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Inbreeding exposes deleterious recessive alleles in homozygotes, lowering fitness and generating inbreeding depression (ID). Both purging (via selection) and fixation (via drift) should reduce segregating deleterious mutations and ID in more inbred populations. These theoretical predictions are not well-tested in wild populations, which is concerning given purging/fixation have opposite fitness outcomes. We examined how individual- and population-level inbreeding and genomic heterozygosity affected maternal and progeny fitness within and among 12 wild populations of Impatiens capensis. We quantified maternal fitness in home sites, maternal multilocus heterozygosity (using 12,560 single-nucleotide polymorphisms), and lifetime fitness of selfed and predominantly outcrossed progeny in a common garden. These populations spanned a broad range of individual-level (f i = −0.17–0.98) and population-level inbreeding (F IS = 0.25–0.87). More inbred populations contained fewer polymorphic loci, less fecund mothers, and smaller progeny, suggesting higher fixed loads. However, despite appreciable ID (mean: 8.8 lethal equivalents per gamete), ID did not systematically decline in more inbred population. More heterozygous mothers were more fecund and produced fitter progeny in outcrossed populations, but this pattern unexpectedly reversed in highly inbred populations. These observations suggest that persistent overdominance or some other force acts to forestall purging and fixation in these populations. [ABSTRACT FROM AUTHOR]

Incompletely reproductively isolated species often segregate into different microhabitats, even when they are able to survive and reproduce in both habitats. Longer term evolutionary factors may contribute to this lack of cross‐habitat persistence. When reproductive interference reduces immigrant fitness, assortative mating, including self‐fertilization, increases immigrants' fitness in a single generation, but longer term, inbreeding depression may reduce the chance of population persistence. Two California monkeyflower species repeatedly segregate into drier and wetter areas in their zone of sympatry. To test whether inbreeding depression may contribute to the maintenance of this segregation pattern, we transplanted outbred and successively inbred Mimulus guttatus and Mimulus nudatus into their native habitats and heterospecific habitats. We measured germination, survival, and seed set and found that recurrent selfing reduced all aspects of fitness in both species, most strongly in foreign habitats. A simulation model, parameterized from the transplant experiment, found that inbreeding reduced fitness to such an extent that sequentially inbred populations of either species would be unable to persist in heterospecific‐occupied habitats in the absence of continued gene flow. These results demonstrate that individual immigrants are unlikely to form persistent populations and thus, inbreeding depression contributes to the absence of fine‐scale coexistence in this species pair. [ABSTRACT FROM AUTHOR]

Inbreeding depression resulting from partially recessive deleterious alleles is thought to be the main genetic factor preventing self‐fertilizing mutants from spreading in outcrossing hermaphroditic populations. However, deleterious alleles may also generate an advantage to selfers in terms of more efficient purging, while the effects of epistasis among those alleles on inbreeding depression and mating system evolution remain little explored. In this article, we use a general model of selection to disentangle the effects of different forms of epistasis (additive‐by‐additive, additive‐by‐dominance, and dominance‐by‐dominance) on inbreeding depression and on the strength of selection for selfing. Models with fixed epistasis across loci, and models of stabilizing selection acting on quantitative traits (generating distributions of epistasis) are considered as special cases. Besides its effects on inbreeding depression, epistasis may increase the purging advantage associated with selfing (when it is negative on average), while the variance in epistasis favors selfing through the generation of linkage disequilibria that increase mean fitness. Approximations for the strengths of these effects are derived, and compared with individual‐based simulation results. [ABSTRACT FROM AUTHOR]

Most flowering plants are hermaphroditic and experience strong pressures to evolve self‐pollination (automatic selection and reproductive assurance). Inbreeding depression (ID) can oppose selection for selfing, but it remains unclear if ID is typically strong enough to maintain outcrossing. To measure the full cost of sustained inbreeding on fitness, and its genomic basis, we planted highly homozygous, fully genome‐sequenced inbred lines of yellow monkeyflower (Mimulus guttatus) in the field next to outbred plants from crosses between the same lines. The cost of full homozygosity is severe: 65% for survival and 86% for lifetime seed production. Accounting for the unmeasured effect of lethal and sterile mutations, we estimate that the average fitness of fully inbred genotypes is only 3–4% that of outbred competitors. The genome sequence data provide no indication of simple overdominance, but the number of rare alleles carried by a line, especially within rare allele clusters nonrandomly distributed across the genome, is a significant negative predictor of fitness measurements. These findings are consistent with a deleterious allele model for ID. High variance in rare allele load among lines and the genomic distribution of rare alleles both suggest that migration might be an important source of deleterious alleles to local populations. [ABSTRACT FROM AUTHOR]

