Testing alternative methods for purging genetic load using the housefly (Musca domestica L.)

When a population faces long-term inbreeding, artificial selection, in principle, can enhance natural selection processes for purging the exposed genetic load. However, strong purge pressures might actually decrease fitness through the inadvertent fixation of deleterious alleles and allelic combinations. We tested lines of the housefly (Musca domestica L.) for the effectiveness of artificial selection to promote the adaptation to small population size. Specifically, replicate populations were held at average census sizes of 54 for nine generations or 30 for 14 generations while being subjected to artificial selection pressure for increased fitness in overall mating propensity (i.e., the proportion of virgin male–female pairs initiating copulation within 30 min), while also undergoing selection to create differences among lines in multivariate components of courtship performance. In the 14-generation experiment, a subset of the lines were derived from a founder-flush population (i.e., derived from three male–female pairs). In both experiments, we also maintained parallel non-selection lines to assess the potential for natural purging through serial inbreeding alone. Sub-populations derived from a stock newly derived from the wild responded to artificial selection for increased mating propensity, but only in the short-term, with eventual rebounds back to the original levels. Serial inbreeding in these lines simply reduced mating propensity. In sub-populations derived from the same base population, but 36 generations later, both artificial selection and serial inbreeding increased mating propensity, but mainly to restore the level found upon establishment in the laboratory. Founder-flush lines responded as well as the non-bottlenecked controls, so we base our major conclusions on the comparisons between fresh-caught and long-term laboratory stocks. We suggest that the effectiveness of the alternative purge protocols depended upon the amount of genetic load already exposed, such that prolonged periods of relaxed or altered selection pressures of the laboratory rendered a population more responsive to purging protocols.     

Long-term exhaustion of the inbreeding load in Drosophila melanogaster

Inbreeding depression, the decline in fitness of inbred individuals, is a ubiquitous phenomenon of great relevance in evolutionary biology and in the fields of animal and plant breeding and conservation. Inbreeding depression is due to the expression of recessive deleterious alleles that are concealed in heterozygous state in noninbred individuals, the so-called inbreeding load. Genetic purging reduces inbreeding depression by removing these alleles when expressed in homozygosis due to inbreeding. It is generally thought that fast inbreeding (such as that generated by full-sib mating lines) removes only highly deleterious recessive alleles, while slow inbreeding can also remove mildly deleterious ones. However, a question remains regarding which proportion of the inbreeding load can be removed by purging under slow inbreeding in moderately large populations. We report results of two long-term slow inbreeding Drosophila experiments (125–234 generations), each using a large population and a number of derived lines with effective sizes about 1000 and 50, respectively. The inbreeding load was virtually exhausted after more than one hundred generations in large populations and between a few tens and over one hundred generations in the lines. This result is not expected from genetic drift alone, and is in agreement with the theoretical purging predictions. Computer simulations suggest that these results are consistent with a model of relatively few deleterious mutations of large homozygous effects and partially recessive gene action.Subject terms: Quantitative trait, Inbreeding     

Effects of population structures and selection strategies on the purging of inbreeding depression due to deleterious mutations

Stochastic simulations were run to compare the effects of nine breeding schemes, using full-sib mating, on the rate of purging of inbreeding depression due to mutations with equal deleterious effect on viability at unlinked loci in an outbred population. A number of full-sib mating lines were initiated from a large outbred population and maintained for 20 generations (if not extinct). Selection against deleterious mutations was allowed to occur within lines only, between lines or equal within and between lines, and surviving lines were either not crossed or crossed following every one or three generations of full-sib mating. The effectiveness of purging was indicated by the decreased number of lethal equivalents and the increased fitness of the purged population formed from crossing surviving lines after 20 generations under a given breeding scheme. The results show that the effectiveness of purging, the survival of the inbred lines and the inbreeding level attained are generally highest with between-line selection and lowest with within-line selection. Compared with no crossing, line crossing could lower the risk of extinction and the inbreeding coefficient of the purged population substantially with little loss of the effectiveness of purging. Compromising between the effectiveness of purging, and the risk of extinction and inbreeding coefficient, the breeding scheme with equal within- and between-line selection and crossing alternatively with full-sib mating is generally the most desirable scheme for purging deleterious mutations. Unless most deleterious mutations have relatively large effects on fitness in species with reproductive ability high enough to cope with the depressed fitness and thus increased risk of extinction with inbreeding, it is not justified to apply a breeding programme aimed at purging inbreeding depression by inbreeding and selection to a population of conservation concern.     

