Reduced inbreeding depression due to historical inbreeding in Drosophila melanogaster: evidence for purging

An important issue in conservation biology and the study of evolution is the extent to which inbreeding depression can be reduced or reversed by natural selection. If the deleterious recessive alleles causing inbreeding depression can be 'purged' by natural selection, outbred populations that have a history of inbreeding are expected to be less susceptible to inbreeding depression. This expectation, however, has not been realized in previous laboratory experiments. In the present study, we used Drosophila melanogaster as a model system to test for an association between inbreeding history and inbreeding depression. We created six 'purged' populations from experimental lineages that had been maintained at a population size of 10 male-female pairs for 19 generations. We then measured the inbreeding depression that resulted from one generation of full-sib mating in the purged populations and in the original base population. The magnitude of inbreeding depression in the purged populations was approximately one-third of that observed in the original base population. In contrast to previous laboratory experiments, therefore, we found that inbreeding depression was reduced in populations that have a history of inbreeding. The large purging effects observed in this study may be attributable to the rate of historical inbreeding examined, which was slower than that considered in previous experiments.     

Effects of inbreeding and rate of inbreeding in Drosophila melanogaster- Hsp70 expression and fitness

Induction of heat shock proteins (Hsp) is a well-known mechanism through which cells cope with stressful conditions. Hsp are induced by a variety of extrinsic stressors. However, recently intrinsic stressors (aging and inbreeding) have been shown to affect expression of Hsp. Increased homozygosity due to inbreeding may disrupt cellular homeostasis by causing increased expression of recessive deleterious mutations and breakdown of epistatic interactions. We investigated the effect of inbreeding and the rate of inbreeding on the expression of Hsp70, larval heat resistance and fecundity. In Drosophila melanogaster we found that inbred lines (F approximately 0.67) had significantly up-regulated expression of Hsp70, and reduced heat resistance and fecundity as compared with outbred control lines. A significant negative correlation was observed between Hsp70 expression and resistance to an extreme heat stress in inbred lines. We interpreted this as an increased requirement for Hsp70 in the lines suffering most from inbreeding depression. Inbreeding depression for fecundity was reduced with a slower rate of inbreeding compared with a fast rate of inbreeding. Thus, the effectiveness of purging seems to be improved with a slower rate of inbreeding.     

Inbreeding and extinction: Effects of rate of inbreeding

Deleterious alleles may be removed (purged) bynatural selection in populations undergoinginbreeding. However, there is controversyregarding the effectiveness of selection inreducing the risk of extinction due toinbreeding, especially in relation to the rateof inbreeding. We evaluated the effect of therate of inbreeding on reducing extinction risk,in populations of Drosophila melanogastermaintained using full-sib mating (160replicates), or at effective population sizes(N _e) of 10 (80) or 20 (80).Extinction rates in the populations maintainedusing full-sib mating occurred at lower levelsof inbreeding than in the larger populations,whereas the two larger populations did notdiffer significantly from each other.Inbreeding coefficients at 50% extinction were0.62, 0.79 and 0.77 for the full-sib (N _e = 2.6), N _e = 10 and N _e = 20 treatments, respectively. Populations of N _e = 20 that remained extant after 60 generations, showed inbreeding depression, with the mean fitness of these populations being only 45% of the outbredcontrols. There was considerable variationamong the 31 inbred populations in fitness, butnone of the N _e = 20 populations hadfitness that was higher than the outbredcontrol. We conclude that purging may slow therate of extinction slightly, but it cannot berelied on to eliminate the deleterious effectsof 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.     

Effects of experimental inbreeding on herbivore resistance and plant fitness: the role of history of inbreeding, herbivory and abiotic factors

Inbreeding is common in plant populations and can affect plant fitness and resistance against herbivores. These effects are likely to depend on population history. In a greenhouse experiment with plants from 17 populations of Lychnis flos-cuculi, we studied the effects of experimental inbreeding on resistance and plant fitness. Depending on the levels of past herbivory and abiotic factors at the site of plant origin, we found either inbreeding or outbreeding depression in herbivore resistance. Furthermore, when not damaged experimentally by snail herbivores, plants from populations with higher heterozygosity suffered from inbreeding depression and those from populations with lower heterozygosity suffered from outbreeding depression. These effects of inbreeding and outbreeding were not apparent under experimental snail herbivory. We conclude that inbreeding effects on resistance and plant fitness depend on population history. Moreover, herbivory can mask inbreeding effects on plant fitness. Thus, understanding inbreeding effects on plant fitness requires studying multiple populations and considering population history and biotic interactions.     

