populations of the biennial Sabatia angularis as well as present novel models of inbreeding depression and heterosis under varying demographic scenarios at disequilibrium (fragmentation, bottlenecks, disturbances). Our experimental study reveals high average inbreeding depression and heterosis across populations. Across our small sample, heterosis declined with H_e, as predicted, whereas inbreeding depression did not vary with H_e and actually decreased with N_c. Our theoretical results demonstrate that inbreeding depression and heterosis levels can vary widely across populations at disequilibrium despite similar H_e and highlight that joint demographic and genetic dynamics are key to predicting patterns of genetic load in nonequilibrium systems.     

Inbreeding depression is high in a self‐incompatible perennial herb population but absent in a self‐compatible population showing mixed mating

High inbreeding depression is thought to be one of the major factors preventing evolutionary transitions in hermaphroditic plants from self‐incompatibility (SI) and outcrossing toward self‐compatibility (SC) and selfing. However, when selfing does evolve, inbreeding depression can be quickly purged, allowing the evolution of complete self‐fertilization. In contrast, populations that show intermediate selfing rates (a mixed‐mating system) typically show levels of inbreeding depression similar to those in outcrossing species, suggesting that selection against inbreeding might be responsible for preventing the transition toward complete self‐fertilization. By implication, crosses among populations should reveal patterns of heterosis for mixed‐mating populations that are similar to those expected for outcrossing populations. Using hand‐pollination crosses, we compared levels of inbreeding depression and heterosis between populations of Linaria cavanillesii (Plantaginaceae), a perennial herb showing contrasting mating systems. The SI population showed high inbreeding depression, whereas the SC population displaying mixed mating showed no inbreeding depression. In contrast, we found that heterosis based on between‐population crosses was similar for SI and SC populations. Our results are consistent with the rapid purging of inbreeding depression in the derived SC population, despite the persistence of mixed mating. However, the maintenance of outcrossing after a transition to SC is inconsistent with the prediction that populations that have purged their inbreeding depression should evolve toward complete selfing, suggesting that the transition to SC in L. cavanillesii has been recent. SC in L. cavanillesii thus exemplifies a situation in which the mating system is likely not at an equilibrium with inbreeding depression.     

Close‐kin mating,but not inbred parents,reduces hatching rates and offspring quality in a threatened tortoise

Inbreeding depression, the reduction in fitness due to mating of related individuals, is of particular conservation concern in species with small, isolated populations. Although inbreeding depression is widespread in natural populations, long‐lived species may be buffered from its effects during population declines due to long generation times and thus are less likely to have evolved mechanisms of inbreeding avoidance than species with shorter generation times. However, empirical evidence of the consequences of inbreeding in threatened, long‐lived species is limited. In this study, we leverage a well‐studied population of gopher tortoises, Gopherus polyphemus, to examine the role of inbreeding depression and the potential for behavioural inbreeding avoidance in a natural population of a long‐lived species. We tested the hypothesis that increased parental inbreeding leads to reduced hatching rates and offspring quality. Additionally, we tested for evidence of inbreeding avoidance. We found that high parental relatedness results in offspring with lower quality and that high parental relatedness is correlated with reduced hatching success. However, we found that hatching success and offspring quality increase with maternal inbreeding, likely due to highly inbred females mating with more distantly related males. We did not find evidence for inbreeding avoidance in males and outbred females, suggesting sex‐specific evolutionary trade‐offs may have driven the evolution of mating behaviour. Our results demonstrate inbreeding depression in a long‐lived species and that the evolution of inbreeding avoidance is shaped by multiple selective forces.     

Limited inbreeding depression in a bottlenecked population is age but not environment dependent

Inbreeding depression is known to vary greatly between populations and among species. Some of this variation is due to differences in genetic load between populations, while some is due to differences in the environment (e.g. local weather conditions) or demography of the population (e.g. age structure and breeding experience) in which inbreeding is expressed. Although the effects of these factors in isolation are well understood, there is still relatively little known about the interface between inbreeding on one hand, and environment and demography on the other in wild populations. We examined how environmental and demographic factors mediated the effects of inbreeding in a threatened species of bird. The Stewart Island robin, Petroica australis rakiura, has been subjected to a prolonged bottleneck for over 150 years. A complete pedigree of a reintroduced island population, extending back seven seasons to its founding, was available for analysis along with survival data (at the level of the brood) obtained from intensive monitoring over two breeding seasons. We found no strong support that the degree to which a brood was inbred affected its survival at either the hatching, fledging or recruitment stages. The inbreeding coefficient of the mother did have an effect on brood survival when analysed over all three life history stages, but only as a result of an interaction with female age, with broods of one‐year‐old inbred females suffering greater mortality than those of older inbred females. Although habitat type, temperature, rainfall and year were the best predictors of brood survival for most life history stages, their effects were weak and there were no interactions with inbreeding. Furthermore, there was no strong evidence of inbreeding depression associated with two periods of severe weather. This population is atypical in that inbreeding depression appears to be weak even under severe environmental conditions, and may be indicative that this bottlenecked population has either reduced genetic load or has fixed deleterious alleles.     

