Effects of inbreeding on variation in diapause duration and early fecundity in the Kanzawa spider mite

In arthropods, both diapause duration and ability to produce eggs in early adult life (early fecundity) are important life‐history traits for successful settling in a new habitat. In herbivorous spider mites (Acari: Tetranychidae), inbreeding frequently occurs because new colonies are established by only one or a few females. In the present study, we investigated the impact of inbreeding on the phenotypic variance in diapause duration and early fecundity in the Kanzawa spider mite, Tetranychus kanzawai Kishida. Diapause duration was compared between the inbreeding treatment (strongly inbred strains) and the control (strains immediately taken from the stock culture) under winter‐mimicking laboratory conditions. The variance in diapause duration was smaller in the inbreeding treatment than in the control, though the magnitude of decrease of variance was less than expected. On the other hand, early fecundity did not show any reduction of variance. The present results revealed that inbreeding reduces phenotypic variation, as expected in theory.     

Life-history plasticity and inbreeding depression under mate limitation and predation risk: cumulative lifetime fitness dissected with a life table response experiment

Environmental effects on the evolution of mating systems are increasingly discussed, but we lack many examples of how environmental conditions affect the expression and consequences of alternative mating systems. Variation in mate availability sets up a trade-off between reproductive assurance and inbreeding depression, but the consequences of both mate limitation and inbreeding may depend on other environmental conditions. Predation risk is common under natural conditions, and known to affect allocation to reproduction, but we know little about the effects of isolation and inbreeding under predation risk. We reared selfed and outcrossed hermaphroditic freshwater snails (Physa acuta) in four environments (predator cues present or absent crossed with mating partners available or not) and quantified life-history traits and cumulative lifetime fitness. Our results confirm that isolation from mates can increase longevity and growth, resulting in higher lifetime fecundity. Thus, we observed no evidence for mate limitation of reproduction. However, reproduction under isolation (i.e., selfing) resulted in inbreeding depression, which should counteract the benefits of selfing. Inbreeding depression in fitness occurred in both predator and no-predator environments, but there was no overall change in inbreeding depression with predator cues. This represents, to our knowledge, the first empirical estimate of the effect of predation risk on inbreeding depression in an animal. Cumulative fitness was most influenced by early survival and especially early fecundity. As predation risk and inbreeding (both ancestral and due to a lack of mates) reduced early fecundity, these effect are predicted to have important contributions to population growth under natural conditions. Therefore life-history plasticity (e.g., delayed reproduction) is likely to be very important to overall fitness.     

A genetic perspective on mating systems and sex ratios of parasitoid wasps

Parasitoid sex ratios are influenced by mating systems, whether complete inbreeding, partial inbreeding, complete inbreeding avoidance, or production of all-male broods by unmated females. Population genetic theory demonstrates that inbreeding is possible in haplodiploids because the purging of deleterious and lethal mutations through haploid males reduces inbreeding depression. However, this purging does not act quickly for deleterious mutations or female-limited traits (e.g., fecundity, host searching, sex ratio). The relationship between sex ratio, inbreeding, and inbreeding depression has not been explored in depth in parasitoids. The gregarious egg parasitoid, Trichogramma pretiosum Riley, collected from Riverside, CA (USA) produced a female-biased sex ratio of 0.24 (proportion of males). Six generations of sibling mating in the laboratory uncovered considerable inbreeding depression (∼ 20%) in fecundity and sex ratio. A population genetic study (based upon allozymes) showed the population was inbred (F _it = 0.246), which corresponds to 56.6% sib-mating. However, average relatedness among females emerging from the same host egg was only 0.646, which is less than expected (0.75) if ovipositing females mate randomly. This lower relatedness could arise from inbreeding avoidance, multiple mating by females, or superparasitism. A review of the literature in general shows relatively low inbreeding depression in haplodiploid species, but indicates that inbreeding depression can be as high as that found in Drosophila. Finally, mating systems and inbreeding depression are thought to evolve in concert (in plants), but similar dynamic models of the joint evolution of sex ratio, mating systems, and inbreeding depression have not been developed for parasitoid wasps. Received: November 13, 1998 /Accepted: January 8, 1999     

Breeding systems in Clivia (Amaryllidaceae): late‐acting self‐incompatibility and its functional consequences