In many hermaphroditic flowering plants, self‐fertilization is prevented by self‐incompatibility (SI), often controlled by a single locus, the S‐locus. In single isolated populations, the maintenance of SI depends chiefly on inbreeding depression and the number of SI alleles at the S‐locus. In subdivided populations, however, population subdivision has complicated effects on both the number of SI alleles and the level of inbreeding depression, rendering the maintenance of SI difficult to predict. Here, we explore the conditions for the invasion of a self‐compatible mutant in a structured population. We find that the maintenance of SI is strongly compromised when a population becomes subdivided. We show that this effect is mainly caused by the decrease in the local diversity of SI alleles rather than by a change in the dynamics of inbreeding depression. Strikingly, we also find that the diversity of SI alleles at the whole population level is a poor predictor of the maintenance of SI. We discuss the implications of our results for the interpretation of empirical data on the loss of SI in natural populations. [ABSTRACT FROM AUTHOR]

This article analyzes how partial selfing in a large source population influences its ability to colonize a new habitat via the introduction of a few founder individuals. Founders experience inbreeding depression due to partially recessive deleterious alleles as well as maladaptation to the new environment due to selection on a large number of additive loci. I first introduce a simplified version of the inbreeding history model to characterize mutation‐selection balance in a large, partially selfing source population under selection involving multiple nonidentical loci. I then use individual‐based simulations to study the eco‐evolutionary dynamics of founders establishing in the new habitat under a model of hard selection. The study explores how selfing rate shapes establishment probabilities of founders via effects on both inbreeding depression and adaptability to the new environment, and also distinguishes the effects of selfing on the initial fitness of founders from its effects on the long‐term adaptive response of the populations they found. A high rate of (but not complete) selfing is found to aid establishment over a wide range of parameters, even in the absence of mate limitation. The sensitivity of the results to assumptions about the nature of polygenic selection is discussed. [ABSTRACT FROM AUTHOR]

In flowering plants, the evolution of females is widely hypothesized to be the first step in the evolutionary pathway to separate male and female sexes, or dioecy. Natural enemies have the potential to drive this evolution if they preferentially attack hermaphrodites over females. We studied sex‐based differences in exposure to anther‐smut (Microbotryum), a sterilizing pollinator‐transmitted disease, in Dianthus pavonius, a gynodioecious perennial herb. We found that within a heavily diseased population, females consistently had lower levels of Microbotryum spore deposition relative to hermaphrodites and that this difference was driven by rapid floral closing in females following successful pollination. We further show that this protective closing behavior is frequency dependent; females close faster when they are rare. These results indicate that anther‐smut disease is an important source of selection for females, especially since we found in a common garden experiment no evidence that females have any inherent fecundity advantages over hermaphrodites. Finally, we show that among populations, those where anther‐smut is present have a significantly higher frequency of females than those where the disease is absent. Taken together our results indicate that anther‐smut disease is likely an important biotic factor driving the evolution and maintenance of females in this gynodioecious species. [ABSTRACT FROM AUTHOR]

Quantifying sex‐specific additive genetic variance (VA) in fitness, and the cross‐sex genetic correlation (rA), is prerequisite to predicting evolutionary dynamics and the magnitude of sexual conflict. Further, quantifying VA and rA in underlying fitness components, and genetic consequences of immigration and resulting gene flow, is required to identify mechanisms that maintain VA in fitness. However, these key parameters have rarely been estimated in wild populations experiencing natural environmental variation and immigration. We used comprehensive pedigree and life‐history data from song sparrows (Melospiza melodia) to estimate VA and rA in sex‐specific fitness and underlying fitness components, and to estimate additive genetic effects of immigrants alongside inbreeding depression. We found evidence of substantial VA in female and male fitness, with a moderate positive cross‐sex rA. There was also substantial VA in male but not female adult reproductive success, and moderate VA in juvenile survival but not adult annual survival. Immigrants introduced alleles with negative additive genetic effects on local fitness, potentially reducing population mean fitness through migration load, but alleviating expression of inbreeding depression. Our results show that VA for fitness can be maintained in the wild, and be broadly concordant between the sexes despite marked sex‐specific VA in reproductive success. [ABSTRACT FROM AUTHOR]

Abstract: Inbreeding depression is often intensified under environmental stress (i.e., inbreeding–stress interaction). Although the fitness consequences of this phenomenon are well‐described, underlying mechanisms such as an increased expression of deleterious alleles under stress, or a lower capacity for adaptive responses to stress with inbreeding, have rarely been investigated. We investigated a fitness component (egg‐to‐adult viability) and gene‐expression patterns using RNA‐seq analyses in noninbred control lines and in inbred lines of Drosophila melanogaster exposed to benign temperature or heat stress. We find little support for an increase in the cumulative expression of deleterious alleles under stress. Instead, inbred individuals had a reduced ability to induce an adaptive gene regulatory stress response compared to controls. The decrease in egg‐to‐adult viability due to stress was most pronounced in the lines with the largest deviation in the adaptive stress response (R2 = 0.48). Thus, we find strong evidence for a lower capacity of inbred individuals to respond by gene regulation to stress and that this is the main driver of inbreeding‐stress interactions. In comparison, the altered gene expression due to inbreeding at benign temperature showed no correlation with fitness and was pronounced in genomic regions experiencing the highest increase in homozygosity. [ABSTRACT FROM AUTHOR]