Salmonid inbreeding: a review

We review the published literature oninbreeding and its consequences in salmonidfishes. Inbreeding reduces genetic variationwithin populations by decreasingheterozygosity, either through an increasedchance of sharing parental genes or a loss ofalleles from random genetic drift. Increasedinbreeding is often associated with a reductionin mean phenotypic value of one or more traitswith respect to fitness (inbreedingdepression). We identify several sources ofinbreeding in salmonids. Although inbreedingoccurs naturally, much of the evidence forinbreeding stems from direct or indirectresults of human activity. The potentialconsequences of inbreeding highlight theimportance of maintaining genetic diversity insalmonid populations. Our weak understandingof genetic interactions between cultured andwild salmonids has allowed widespread practicesthat can reduce genetic variability in naturalpopulations. Although studies have detectedinbreeding depression in salmonids, its geneticbasis has rarely been addressed in wild,anadromous salmon. The genetic basis ofinbreeding depression is complex, andevaluating its effects over the entire lifecycle remains challenging. The experimentalevidence nevertheless reinforces the importanceof maintaining genetic variation withinpopulations as a primary goal of conservationand management.     

Multigenerational inbreeding in <Emphasis Type="Italic">Succisa pratensis</Emphasis>: Effects on fitness components

We examined the effects of repeated inbreeding on fitness components of the long-lived perennial Succisa pratensis (Dipsacaceae). Plants from six populations differing in size were used to establish lines with expected inbreeding coefficients f of 0, 0.5 and 0.75. The effects of different inbreeding levels were measured for seed set, seed mass, percentage germination and seedling relative growth rate. Seed set decreased following one generation of inbreeding and seedling growth rate decreased after two generations of inbreeding. Our study indicated that the mutational load is difficult to purge and that continued inbreeding tends to affect important traits in S. pratensis. Although the partial dominance hypothesis for inbreeding depression seems to account for the results, the overdominance hypothesis cannot be ruled out completely. Overall, we conclude that the response of a long-lived plant, such as S. pratensis, to repeated inbreeding does not differ from that of other plant species with shorter life spans, surely because the mechanisms that account for inbreeding depression are universal for all plant species.     

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.     

Viral epizootic reveals inbreeding depression in a habitually inbreeding mammal

Inbreeding is typically detrimental to fitness. However, some animal populations are reported to inbreed without incurring inbreeding depression, ostensibly due to past "purging" of deleterious alleles. Challenging this is the position that purging can, at best, only adapt a population to a particular environment; novel selective regimes will always uncover additional inbreeding load. We consider this in a prominent test case: the eusocial naked mole-rat (Heterocephalus glaber), one of the most inbred of all free-living mammals. We investigated factors affecting mortality in a population of naked mole-rats struck by a spontaneous, lethal coronavirus outbreak. In a multivariate model, inbreeding coefficient strongly predicted mortality, with closely inbred mole-rats (F> or = 0.25) over 300% more likely to die than their outbred counterparts. We demonstrate that, contrary to common assertions, strong inbreeding depression is evident in this species. Our results suggest that loss of genetic diversity through inbreeding may render populations vulnerable to local extinction from emerging infectious diseases even when other inbreeding depression symptoms are absent.     

Royal dynasties as human inbreeding laboratories: the Habsburgs

The European royal dynasties of the Early Modern Age provide a useful framework for human inbreeding research. In this article, consanguineous marriage, inbreeding depression and the purging of deleterious alleles within a consanguineous population are investigated in the Habsburgs, a royal dynasty with a long history of consanguinity over generations. Genealogical information from a number of historical sources was used to compute kinship and inbreeding coefficients for the Habsburgs. The marriages contracted by the Habsburgs from 1450 to 1750 presented an extremely high mean kinship (0.0628±0.009), which was the result of the matrimonial policy conducted by the dynasty to establish political alliances through marriage. A strong inbreeding depression for both infant and child survival was detected in the progeny of 71 Habsburg marriages in the period 1450–1800. The inbreeding load for child survival experienced a pronounced decrease from 3.98±0.87 in the period 1450–1600 to 0.93±0.62 in the period 1600–1800, but temporal changes in the inbreeding depression for infant survival were not detected. Such a reduction of inbreeding depression for child survival in a relatively small number of generations could be caused by elimination of deleterious alleles of a large effect according with predictions from purging models. The differential purging of the infant and child inbreeding loads suggest that the genetic basis of inbreeding depression was probably very different for infant and child survival in the Habsburg lineage. Our findings provide empirical support that human inbreeding depression for some fitness components might be purged by selection within consanguineous populations.     

