Mate choice, genetic incompatibility, and outbreeding in the ornate dragon lizard, Ctenophorus ornatus

There has been recent interest in the role genetic incompatibility may play in mate or sperm choice. One source of incompatibility may be too similar or disparate genomes. An isolated population of the ornate dragon lizard, Ctenophorus ornatus, was followed over a breeding season and parentage assigned to the offspring using microsatellites. It was found that the adults in the population had an eight per cent chance of mating with a relative. I examined whether C. ornatus mate (or fertilize their eggs) with respect to genetic similarity. There was no difference in a female's relatedness to the male in whose territory she resided and her average relatedness to all other males. Neither was there a difference in the relatedness of the male that sired a female's offspring and the female's average relatedness to all other males. There was no evidence of a cost to mating with a more genetically similar mate, because offspring survival was not influenced by degree of inbreeding or outbreeding. However, females that were more outbred had fewer offspring survive. In this small population there are two possible explanations for the decreased fitness associated with outbreeding. First, the species may have an evolutionary history of inbreeding and thus may be susceptible to outbreeding depression. Second. as fitter individuals produce more offspring, these offspring have an increased probability of breeding with relatives, leading to an indirect relationship between fitness and outbreeding.     

Outbreeding depression, but no inbreeding depression in haplodiploid Ambrosia beetles with regular sibling mating

In sexual reproduction the genetic similarity or dissimilarity between mates strongly affects offspring fitness. When mating partners are too closely related, increased homozygosity generally causes inbreeding depression, whereas crossing between too distantly related individuals may disrupt local adaptations or coadaptations within the genome and result in outbreeding depression. The optimal degree of inbreeding or outbreeding depends on population structure. A long history of inbreeding is expected to reduce inbreeding depression due to purging of deleterious alleles, and to promote outbreeding depression because of increased genetic variation between lineages. Ambrosia beetles (Xyleborini) are bark beetles with haplodiploid sex determination, strong local mate competition due to regular sibling mating within the natal chamber, and heavily biased sex ratios. We experimentally mated females of Xylosandrus germanus to brothers and unrelated males and measured offspring fitness. Inbred matings did not produce offspring with reduced fitness in any of the examined life-history traits. In contrast, outcrossed offspring suffered from reduced hatching rates. Reduction in inbreeding depression is usually attributed to purging of deleterious alleles, and the absence of inbreeding depression in X. germanus may represent the highest degree of purging of all examined species so far. Outbreeding depression within the same population has previously only been reported from plants. The causes and consequences of our findings are discussed with respect to mating strategies, sex ratios, and speciation in this unusual system.     

Active inbreeding in a cichlid fish and its adaptive significance

Levels of inbreeding are highly variable in natural populations. Inbreeding can be due to random factors (like population size), limited dispersal, or active mate choice for relatives. Because of inbreeding depression, mating with kin is often avoided, although sometimes intermediately related individuals are preferred (optimal outbreeding). However, theory predicts that the advantages of mating with close kin can override the effects of inbreeding depression, but in the animal kingdom, empirical evidence for this is scarce. Here we show that both sexes of Pelvicachromis taeniatus, an African cichlid with biparental brood care, prefer mating with unfamiliar close kin over nonkin, suggesting inclusive fitness advantages for inbreeding individuals. Biparental care requires synchronous behavior among parents. Since parental care is costly, there is a conflict between parents over care, which can reduce offspring fitness. Relatedness is expected to enhance cooperation among individuals. The comparison of the parental behavior of in- and outbreeding pairs showed that related parents were more cooperative and invested more than unrelated parents. Since we found no evidence for inbreeding depression, our results suggest that in P. taeniatus, inbreeding is an advantageous strategy.     

Outbreeding depression and breeding system evolution in small,remnant populations of <Emphasis Type="Italic">Primula vulgaris</Emphasis>: consequences for genetic rescue