Late‐acting (ovarian) self‐incompatibility, characterized by minimal or zero seed production following self‐pollen tube growth to the ovules, is expected to show phylogenetic clustering, but can otherwise be difficult to distinguish from early‐acting inbreeding depression. In Amaryllidaceae, late‐acting self‐incompatibility has been proposed for Narcissus (Narcisseae) and Cyrtanthus (Cyrtantheae). Here, we investigate whether it occurs in the horticulturally important genus Clivia (Haemantheae) and test whether species in this genus experience ovule discounting in wild populations. Seed‐set results following controlled hand pollinations revealed that Clivia miniata and C. gardenii are largely self‐sterile. Self‐ and cross‐pollinated flowers of both species had similar proportions of pollen tubes entering the ovary, and those of C. gardenii also did not differ in the proportions of pollen tubes that penetrated ovules, thus ruling out classical gametophytic self‐incompatibility acting in the style, but not early inbreeding depression. Flowers that received equal mixtures of self‐ and cross‐pollen set fewer seeds than those that received cross‐pollen only, but it was unclear whether this effect was a result of ovule discounting or interactions on the stigma. The prevention of self‐pollination by the emasculation of either single flowers or whole inflorescences in wild populations did not affect seed set, suggesting that ovule discounting is not a major natural limitation on seed production. Flowers typically produce one to three large fleshy seeds from approximately 16 available ovules, even when supplementally hand pollinated, suggesting that fecundity is mostly resource limited. The results of this study suggest that Clivia spp. are largely self‐sterile as a result of either a late‐acting self‐incompatibility system or severe early inbreeding depression, but ovule discounting caused by self‐pollination is not a major constraint on fecundity. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 155–168.     

Effect of rate of inbreeding on inbreeding depression in Drosophila melanogaster

Summary This experiment was designed to study the relationship between rate of inbreeding and observed inbreeding depression of larval viability, adult fecundity and cold shock mortality in Drosophila melanogaster. Rates of inbreeding used were full-sib mating and closed lines of N=4 and N=20. Eight generations of mating in the N=20 lines, three generations in the N=4 lines and one generation of full-sib mating were synchronised to simultaneously produce individuals with an expected level of inbreeding coefficient (F) of approximately 0.25. Inbreeding depression for the three traits was significant at F=0.25. N=20 lines showed significantly less inbreeding depression than full-sib mated lines for larval viability at approximately the same level of F. A similar trend was observed for fecundity. No effect of rate of inbreeding depression was found for cold shock mortality, but this trait was measured with less precision than the other two. Natural selection acting on loci influencing larval viability and fecundity during the process of inbreeding could explain these results. Selection is expected to be more effective with slow rates of inbreeding because there are more generations and greater opportunity for selection to act before F=0.25 is reached. Selection intensities seem to have been different in the three traits measured. Selection was most intense for larval viability, less intense for fecundity and, perhaps, negligible at loci influencing cold shock mortality.     

LIFETIME ESTIMATES OF BIPARENTAL INBREEDING DEPRESSION IN THE SELF-INCOMPATIBLE ANNUAL PLANT RAPHANUS SATIVUS

Studies of inbreeding depression in plant populations have focused primarily on comparisons of selfing versus outcrossing in self-compatible species. Here we examine the effect of five naturally occurring levels of inbreeding (f ranging from 0 to 0.25 by pedigree) on components of lifetime fitness in a field population of the self-incompatible annual, Raphanus sativus. Pre- and postgermination survival and reproductive success were examined for offspring resulting from compatible cross-pollinations. Multiple linear regression of inbreeding level on rates of fruit and seed abortion as well as seed weight and total seed weight per fruit were not significant. Inbreeding level was not found to affect seed germination, offspring survival in the field, date of first flowering, or plant biomass (dry weight minus fruit). The effect of inbreeding on seedling viability in the greenhouse and viability to flowering was significant but small and inconsistently correlated with inbreeding level. Maternal fecundity, however, a measure of seed yield, was reduced almost 60% in offspring from full-sib crosses (f = 0.25) relative to offspring resulting from experimental outcross pollinations (f = 0). Water availability, a form of physiological stress, affected plant biomass but did not affect maternal fecundity, nor did it interact with inbreeding level to influence these characters. The delayed expression of strong inbreeding depression suggests that highly deleterious recessive alleles were not a primary cause of fitness loss with inbreeding. Highly deleterious recessives may have been purged by bottlenecks in population size associated with the introduction of Raphanus and its recent range expansions. In general, reductions in total relative fitness of greater than 50% associated with full-sib crosses should be sufficient to prohibit the evolution of self-compatibility via transmission advantage in Raphanus.     