Abstract: Inbreeding depression plays a major role in shaping mating systems: in particular, inbreeding avoidance is often proposed as a mechanism explaining extra‐pair reproduction in socially monogamous species. This suggestion relies on assumptions that are rarely comprehensively tested: that inbreeding depression is present, that higher kinship between social partners increases infidelity, and that infidelity reduces the frequency of inbreeding. Here, we test these assumptions using 26 years of data for a cooperatively breeding, socially monogamous bird with high female infidelity, the superb fairy‐wren (Malurus cyaneus). Although inbred individuals were rare (∼6% of offspring), we found evidence of inbreeding depression in nestling mass (but not in fledgling survival). Mother–son social pairings resulted in 100% infidelity, but kinship between a social pair did not otherwise predict female infidelity. Nevertheless, extra‐pair offspring were less likely to be inbred than within‐pair offspring. Finally, the social environment (the number of helpers in a group) did not affect offspring inbreeding coefficients or inbreeding depression levels. In conclusion, despite some agreement with the assumptions that are necessary for inbreeding avoidance to drive infidelity, the apparent scarcity of inbreeding events and the observed levels of inbreeding depression seem insufficient to explain the ubiquitous infidelity in this system, beyond the mother–son mating avoidance. [ABSTRACT FROM AUTHOR]

Mating with relatives has often been shown to negatively affect offspring fitness (inbreeding depression). There is considerable evidence for inbreeding depression due to effects on naturally selected traits, particularly those expressed early in life, but there is less evidence of it for sexually selected traits. This is surprising because sexually selected traits are expected to exhibit strong inbreeding depression. Here, we experimentally created inbred and outbred male mosquitofish ( Gambusia holbrooki). Inbred males were the offspring of matings between full siblings. We then investigated how inbreeding influenced a number of sexually selected male traits, specifically: attractiveness, sperm number and velocity, as well as sperm competitiveness based on a male's share of paternity. We found no inbreeding depression for male attractiveness or sperm traits. There was, however, evidence that lower heterozygosity decreased paternity due to reduced sperm competitiveness. Our results add to the growing evidence that competitive interactions exacerbate the negative effects of the increased homozygosity that arises when there is inbreeding. [ABSTRACT FROM AUTHOR]

I present analytical predictions for the equilibrium inbreeding load expected in a population under mutation, selection, and a regular mating system for any population size and for any magnitude and recessivity of the deleterious effects. Using this prediction, I deduce the relative fitness of mutant alleles with small effect on selfing to explore the situations where selfing or outcrossing are expected to evolve. The results obtained are in agreement with previous literature, showing that natural selection is expected to lead to stable equilibria where populations show either complete outcrossing or complete selfing, and that selfing is promoted by large deleterious mutation rates. I find that the evolution of selfing is favored by a large recessivity of deleterious effects, while the magnitude of homozygous deleterious effects only becomes relevant in relatively small populations. This result contradicts the standard assumption that purging in large populations will only promote selfing when homozygous deleterious effects are large, and implies that previously published results obtained assuming lethal mutations in large populations can be extrapolated to nonlethal alleles of similar recessivity. This conclusion and the general approach used in this analysis can be useful in the study of the evolution of mating systems. [ABSTRACT FROM AUTHOR]

The bimodal distribution of fitness effects of new mutations and standing genetic variation, due to early-acting strongly deleterious recessive mutations and late-acting mildly deleterious mutations, is analyzed using the Kondrashov model for lethals (K), with either the infinitesimal model for selfing (IMS) or the Gaussian allele model (GAM) for quantitative genetic variance under stabilizing selection. In the combined models (KIMS and KGAM) high genomic mutation rates to lethals and weak stabilizing selection on many characters create strong interactions between early and late inbreeding depression, by changing the distribution of lineages selfed consecutively for different numbers of generations. Alternative stable equilibria can exist at intermediate selfing rates for a given set of parameters. Evolution of quantitative genetic variance under multivariate stabilizing selection can strongly influence the purging of nearly recessive lethals, and sometimes vice versa. If the selfing rate at the purging threshold for quantitative genetic variance in IMS or GAM alone exceeds that for nearly recessive lethals in K alone, then in KIMS and KGAM stabilizing selection causes selective interference with purging of lethals, increasing the mean number of lethals compared to K; otherwise, stabilizing selection causes selective facilitation in purging of lethals, decreasing the mean number of lethals. [ABSTRACT FROM AUTHOR]

In flowering plants, shifts from outcrossing to partial or complete self-fertilization have occurred independently thousands of times, yet the underlying adaptive processes are difficult to discern. Selfing's ability to provide reproductive assurance when pollination is uncertain is an oft-cited ecological explanation for its evolution, but this benefit may be outweighed by costs diminishing its selective advantage over outcrossing. We directly studied the fitness effects of a self-compatibility mutation that was backcrossed into a self-incompatible (SI) population of Leavenworthia alabamica, illuminating the direction and magnitude of selection on the mating-system modifier. In array experiments conducted in two years, self-compatible (SC) plants produced 17-26% more seed, but this advantage was counteracted by extensive seed discounting-the replacement of high-quality outcrossed seeds by selfed seeds. Using a simple model and simulations, we demonstrate that SC mutations with these attributes rarely spread to high frequency in natural populations, unless inbreeding depression falls below a threshold value (0.57 ≤ δthreshold ≤ 0.70) in SI populations. A combination of heavy seed discounting and inbreeding depression likely explains why outcrossing adaptations such as self-incompatibility are maintained generally, despite persistent input of selfing mutations, and frequent limits on outcross seed production in nature. [ABSTRACT FROM AUTHOR]

