How relatedness between mates influences reproductive success: An experimental analysis of self‐fertilization and biparental inbreeding in a marine bryozoan

Kin associations increase the potential for inbreeding. The potential for inbreeding does not, however, make inbreeding inevitable. Numerous factors influence whether inbreeding preference, avoidance, or tolerance evolves, and, in hermaphrodites where both self‐fertilization and biparental inbreeding are possible, it remains particularly difficult to predict how selection acts on the overall inbreeding strategy, and to distinguish the type of inbreeding when making inferences from genetic markers. Therefore, we undertook an empirical analysis on an understudied type of mating system (spermcast mating in the marine bryozoan, Bugula neritina) that provides numerous opportunities for inbreeding preference, avoidance, and tolerance. We created experimental crosses, containing three generations from two populations to estimate how parental reproductive success varies across parental relatedness, ranging from self, siblings, and nonsiblings from within the same population. We found that the production of viable selfed offspring was extremely rare (only one colony produced three selfed offspring) and biparental inbreeding more common. Paternity analysis using 16 microsatellite markers confirmed outcrossing. The production of juveniles was lower for sib mating compared with nonsib mating. We found little evidence for consistent inbreeding, in terms of nonrandom mating, in adult samples collected from three populations, using multiple population genetic inferences. Our results suggest several testable hypotheses that potentially explain the overall mating and dispersal strategy in this species, including early inbreeding depression, inbreeding avoidance through cryptic mate choice, and differential dispersal distances of sperm and larvae.     

Life history mediates mate limitation and population viability in self‐incompatible plant species

Genetically controlled self‐incompatibility systems represent links between genetic diversity and plant demography with the potential to directly impact on population dynamics. We use an individual‐based spatial simulation to investigate the demographic and genetic consequences of different self‐incompatibility systems for plants that vary in reproductive capacity and lifespan. The results support the idea that, in the absence of inbreeding effects, populations of self‐incompatible species will often be smaller and less viable than self‐compatible species, particularly for shorter‐lived organisms or where potential fecundity is low. At high ovule production and low mortality, self‐incompatible and self‐compatible species are demographically similar, thus self‐incompatibility does not automatically lead to reduced mate availability or population viability. Overall, sporophytic codominant self‐incompatibility was more limiting than gametophytic or sporophytic dominant systems, which generally behaved in a similar fashion. Under a narrow range of conditions, the sporophytic dominant system maintained marginally greater mate availability owing to the production of S locus homozygotes. While self‐incompatibility reduces population size and persistence for a broad range of conditions, the actual number of S alleles, beyond that required for reproduction, is important for only a subset of life histories. For these situations, results suggest that addition of new S alleles may result in significant demographic rescue.     

Electrostatic potentials of the S‐locus F‐box proteins contribute to the pollen S specificity in self‐incompatibility in Petunia hybrida

Self‐incompatibility (SI) is a self/non‐self discrimination system found widely in angiosperms and, in many species, is controlled by a single polymorphic S‐locus. In the Solanaceae, Rosaceae and Plantaginaceae, the S‐locus encodes a single S‐RNase and a cluster of S‐locus F‐box (SLF) proteins to control the pistil and pollen expression of SI, respectively. Previous studies have shown that their cytosolic interactions determine their recognition specificity, but the physical force between their interactions remains unclear. In this study, we show that the electrostatic potentials of SLF contribute to the pollen S specificity through a physical mechanism of ‘like charges repel and unlike charges attract’ between SLFs and S‐RNases in Petunia hybrida. Strikingly, the alteration of a single C‐terminal amino acid of SLF reversed its surface electrostatic potentials and subsequently the pollen S specificity. Collectively, our results reveal that the electrostatic potentials act as a major physical force between cytosolic SLFs and S‐RNases, providing a mechanistic insight into the self/non‐self discrimination between cytosolic proteins in angiosperms.     

STRONG INBREEDING DEPRESSION IN TWO SCANDINAVIAN POPULATIONS OF THE SELF‐INCOMPATIBLE PERENNIAL HERB ARABIDOPSIS LYRATA

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.     