Inbreeding and incompatibility in Trichogramma nr. brassicae: evidence and implications for quality control

Inbreeding effects and incompatibility relationships were examined in strains of the egg parasitoid Trichogramma nr brassicae (Hymenoptera: Trichogrammatidae) from southeastern Australia. Crosses between strains provided weak evidence of incompatibility in a few cases. However sex ratio in crosses within strains tended to be more female-biased than in crosses between strains. Inbreeding was imposed for four generations (F>0.59) of sib mating. The fitness of inbred strains was compared to that of outbred strains generated by crossing the inbred strains. No effects of inbreeding were found for any of the four female traits examined (fecundity, body length, head width and hind tibia length), indicating that T. nr. brassicae is not subjected to inbreeding depression. Inbreeding effects were also not found for male mating success as expected for the haploid sex. There were differences among strains for all traits apart from fecundity, indicating heritable variation. Strain differences for fitness measures were uncorrelated with wasp size. The potential use of inbreeding in the quality control of Trichogramma for mass-release is discussed. Inbreeding may be a useful tool in minimising the effects of laboratory adaptation, thereby extending the useful life of a strain.     

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.     

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.     

Selection responses of means and inbreeding depression for female fecundity in Drosophila melanogaster suggest contributions from intermediate-frequency alleles to quantitative trait variation

The extent to which quantitative trait variability is caused by rare alleles maintained by mutation, versus intermediate-frequency alleles maintained by balancing selection, is an unsolved problem of evolutionary genetics. We describe the results of an experiment to examine the effects of selection on the mean and extent of inbreeding depression for early female fecundity in Drosophila melanogaster. Theory predicts that rare, partially recessive deleterious alleles should cause a much larger change in the effect of inbreeding than in the mean of the outbred population, with the change in inbreeding effect having an opposite sign to the change in mean. The present experiment fails to support this prediction, suggesting that intermediate-frequency alleles contribute substantially to genetic variation in early fecundity.     

INBREEDING DEPRESSION IN SCHIEDEA GLOBOSA AND S. SALICARIA (CARYOPHYLLACEAE), SUBDIOECIOUS AND GYNODIOECIOUS HAWAIIAN SPECIES

Progeny produced by inbreeding were compared to progeny derived from outcrosses for gynodioecious Schiedea salicaria and subdioecious S. globosa to assess fitness consequences of breeding system on parental fecundity (seeds per capsule) and progeny measures of fitness (germination, survival, biomass, and number of flowers). Results from both species indicated that inbreeding depression occurred at all measured stages of the life history. In both species, different females showed different levels of inbreeding depression. Multiplicative fitness functions of the ratio of values for selfed and outcrossed progeny in S. salicaria resulted in inbreeding depression values of 0.62–0.94. Within- vs. between-family crosses of S. globosa also resulted in inbreeding depression values as high as 0.49. These values suggest that inbreeding depression may promote the evolution of dioecy within S. globosa and S. salicaria, depending on the levels of natural outcrossing.     

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.     

SEX RATIO VARIATION IN A PARASITIC WASP II. DIALLEL CROSS

Sex ratio has been studied from many theoretical and empirical perspectives, but a general assumption in sex ratio research is that changes in sex ratio occur because of selection on sex ratio itself. I carried out a quantitative genetic experiment—a diallel cross among three strains—on a parasitic wasp, Muscidifurax raptor (Hymenoptera: Pteromalidae), to measure genetic variation for sex ratio. I also tested whether sex ratio may change as a consequence of selection on other life-history traits by estimating genetic covariances between sex ratio, fecundity, longevity, and development time. Most of the variation among strains could be accounted for by a maternal effect, likely caused by a microsporidian parasite that was transmitted through the West Germany (WG) strain. Genetic variation was small by comparison, but almost all traits were affected by dominance. The only significant additive genetic effect was for fecundity early in life. Upon crossing, all traits displayed heterosis: more female-biased sex ratio, greater fecundity, longer life, and faster development time. All life-history traits were correlated phenotypically, but the correlations were mainly the result of decreased performance in crosses with the WG strain that carried the microsporidian parasite. Dominance genetic correlations were also found between sex ratio, fecundity, and longevity. How the correlation between sex ratio and other life-history traits would affect sex ratio evolution depends upon the frequencies of sex-ratio genotypes within a population as well as the signs of the correlations, because sex ratio is under frequency-dependent selection whereas other traits are generally under directional selection. Although the results from crosses among laboratory populations should be approached with caution, the inbreeding depression (the difference between inbred and outcrossed progeny) found in M. raptor implies that the evolution of a female-biased sex ratio could be affected by selection for inbreeding avoidance.     