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 ( dNL ≈ 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. [ABSTRACT FROM AUTHOR]

Basic models of mating-system evolution predict that hermaphroditic organisms should mostly either cross-fertilize, or self-fertilize, due to self-reinforcing coevolution of inbreeding depression and outcrossing rates. However transitions between mating systems occur. A plausible scenario for such transitions assumes that a decrease in pollinator or mate availability temporarily constrains outcrossing populations to self-fertilize as a reproductive assurance strategy. This should trigger a purge of inbreeding depression, which in turn encourages individuals to self-fertilize more often and finally to reduce male allocation. We tested the predictions of this scenario using the freshwater snail Physa acuta, a self-compatible hermaphrodite that preferentially outcrosses and exhibits high inbreeding depression in natural populations. From an outbred population, we built two types of experimental evolution lines, controls (outcrossing every generation) and constrained lines (in which mates were often unavailable, forcing individuals to self-fertilize). After ca. 20 generations, individuals from constrained lines initiated self-fertilization earlier in life and had purged most of their inbreeding depression compared to controls. However, their male allocation remained unchanged. Our study suggests that the mating system can rapidly evolve as a response to reduced mating opportunities, supporting the reproductive assurance scenario of transitions from outcrossing to selfing. [ABSTRACT FROM AUTHOR]

Inbreeding depression, the reduced fitness of offspring of related individuals, is a central theme in evolutionary biology. Inbreeding effects are influenced by the genetic makeup of a population, which is driven by any history of genetic bottlenecks and genetic drift. The Chatham Island black robin represents a case of extreme inbreeding following two severe population bottlenecks. We tested whether inbreeding measured by a 20-year pedigree predicted variation in fitness among individuals, despite the high mean level of inbreeding and low genetic diversity in this species. We found that paternal and maternal inbreeding reduced fledgling survival and individual inbreeding reduced juvenile survival, indicating that inbreeding depression affects even this highly inbred population. Close inbreeding also reduced survival for fledglings with less-inbred mothers, but unexpectedly improved survival for fledglings with highly inbred mothers. This counterintuitive interaction could not be explained by various potentially confounding variables. We propose a genetic mechanism, whereby a highly inbred chick with a highly inbred parent inherits a 'proven' genotype and thus experiences a fitness advantage, which could explain the interaction. The positive and negative effects we found emphasize that continuing inbreeding can have important effects on individual fitness, even in populations that are already highly inbred. [ABSTRACT FROM AUTHOR]

Extra-pair reproduction is widely hypothesized to allow females to avoid inbreeding with related socially paired males. Consequently, numerous field studies have tested the key predictions that extra-pair offspring are less inbred than females' alternative within-pair offspring, and that the probability of extra-pair reproduction increases with a female's relatedness to her socially paired male. However, such studies rarely measure inbreeding or relatedness sufficiently precisely to detect subtle effects, or consider biases stemming from failure to observe inbred offspring that die during early development. Analyses of multigenerational song sparrow ( Melospiza melodia) pedigree data showed that most females had opportunity to increase or decrease the coefficient of inbreeding of their offspring through extra-pair reproduction with neighboring males. In practice, observed extra-pair offspring had lower inbreeding coefficients than females' within-pair offspring on average, while the probability of extra-pair reproduction increased substantially with the coefficient of kinship between a female and her socially paired male. However, simulations showed that such effects could simply reflect bias stemming from inbreeding depression in early offspring survival. The null hypothesis that extra-pair reproduction is random with respect to kinship therefore cannot be definitively rejected in song sparrows, and existing general evidence that females avoid inbreeding through extra-pair reproduction requires reevaluation given such biases. [ABSTRACT FROM AUTHOR]

The mode of pollination is often neglected regarding the evolution of selfing. Yet the distribution of mating systems seems to depend on the mode of pollination, and pollinators are likely to interfere with selfing evolution, since they can cause strong selective pressures on floral traits. Most selfing species reduce their investment in reproduction, and display smaller flowers, with less nectar and scents (referred to as selfing syndrome). We model the evolution of prior selfing when it affects both the demography of plants and pollinators and the investment of plants in pollination. Including the selfing syndrome in the model allows to predict several outcomes: plants can evolve either toward complete outcrossing, complete selfing, or to a stable mixed-mating system, even when inbreeding depression is high. We predict that the evolution to high prior selfing could lead to evolutionary suicides, highlighting the importance of merging demography and evolution in models. The consequence of the selfing syndrome on plant-pollinator interactions could be a widespread mechanism driving the evolution of selfing in animal-pollinated taxa. [ABSTRACT FROM AUTHOR]