The loss of self‐incompatibility in a range expansion

It is commonly observed that plant species' range margins are enriched for increased selfing rates and, in otherwise self‐incompatible species, for self‐compatibility (SC). This has often been attributed to a response to selection under mate and/or pollinator limitation. However, range expansion can also cause reduced inbreeding depression, and this could facilitate the evolution of selfing in the absence of mate or pollinator limitation. Here, we explore this idea using spatially explicit individual‐based simulations of a range expansion, in which inbreeding depression, variation in self‐incompatibility (SI), and mate availability evolve. Under a wide range of conditions, the simulated range expansion brought about the evolution of selfing after the loss of SI in range‐marginal populations. Under conditions of high recombination between the self‐incompatibility locus (S‐locus) and viability loci, SC remained marginal in the expanded metapopulation and could not invade the range core, which remained self‐incompatible. In contrast, under low recombination and migration rates, SC was frequently able to displace SI in the range core by maintaining its association with a genomic background with purged genetic load. We conclude that the evolution of inbreeding depression during a range expansion promotes the evolution of SC at range margins, especially under high rates of recombination.?     

GENETIC ARCHITECTURE OF INBREEDING DEPRESSION AND THE MAINTENANCE OF GAMETOPHYTIC SELF‐INCOMPATIBILITY

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.     

Consequences of genetic incompatibility on fitness and mate choice: the male point of view

Hymenopterans under single‐locus complementary sex determination (sl‐CSD) face inbreeding costs due to this sex determination mode. Under sl‐CSD, homozygote eggs at the sl‐CSD locus usually develop into unviable or sterile diploid males. Production of such costly males increases when sib‐mating happens because related individuals share half of their genome. In the hymenopteran Venturia canescens (a solitary parasitoid wasp), diploid males are sterile, leading to fitness costs through genetic incompatibility between parents. Whereas the costs of producing diploid males and behavioural strategies that would reduce such costs have been studied in females, the potential fitness costs faced by males have not. Here, we aimed to investigate fitness costs that males endure after a single sib‐mating and tested whether they have the ability to avoid sib‐mating through kin recognition. Our results show that males have a reduced fitness (i.e. they produce fewer daughters) when mating with their sibs. We also show that males have the ability to distinguish between non‐sib and sib females (i.e. kin). They use chemical marks emitted by the females to discriminate kin from non‐kin. We discuss the evolution of kin recognition in males in the context of mate choice for genetic compatibility. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 279–286.     

Ubiquitin–proteasome‐mediated degradation of S‐RNase in a solanaceous cross‐compatibility reaction

Many plants have a self‐incompatibility (SI) system in which the rejection of self‐pollen is determined by multiple haplotypes at a single locus, termed S. In the Solanaceae, each haplotype encodes a single ribonuclease (S‐RNase) and multiple S‐locus F‐box proteins (SLFs), which function as the pistil and pollen SI determinants, respectively. S‐RNase is cytotoxic to self‐pollen, whereas SLFs are thought to collaboratively recognize non‐self S‐RNases in cross‐pollen and detoxify them via the ubiquitination pathway. However, the actual mechanism of detoxification remains unknown. Here we isolate the components of a SCF^SLF (SCF = SKP1‐CUL1‐F‐box‐RBX1) from Petunia pollen. The SCF^SLF polyubiquitinates a subset of non‐self S‐RNases in vitro. The polyubiquitinated S‐RNases are degraded in the pollen extract, which is attenuated by a proteasome inhibitor. Our findings suggest that multiple SCF^SLF complexes in cross‐pollen polyubiquitinate non‐self S‐RNases, resulting in their degradation by the proteasome.     