The decline in fitness with inbreeding: evidence for negative dominance‐by‐dominance epistasis in Drosophila melanogaster

Genetic interactions can play an important role in the evolution of reproductive strategies. In particular, negative dominance‐by‐dominance epistasis for fitness can theoretically favour sex and recombination. This form of epistasis can be detected statistically because it generates nonlinearity in the relationship between fitness and inbreeding coefficient. Measures of fitness in progressively inbred lines tend to show limited evidence for epistasis. However, tests of this kind can be biased against detecting an accelerating decline due to line losses at higher inbreeding levels. We tested for dominance‐by‐dominance epistasis in Drosophila melanogaster by examining viability at five inbreeding levels that were generated simultaneously, avoiding the bias against detecting nonlinearity that has affected previous studies. We find an accelerating rate of fitness decline with inbreeding, indicating that dominance‐by‐dominance epistasis is negative on average, which should favour sex and recombination.     

Role of inbreeding depression and purging in captive breeding and restoration programmes

Inbreeding depression is a major force affecting the evolution and viability of small populations in captive breeding and restoration programmes. Populations that experience small sizes may be less susceptible to future inbreeding depression because they have been purged of deleterious recessive alleles. We review issues related to purging, as they apply to the management of small populations, and discuss an experiment we conducted examining purging in populations of mosquitofish (Gambusia affinis). Purging is an important process in many small populations, but the literature contains a diversity of responses to purging both within and among studies. With the exception that slow inbreeding results in more purging and less threat to population viability, there seem to be few consistent trends that aid in prediction of how a purging event will affect a population. In our examination of purging on population viability in mosquitofish, single or multiple bottlenecks do not appear to have resulted in any purging of the influence of genetic load on population growth. Rather, serial bottlenecks resulted in a marked decline in population growth and an increase in extinction. Our results, taken together with those of reviewed studies, suggest that in small populations there is great uncertainty regarding the success of any single purging event in eliminating inbreeding depression, together with the high likelihood that purging will depress population viability through the fixation of deleterious alleles. In management of captive breeding and restoration programmes, the common practice of avoiding inbreeding and small population sizes should be followed whenever possible.     

Mutation-selection balance accounting for genetic variation for viability in Drosophila melanogaster as deduced from an inbreeding and artificial selection experiment

We carried out an experiment of inbreeding and upward artificial selection for egg-to-adult viability in a recently captured population of Drosophila melanogaster, as well as computer simulations of the experimental design, in order to obtain information on the nature of genetic variation for this important fitness component. The inbreeding depression was linear with a rate of 0.70 +/- 0.11% of the initial mean per 1% increase in inbreeding coefficient, and the realized heritability was 0.06 +/- 0.07. We compared the empirical observations of inbreeding depression and selection response with computer simulations assuming a balance between the occurrence of partially recessive deleterious mutations and their elimination by selection. Our results suggest that a model assuming mutation-selection balance with realistic mutational parameters can explain the genetic variation for viability in the natural population studied. Several mutational models are incompatible with some observations and can be discarded. Mutational models assuming a low rate of mutations of large average effect and highly recessive gene action, and others assuming a high rate of mutations of small average effect and close to additive gene action, are compatible with all the observations.     

Lack of nonadditive genetic effects on early fecundity in Drosophila melanogaster

Fecundity is usually considered as a trait closely connected to fitness and is expected to exhibit substantial nonadditive genetic variation and inbreeding depression. However, two independent experiments, using populations of different geographical origin, indicate that early fecundity in Drosophila melanogaster behaves as a typical additive trait of low heritability. The first experiment involved artificial selection in inbred and non-inbred lines, all of them started from a common base population previously maintained in the laboratory for about 35 generations. The realized heritability estimate was 0.151 +/- 0.075 and the inbreeding depression was very small and nonsignificant (0.09 +/- 0.09% of the non-inbred mean per 1% increase in inbreeding coefficient). With inbreeding, the observed decrease in the within-line additive genetic variance and the corresponding increase of the between-line variance were very close to their expected values for pure additive gene action. This result is at odds with previous studies showing inbreeding depression and, therefore, directional dominance for the same trait and species. All experiments, however, used laboratory populations, and it is possible that the original genetic architecture of the trait in nature was subsequently altered by the joint action of random drift and adaptation to captivity. Thus, we carried out a second experiment, involving inbreeding without artificial selection in a population recently collected from the wild. In this case we obtained, again, a maximum-likelihood heritability estimate of 0.210 +/- 0.027 and very little nonsignificant inbreeding depression (0.06 +/- 0.12%). The results suggest that, for fitness-component traits, low levels of additive genetic variance are not necessarily associated with large inbreeding depression or high levels of nonadditive genetic variance.     