Perspective: purging the genetic load: a review of the experimental evidence

Inbreeding depression, the reduction in fitness that accompanies inbreeding, is one of the most important topics of research in evolutionary and conservation genetics. In the recent literature, much attention has been paid to the possibility of purging the genetic load. If inbreeding depression is due to deleterious alleles, whose effect on fitness are negative when in a homozygous state, then successive generations of inbreeding may result in a rebound in fitness due to the selective decrease in frequency of deleterious alleles. Here we examine the experimental evidence for purging of the genetic load by collating empirical tests of rebounds in fitness-related traits with inbreeding in animals and plants. We gathered data from 28 studies including five mammal, three insect, one mollusc, and 13 plant species. We tested for purging by examining three measures of fitness-component variation with serial generations of inbreeding: (1) changes in inbreeding depression, (2) changes in fitness components of inbred lines relative to the original outbred line, and (3) purged population (outcrossed inbred lines) trait means as a function of ancestral outbred trait means. Frequent and substantial purging was found using all three measures, but was particularly pronounced when tracking changes in inbreeding depression. Despite this, we found little correspondence between the three measures of purging within individual studies, indicating that the manner in which a researcher chooses to estimate purging will affect interpretation of the results obtained. The discrepancy suggests an alternative hypothesis: rebounds in fitness with inbreeding may have resulted from adaptation to laboratory conditions and not to purging when using outcrossed inbred lines. However, the pronounced reduction in inbreeding depression for a number of studies provides evidence for purging, as the measure is likely less affected by selection for laboratory conditions. Unlike other taxon-specific reviews on this topic, our results provide support for the purging hypothesis, but firm predictions about the situations in which purging is likely or the magnitude of fitness rebound possible when populations are inbred remain difficult. Further research is required to resolve the discrepancy between the results obtained using different experimental approaches.     

Inbreeding rate modifies the dynamics of genetic load in small populations

The negative fitness consequences of close inbreeding are widely recognized, but predicting the long-term effects of inbreeding and genetic drift due to limited population size is not straightforward. As the frequency and homozygosity of recessive deleterious alleles increase, selection can remove (purge) them from a population, reducing the genetic load. At the same time, small population size relaxes selection against mildly harmful mutations, which may lead to accumulation of genetic load. The efficiency of purging and the accumulation of mutations both depend on the rate of inbreeding (i.e., population size) and on the nature of mutations. We studied how increasing levels of inbreeding affect offspring production and extinction in experimental Drosophila littoralis populations replicated in two sizes, N = 10 and N = 40. Offspring production and extinction were measured over 25 generations concurrently with a large control population. In the N = 10 populations, offspring production decreased strongly at low levels of inbreeding, then recovered only to show a consistent subsequent decline, suggesting early expression and purging of recessive highly deleterious alleles and subsequent accumulation of mildly harmful mutations. In the N = 40 populations, offspring production declined only after inbreeding reached higher levels, suggesting that inbreeding and genetic drift pose a smaller threat to population fitness when inbreeding is slow. Our results suggest that highly deleterious alleles can be purged in small populations already at low levels of inbreeding, but that purging does not protect the small populations from eventual genetic deterioration and extinction.     

Population type can influence the magnitude of inbreeding depression in Clarkia concinna (Onagraceae)

