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.     

Egg-hatching benefits gained by polyandrous female locusts are not due to the fertilization advantage of nonsibling males

Several studies suggest that polyandrous females bias paternity in favor of unrelated males to avoid inbreeding depression. Here we tested whether the migratory locust biases sperm usage toward unrelated males by analyzing the paternity of offspring from females mated with either two siblings, or two nonsiblings, or a sibling and a nonsibling in either order. We found that the eggs of females mated only with siblings had decreased hatching success. When females mated with both a nonsibling and a sibling, egg hatchability was significantly increased. Subsequent paternity analyses found no evidence that females could avoid fertilization by sibling males. Therefore, improvement of the hatchability of eggs sired by siblings suggests that rather than biased fertilization by females toward genetically compatible or superior males, male-induced maternal effects or direct effects of male ejaculates might influence the survival of offspring sired by related males.     

BENEFITS OF MULTIPLE MATES IN THE CRICKET GRYLLUS BIMACULATUS

Despite the importance of polyandry for sexual selection, the reasons why females frequently mate with several males remain poorly understood. A number of genetic benefits have been proposed, based on the idea that by taking multiple mates, females increase the likelihood that their offspring will be sired by genetically more compatible or superior males. If certain males have intrinsically “good genes,” any female mating with them will produce superior offspring. Alternatively, if some males have genetic elements that are incompatible with a particular female, then she may benefit from polyandry if the sperm of such males are less likely to fertilize her eggs. We examined these hypotheses in the field cricket Gryllus bimaculatus (Orthoptera: Gryllidae). By allocating females identical numbers of matings but different numbers of mates we investigated the influence of number of mates on female fecundity, and both short- and long-term offspring fitness. This revealed no effect of number of mates on number of eggs laid. However, hatching success of eggs increased with number of mates. This effect could not be attributed to nongenetic effects such as the possibility that polyandry reduces variance in the quantity or fertilizing ability of sperm females receive, because a control group receiving half the number of copulations showed no drop in hatching success. Offspring did not differ in survival, adult mass, size, or development time with treatment. When males were mated to several different females there were no repeatable differences between individual males in the hatching success of their mate's eggs. This suggests that improved hatching success of polyandrous females is not due to certain males having genes that improve egg viability regardless of their mate. Instead, our results support the hypothesis that certain males are genetically more compatible with certain females, and that this drives polyandry through differential fertilization success of sperm from more compatible males.     

Postcopulatory inbreeding avoidance by female crickets only revealed by molecular markers

Multiple mating is thought to provide an opportunity for females to avoid the costs of genetic incompatibility by postcopulatory selection of compatible sperm haplotypes. Few studies have tested the genetic incompatibility hypothesis directly. Here we experimentally manipulated the compatibility of females with their mates using the gryllid cricket Teleogryllus oceanicus. We recorded the hatching success of eggs laid by females mated with two nonsibling males, two siblings, or one nonsibling male and one sibling. In contrast with two previous studies on crickets that have adopted this approach, the hatching success of eggs did not differ between females mated with two full siblings and females mated with two unrelated males, indicating that embryo viability was not a cost of inbreeding in this species. We assigned paternity to offspring produced by females mated to both a sibling and a nonsibling male using microsatellite markers. As in previous studies of this species, we were unable to detect any difference in the proportion of offspring sired by the 1st and the 2nd male to mate with a female when females were unrelated to their mates. However, in our experimental matings the proportion of offspring sired by the nonsibling male depended on his sequence position. Paternity was biased toward the nonsibling male when he mated first. Our data show that molecular analyses of paternity are essential to detect subtle mechanisms of postcopulatory sexual selection.     