Insects and incest: Sib‐mating tolerance in natural populations of a parasitoid wasp

Sib‐mating avoidance is a pervasive behaviour that is expected to evolve in species subject to inbreeding depression. Although laboratory studies provide elegant demonstrations, small‐scaled bioassays minimize the costs of mate finding and choice, and thus may produce spurious findings. We therefore combined laboratory experiments with field observations to examine the existence of inbreeding avoidance using the parasitoid wasp Venturia canescens. In the laboratory, our approach consisted of mate‐choice experiments to assess kin discrimination in population cages with competitive interactions. A higher mating probability after sib rejections suggested that females could discriminate their sibs; however, in contrast to previous findings, sib‐mating avoidance was not observed. To compare our laboratory results to field data, we captured 241 individuals from two populations. Females laid eggs in the lab, and 226 daughters were obtained. All individuals were genotyped at 18 microsatellite loci, which allowed inference of the genotype of each female's mate and subsequently the relatedness within each mating pair. We found that the observed rate of sib‐mating did not differ from the probability that sibs encountered one another at random in the field, which is consistent with an absence of sib‐mating avoidance. In addition, we detected a weak but significant male‐biased dispersal, which could reduce encounters between sibs. We also found weak fitness costs associated with sib‐mating. As such, the sex‐biased dispersal that we found is probably sufficient to mitigate these costs. These results imply that kin discrimination has probably evolved for purposes other than mate choice, such as superparasitism avoidance.     

Pollen S–locus F–box proteins of Petunia involved in S–RNase‐based self‐incompatibility are themselves subject to ubiquitin‐mediated degradation

Many flowering plants show self‐incompatibility, an intra‐specific reproductive barrier by which pistils reject self‐pollen to prevent inbreeding and accept non‐self pollen to promote out‐crossing. In Petunia, the polymorphic S–locus determines self/non‐self recognition. The locus contains a gene encoding an S–RNase, which controls pistil specificity, and multiple S‐locus F‐box (SLF) genes that collectively control pollen specificity. Each SLF is a component of an SCF (Skp1/Cullin/F‐box) complex that is responsible for mediating degradation of non‐self S‐RNase(s), with which the SLF interacts, via the ubiquitin–26S proteasome pathway. A complete set of SLFs is required to detoxify all non‐self S‐RNases to allow cross‐compatible pollination. Here, we show that SLF1 of Petunia inflata is itself subject to degradation via the ubiquitin–26S proteasome pathway, and identify an 18 amino acid sequence in the C‐terminal region of S₂‐SLF1 (SLF1 of S₂ haplotype) that contains a degradation motif. Seven of the 18 amino acids are conserved among all 17 SLF proteins of S₂ haplotype and S₃ haplotype involved in pollen specificity, suggesting that all SLF proteins are probably subject to similar degradation. Deleting the 18 amino acid sequence from S₂‐SLF1 stabilized the protein but abolished its function in self‐incompatibility, suggesting that dynamic cycling of SLF proteins is an integral part of their function in self‐incompatibility.     

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

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

THE POPULATION GENETICS OF SPOROPHYTIC SELF‐INCOMPATIBILITY IN THREE HYBRIDIZING SENECIO (ASTERACEAE) SPECIES WITH CONTRASTING POPULATION HISTORIES

Hybridization generates evolutionary novelty and spreads adaptive variation. By promoting outcrossing, plant self‐incompatibility (SI) systems also favor interspecific hybridization because the S locus is under strong negative frequency‐dependent balancing selection. This study investigates the SI mating systems of three hybridizing Senecio species with contrasting population histories. Senecio aethnensis and S. chrysanthemifolius native to Sicily, form a hybrid zone at intermediate altitudes on Mount Etna, and their neo‐homoploid hybrid species, S. squalidus, has colonized disturbed urban habitats in the UK during the last 150 years. We show that all three species express sporophytic SI (SSI), where pollen incompatibility is controlled by the diploid parental genome, and that SSI is inherited and functions normally in hybrids. Large‐scale crossing studies of wild sampled populations allowed direct comparison of SSI between species and found that the main impacts of colonization in S. squalidus compared to Sicilian Senecio was a reduced number of S alleles, increased S allele frequencies, and increased interpopulation S allele sharing. In general, many S alleles were shared between species and the S locus showed reduced intra‐ and interspecific population genetic structure compared to molecular genetic markers, indicative of enhanced effective gene flow due to balancing selection.     