Reduced population size can induce quick evolution of inbreeding depression in the invasive ladybird <Emphasis Type="Italic">Harmonia axyridis</Emphasis>

Understanding biological invasion is currently one of the main scientific challenges for ecologists. The introduction process is crucial for the success of an invasion, especially when it involves a demographic bottleneck. A small introduced population is expected to face a higher risk of extinction before the first stage of invasion is complete if inbreeding depression, caused by the expression of deleterious alleles, is important. Changes in mating regimes or in population size can induce the evolution of deleterious allele frequencies, either by selection or by drift, possibly resulting in the purging or the fixation of such alleles within the population. The harlequin ladybird Harmonia axyridis became invasive on several continents following a scenario including at least one event of demographic bottleneck. Although native populations suffered from severe inbreeding depression, it was greatly reduced in invasive ones suggesting that deleterious alleles were purged during the invasion process. In this study, we performed an experiment designed to manipulate the effective population size of H. axyridis across successive generations to mimic contrasting introduction events. We used the measurement of two fitness-related phenotypic traits in order to test (1) if inbreeding depression can evolve at the time-scale of an invasion; and (2) if the changes in inbreeding depression following a bottleneck in laboratory conditions are compatible with the purging of deleterious alleles observed in this species. We found that two generations of very low population size are enough to induce a substantial change in inbreeding depression. Although the genetic changes mostly consisted in fixation of deleterious alleles, purging did also occur, sometimes simultaneously with fixation.     

The different sources of variation in inbreeding depression, heterosis and outbreeding depression in a metapopulation of Physa acuta

Understanding how parental distance affects offspring fitness, i.e., the effects of inbreeding and outbreeding in natural populations, is a major goal in evolutionary biology. While inbreeding is often associated with fitness reduction (inbreeding depression), interpopulation outcrossing may have either positive (heterosis) or negative (outbreeding depression) effects. Within a metapopulation, all phenomena may occur with various intensities depending on the focal population (especially its effective size) and the trait studied. However, little is known about interpopulation variation at this scale. We here examine variation in inbreeding depression, heterosis, and outbreeding depression on life-history traits across a full-life cycle, within a metapopulation of the hermaphroditic snail Physa acuta. We show that all three phenomena can co-occur at this scale, although they are not always expressed on the same traits. A large variation in inbreeding depression, heterosis, and outbreeding depression is observed among local populations. We provide evidence that, as expected from theory, small and isolated populations enjoy higher heterosis upon outcrossing than do large, open populations. These results emphasize the need for an integrated theory accounting for the effects of both deleterious mutations and genetic incompatibilities within metapopulations and to take into account the variability of the focal population to understand the genetic consequences of inbreeding and outbreeding at this scale.     

Inbreeding depression by recessive deleterious genes affecting female fecundity of a haplo‐diploid mite

The effect of inbreeding on haplo‐diploid organisms has been regarded as very low, because deleterious recessive genes on hemizygous (haploid) males were immediately purged generation by generation. However, we determined such recessive genes to decrease female fecundity in a population of Schizotetranychus miscanthi Saito which is known in the Acari as a subsocial species with haplo‐diploidy. In mother–son inbreeding experiments, there was no depression in egg hatchability nor in the larval survival of progeny over four generations. There was, on the other hand, significant inbreeding depression in the fecundity with increasing f‐value. Crosses between two lineages, one having deleterious effects on the fecundity and the other having no such effects, established during the inbreeding, revealed heterosis, and backcrosses showed that the depression was caused by deleterious recessive(s). These results strongly suggest the existence of some deleterious genes governing only the traits of adult females in wild populations of haplo‐diploid organisms.     

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.     