In what types of environments should we expect to find strong inbreeding depression? Previous studies indicate that inbreeding depression, δ, is positively correlated with the stressfulness of the environment in which it is measured. However, it remains unclear why stress, per se, should increase δ. To our knowledge, only 'competitive stress' has a logical connection to δ. Through competition for resources, better quality (outbred) individuals make the environment worse for lower quality (inbred) individuals, accentuating the differences between them. For this reason, we expect inbreeding depression to be stronger in environments where the fitness of individuals is more sensitive to the presence of conspecifics (i.e., where fitness is more density dependent). Indeed, some studies suggest a role for competition within environments, but this idea has not been tested in the context of understanding variation in δ across environments. Using Drosophila melanogaster, we estimated δ for viability in 22 different environments. These environments were simultaneously characterized for (1) stressfulness and (2) density dependence. Although stress and density dependence are moderately correlated with each other, inbreeding depression is much more strongly correlated with density dependence. These results suggest that mean selection across the genome is stronger in environments where competition is intense, rather than in environments that are stressful for other reasons. [ABSTRACT FROM AUTHOR]

Gametophytic self-incompatibility (GSI) is a widespread genetic system, which enables hermaphroditic plants to avoid self-fertilization and mating with close relatives. Inbreeding depression is thought to be the major force maintaining SI; however, inbreeding depression is a dynamical variable that depends in particular on the mating system. In this article we use multilocus, individual-based simulations to examine the coevolution of SI and inbreeding depression within finite populations. We focus on the conditions for the maintenance of SI when self-compatible (SC) mutants are introduced in the population by recurrent mutation, and compare simulation results with predictions from an analytical model treating inbreeding depression as a fixed parameter (thereby neglecting effects of purging within the SC subpopulation). In agreement with previous models, we observe that the maintenance of SI is associated with high inbreeding depression and is facilitated by high rates of self-pollination. Purging of deleterious mutations by SC mutants has little effect on the spread of those mutants as long as most deleterious alleles have weak fitness effects: in this case, the genetic architecture of inbreeding depression has little effect on the maintenance of SI. By contrast, purging may greatly enhance the spread of SC mutants when deleterious alleles have strong fitness effects. [ABSTRACT FROM AUTHOR]

Inbreeding mating systems are uncommon because of inbreeding depression. Mating among close relatives can evolve, however, when outcrossing is constrained. Social spiders show obligatory mating among siblings. In combination with a female-biased sex ratio, sib-mating results in small effective populations. In such a system, high genetic homozygosity is expected, and drift may cause population divergence. We tested the effect of outcrossing in the social spider Stegodyphus dumicola. Females were mated to sib-males, to a non-nestmate within the population, or to a male from a distant population, and fitness traits of F1s were compared. We found reduced hatching success of broods from between-population crosses, suggesting the presence of population divergence at a large geographical scale that may result in population incompatibility. However, a lack of a difference in offspring performance between inbred and outbred crosses indicates little genetic variation between populations, and could suggest recent colonization by a common ancestor. This is consistent with population dynamics of frequent colonizations by single sib-mated females of common origin, and extinctions of populations after few generations. Although drift or single mutations can lead to population divergence at a relatively short time scale, it is possible that dynamic population processes homogenize these effects at longer time scales. [ABSTRACT FROM AUTHOR]

In principle, parental relatedness, parental age, and the age of parental gametes can all influence offspring fitness through inbreeding depression and the parental effects of organismal and postmeiotic gametic senescence. However, little is known about the extent to which these factors interact and contribute to fitness variation. Here, we show that, in Drosophila melanogaster, offspring viability is strongly affected by a three-way interaction between parental relatedness, parental age, and gametic age at successive developmental stages. Overall egg-to-adult viability was lowest for offspring produced with old gametes of related, young parents. This overall effect was largely determined at the pupa-adult stage, although three-way interactions between parental relatedness, parental age and gametic age also explained variation in egg hatchability and larva-pupa survival. Controlling for the influence of parental and gametic age, we show that inbreeding depression is negligible for egg hatchability but significant at the larva-pupa and pupa-adult stages. At the pupa-adult stage, where offspring could be sexed, parental relatedness, parental age, and gametic age interacted differently in male and female offspring, with daughters suffering higher inbreeding depression than sons. Collectively, our results demonstrate that the architecture of offspring fitness is strongly influenced by a complex interaction between parental effects, inbreeding depression and offspring sex. [ABSTRACT FROM AUTHOR]

Inbreeding depression is a key factor influencing mating system evolution in plants, but current understanding of its relationship with selfing rate is limited by a sampling bias with few estimates for self-incompatible species. We quantified inbreeding depression (δ) over two growing seasons in two populations of the self-incompatible perennial herb Arabidopsis lyrata ssp. petraea in Scandinavia. Inbreeding depression was strong and of similar magnitude in both populations. Inbreeding depression for overall fitness across two seasons (the product of number of seeds, offspring viability, and offspring biomass) was 81% and 78% in the two populations. Chlorophyll deficiency accounted for 81% of seedling mortality in the selfing treatment, and was not observed among offspring resulting from outcrossing. The strong reduction in both early viability and late quantitative traits suggests that inbreeding depression is due to deleterious alleles of both large and small effect, and that both populations experience strong selection against the loss of self-incompatibility. A review of available estimates suggested that inbreeding depression tends to be stronger in self-incompatible than in self-compatible highly outcrossing species, implying that undersampling of self-incompatible taxa may bias estimates of the relationship between mating system and inbreeding depression. [ABSTRACT FROM AUTHOR]