Fragmented populations may face high risk of extinction due to the deleterious consequences of increased inbreeding or of genetic drift in small and isolated populations. Theories on the mechanisms of inbreeding depression predict that the severity of inbreeding depression can eventually decrease in populations that persistently inbreed, and hence populations that are isolated through habitat fragmentation might experience a decrease in inbreeding depression over time. In this study, we tested this hypothesis using the patchily distributed, outcrossing annual plant, Clarkia concinna concinna (Onagraceae), which naturally experiences many fragmentation effects. We collected seeds from isolated and central subpopulations and created artificially inbred and outcrossed lines. Progeny from these crosses were planted into the field and greenhouse and assayed for fitness traits over the course of a growing season. Overall, inbreeding depression was substantial, ranging as high as 0.76 (for cumulative fitness in the field), and significant for plant height, fecundity, and above-ground biomass in all experiments. No inbreeding depression was detected for germination or survival rates in the greenhouse experiments, but in the field, survival was significantly depressed for inbred progeny. There was no evidence to support our hypothesis that increased inbreeding in isolated populations would lead to the purging of deleterious alleles and a decrease in the severity inbreeding depression. The most likely hypothesis to explain our results is that purging is not occurring more strongly in the isolated populations due to details of a number of genetic factors (e.g., selection against deleterious alleles is inconsistent or insufficient, or drift has caused fixation of deleterious alleles in these populations). This study supports the view that even when inbreeding depression is predicted to be less problematic, it may still be an important force influencing the fitness of populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of population size on performance and inbreeding depression in <Emphasis Type="Italic">Lupinus perennis</Emphasis>

We investigated the relationships among population size, offspring performance, and inbreeding depression (δ) in Lupinus perennis by examining the effect of population size category (large vs. small) on seed production and offspring performance for three pollination treatments (open pollination, hand crossing and hand selfing). In each of our four pairs of populations, one member of the pair was substantially larger than the other. We then grew seeds from this factorial design (2 sizes × 4 pairs × 3 pollination treatments) in the greenhouse to investigate whether population size affects offspring performance in a common environment, and how small size affects purging of the inbreeding load. Multiplicative performance across four early life-stage components (seed production, seedling emergence, seedling survival and seedling growth) of smaller populations was not significantly lower, although biomass of seedlings declined in smaller populations. Self-pollination reduced seed production, seedling emergence and seedling growth, reflecting substantial inbreeding depression (δ = 0.404 ± 0.043). Population size categories did not consistently differ in levels of inbreeding depression, suggesting that purging of genetic load in smaller populations has been limited, and that all populations still harbor inbreeding load. We also found a significant decrease in log performance with increases in the population inbreeding coefficient. These results indicate that even in seemingly large populations, lupines are susceptible to considerable fitness declines through both inbreeding load within populations, and drift load via genetic erosion and fixation of deleterious alleles between populations.     

INBREEDING: ITS EFFECT ON RESPONSE TO SELECTION FOR PUPAL WEIGHT AND THE HERITABLE VARIANCE IN FITNESS IN THE FLOUR BEETLE,TRIBOLIUM CASTANEUM

We report our studies of the effect of inbreeding on the response to selection for increased pupal weight in the flour beetle, Tribolium castaneum. We also report the effects of inbreeding and selection for pupal weight on the heritable variation in fitness and fitness components. We created replicate and independent inbred lines with F-values of 0.00, 0.375, and 0.672, by 0, 2, and 5 generations, respectively, of brother-sister mating of adult beetles from an outbred stock population. Subsequently, we imposed artificial within-family selection for increased pupal weight in each of 15 inbred lines for eight generations; each line had its own paired, unselected control. We compared the response to selection across the three levels of inbreeding with theoretical expectation, and investigated the effects of inbreeding and selection on fitness variation among families within all 30 selected and control lines. Among-line variation in pupal weight increased with increased inbreeding prior to selection but diminished with directional selection. Inbreeding reduced the realized heritability of pupal weight concordant with quantitative predictions of additive theory. Mean fitness, measured in several ways, declined with inbreeding and declined further with selection. In contrast, the genetic variation for fitness in the inbred and selected lines lines equalled or exceeded that of the outbred controls. Our results suggest that inbreeding and selection may affect traits in different ways depending on the relative amounts of additive and nonadditive genetic variation.     

Experimental evolution of the genetic load and its implications for the genetic basis of inbreeding depression