Investigating the genetic load of an emblematic invasive species: the case of the invasive harlequin ladybird Harmonia axyridis

Introduction events can lead to admixture between genetically differentiated populations and bottlenecks in population size. These processes can alter the adaptive potential of invasive species by shaping genetic variation, but more importantly, they can also directly affect mean population fitness either increasing it or decreasing it. Which outcome is observed depends on the structure of the genetic load of the species. The ladybird Harmonia axyridis is a good example of invasive species where introduced populations have gone through admixture and bottleneck events. We used laboratory experiments to manipulate the relatedness among H. axyridis parental individuals to assess the possibility for heterosis or outbreeding depression in F₁ generation offspring for two traits related to fitness (lifetime performance and generation time). We found that inter‐populations crosses had no major impact on the lifetime performance of the offspring produced by individuals from either native or invasive populations. Significant outbreeding depression was observed only for crosses between native populations for generation time. The absence of observed heterosis is indicative of a low occurrence of fixed deleterious mutations within both the native and invasive populations of H. axyridis. The observed deterioration of fitness in native inter‐population crosses most likely results from genetic incompatibilities between native genomic backgrounds. We discuss the implications of these results for the structure of genetic load in H. axyridis in the light of the available information regarding the introduction history of this species.     

EFFECTIVENESS OF SELECTION IN REDUCING THE GENETIC LOAD IN POPULATIONS OF PEROMYSCUS POLIONOTUS DURING GENERATIONS OF INBREEDING

It has been hypothesized that natural selection reduces the “genetic load” of deleterious alleles from populations that inbreed during bottlenecks, thereby ameliorating impacts of future inbreeding. We tested the efficiency with which natural selection purges deleterious alleles from three subspecies of Peromyscus polionotus during 10 generations of laboratory inbreeding by monitoring pairing success, litter size, viability, and growth in 3604 litters produced from 3058 pairs. In P. p. subgriseus, there was no reduction across generations in inbreeding depression in any of the fitness components. Strongly deleterious recessive alleles may have been removed previously during episodes of local inbreeding in the wild, and the residual genetic load in this population was not further reduced by selection in the lab. In P. p. rhoadsi, four of seven fitness components did show a reduction of the genetic load with continued inbreeding. The average reduction in the genetic load was as expected if inbreeding depression in this population is caused by highly deleterious recessive alleles that are efficiently removed by selection. For P. p. leucocephalus a population that experiences periodic bottlenecks in the wild, the effect of further inbreeding in the laboratory was to exacerbate rather than reduce the genetic load. Recessive deleterious alleles may have been removed from this population during repeated bottlenecks in the wild; the population may be close to a threshold level of heterozygosity below which fitness declines rapidly. Thus, the effects of selection on inbreeding depression varied substantially among populations, perhaps due to different histories of inbreeding and selection.     

On the expected relationship between inbreeding,fitness, and extinction

We assessed the expected relationship between the level and the cost of inbreeding, measured either in terms of fitness, inbreeding depression or probability of extinction. First, we show that the assumption of frequent, slightly deleterious mutations do agree with observations and experiments, on the contrary to the assumption of few, moderately deleterious mutations. For the same inbreeding coefficient, populations can greatly differ in fitness according to the following: (i) population size; larger populations show higher fitness (ii) the history of population size; in a population that recovers after a bottleneck, higher inbreeding can lead to higher fitness and (iii) population demography; population growth rate and carrying capacity determine the relationship between inbreeding and extinction. With regards to the relationship between inbreeding depression and inbreeding coefficient, the population size that minimizes inbreeding depression depends on the level of inbreeding: inbreeding depression can even decrease when population size increases. It is therefore clear that to infer the costs of inbreeding, one must know both the history of inbreeding (e.g. past bottlenecks) and population demography.     

Inbreeding and inbreeding depression in endangered red wolves (Canis rufus)

In natural populations, the expression and severity of inbreeding depression can vary widely across taxa. Describing processes that influence the extent of inbreeding and inbreeding depression aid in our understanding of the evolutionary history of mating systems such as cooperative breeding and nonrandom mate selection. Such findings also help shape wildlife conservation theory because inbreeding depression reduces the viability of small populations. We evaluated the extent of inbreeding and inbreeding depression in a small, re‐introduced population of red wolves (Canis rufus) in North Carolina. Since red wolves were first re‐introduced in 1987, pedigree inbreeding coefficients (f) increased considerably and almost every wild born wolf was inbred (average f = 0.154 and max f = 0.383). The large inbreeding coefficients were due to both background relatedness associated with few founders and numerous close relative matings. Inbreeding depression was most evident for adult body size and generally absent for direct fitness measures such as reproductive success and survival; no lethal equivalents (LE = 0.00) were detected in juvenile survival. The lack of strong inbreeding depression in direct measures of fitness could be due to a founder effect or because there were no outbred individuals for comparison. Our results highlight the variable expression of inbreeding depression across traits and the need to measure a number of different traits when evaluating inbreeding depression in a wild population.     