Many species suffer from anthropogenic habitat fragmentation. The resulting small and isolated populations are more prone to extinction due to, amongst others, genetic erosion, inbreeding depression and Allee-effects. Genetic rescue can help mitigate such problems, but might result in outbreeding depression. We evaluated offspring fitness after selfing and outcrossing within and among three very small and isolated remnant populations of the heterostylous plant Primula vulgaris. We used greenhouse-grown offspring from these populations to test several fitness components. One population was fixed for the pin-morph, and was outcrossed with another population in the field to obtain seeds. Genetic diversity of parent and offspring populations was studied using microsatellites. Morph and population-specific heterosis, inbreeding and outbreeding depression were observed for fruit and seed set, seed weight and cumulative fitness. Highest fitness was observed in the field-outcrossed F1-population, which also showed outbreeding depression following subsequent between-population (back)crossing. Despite outbreeding depression, fitness was still relatively high. Inbreeding coefficients indicated that the offspring were more inbred than their parent populations. Offspring heterozygosity and inbreeding coefficients correlated with observed fitness. One population is evolving homostyly, showing a thrum morph with an elongated style and high autonomous fruit and seed set. This has important implications for conservation strategies such as genetic rescue, as the mating system will be altered by the introduction of homostyles.     

Balancing in- and out-breeding by the predatory mite <Emphasis Type="Italic">Phytoseiulus persimilis</Emphasis>

In- and out-breeding depressions are commonly observed phenomena in sexually reproducing organisms with a patchy distribution pattern, and spatial segmentation and/or isolation of groups. At the genetic level, inbreeding depression is due to increased homozygosity, whereas outbreeding depression is due to inferior genetic compatibility of mates. Optimal outbreeding theory suggests that intermediate levels of mate relatedness should provide for the highest fitness gains. Here, we assessed the fitness consequences of genetic relatedness between mates in plant-inhabiting predatory mites Phytoseiulus persimilis, which are obligatory sexually reproducing but haplo-diploid. Both females and males arise from fertilized eggs but males lose the paternal chromosome set during embryogenesis, dubbed pseudo-arrhenotoky. Phytoseiulus persimilis are highly efficacious in reducing crop-damaging spider mite populations and widely used in biological control. Using iso-female lines of two populations, from Sicily and Greece, we assessed the fecundity of females, and sex ratio of their offspring, that mated with either a sibling, a male from the same population or a male from the other population. Additionally, we recorded mating latency and duration. Females mating with a male from the same population produced more eggs, with a lower female bias, over a longer time than females mating with a sibling or with a male from the other population. Mating latency was unaffected by mate relatedness; mating duration was disproportionally long in sibling couples, likely indicating female reluctance to mate and sub-optimal spermatophore transfer. Our study provides a rare example of in- and out-breeding depression in a haplo-diploid arthropod, supporting the optimal outbreeding theory.     

Inbreeding,inbreeding depression,and infidelity in a cooperatively breeding bird*

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.     

The evolution of cytoplasmic incompatibility types: integrating segregation, inbreeding and outbreeding

Cytoplasmic incompatibility (CI) is a reproductive incompatibility induced by maternally transmitted bacteria of the genera Wolbachia and Cardinium. In the simplest form of CI, offspring from infected males and uninfected females suffer from increased mortality. However, it has been noted that crosses between males and females carrying different strains of infection are often also incompatible. The evolutionary processes leading to the emergence of new CI-compatibility types are still not resolved. Here, we develop a model that extends previous theoretical approaches by including segregation of bacterial strains during transmission as well as a continuum of breeding systems ranging from inbreeding (complete sib mating) to outbreeding (complete sib-mating avoidance). Our results demonstrate that (1) with segregation of strains, evolution is unlikely to lead to new CI types that co-occur as a double infection with the preexisting one, (2) inbreeding substantially hampers the evolution of new CI types, and (3) outbreeding facilitates the evolution of new CI types. Our model also provides a hypothesis on the evolutionary origin of CI.     

Inbreeding in the Seychelles warbler: environment-dependent maternal effects

The deleterious effects of inbreeding can be substantial in wild populations and mechanisms to avoid such matings have evolved in many organisms. In situations where social mate choice is restricted, extrapair paternity may be a strategy used by females to avoid inbreeding and increase offspring heterozygosity. In the cooperatively breeding Seychelles warbler, Acrocephalus sechellensis, neither social nor extrapair mate choice was used to avoid inbreeding facultatively, and close inbreeding occurred in approximately 5% of matings. However, a higher frequency of extra-group paternity may be selected for in female subordinates because this did reduce the frequency of mating between close relatives. Inbreeding resulted in reduced individual heterozygosity, which, against expectation, had an almost significant (P = 0.052), positive effect on survival. Conversely, low heterozygosity in the genetic mother was linked to reduced offspring survival, and the magnitude of this intergenerational inbreeding depression effect was environment-dependent. Because we controlled for genetic effects and most environmental effects (through the experimental cross-fostering of nestlings), we conclude that the reduced survival was a result of maternal effects. Our results show that inbreeding can have complicated effects even within a genetic bottlenecked population where the "purging" of recessive alleles is expected to reduce the effects of inbreeding depression.     