Inbreeding depression has become a central theme in evolutionary biology and is considered to be a driving force for the evolution of reproductive morphology, physiology, behavior, and mating systems. Despite the overwhelming body of empirical work on the reproductive consequences of inbreeding, relatively little is known on whether inbreeding depresses male and female fitness to the same extent. However, sex-specific inbreeding depression has been argued to affect the evolution of selfing rates in simultaneous hermaphrodites and provides a powerful approach to test whether selection is stronger in males than in females, which is predicted to be the consequence of sexual selection. We tested for sex-specific inbreeding depression in the simultaneously hermaphroditic freshwater snail Physa acuta by comparing the reproductive performance of both sex functions between selfed and outcrossed focal individuals under different levels of male-male competition. We found that inbreeding impaired both male and female reproductive success and that the magnitude of male inbreeding depression exceeded female inbreeding depression when the opportunity for sperm competition was highest. Our study provides the first evidence for sex-specific inbreeding depression in a hermaphroditic animal and highlights the importance of considering the level of male-male competition when assessing sex differences in inbreeding depression. [ABSTRACT FROM AUTHOR]

Gynodioecy, the co-occurrence of females and hermaphrodites, is often due to conflicting interactions between cytoplasmic male sterility genes and nuclear restorers. Although gynodioecy often occurs in self-compatible species, the effect of self-pollination, inbreeding depression, and pollen limitation acting differently on females and hermaphrodites remains poorly known in the case of nuclear-cytoplasmic gynodioecy (NCG). In this study, we model NCG in an infinite population and we study the effect of selfing rate, inbreeding depression, and pollen limitation on the maintenance of gynodioecy and on sex ratios at equilibrium. We found that selfing and inbreeding depression have a strong impact, which depends on whether restorer cost acts on male or female fitness. When cost affects male fitness, the strength of cost has no effect, whereas selfing and inbreeding depression only impact gynodioecy by modifying the value of the female advantage. When cost affects female fitness, selfing facilitates NCG and reduces the role of strength of the cost, even when no inbreeding depression occurs, whereas inbreeding depression globally restricts the maintenance of the polymorphism. Finally, we found that pollen limitation could additionally strongly modify the dynamic of gynodioecy. We discuss our findings in the light of empirical data available in gynodioecious species. [ABSTRACT FROM AUTHOR]

The handicap principle predicts that sexual traits are more susceptible to inbreeding depression than nonsexual traits. However, this hypothesis has received little testing and results are inconsistent. We used 11 generations of full-sibling mating to test the effect of inbreeding on sexual and nonsexual traits in the stalk-eyed fly Diasemopsis meigenii. Consistent with the theoretical predictions, the male sexual trait (eyespan) decreased more than nonsexual traits (female eyespan and male wing length), even after controlling for body size variation. In addition, male eyespan was a reliable predictor of line extinction, unlike other nonsexual traits. After 11 generations, inbred lines were crossed to generate inbred and outbred families. All morphological traits were larger in outbred individuals than inbred individuals. This heterosis was greater in male eyespan than in male wing length, but not female eyespan. The elevated response in male eyespan to genetic stress mirrored the result found using environmental stress during larval development and suggests that common mechanisms underlie the patterns observed. Overall, these results support the hypothesis that male sexual traits suffer more from inbreeding depression than nonsexual traits and are in line with predictions based on the handicap principle. [ABSTRACT FROM AUTHOR]

Molecular estimates of inbreeding may be made using genetic markers such as microsatellites, however the interpretation of resulting heterozygosity-fitness correlations (HFCs) with respect to inbreeding depression is not straightforward. We investigated the relationship between pedigree-determined inbreeding coefficients ( f) and HFCs in a closely monitored, reintroduced population of Stewart Island robins ( Petroica australis rakiura) on Ulva Island, New Zealand. Using a full sibling design, we focused on differences in juvenile survival associated specifically with individual sibling variation in standardized multilocus heterozygosity (SH) when expected f was identical. We found that within broods, siblings with higher SH at microsatellite loci experienced a higher probability of juvenile survival. This effect, however, was detected primarily within broods that experienced inbreeding or when inbreeding had occurred in their pedigree histories (i.e., at the parents' level). Thus we show, for the first time in a wild population, that the strength of an HFC is partially dependent on the presence of inbreeding events in the recent pedigree history. Our results illustrate the importance of realized effects of inbreeding on genetic variation and fitness and the value of full-sibling designs for the study of HFCs in the context of small, inbred populations. [ABSTRACT FROM AUTHOR]