The degree to which, and rapidity with which, inbreeding depression can be purged from a population has important implications for conservation biology, captive breeding practices, and invasive species biology. The degree and rate of purging also informs us regarding the genetic mechanisms underlying inbreeding depression. We examine the evolution of mean survival and inbreeding depression in survival following serial inbreeding in a seed-feeding beetle, Stator limbatus, which shows substantial inbreeding depression at all stages of development. We created two replicate serially inbred populations perpetuated by full-sib matings and paired with outbred controls. The genetic load for the probability that an egg produces an adult was purged at approximately 0.45-0.50 lethal equivalents/generation, a reduction of more than half after only three generations of sib-mating. After serial inbreeding we outcrossed all beetles then measured (1) larval survival of outcrossed beetles and (2) inbreeding depression. Survival of outcrossed beetles evolved to be higher in the serially inbred populations for all periods of development. Inbreeding depression and the genetic load were significantly lower in the serially inbred than control populations. Inbreeding depression affecting larval survival of S. limbatus is largely due to recessive deleterious alleles of large effect that can be rapidly purged from a population by serial sib-mating. However, the effectiveness of purging varied among the periods of egg/larval survival and likely varies among other unstudied fitness components. This study presents novel results showing rapid and extensive purging of the genetic load, specifically a reduction of as much as 72% in only three generations of sib-mating. However, the high rate of extinction of inbred lines, despite the lines being reared in a benign laboratory environment, indicates that intentional purging of the genetic load of captive endangered species will not be practical due to high rates of subpopulation extinction.     

No genetic correlation between the sexes in mating frequency in the bean beetle, Callosobruchus chinensis

Female multiple mating, which is common in animals, may have evolved not in response to fitness advantages to females but as a genetic corollary to selection on males to mate frequently. This nonadaptive hypothesis assumes a genetic correlation between females and males in mating frequency, which has received a few empirical investigations. We tested this hypothesis by observing the correlated response in male mating frequency in the adzuki bean beetle, Callosobruchus chinensis to artificial selection on female propensity to remate. Compared to control females, females from lines selected for increased or decreased female propensity to remate had, respectively, higher or lower mating frequency measured by the number of mating within a given period. This indicates that female receptivity to remating is genetically correlated with female mating frequency, and thus the artificial selection for female propensity to remate influenced female mating frequency. In contrast, males from the selected lines that diverged in female mating frequency did not vary significantly in their mating frequency. These results indicate that there is no genetic correlation between the sexes in mating frequency in C. chinensis. This study shows that the reason why females in C. chinensis remate despite suffering fitness costs cannot be explained by indirect selection resulting from selection on males to mate multiple times.     

Patterns of the purging of deleterious genes with synergistic interactions in different breeding schemes

 The purging of deleterious genes for increasing progeny fitness and/or avoiding extinction in breeding programs, particularly with endangered species, has become of increased interest in recent years. Some studies have shown that purging can be effective only for deleterious genes of multiplicative, large effects, such as lethal or sublethal genes. In the present study, we examine the effectiveness of purging viability genes of synergistic, small effect with continuous selfing, full-sib mating, or half-sib mating, by computer simulation. A diploid breeding population with a constant progeny size of 10, 50 or 200 is simulated, one of the three breeding schemes referred to above is practiced over ten generations, and the patterns of purging, progeny survival and population extinction are examined. The rate and amount of purging generally increase with high dominance, strong synergism, high genetic load and low inbreeding. Progeny survival can increase only for a progeny size larger than 50 using schemes of mild inbreeding when there is a high level of dominance and strong synergism. The probability of extinction could greatly increase up to 100% for a progeny size of 10, but mostly is less than 10% for a progeny size larger than 50 in terms of the genetic load examined. The implications of these simulated results for purging deleterious genes of small effect in small breeding populations are discussed. Received: 5 July 1998 / Accepted: 5 August 1998     

Dynamics of inbreeding depression due to deleterious mutations in small populations: mutation parameters and inbreeding rate

A multilocus stochastic model is developed to simulate the dynamics of mutational load in small populations of various sizes. Old mutations sampled from a large ancestral population at mutation-selection balance and new mutations arising each generation are considered jointly, using biologically plausible lethal and deleterious mutation parameters. The results show that inbreeding depression and the number of lethal equivalents due to partially recessive mutations can be partly purged from the population by inbreeding, and that this purging mainly involves lethals or detrimentals of large effect. However, fitness decreases continuously with inbreeding, due to increased fixation and homozygosity of mildly deleterious mutants, resulting in extinctions of very small populations with low reproductive rates. No optimum inbreeding rate or population size exists for purging with respect to fitness (viability) changes, but there is an optimum inbreeding rate at a given final level of inbreeding for reducing inbreeding depression or the number of lethal equivalents. The interaction between selection against partially recessive mutations and genetic drift in small populations also influences the rate of decay of neutral variation. Weak selection against mutants relative to genetic drift results in apparent overdominance and thus an increase in effective size (Ne) at neutral loci, and strong selection relative to drift leads to a decrease in Ne due to the increased variance in family size. The simulation results and their implications are discussed in the context of biological conservation and tests for purging.     