Differential allocation in a lekking bird: females lay larger eggs and are more likely to have male chicks when they mate with less related males

The differential allocation hypothesis predicts increased investment in offspring when females mate with high-quality males. Few studies have tested whether investment varies with mate relatedness, despite evidence that non-additive gene action influences mate and offspring genetic quality. We tested whether female lekking lance-tailed manakins (Chiroxiphia lanceolata) adjust offspring sex and egg volume in response to mate attractiveness (annual reproductive success, ARS), heterozygosity and relatedness. Across 968 offspring, the probability of being male decreased with increasing parental relatedness but not father ARS or heterozygosity. This correlation tended to diminish with increasing lay-date. Across 162 offspring, egg volume correlated negatively with parental relatedness and varied with lay-date, but was unrelated to father ARS or heterozygosity. Offspring sex and egg size were unrelated to maternal age. Comparisons of maternal half-siblings in broods with no mortality produced similar results, indicating differential allocation rather than covariation between female quality and relatedness or sex-specific inbreeding depression in survival. As males suffer greater inbreeding depression, overproducing females after mating with related males may reduce fitness costs of inbreeding in a system with no inbreeding avoidance, while biasing the sex of outbred offspring towards males may maximize fitness via increased mating success of outbred sons.     

The transition to social inbred mating systems in spiders: role of inbreeding tolerance in a subsocial predecessor

The social spiders are unusual among cooperatively breeding animals in being highly inbred. In contrast, most other social organisms are outbred owing to inbreeding avoidance mechanisms. The social spiders appear to originate from solitary subsocial ancestors, implying a transition from outbreeding to inbreeding mating systems. Such a transition may be constrained by inbreeding avoidance tactics or fitness loss due to inbreeding depression. We examined whether the mating system of a subsocial spider, in a genus with three social congeners, is likely to facilitate or hinder the transition to inbreeding social systems. Populations of subsocial Stegodyphus lineatus are substructured and spiders occur in patches, which may consist of kin groups. We investigated whether male mating dispersal prevents matings within kin groups in natural populations. Approximately half of the marked males that were recovered made short moves (< 5m) and mated within their natal patch. This potential for inbreeding was counterbalanced by a relatively high proportion of immigrant males. In mating experiments, we tested whether inbreeding actually results in lower offspring fitness. Two levels of inbreeding were tested: full sibling versus non-sib matings and matings of individuals within and between naturally occurring patches of spiders. Neither full siblings nor patch mates were discriminated against as mates. Sibling matings had no effect on direct fitness traits such as fecundity, hatching success, time to hatching and survival of the offspring, but negatively affected offspring growth rates and adult body size of both males and females. Neither direct nor indirect fitness measures differed significantly between within patch and between-patch pairs. We tested the relatedness between patch mates and nonpatch mates using DNA fingerprinting (TE-AFLP). Kinship explained 30% of the genetic variation among patches, confirming that patches are often composed of kin. Overall, we found limited male dispersal, lack of kin discrimination, and tolerance to low levels of inbreeding. These results suggest a history of inbreeding which may reduce the frequency of deleterious recessive alleles in the population and promote the evolution of inbreeding tolerance. It is likely that the lack of inbreeding avoidance in subsocial predecessors has facilitated the transition to regular inbreeding social systems.     

Inclusive fitness of 'kissing cousins': new evidence of a role for kin selection in the evolution of extra-pair mating in birds