The origin and maintenance of separate sexes (dioecy) is an enduring evolutionary puzzle. Although both hermaphroditism and dioecy occur in many diverse clades, we know little about the long-term evolutionary consequences of changing sexual system. Here we find evidence for at least 133 transitions between sexual systems in mosses, representing an almost unparalleled lability in the evolution of their sexual systems. Furthermore, in contrast to predictions, the transition rate from hermaphroditism to dioecy was approximately twice as high as the reverse transition. Our results also suggest that hermaphrodites may have higher rates of diversification than dioecious mosses. These results illustrate the utility of mosses for understanding the genomic and macroevolutionary consequences of hermaphroditism and dioecy. [ABSTRACT FROM AUTHOR]

Theory predicts that positive heterozygosity-fitness correlations (HFCs) arise as a consequence of inbreeding, which is often assumed to have a strong impact in small, fragmented populations. Yet according to empirical data, HFC in such populations seem highly variable and unpredictable. We here discuss two overlooked phenomena that may contribute to this variation. First, in a small population, each generation may consist of a few families. This generates random correlations between particular alleles and fitness (AFCs, allele-fitness correlations) and results in too liberal tests for HFC. Second, in some contexts, small populations receiving immigrants may be more impacted by outbreeding depression than by inbreeding depression, resulting in negative rather than positive HFC. We investigated these processes through a case study in tadpole cohorts of Pelodytes punctatus living in small ponds. We provide evidence for a strong family structure and significant AFC in this system, as well as an example of negative HFC. By simulations, we show that this negative HFC cannot be a spurious effect of family structure, and therefore reflects outbreeding depression in the studied population. Our example suggests that a detailed examination of AFC and HFC patterns can provide valuable insights into the internal genetic structure and sources of fitness variation in small populations. [ABSTRACT FROM AUTHOR]

Inbreeding depression can reduce the performance of offspring produced by mating between relatives, with consequences for population dynamics and sexual-system evolution. In flowering plants, inbreeding depression commonly acts most intensely during seed development. This predispersal component is typically estimated by comparing seed production following exclusive self- and cross-pollination, but such estimates are unbiased only if seed production is limited by ovule availability, rather than by pollen receipt or seed-development resources. To overcome this problem, we propose experimental and statistical methods based on a model of ovule fertilization and seed development that accounts for differential fertilization by self- and cross-pollen, limited ovule viability or receptivity, differential survival of self- and cross-zygotes and limited resource availability. Simulations illustrate that the proposed methods eliminate bias in estimated predispersal inbreeding depression caused by pollen limitation and can improve estimates under resource limitation. Application of these methods to two orchid species further demonstrates their utility in identifying and estimating diverse influences on reproductive performance under typical conditions. Although our theoretical results raise questions about the reported intensity of predispersal inbreeding depression, our proposed methods guard against bias while also providing insight into plant reproduction. [ABSTRACT FROM AUTHOR]

We model the impact of pollinator visitation rate and behavior on the short-term evolution of population flowering phenologies determined by the distributions of flowering times within and among individual plants. Evolution of population flowering phenologies depends on the phenotypic variances and heritabilities of the within-individual mean and variance of flowering time. In the ecological scenarios we investigate selection does not produce a correlation of the mean and variance of individual flowering time. Self-incompatibility causes weak stabilizing selection on flowering time that acts to reduce the within-individual variance in flowering time. Disruptive selection due to pollinator limitation acts mostly to increase the among-individual variance in flowering time. Stabilizing selection due to pollinator attraction, or short reproductive season, acts mostly to decrease the within-individual variance in flowering time. Temporal autocorrelation of environmental stochasticity in pollinator visitation rate strongly selects to increase the within-individual variance in flowering time. These predictions can be tested by measuring the causal factors described above, partitioning the variance in population phenology within and among individuals, and estimating the inheritance of, and selection on, within-individual mean and variance of flowering time. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Despite fundamental importance to population dynamics, mating system evolution, and conservation management, the fitness consequences of breeding patterns in natural settings are rarely directly and rigorously evaluated. We experimentally crossed Echinacea angustifolia, a widespread, perennial prairie plant undergoing radical changes in distribution and abundance due to habitat fragmentation. We quantified the effects of both biparental inbreeding and crossing between remnant populations on progeny survival and reproduction in the field over the first eight years. Lifetime fitness is notoriously difficult to assess particularly for iteroparous species because of the long sequence and episodic nature of selection events. Even with fitness data in hand, analysis is typically plagued by nonnormal distributions of overall fitness that violate the assumptions of the usual parametric statistical approaches. We applied aster modeling, which integrates the measurements of separate, sequential, nonnormally distributed annual fitness components, and estimated current biparental inbreeding depression at 68% in progeny of sibling-mating. The effect of between-remnant crossing on fitness was negligible. Given that relatedness among individuals in remnant populations is already high and dispersal very limited, inbreeding depression may profoundly affect future dynamics and persistence of these populations, as well as their genetic composition. [ABSTRACT FROM AUTHOR]