Estimation of genetic purging under competitive conditions

Inbreeding depression for fitness traits is a key issue in evolutionary biology and conservation genetics. The magnitude of inbreeding depression, though, may critically depend on the efficiency of genetic purging, the elimination or recessive deleterious mutations by natural selection after they are exposed by inbreeding. However, the detection and quantification of genetic purging for nonlethal mutations is a rather difficult task. Here, we present two comprehensive sets of experiments with Drosophila aimed at detecting genetic purging in competitive conditions and quantifying its magnitude. We obtain, for the first time in competitive conditions, an estimate for the predictive parameter, the purging coefficient (d), that quantifies the magnitude of genetic purging, either against overall inbreeding depression (d ≈ 0.3), or against the component ascribed to nonlethal alleles (d_NL ≈ 0.2). We find that competitive fitness declines at a high rate when inbreeding increases in the absence of purging. However, in moderate size populations under competitive conditions, inbreeding depression need not be too dramatic in the medium to short term, as the efficiency of purging is also very high. Furthermore, we find that purging occurred under competitive conditions also reduced the inbreeding depression that is expressed in the absence of competition.     

Response to selection on cold tolerance is constrained by inbreeding

The evolutionary potential of any given population is of fundamental importance for its longer term prospects. Modern land-use practices often result in small and isolated populations, increasing the risk of extinction through reduced genetic diversity as a consequence of inbreeding or drift. Such genetic erosion may also interfere with a population's evolutionary potential. In this study, we investigate the consequences of inbreeding on evolutionary potential (the ability to increase cold resistance) in a laboratory population of the tropical butterfly Bicyclus anynana. To explore constraints on evolution, we applied artificial selection to chill-coma recovery time, starting from three levels of inbreeding (outbred control, one or two full-sibling matings). Ten generations of selection produced highly divergent phenotypes, with the lines selected for increased cold tolerance showing about 28% shorter recovery times after cold exposure relative to unselected controls. Correlated responses to selection in 10 different life-history and stress-resistance traits were essentially absent. Inbred lines showed a weaker response to selection, indicating reduced evolutionary potential and thereby constraints on evolution. Inbreeding depression was still measurable in some traits after the course of selection. Traits more closely related to fitness showed a clear fitness rebound, suggesting a trait-specific impact of purging. Our findings have important implications for the longer term survival of small populations in fragmented landscapes.     

Purging of inbreeding depression and fitness decline in bottlenecked populations of Drosophila melanogaster

Drastic reductions in population size, or bottlenecks, are thought to significantly erode genetic variability and reduce fitness. However, it has been suggested that a population can be purged of the genetic load responsible for reduced fitness when subjected to bottlenecks. To investigate this phenomenon, we put a number of Drosophila melanogaster isofemale lines known to differ in inbreeding depression through four ‘founder‐flush’ bottleneck cycles with flush sizes of 5 or 100 pairs and assayed for relative fitness (single‐pair productivity) after each cycle. Following the founder‐flush phase, the isofemale lines, with a large flush size and a history of inbreeding depression, recovered most of the fitness lost from early inbreeding, consistent with purging. The same isofemale lines, with a small flush size, did not regain fitness, consistent with the greater effect of genetic drift in these isofemale lines. On the other hand, the isofemale lines that did not show initial inbreeding depression declined in fitness after repeated bottlenecks, independent of the flush size. These results suggest that the nature of genetic variation in fitness may greatly influence the way in which populations respond to bottlenecks and that stochastic processes play an important role. Consequently, an attempt intentionally to purge a population of detrimental variation through inbreeding appears to be a risky strategy, particularly in the genetic management of endangered species.     