Social monogamy is nearly ubiquitous across avian taxa,but evidence from a proliferation of studies utilizing molecular paternity analysis suggests that sexual monogamy is the rare exception rather than the rule (Griffith et al. 2002). Efforts to explain the prevalence of extra-pair paternity (EPP) have largely focused on the potential fitness benefits for offspring genetic quality, as females are less likely to benefit directly from seeking extra-pair mates. In particular, there has been considerable interest in the degree to which EPP may represent an adaptive female strategy to avoid inbreeding (or outbreeding)depression when paired with a highly related (or unrelated)social mate (Kempenaers 2007). Others have argued that, because relatives share many genes identical by descent,females might increase their own inclusive fitness by providing additional breeding opportunities to genetically related males (Waser et al. 1986; Kokko & Ots 2006). Thus, in the absence of significant inbreeding depression, pursuing EPP with relatives should be favoured by kin selection, although there exist few unambiguous empirical examples of such preferences in the literature. In this issue of Molecular Ecology, Wang &Lu (2011) present an analysis of mating patterns with respect to genetic relatedness of social and extra-pair partners in the ground tit (Parus humilis), a facultative cooperative breeder in which socially monogamous pairs occasionally form cooperative groups with unpaired helper males (Fig. 1). Consistent with the predictions of the kin-selection hypothesis, females in both bi-parental and cooperative groups preferentially engaged in extra-pair matings with relatives, irrespective of relatedness to their social mates, and while suffering no apparent costs of inbreeding depression in their progeny. These finding shave several exciting implications for our understanding of avian mating system diversity and the evolution of cooperative breeding.     

Lack of inbreeding avoidance in the Argentine ant Linepithema humile

Although workers might increase their inclusive fitness by favoringcloser over more distant kin, evidence suggest that nepotismgenerally does not occur within colonies of social insects.It has been suggested that this may be due to the cost of recognitionerrors. We tested whether recognition occurs in a system wherea better than random ability to recognize kin should be selected for. Using DNA microsatellites, we show that sexuals of theArgentine ant Linepithema humile fail to use genetic cues toavoid sib-mating. When offspring of two queens were allowedto mate, the percentage of matings among siblings was not significantlylower than expected under the hypothesis of random mating.The finding that sexuals fail to use genetic cues to avoid sib-matings cannot be attributed to the cost of recognitionerrors because any recognition system that would lead to abetter than random ability to avoid sib-mating should be selectedfor when there are costs to inbreeding. These data are thusconsistent with the view that kin recognition mediated solelyby genetic cues might be intrinsically error prone within coloniesof social insects.     

Preference for Outbreeding in Inbred Littledale’s Whistling Rats <Emphasis Type="Italic">Parotomys littledalei</Emphasis>

Inbreeding is sometimes tolerated in species occupying patchy, unpredictable habitats, when it becomes too costly to outbreed. Although traits leading to inbreeding depression may be selected against, the accumulation of deleterious alleles from constant inbreeding might promote outbreeding. Littledale’s whistling rat Parotomys littledalei occupies patchy habitats in arid western South Africa, and inbreeds in captivity without suffering inbreeding depression. We tested whether female whistling rats make different mate choice decisions based on their inbreeding status, by providing inbred and outbred females with male odours in four pairwise combinations: unrelated outbred (r = 0) versus either familiar littermate sibling (r = 0.5), half-sibling (r = 0.25), first cousin (r = 0.125), or unrelated inbred (r = 0). Inbred females generally preferred unrelated males, but did not show a preference between unrelated males and first cousins. In contrast, outbred females did not generally show a preference, but preferred first cousins to unrelated males. In breeding trials, reproductive success (output and viability of young) of outbred females paired with unrelated or related males was equally high, whereas inbred females suffered reduced reproductive success when paired with closely-related males. Our results indicate that the inbreeding status of female whistling rats influences their decision to outbreed or inbreed, which might influence their fitness. While outbred whistling rats do not suffer inbreeding depression, the costs of constant inbreeding could promote outbreeding by inbred females. We propose that the choice to inbreed or outbreed in female whistling rats depends on their inbreeding status and the genetic relatedness of available mates, with outbreeding more likely to occur unless unrelated mates are not available.     

Evidence for Inbreeding Depression and Pre-Copulatory,but Not Post Copulatory Inbreeding Avoidance in the Cabbage Beetle Colaphellus bowringi