The evolution of trade-offs: effects of inbreeding on fecundity relationships in the cricket Gryllus firmus

The evolution of traits is modulated by their interrelationships with each other, particularly when those relationships result in a fitness trade-off. In this paper we explore the consequences of genetic architecture on functional relationships between traits. Specifically, we address the consequences of inbreeding on these relationships. We show that the linear regression between two traits will not be affected if there is no dominance genetic variance in either trait, whereas the intercept but not the slope of the regression will change if there is dominance genetic variance in one trait only. We test the latter hypothesis using fecundity relationships in the cricket Gryllus firmus. Data from pedigree analysis and an inbreeding experiment show that there is significant dominance genetic variance in fecundity, but not head width (an index of body size) or dorsal longitudinal muscle (DLM) mass. Fecundity increases with head width, but decreases with DLM mass. As predicted, the intercepts of the regressions of fecundity on these two morphological traits decrease with inbreeding, but there is little or no change in slope. Gryllus firmus is wing dimorphic, with the macropterous (LW) morph having a lower fecundity than the micropterous (SW) morph. We hypothesize that the difference in fecundity arises primarily because of a competition for resources in the LW females between DLM maintenance (i.e., mass) and egg production. As a consequence, we predict that the fecundity within each morph should decline linearly with the inbreeding coefficient at the same rate in both morphs. The result of this will be a change in the relative fitness of the two morphs, that of the SW morph increasing with inbreeding. This prediction is supported. These results indicate that trade-offs will evolve and such changes will affect evolutionary trajectories by altering the pattern of relationships among fitness components.     

Reproductive and post‐reproductive life history of wild‐caught Drosophila melanogaster under laboratory conditions

The life history of the fruit fly (Drosophila melanogaster) is well understood, but fitness components are rarely measured by following single individuals over their lifetime, thereby limiting insights into lifetime reproductive success, reproductive senescence and post‐reproductive lifespan. Moreover, most studies have examined long‐established laboratory strains rather than freshly caught individuals and may thus be confounded by adaptation to laboratory culture, inbreeding or mutation accumulation. Here, we have followed the life histories of individual females from three recently caught, non‐laboratory‐adapted wild populations of D. melanogaster. Populations varied in a number of life‐history traits, including ovariole number, fecundity, hatchability and lifespan. To describe individual patterns of age‐specific fecundity, we developed a new model that allowed us to distinguish four phases during a female's life: a phase of reproductive maturation, followed by a period of linear and then exponential decline in fecundity and, finally, a post‐ovipository period. Individual females exhibited clear‐cut fecundity peaks, which contrasts with previous analyses, and post‐peak levels of fecundity declined independently of how long females lived. Notably, females had a pronounced post‐reproductive lifespan, which on average made up 40% of total lifespan. Post‐reproductive lifespan did not differ among populations and was not correlated with reproductive fitness components, supporting the hypothesis that this period is a highly variable, random ‘add‐on’ at the end of reproductive life rather than a correlate of selection on reproductive fitness. Most life‐history traits were positively correlated, a pattern that might be due to genotype by environment interactions when wild flies are brought into a novel laboratory environment but that is unlikely explained by inbreeding or positive mutational covariance caused by mutation accumulation.     

Sustainability of the South China tiger: implications of inbreeding depression and introgression

The South China tiger (Panther tigris amoyensis) is critically endangered with 73 remaining individuals living in captivity, all derived from six wild founders since 1963. The population shows a low level of juvenile survivorship and reproductive difficulties, and faces a huge conservation challenge. In this study, inbreeding depression and genetic diversity decline were examined by using pedigree data and 17 microsatellites. The constant B, which is related to the number of lethal equivalents, was estimated to be 0 for the offspring of noninbred parents, but was >0 for the offspring of inbred parents and for all offspring. Percentage of successfully breeding tigers inversely correlated with inbreeding level (r = −0.626, α = 0.05). Taken together, these findings suggest the population is suffering from inbreeding depression in juvenile survivorship and fecundity. No significant correlation was detectable for the mean litter size with f of either dams (r = −0.305, α = 0.46) or kittens (r = 0.105, α = 0.71), indicating litter size was not strongly subject to inbreeding depression. The average number of alleles per locus was 4.24 ± 1.03 (SE), but effective number of alleles was only 2.53 ± 0.91. Twenty-one alleles carried by early breeders at 13 loci were absent in the present breeders and potential breeders. Multilocus heterozygosity was inversely correlated with inbreeding levels (r = −0.601, α = 0.004). These findings suggest rapid allelic diversity loss is occurring in this small captive population and that heterozygosity is being lost as it becomes more inbred. Our phylogenetic analysis supports past work indicating introgression from northern Indochinese tigers in the population. As no wild representatives of the South China tiger can be added to the captive population, we may consider the alternate scenario of further introgression in the interest of countering inbreeding depression and declining genetic diversity.     