Inbreeding depression for various traits in two cultured populations of the American bay scallop, Argopecten irradians irradians Lamarck (1819) introduced into China

This paper examines the effect of inbreeding level of population on the magnitude of inbreeding depression expressed by comparing them between two cultured populations (A and B) in the hermaphroditic animal of the bay scallop Argopecten irradians irradians. Population A is expected to have less genetic variations and higher inbreeding level due to longer cultured history (20 generations) and less “ancestral” individuals (26 individuals) than population B due to shorter cultured history (4 generations) and more “ancestral” individuals (406 individuals). Two groups within each population were produced, one using self-fertilization and one using mass-mating within the same population. Selfed offspring (AS and BS) from two populations both had lower fitness components than their mass-mated counterparts (AM and BM) and exhibited inbreeding depression for all examined traits, e.g. lower hatching, less viability and slower growth, indicating that inbreeding depression is a common feature in this animal. Fitness components in all traits of offspring from population A significantly differed those from population B and the magnitude of inbreeding depression for all traits in population A with higher inbreeding level was significantly smaller than that in population B with lower inbreeding level, indicating that both fitness components and magnitude of inbreeding depression were significantly affected by inbreeding level of populations and genetic load harbored in population A may be partially purged through inbreeding. Moreover, the magnitude of inbreeding depression in the two populations both varied among traits and life history stages. The present results support the partial-dominance hypothesis of inbreeding depression.     

On the genetic parameter determining the efficiency of purging: an estimate for Drosophila egg‐to‐pupae viability

The consequences of inbreeding on fitness can be crucial in evolutionary and conservation grounds and depend upon the efficiency of purging against deleterious recessive alleles. Recently, analytical expressions have been derived to predict the evolution of mean fitness, taking into account both inbreeding and purging, which depend on an ‘effective purging coefficient (d_e)’. Here, we explore the validity of that predictive approach and assay the strength of purging by estimating d_e for egg‐to‐pupae viability (EPV) after a drastic reduction in population size in a recently captured base population of Drosophila melanogaster. For this purpose, we first obtained estimates of the inbreeding depression rate (δ) for EPV in the base population, and we found that about 40% was due to segregating recessive lethals. Then, two sets of lines were founded from this base population and were maintained with different effective size throughout the rest of the experiment (N = 6; N = 12), their mean EPV being assayed at different generations. Due to purging, the reductions in mean EPV experienced by these lines were considerably smaller than the corresponding neutral predictions. For the 60% of δ attributable to nonlethal deleterious alleles, our results suggest an effective purging coefficient d_e > 0.02. Similarly, we obtain that d_e > 0.09 is required to roughly account for purging against the pooled inbreeding depression from lethal and nonlethal deleterious alleles. This implies that purging should be efficient for population sizes of the order of a few tens and larger, but might be inefficient against nonlethal deleterious alleles in smaller populations.     

Inbreeding at the edge: does inbreeding depression increase under more stressful conditions?

Edge populations are frequently small and subject to stressful conditions that may compromise their long‐term viability. Inbreeding can play an important role in small populations by reducing genetic diversity, leading to the fixation of deleterious mutations and, finally, carrying populations to an extinction vortex through inbreeding depression. Although stressful conditions may enhance the intensity of inbreeding depression, evidence to date is inconclusive in marginal habitats. Local adaptation, promoting native genotypes, and gene flow, reducing allele fixation, are two factors that can have different effects on the intensity of inbreeding depression. Three populations of Silene ciliata distributed across an elevation gradient at the southernmost edge of the species distribution were used for this study. Several fitness components – germination, survival and growth rate – were compared between inbred seedlings and seedlings from within‐ and between‐population outcrosses, both in the field and controlled conditions. Overall, inbred seedlings had lower fitness than outcrossed seedlings. For most of the variables analysed, similar inbreeding depression effects were found in all three populations, but, for seed weight and seedling survival curve, inbreeding depression was only found in the low altitude population. Similarly, inbreeding depression was more evident in the field than in controlled chamber conditions. Outcrosses between populations contributed to an increase in most fitness estimates and populations, suggesting that the benefits of reducing inbreeding depression overrode the potentially deleterious effects of disrupting local adaptation. Our results suggest that inbreeding depression plays an important role in the fitness of early life stages of Silene ciliata at its southernmost distribution limit, but only provided partial support to the hypothesis that stressful conditions enhance the expression of inbreeding depression.     