Recent theoretical advances have suggested that various forms of balancing selection may promote the evolution of dominance through an increase of the proportion of heterozygote genotypes. We test whether dominance can evolve in the sporophytic self-incompatibility (SSI) system in plants. SSI prevents mating between individuals expressing identical SI phenotypes by recognition of pollen by pistils, which avoids selfing and inbreeding depression. SI phenotypes depend on a complex network of dominance relationships between alleles at the self-incompatibility locus ( S-locus). Empirical studies suggest that these relationships are not random, but the exact evolutionary processes shaping these relationships remain unclear. We investigate the expected patterns of dominance under the hypothesis that dominance is a direct target of natural selection. We follow the fate of a mutant allele at the S-locus whose dominance relationships are changed but whose specificity remains unaltered. We show that strict codominance is not evolutionarily stable in SSI, and that inbreeding depression due to deleterious mutations linked or unlinked to the S-locus exerts strong constraints on changes in relative levels of dominance in pollen and pistil. Our results provide a general adaptive explanation for most patterns of dominance relationships empirically observed in natural plant populations. [ABSTRACT FROM AUTHOR]

After over a half century of empirical and theoretical research regarding the evolution and maintenance of gynodioecy in plants, unexplored factors influencing the relative fitnesses of females and hermaphrodites remain. Theoretical studies suggest that hermaphrodite self-fertilization (selfing) rate influences the maintenance of gynodioecy and we hypothesized that population sex ratio may influence hermaphrodite selfing rate. An experimental test for frequency-dependent self-fertilization was conducted using replicated populations constructed with different sex ratios of the gynodioecious plant Silene vulgaris. We found that hermaphrodite selfing increased with decreased hermaphrodite frequency, whereas evidence for increased inbreeding depression was equivocal. We argue that incorporation of context dependent inbreeding into future models of the evolution of gynodioecy is likely to yield novel insights into sex ratio evolution. [ABSTRACT FROM AUTHOR]

The extent to which indirect genetic benefits can drive the evolution of directional mating preferences for more ornamented mates, and the mechanisms that maintain such preferences without depleting genetic variance, remain key questions in evolutionary ecology. We used an individual-based genetic model to examine whether a directional preference for mates with higher genome-wide heterozygosity ( H), and consequently greater ornamentation, could evolve and be maintained in the absence of direct fitness benefits of mate choice. We specifically considered finite populations of varying size and spatial genetic structure, in which parent–offspring resemblance in heterozygosity could provide an indirect benefit of mate choice. A directional preference for heterozygous mates evolved under broad conditions, even given a substantial direct cost of mate choice, low mutation rate, and stochastic variation in the link between individual heterozygosity and ornamentation. Furthermore, genetic variance was retained under directional sexual selection. Preference evolution was strongest in smaller populations, but weaker in populations with greater internal genetic structure in which restricted dispersal increased local inbreeding among offspring of neighboring females that all preferentially mated with the same male. These results suggest that directional preferences for heterozygous or outbred mates could evolve and be maintained in finite populations in the absence of direct fitness benefits, suggesting a novel resolution to the lek paradox. [ABSTRACT FROM AUTHOR]

Stressful environments are often said to increase the expression of inbreeding depression. Alternatively, Crow's “opportunity for selection” (the squared phenotypic coefficient of variation) sets a limit to how much selection can occur, constraining the magnitude of inbreeding depression. To test these hypotheses, we planted self- and cross-fertilized seeds of Brassica rapa into a factorial experiment that varied plant density and saline watering stresses. We then repeated the experiment, reducing the salt concentration. We observed considerable inbreeding depression, particularly for survival in the first experiment and growth in the second. Both stresses independently depressed plant performance. Families differed in their amounts of inbreeding depression and reaction norms across environments. Outcrossed progeny were sometimes more variable. Stresses had small and inconsistent effects on inbreeding depression and, when significant, tended to diminish it. Levels of phenotypic variability often predicted whether inbreeding depression would increase or decrease across environments and were particularly effective in predicting which traits display the most inbreeding depression. Thus, we find little support for the stress hypothesis and mixed support for the phenotypic variability hypothesis. Variable levels of phenotypic variation provide a parsimonious explanation for shifts in inbreeding depression that should be tested before invoking more complex hypotheses. [ABSTRACT FROM AUTHOR]

Inbreeding load, a key parameter in evolutionary ecology, is frequently estimated by regressing fitness (or related traits) on inbreeding coefficient across population members. This approach assumes that inbreeding occurs randomly with respect to an individual's intrinsic ability to produce fit offspring; estimated loads might otherwise be biased by covariation between inbreeding and individual quality. This assumption, however, is rarely validated. We tested whether, in free-living song sparrows Melospiza melodia, an individual's observed kinship with its social mate (and hence the degree of inbreeding in which an individual participated) was correlated with specific phenotypic traits that are likely to indicate individual quality. Males (and to some extent females) that hatched earlier within their cohort, had shorter tarsi, bred earlier during their first year, or survived fewer years paired with more closely related mates and therefore produced relatively inbred offspring. These correlations arose because males with specific phenotypes were more closely related to the female population (and therefore more likely to pair with closer relatives under random pairing), and because males with specific phenotypes paired with closer relatives than expected. Such correlations could bias estimated inbreeding loads, and should be considered in quantitative genetic analyses of phenotypic variance in populations in which inbreeding occurs. [ABSTRACT FROM AUTHOR]