Inbreeding is known to have adverse effects on fitness-related traits in a range of insect species. A series of theoretical and experimental studies have suggested that polyandrous insects could avoid the cost of inbreeding via pre-copulatory mate choice and/or post-copulatory mechanisms. We looked for evidence of pre-copulatory inbreeding avoidance using female mate preference trials, in which females were given the choice of mating with either of two males, a sibling and a non-sibling. We also tested for evidence of post-copulatory inbreeding avoidance by conducting double mating experiments, in which four sibling females were mated with two males sequentially, either two siblings, two non-siblings or a sibling and a non-sibling in either order. We identified substantial inbreeding depression: offspring of females mated to full siblings had lower hatching success, slower development time from egg to adult, lower survival of larval and pupal stages, and lower adult body mass than the offspring of females mated to non-sibling males. We also found evidence of pre-copulatory inbreeding avoidance, as females preferred to mate with non-sibling males. However, we did not find any evidence of post-copulatory inbreeding avoidance: egg hatching success of females mating to both sibling and non-sibling males were consistent with sperm being used without bias in relation to mate relatedness. Our results suggest that this cabbage beetle has evolved a pre-copulatory mechanism to avoid matings between close relative, but that polyandry is apparently not an inbreeding avoidance mechanism in C. bowringi.     

Identification at the larval stage of four Curculio species coexisting on oak trees using PCR‐RFLP

Several species in the family Cecidomyiidae (Diptera) are known by a particular reproduction mechanism called monogeny, in which all offspring of each individual female are either exclusively male or exclusively female. As a consequence, no full‐sib mating occurs in monogenous populations. So offspring of each female must inevitably mate with those of other females, and in the worst condition, mating may occur between individuals with a common sire but different mothers, where they are only half siblings. Simulation of monogenous gall midge pedigrees, calculation of inbreeding coefficients of individuals for 20 generations, and comparing the results with those of a non‐monogenous species revealed that the total average inbreeding coefficients of individuals were not dramatically different between these two populations. However, the maximum inbreeding coefficient of individuals per generation in non‐monogenous populations was significantly higher than in monogenous populations. Despite the effect of monogeny on inbreeding, it is doubted whether inbreeding avoidance is the only force that drives the evolution of monogeny in gall midges.     

Sib‐mating in the ant Plagiolepis pygmaea: adaptative inbreeding?

Multiple functional queens in a colony (polygyny) and multiple mating by queens (polyandry) in social insects challenge kin selection, because they dilute inclusive fitness benefits from helping. Colonies of the ant Plagiolepis pygmaea brash contain several hundreds of multiply mated queens. Yet, within‐colony relatedness remains unexpectedly high. This stems from low male dispersal, extensive mating among relatives and adoption of young queens in the natal colony. We investigated whether inbreeding results from workers expelling foreign males, and/or from preferential mating between related partners. Our data show that workers actively repel unrelated males entering their colony, and that queens preferentially mate with related males. These results are consistent with inclusive fitness being a driving force for inbreeding: by preventing outbreeding, workers reduce erosion of relatedness within colonies due to polygyny and polyandry. That virgin queens mate preferentially with related males could result from a long history of inbreeding, which is expected to reduce depression in species with regular sibmating.     

Female choice and the relatedness of mates in the guppy (<Emphasis Type="Italic">Poecilia reticulata</Emphasis>)

Several studies suggest that females may offset the costs of genetic incompatibility by exercising pre-copulatory or post-copulatory mate choice to bias paternity toward more compatible males. One source of genetic incompatibility is the degree of relatedness among mates; unrelated males are expected to be genetically more compatible with a female than her relatives. To address this idea, we investigated the potential for inbreeding depression and paternity biasing mechanisms (pre- and post-copulatory) of inbreeding avoidance in the guppy, Poecilia reticulata. Inbreeding resulted in a reduction in offspring number and quality. Females mated to siblings gave birth to significantly fewer offspring compared to females mated to non-siblings and inbred male offspring took longer to reach sexual maturity. There was no evidence of inbreeding avoidance in pre-copulatory behaviors of females or males. Sexual responsiveness of females to courting males and the number of sexual behaviors males directed at females did not decrease as a function of the relatedness of the two individuals. We also tested whether female guppies can use post-copulatory mechanisms to bias sperm usage toward unrelated males by comparing the number of offspring produced by females mated to two of their siblings (SS), two males unrelated to the female (NN), or to one unrelated male and a sibling male (NS). We found that NS females produced a number of offspring not significantly different than what would be expected if fertilization success were halfway between completely outbreeding (NN) and completely inbreeding (SS) females. This suggests that there is no significant improvement in the number of offspring produced by females mating to both related and unrelated males, relative to that which would be expected if sperm from both males were used equally. Our results suggest that female guppies do not discriminate against closely related males or their sperm.     

Males Benefit from Mating with Outbred Females in Drosophila littoralis: Male Choice for Female Genetic Quality?