Effects of inbreeding and environmental stress on fitness – using Drosophila buzzatii as a model organism

Wild endangered populations can suffer fromadverse effects on fitness due to inbreedingand environmental stress. Often, both geneticand environmental stress factors may be presentin populations at the same time. Thereforeknowledge on the potential interactions betweenthese factors is important for the conservationof wild populations. When measuring fitness(e.g. survival and reproductive potential) ofindividuals in the laboratory, and in nature,inbreeding by environment interactions are nowbeing reported more often. The increased focuson environmental dependency of inbreedingdepression will therefore enable conservationbiologists to include this knowledge in themanagement of endangered populations in thewild. In this study, the effects ofenvironmental stress and inbreeding on fitnessare estimated in a laboratory population ofDrosophila buzzatii. Random- or full-sibmating were used to generate independentreplicate lines of four different inbreedinglevels (F = 0, F = 0.25, F = 0.50, F = 0.672)in four different environments. Theenvironments were thermal and dimethoate stressseparately and in combination, as well as anon-stressful control environment. Twoexperiments were carried out to measureproductivity (a multiplicative measure offecundity and viability) using a full factorialdesign. In the first experiment, productivitywas estimated for all lines and inbreedinglevels in the environment in which flies wereinbred and reared for several generations. Inthe second experiment, productivity of thelines reared in the control environment wastested in all four environments and for allinbreeding levels. Our results show asignificant effect of inbreeding andenvironmental stress on productivity in bothexperiments and the effect increased when flieswere exposed to novel environmental conditions.Productivity was not affected by theinteraction between inbreeding andenvironmental stress when flies were tested inthe environments in which they were reared,whereas there was a tendency towards a stressby inbreeding interaction when flies wereexposed to novel environments. The variance andthe coefficient of variation in productivitywere each affected by environmental stress andinbreeding, indicating that environmentalconditions as well as genetic background areimportant for variation in productivity.However, the two measures of variation oftenshowed opposite trends. The results obtained inthis study indicate that the environmentalconditions under which inbreeding occurs areimportant. This is relevant for the maintenanceand management of populations in captivity andin relation to reintroduction of endangeredspecies in nature.     

The effects of stress intensity and stress type on inbreeding depression in Silene vulgaris

Inbreeding depression (ID) is generally assumed to increase under stressful conditions, but a number of studies have found the opposite pattern, that is that crossed offspring were more capable of exploiting benign conditions. Alternatively, the phenotypic variation hypothesis predicts that not stress intensity, but enhanced phenotypic variation in an environment leads to increased ID. We subjected inbred and crossed offspring of Silene vulgaris to drought, simulated herbivory, copper contamination, and two levels of nutrient deficiency and shade. In contrast to the predominant expectation, most stress treatments decreased inbreeding depression. With increasing nutrient limitation, ID decreased strongly, whereas under increasing shade ID did not change. These differences may be due to purging in the population of origin where conditions are nutrient‐poor and dry, but not shaded. In contrast to the greenhouse experiment, ID was higher in a field site than in a more benign common garden. However, the predictions of the phenotypic variation hypothesis were met in both the greenhouse and the field versus garden experiment. The results suggest that there may be no general relationship between ID and stress intensity, but specific effects of stress type and the novelty and variability of the environment.     

No evidence for optimal fitness at intermediate levels of inbreeding in Drosophila melanogaster

Optimal outbreeding theory predicts fitness benefits to intermediate levels of inbreeding. In the present study, we test for linear (consistent with inbreeding depression) and nonlinear (consistent with optimal outbreeding) effects of inbreeding on reproductive fitness in male and female Drosophila melanogaster . We found linear declines in fitness associated with increased inbreeding for egg-to-adult viability, but not the number of eggs laid or sperm competitive ability. Egg-to-adult viability was also lower in the progeny of inbred males and females mated to unrelated individuals. However, there was no evidence for optimal fitness at intermediate levels of inbreeding for any trait. The present study highlights the importance of considering biologically realistic levels of inbreeding and cross-generational effects when investigating the costs and benefits of mating with relatives.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 501–510.     

Host inbreeding increases susceptibility to ectoparasitism

Inbreeding, which increases homozygosity throughout the genome by increasing the proportion of alleles that are identical by descent, is expected to compromise resistance against parasitism. Here, we demonstrate that host inbreeding increases susceptibility to ectoparasitism in a natural fruit fly (Drosophila nigrospiracula) - mite (Macrocheles subbadius) association, and that this effect depends on host genetic background. Moreover, flies generated from reciprocal crosses between susceptible inbred lines exhibited elevated levels of resistance similar to that in the mass-bred base population, confirming in reverse direction the causative link between expected heterozygosity and resistance. We also show that inbreeding reduces the host's ability to sustain energetically expensive behaviours, and that host exhaustion dramatically increases susceptibility. These findings suggest that inbreeding depression for resistance results from an inability to sustain defensive behaviours because of compromised physiological competence.     

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.