The benefits of polyandry in the free-spawning polychaete Galeolaria caespitosa

In many species, females are thought to benefit from polyandry due to the reduced risks of fertilization by genetically incompatible sperm. However, few studies that have reported such benefits have directly attributed variation in female reproductive success to the interacting effects of males and females at fertilization. In this paper, we determine whether male x female interactions influence fertilization in vitro in the free-spawning, sessile polychaete Galeolaria caespitosa. Furthermore, we determined whether polyandry results in direct fertilization benefits for females by experimentally manipulating the number of males contributing towards staged spawning events. To test for male x female interaction effects we performed an initial experiment that crossed seven males with six females (in all 42 combinations), enabling us to assess fertilization rates for each specific male-female pairing and attribute variation in fertilization success to males, females and their interaction. This initial experiment revealed a strong interaction between males and females at fertilization, confirming that certain male-female combinations were more compatible than others. A second experiment tested the hypothesis that polyandry enhances female reproductive success by exposing each female's eggs to either a single male's sperm (monandry) or the sperm from three males simultaneously (polyandry). We performed this second experiment at two ecologically relevant sperm concentrations. This latter experiment revealed a strong fertilization benefit of polyandry, independent of the effects of sperm concentration (which were also significant). We suggest that these direct fertilization gains arising from polyandry will constitute an important source of selection on females to mate multiply in nature.     

Sperm selection and genetic incompatibility: does relatedness of mates affect male success in sperm competition?

Sperm selection may be said to occur if females influence the relative success of ejaculates competing to fertilize their ova. Most evidence that female animals or their ova are capable of sperm selection relates to male genetic incompatibility, although relatively few studies focus on competition between conspecific males. Here I look for evidence of sperm selection with respect to relatedness of mates. Reduced fitness or inbreeding effects in offspring resulting from copulations between close relatives are well documented. If females are capable of sperm selection, they might therefore be expected to discriminate against the sperm of sibling males during sperm competition. I describe an experimental protocol designed to test for evidence of sperm selection while controlling for inbreeding effects. Using decorated field crickets (Gryllodes supplicans), I found that sibling males achieved lower fertilization success in competition with a male unrelated to the female than in competition with another sibling more frequently than expected by chance, although the mean paternity values did not differ significantly between treatments. The tendancy for sibling males to achieve relatively lower fertilization success in competition with males unrelated to the female could not be explained by the effects of increased ejaculate allocation, female control of sperm transfer or inbreeding. This study therefore provides some evidence in support of the idea that female insects (or their ova) may be capable of selection against sperm on the basis of genetic similarity of conspecific males.     

Male-by-female interactions influence fertilization success and mediate the benefits of polyandry in the sea urchin Heliocidaris erythrogramma

Numerous studies have reported that females benefit from mating with multiple males (polyandry) by minimizing the probability of fertilization by genetically incompatible sperm. Few, however, have directly attributed variation in female reproductive success to the fertilizing capacity of sperm. In this study we report on two experiments that investigated the benefits of polyandry and the interacting effects of males and females at fertilization in the free-spawning Australian sea urchin Heliocidaris erythrogramma. In the first experiment we used a paired (split clutch) experimental design and compared fertilization rates within female egg clutches under polyandry (eggs exposed to the sperm from two males simultaneously) and monandry (eggs from the same female exposed to sperm from each of the same two males separately). Our analysis revealed a significant fertilization benefit of polyandry and strong interacting effects of males and females at fertilization. Further analysis of these data strongly suggested that the higher rates of fertilization in the polyandry treatment were due to an overrepresentation of fertilizations due to the most compatible male. To further explore the interacting effects of males and females at fertilization we performed a second factorial experiment in which four males were crossed with two females (in all eight combinations). In addition to confirming that fertilization success is influenced by male x female interactions, this latter experiment revealed that both sexes contributed significant variance to the observed patterns of fertilization. Taken together, these findings highlight the importance of male x female interactions at fertilization and suggest that polyandry will enable females to reduce the cost of fertilization by incompatible gametes.     

The evolution of polyandry: an examination of the genetic incompatibility and good‐sperm hypotheses

I have examined the adaptive significance of polyandry using the Australian field cricket Teleogryllus oceanicus. Previous studies of polyandry have examined differences in offspring production by females mated multiply to a single male or females mated multiply to different males. Here I combine this approach with a study of parentage of offspring produced in the later group. Females mated to two different males had a higher proportion of their eggs hatching than did females mating twice with a single male. Offspring fitness parameters were not effected. There was little evidence to suggest that females elevate their hatching success via fertilizing their eggs with sperm from genetically compatible males. Although the average paternity points towards random sperm mixing, there was considerable individual variation in sperm competition success. Patterns of parentage were consistent across females mating twice or four times. Sperm competition success was not related to offspring viability or performance. Thus, the notion that competitively superior sperm produce competitively superior offspring is not supported either. The mechanism underlying increased hatching success with polyandry requires further study.     