Effects of inbreeding and temperature stress on life history and immune function in a butterfly

Theory predicts that inbreeding depression should be more pronounced under environmental stress due to an increase in the expression of recessive deleterious alleles. If so, inbred populations may be especially vulnerable to environmental change. Against this background, we here investigate effects of inbreeding, temperature stress and its interactions with inbreeding in the tropical butterfly Bicyclus anynana. We use a full‐factorial design with three levels of inbreeding (F = 0/0.25/0.38) and three temperature treatments (2 h exposure to 1, 27 or 39 °C). Despite using relatively low levels of inbreeding significant inbreeding depression was found in pupal mass, pupal time, thorax mass, abdomen fat content, egg hatching success and fecundity. However, stress resistance traits (heat tolerance, immune function) were not affected by inbreeding and interactions with temperature treatments were virtually absent. We thus found no support for an increased sensitivity of inbred individuals to environmental stress, and suspect that such patterns are restricted to harsher conditions. Our temperature treatments evidently imposed stress, significantly reducing longevity, fecundity, egg hatching success and haemocyte numbers, while fat content, protein content and lysozyme activity remained unaffected. Males and females differed in all traits measured except pupal time, protein content and phenoloxidase (PO) activity. Correlation analyses revealed, among others, a trade‐off between PO and lysozyme activity, and negative correlations between fat content and several other traits. We stress that more data are needed on the effects of inbreeding, temperature variation and sexual differences on insect immune function before more general conclusions can be drawn.     

Environmental conditions during early life determine the consequences of inbreeding in Agrostemma githago (Caryophyllaceae)

In an inbred population, selection may reduce the frequency of deleterious recessive alleles through a process known as purging. Empirical studies suggest, however, that the efficacy of purging in natural populations is highly variable. This variation may be due, in part, to variation in the expression of inbreeding depression available for selection to act on. This experiment investigates the roles of life stage and early‐life environment in determining the expression of inbreeding depression in Agrostemma githago. Four population‐level crosses (‘self’, ‘within’, ‘near’ and ‘far’) were conducted on 20 maternal plants from a focal population. Siblings were planted into one of three early environmental treatments with varying stress levels. Within the focal population, evidence for purging of deleterious recessive alleles, as well as for variation in the expression of inbreeding depression across the life cycle was examined. In addition, the effect of early environment on the expression of inbreeding depression and the interaction with cross‐type was measured. We find that deleterious recessive alleles have not been effectively purged from our focal population, the expression of inbreeding depression decreases over the course of the life cycle, and a stressful early environment reduces the variance in inbreeding depression expressed later in life, but does not consistently influence the relative fitness of inbred versus outcrossed individuals.     

Inbreeding depression of female fecundity by genetic factors retained in natural populations of a male-haploid social mite (Acari: Tetranychidae)

We previously determined that certain recessive genes decrease female fecundity in a haplo-diploid spider mite, Stigmaeopsis miscanthi (Saito). However, whether the depression was caused by the breakdown of heterosis or the expression of deleterious genes retained in a population could not be determined, because we had started our inbreeding experiment from a mixture of two isolated populations. In order to answer this basic question, inbreeding effects on survival and fecundity were measured for eight small populations occurring far from the two initial populations. There was little depression of immature survival of inbred lineages in all populations. On the other hand, in two inbred lineages, both originating from the smallest populations, female oviposition decreased significantly with the increase of Wrights f-value, showing that mildly deleterious genes are actually retained even in natural populations of haplo-diploid organisms.     

Inbreeding depression of female fecundity by genetic factors retained in natural populations of a male-haploid social mite (Acari: Tetranychidae)

We previously determined that certain recessive genes decrease female fecundity in a haplo-diploid spider mite, Stigmaeopsis miscanthi (Saito). However, whether the depression was caused by the breakdown of heterosis or the expression of deleterious genes retained in a population could not be determined, because we had started our inbreeding experiment from a mixture of two isolated populations. In order to answer this basic question, inbreeding effects on survival and fecundity were measured for eight small populations occurring far from the two initial populations. There was little depression of immature survival of inbred lineages in all populations. On the other hand, in two inbred lineages, both originating from the smallest populations, female oviposition decreased significantly with the increase of Wrights f-value, showing that mildly deleterious genes are actually retained even in natural populations of haplo-diploid organisms.