OUTCROSSING AND FECUNDITY ADVANTAGE OF FEMALES IN GYNODIOECIOUS CHIONOGRAPHIS JAPONICA VAR. KUROHIMENSIS (LILIACEAE)

The selling rate of hermaphrodites, inbreeding depression, and relative fecundity of females compared to hermaphrodites were estimated for a gynodioecious population of Chionographis japonica var. kurohimensis to test the models for the evolution of gynodioecy. In spite of the high level of selling of hermaphrodites, significant amounts of inbreeding depression were found in seed germination and seedling growth. In addition, females produced more fertilized ovules than hermaphrodites. Nevertheless, the conditions theoretically required for maintaining gynodioecy were not satisfied even if the combined effect of these two factors was considered. Additional causes for the evolution of gynodioecy, including biparental inbreeding, are discussed.     

Inbreeding avoidance in spiders: evidence for rescue effect in fecundity of female spiders with outbreeding opportunity

Selection by inbreeding depression should favour mating biases that reduce the risk of fertilization by related mates. However, equivocal evidence for inbreeding avoidance questions the strength of inbreeding depression as a selective force in the evolution of mating biases. Lack of inbreeding avoidance can be because of low risk of inbreeding, variation in tolerance to inbreeding or high costs of outbreeding. We examined the relationship between inbreeding depression and inbreeding avoidance adaptations under two levels of inbreeding in the spider Oedothorax apicatus, asking whether preference for unrelated sperm via pre- and/or post-copulatory mechanisms could restore female fitness when inbreeding depression increases. Using inbred isofemale lines we provided female spiders with one or two male spiders of different relatedness in five combinations: one male sib; one male nonsib; two male sibs; two male nonsibs; one male sib and one male nonsib. We assessed the effect of mating treatment on fecundity and hatching success of eggs after one and three generations of inbreeding. Inbreeding depression in F1 was not sufficient to detect inbreeding avoidance. In F3, inbreeding depression caused a major decline in fecundity and hatching rates of eggs. This effect was mitigated by complete recovery in fecundity in the sib-nonsib treatment, whereas no rescue effect was detected in the hatching success of eggs. The rescue effect is best explained by post-mating discrimination against kin via differential allocation of resources. The natural history of O. apicatus suggests that the costs of outbreeding may be low which combined with high costs of inbreeding should select for avoidance mechanisms. Direct benefits of post-mating inbreeding avoidance and possibly low costs of female multiple mating can favour polyandry as an inbreeding avoidance mechanism.     

Inbreeding depression in fecundity and inbred line extinction in the bulb mite,Rhizoglyphus robini

This study investigated the magnitude of inbreeding depression in fecundity, and whether the depression is purged during six generations of sib mating in the bulb mite, Rhizoglyphus robini. The progeny resulting from a single generation of brother-sister mating suffered significant inbreeding depression in fecundity. During the following six generations of continuous sib-mating, 58% lines were lost, 45% because of sterility and 13% because of preadult mortality. The lines were then outcrossed, and their inbreeding depression compared with that of the base population. The inbreeding depression for the outcrossed population was 0.15, and for the base population 0.19, but the difference was not significant. The lack of significant purging of inbreeding depression indicates that it was caused either by detrimental genes of small effect, or by the breaking down of overdominant relations between alleles. However, the large proportion of extinct lines points to the former mechanism as a predominant cause of inbreeding depression. Theory predicts that the probability of line extinction with inbreeding increases with its load of mutations. If phenotypic variation in fecundity was partly because of differences in numbers of mutations carried by individuals, the fecundity of the line founder could be expected to correlate with the probability that the line derived from it will survive long-term inbreeding. Indeed, fecundity of founder females was significantly associated with line survival, which suggests that line extinction rate may be used as a method to study individual mutational loads, for example, in studies of sexual selection.