Effects of pollen availability and the mutation bias on the fixation of mutations disabling the male specificity of self‐incompatibility

The evolution of self‐compatibility (SC) by the loss of self‐incompatibility (SI) is regarded as one of the most frequent transitions in flowering plants. SI systems are generally characterized by specific interactions between the male and female specificity genes encoded at the S‐locus. Recent empirical studies have revealed that the evolution of SC is often driven by male SC‐conferring mutations at the S‐locus rather than by female mutations. In this study, using a forward simulation model, we compared the fixation probabilities of male vs. female SC‐conferring mutations at the S‐locus. We explicitly considered the effects of pollen availability in the population and bias in the occurrence of SC‐conferring mutations on the male and female specificity genes. We found that male SC‐conferring mutations were indeed more likely to be fixed than were female SC‐conferring mutations in a wide range of parameters. This pattern was particularly strong when pollen availability was relatively high. Under such a condition, even if the occurrence of mutations was biased strongly towards the female specificity gene, male SC‐conferring mutations were much more often fixed. Our study demonstrates that fixation probabilities of those two types of mutation vary strongly depending on ecological and genetic conditions, although both types result in the same evolutionary consequence—the loss of SI.     

DOES MATE LIMITATION IN SELF‐INCOMPATIBLE SPECIES PROMOTE THE EVOLUTION OF SELFING? THE CASE OF LEAVENWORTHIA ALABAMICA

Genetic diversity at the S‐locus controlling self‐incompatibility (SI) is often high because of negative frequency‐dependent selection. In species with highly patchy spatial distributions, genetic drift can overwhelm balancing selection and cause stochastic loss of S‐alleles. Natural selection may favor the breakdown of SI in populations with few S‐alleles because low S‐allele diversity constrains the seed production of self‐incompatible plants. We estimated S‐allele diversity, effective population sizes, and migration rates in Leavenworthia alabamica, a self‐incompatible mustard species restricted to discrete habitat patches in rocky glades. Patterns of polymorphism were investigated at the S‐locus and 15 neutral microsatellites in three large and three small populations with 100‐fold variation in glade size. Populations on larger glades maintained more S‐alleles, but all populations were estimated to harbor at least 20 S‐alleles, and mate availabilities typically exceeded 0.80, which is consistent with little mate limitation in nature. Estimates of the effective size (N_e) in each population ranged from 600 to 1600, and estimated rates of migration (m) ranged from 3 × 10⁻⁴ to nearly 1 × 10⁻³. According to theoretical models, there is limited opportunity for genetic drift to reduce S‐allele diversity in populations with these attributes. Although pollinators or resources limit seed production in small glades, limited S‐allele diversity does not appear to be a factor promoting the incipient breakdown of SI in populations of this species that were studied.     

THE EFFECTS OF PREDATION RISK ON MATING SYSTEM EXPRESSION IN A FRESHWATER SNAIL

Environmental effects on mating system expression are central to understanding mating system evolution in nature. Here, I report the results from a quantitative‐genetic experiment aimed at understanding the role of predation risk in the expression and evolution of life‐history and mating‐system traits in a hermaphroditic freshwater snail (Physa acuta). I reared 30 full‐sib families in four environments that factorially contrast predation risk and mate availability and measured age/size at first reproduction, growth rate, a morphological defense, and the early survival of outcrossed/selfed eggs that were laid under predator/no‐predator conditions. I evaluated the genetic basis of trade‐offs among traits and the stability of the G matrix across environments. Mating reduced growth while predation risk increased growth, but the effects of mating were weaker for predator‐induced snails and the effects of predation risk were weaker for snails without mates. Predation risk reduced the amount of time that individuals waited before self‐fertilizing and reduced inbreeding depression in the offspring. There was a positive among‐family relationship between the amount of time that individuals delayed selfing under predation risk and the magnitude of inbreeding depression. These results highlight several potential roles of enemies in mating‐system expression and evolution.     

Recent selection for self‐compatibility in a population of Leavenworthia alabamica

The evolution of self‐compatibility (SC) is the first step in the evolutionary transition in plants from outcrossing enforced by self‐incompatibility (SI) to self‐fertilization. In the Brassicaceae, SI is controlled by alleles of two tightly linked genes at the S‐locus. Despite permitting inbreeding, mutations at the S‐locus leading to SC may be selected if they provide reproductive assurance and/or gain a transmission advantage in a population when SC plants self‐ and outcross. Positive selection can leave a genomic signature in the regions physically linked to the focus of selection when selection has occurred recently. From an SC population of Leavenworthia alabamica with a known nonfunctional mutation at the S‐locus, we collected sequence data from a ～690 Kb region surrounding the S‐locus, as well as from regions not linked to the S‐locus. To test for recent positive selection acting at the S‐locus, we examined polymorphism and the site‐frequency spectra. Using forward simulations, we demonstrate that recent selection of the strength expected for SC at a locus formerly under balancing selection can generate patterns similar to those seen in our empirical data.     