Fitness, genetic load and purging in experimental populations of the housefly

The relative effects of purging of the genetic load versus thefixation of deleterious alleles, under inbreeding, will influencea population's probability of extinction. The relative contributionof these two phenomena is expected to depend upon the rate ofinbreeding. A further complication is due to the fact that a purgingof the genetic load in one environment does not necessarily implya purging of the genetic load in other environments. To addressthese two issues, we compare fitness and genetic load in populationsexperiencing similar levels of inbreeding, but occurring as either ashort-term bottleneck or as a consequence of long-term reducedpopulation size, over a range of environments. Inbred populationshave consistently lower fitness than outbred populations acrossall environments tested. However, the bottlenecked populationssuffer less inbreeding depression for a given level of inbreeding,whether or not challenged by novel environments, than populationskept at a constant small size. The results of this study demonstratethat populations initiated from a small number of founders are ableto recover fitness and survive novel environmental challenges,provided that habitat is available for rapid population growth.     

Effects of Interference Between Selected Loci on the Mutation Load,Inbreeding Depression,and Heterosis

A classical prediction from single-locus models is that inbreeding increases the efficiency of selection against partially recessive deleterious alleles (purging), thereby decreasing the mutation load and level of inbreeding depression. However, previous multilocus simulation studies found that increasing the rate of self-fertilization of individuals may not lead to purging and argued that selective interference among loci causes this effect. In this article, I derive simple analytical approximations for the mutation load and inbreeding depression, taking into account the effects of interference between pairs of loci. I consider two classical scenarios of nonrandomly mating populations: a single population undergoing partial selfing and a subdivided population with limited dispersal. In the first case, correlations in homozygosity between loci tend to reduce mean fitness and increase inbreeding depression. These effects are stronger when deleterious alleles are more recessive, but only weakly depend on the strength of selection against deleterious alleles and on recombination rates. In subdivided populations, interference increases inbreeding depression within demes, but decreases heterosis between demes. Comparisons with multilocus, individual-based simulations show that these analytical approximations are accurate as long as the effects of interference stay moderate, but fail for high deleterious mutation rates and low dominance coefficients of deleterious alleles.     

SEXUAL SELECTION COUNTERACTS EXTINCTION OF SMALL POPULATIONS OF THE BULB MITES

Genetic drift in small populations can increase frequency of deleterious recessives and consequently lead to inbreeding depression and population extinction. On the other hand, as homozygosity at deleterious recessives increases, they should be purged from populations more effectively by selection. Sexual selection has been postulated to strengthen selection against deleterious mutations, and should thus decrease extinction rate and intensify purging of inbreeding depression. We tested these predictions in the bulb mite Rhizoglyphus robini. We created 100 replicate lines of small populations (five males and five females) and in half of them experimentally removed sexual selection by enforcing monogamy. The lines were propagated for eight generations and then assayed for purging of inbreeding depression. We found that proportion of lines which went extinct was lower with sexual selection than without. We also found evidence for purging of inbreeding depression in the lines with sexual selection, but not in lines without sexual selection. Our results suggest that purging of inbreeding depression was more effective against mutations with relatively large deleterious effects. Thus, although our data clearly indicate a positive impact of sexual selection on short‐term survival of bottlenecked populations, long‐term consequences are less clear as they may be negatively impacted by accumulation of deleterious mutations of small effect.     

Levels of inbreeding depression over seven generations of selfing in the androdioecious clam shrimp, Eulimnadia texana

Androdioecy (mixtures of males and hermaphrodites) is a rare mating system in both plants and animals. Theory suggests that high levels of inbreeding depression can maintain males in androdioecious populations if hermaphrodites commonly self-fertilize. However, if inbreeding depression (delta) can be 'purged' from selfing populations, maintaining males is more difficult. In the androdioecious clam shrimp, Eulimnadia texana, delta is estimated to be as high as 0.7. Previous work suggests that this high level is maintained in the face of high levels of inbreeding due to an associative overdominance of fitness-related loci with the sex-determining locus. Such associative overdominance would make purging of inbreeding depression difficult to impossible. The current experiment was designed to determine if delta can be purged in these shrimp by tracking fitness across seven generations in selfing and outcrossing treatments. Evidence of purging was found in one of four populations, but the remaining populations demonstrated a consistent pattern of delta across generations. Although the experimental design allowed ample opportunity for purging, the majority of populations were unable to purge their genetic load. Therefore, delta in this species is likely due to associative overdominance caused by deleterious recessive alleles linked to the sex determining locus.     

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.