The evolution and expression of mate choice behaviour in either sex depends on the sex‐specific combination of mating costs, benefits of choice and constraints on choice. If the benefits of choice are larger for one sex, we would expect that sex to be choosier, assuming that the mating costs and constraints on choice are equal between sexes. Because deliberate inbreeding is a powerful genetic method for experimental manipulation of the quality of study organisms, we tested the effects of both male and female inbreeding on egg and offspring production in Drosophila littoralis. Female inbreeding significantly reduced offspring production (mostly due to lower egg‐to‐adult viability), whereas male inbreeding did not affect offspring production (despite a slight effect of paternal inbreeding on egg‐to‐adult viability). As inbreeding depressed female quality more than male quality, the benefits of mate choice were larger for males than for females. In mate choice experiments, inbreeding did not affect male mating success (measured as a probability to be accepted as a mate in a large group), suggesting that females did not discriminate among inbred and outbred males. In contrast, female mating success was affected by inbreeding, with outbred females having higher mating success than inbred females. This result was not explained by lower activity of inbred females. Our results show that D. littoralis males benefit from mating with outbred females of high genetic quality and suggest adaptive male mate choice for female genetic quality in this species. Thus, patterns of mating success in mate choice trials mirrored the benefits of choice: the sex that benefited more from choice (i.e. males) was more choosy.     

Inbreeding depression and purging in a haplodiploid: gender-related effects

Compared with diploid species, haplodiploids suffer less inbreeding depression because male haploidy imposes purifying selection on recessive deleterious alleles. However, alleles of genes only expressed in the diploid females are protected in heterozygous individuals. This leads to the prediction that haplodiploids suffer more from inbreeding effects on life-history traits controlled by genes with female-limited expression. To test this, we used a wild population of the haplodiploid mite Tetranychus urticae. First, negative effects of inbreeding were investigated by comparing maturation rate, juvenile survival, oviposition rate and longevity between lines created by three generations of either outbreeding or mother–son inbreeding. Second, purging through inbreeding was investigated by comparing the intensity of inbreeding depression between outbred families with known inbreeding/outbreeding mating histories. Negative effects of inbreeding and evidence for purging were found for the female trait oviposition rate, but not for juvenile survival and longevity. Both male and female maturation rate were negatively affected by inbreeding, most likely due to maternal effects because inbred offspring of outbred mothers was not affected. These results support the hypothesis that, in haplodiploids inbreeding effects and genetic variation due to deleterious recessive alleles may depend on gender.     

What Happens after Inbreeding Avoidance? Inbreeding by Rejected Relatives and the Inclusive Fitness Benefit of Inbreeding Avoidance

Avoiding inbreeding, and therefore avoiding inbreeding depression in offspring fitness, is widely assumed to be adaptive in systems with biparental reproduction. However, inbreeding can also confer an inclusive fitness benefit stemming from increased relatedness between parents and inbred offspring. Whether or not inbreeding or avoiding inbreeding is adaptive therefore depends on a balance between inbreeding depression and increased parent-offspring relatedness. Existing models of biparental inbreeding predict threshold values of inbreeding depression above which males and females should avoid inbreeding, and predict sexual conflict over inbreeding because these thresholds diverge. However, these models implicitly assume that if a focal individual avoids inbreeding, then both it and its rejected relative will subsequently outbreed. We show that relaxing this assumption of reciprocal outbreeding, and the assumption that focal individuals are themselves outbred, can substantially alter the predicted thresholds for inbreeding avoidance for focal males. Specifically, the magnitude of inbreeding depression below which inbreeding increases a focal male’s inclusive fitness increases with increasing depression in the offspring of a focal female and her alternative mate, and it decreases with increasing relatedness between a focal male and a focal female’s alternative mate, thereby altering the predicted zone of sexual conflict. Furthermore, a focal male’s inclusive fitness gain from avoiding inbreeding is reduced by indirect opportunity costs if his rejected relative breeds with another relative of his. By demonstrating that variation in relatedness and inbreeding can affect intra- and inter-sexual conflict over inbreeding, our models lead to novel predictions for family dynamics. Specifically, parent-offspring conflict over inbreeding might depend on the alternative mates of rejected relatives, and male-male competition over inbreeding might lead to mixed inbreeding strategies. Making testable quantitative predictions regarding inbreeding strategies occurring in nature will therefore require new models that explicitly capture variation in relatedness and inbreeding among interacting population members.