Inbreeding depression in an insect with maternal care: influences of family interactions,life stage and offspring sex

Although inbreeding is commonly known to depress individual fitness, the severity of inbreeding depression varies considerably across species. Among the factors contributing to this variation, family interactions, life stage and sex of offspring have been proposed, but their joint influence on inbreeding depression remains poorly understood. Here, we demonstrate that these three factors jointly shape inbreeding depression in the European earwig, Forficula auricularia. Using a series of cross‐breeding, split‐clutch and brood size manipulation experiments conducted over two generations, we first showed that sib mating (leading to inbred offspring) did not influence the reproductive success of earwig parents. Second, the presence of tending mothers and the strength of sibling competition (i.e. brood size) did not influence the expression of inbreeding depression in the inbred offspring. By contrast, our results revealed that inbreeding dramatically depressed the reproductive success of inbred adult male offspring, but only had little effect on the reproductive success of inbred adult female offspring. Overall, this study demonstrates limited effects of family interactions on inbreeding depression in this species and emphasizes the importance of disentangling effects of sib mating early and late during development to better understand the evolution of mating systems and population dynamics.     

Paternal‐specific S‐allele transmission in sweet cherry (Prunus avium L.): the potential for sexual selection

Homomorphic self‐incompatibility is a well‐studied example of a physiological process that is thought to increase population diversity and reduce the expression of inbreeding depression. Whereas theoretical models predict the presence of a large number of S‐haplotypes with equal frequencies at equilibrium, unequal allele frequencies have been repeatedly reported and attributed to sampling effects, population structure, demographic perturbation, sheltered deleterious mutations or selection pressure on linked genes. However, it is unclear to what extent unequal segregations are the results of gametophytic or sexual selection. Although these two forces are difficult to disentangle, testing S‐alleles in the offspring of controlled crosses provides an opportunity to separate these two phenomena. In this work, segregation and transmission of S‐alleles have been characterized in progenies of mixed donors and fully compatible pollinations under field conditions in Prunus avium. Seed set patterns and pollen performance have also been characterized. The results reveal paternal‐specific distorted transmission of S‐alleles in most of the crosses. Interestingly, S‐allele segregation within any given paternal or maternal S‐locus was random. Observations on pollen germination, pollen tube growth rate, pollen tube cohort size, seed set dynamics and transmission patterns strongly suggest post‐pollination, prezygotic sexual selection, with male–male competition as the most likely mechanism. According to these results, post‐pollination sexual selection takes precedence over frequency‐dependent selection in explaining unequal S‐haplotype frequencies.     

Apple MdABCF assists in the transportation of S‐RNase into pollen tubes

Self‐incompatibility (SI) is a reproductive isolation mechanism in flowering plants. Plants in the Solanaceae, Rosaceae and Plantaginaceae belong to the gametophytic self‐incompatibility type. S‐RNase, which is encoded by a female‐specific gene located at the S locus, degrades RNA in the pollen tube and causes SI. Recent studies have provided evidence that S‐RNase is transported non‐selectively into the pollen tube, but have not specified how this transportation is accomplished. We show here that the apple (Malus domestica) MdABCF protein, which belongs to group F of the ABC transporter family, assists in transportation of S‐RNase into the pollen tube. MdABCF is located in the pollen tube membrane and interacts with S‐RNase. S‐RNase was unable to enter the pollen tube when MdABCF was silenced by antisense oligonucleotide transfection. Our results show that MdABCF assists in transportation of either self or non‐self S‐RNase into the pollen tube. Moreover, MdABCF coordinates with the cytoskeleton to transport S‐RNase. Blockage of S‐RNase transport disrupts self‐incompatibility in this system.     

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

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