The more the better – polyandry and genetic similarity are positively linked to reproductive success in a natural population of terrestrial salamanders (Salamandra salamandra)

Although classically thought to be rare, female polyandry is widespread and may entail significant fitness benefits. If females store sperm over extended periods of time, the consequences of polyandry will depend on the pattern of sperm storage, and some of the potential benefits of polyandry can only be realized if sperm from different males is mixed. Our study aimed to determine patterns and consequences of polyandry in an amphibian species, the fire salamander, under fully natural conditions. Fire salamanders are ideal study objects, because mating, fertilization and larval deposition are temporally decoupled, females store sperm for several months, and larvae are deposited in the order of fertilization. Based on 18 microsatellite loci, we conducted paternity analysis of 24 female‐offspring arrays with, in total, over 600 larvae fertilized under complete natural conditions. More than one‐third of females were polyandrous and up to four males were found as sires. Our data clearly show that sperm from multiple males is mixed in the female's spermatheca. Nevertheless, paternity is biased, and the most successful male sires on average 70% of the larvae, suggesting a ‘topping off’ mechanism with first‐male precedence. Female reproductive success increased with the number of sires, most probably because multiple mating ensured high fertilization success. In contrast, offspring number was unaffected by female condition and genetic characteristics, but surprisingly, it increased with the degree of genetic relatedness between females and their sires. Sires of polyandrous females tended to be genetically similar to each other, indicating a role for active female choice.     

Molecular evidence of post-copulatory inbreeding avoidance in the field cricket Gryllus bimaculatus

Female promiscuity has broad implications for individual behaviour, population genetics and even speciation. In the field cricket Gryllus bimaculatus, females will mate with almost any male presented to them, despite receiving no recorded direct benefits. Previous studies have shown that female crickets can benefit from polyandry through increased hatching success of their eggs. There is evidence that this effect is driven by the potential of polyandrous females to avoid fertilizing eggs with sperm from genetically incompatible males. We provide direct evidence supporting the hypothesis that polyandry is a mechanism to avoid genetic incompatibilities resulting from inbreeding. Using microsatellite markers we examined patterns of paternity in an experiment where each female mated with both a related and an unrelated male in either order. Overall, unrelated males were more successful in gaining paternity than were related males, but this effect was driven by a much greater success of unrelated males when they were the first to mate.     

Delineating the roles of males and females in sperm competition

Disentangling the relative roles of males, females and their interactive effects on competitive fertilization success remains a challenge in sperm competition. In this study, we apply a novel experimental framework to an ideally suited externally fertilizing model system in order to delineate these roles. We focus on the chinook salmon, Oncorhynchus tshawytscha, a species in which ovarian fluid (OF) has been implicated as a potential arbiter of cryptic female choice for genetically compatible mates. We evaluated this predicted sexually selected function of OF using a series of factorial competitive fertilization trials. Our design involved a series of 10 factorial crosses, each involving two ‘focal’ rival males whose sperm competed against those from a single ‘standardized’ (non-focal) rival for a genetically uniform set of eggs in the presence of OF from two focal females. This design enabled us to attribute variation in competitive fertilization success among focal males, females (OF) and their interacting effects, while controlling for variation attributable to differences in the sperm competitive ability of rival males, and male-by-female genotypic interactions. Using this experimental framework, we found that variation in sperm competitiveness could be attributed exclusively to differences in the sperm competitive ability of focal males, a conclusion supported by subsequent analyses revealing that variation in sperm swimming velocity predicts paternity success. Together, these findings provide evidence that variation in paternity success can be attributed to intrinsic differences in the sperm competitive ability of rival males, and reveal that sperm swimming velocity is a key target of sexual selection.     

Cryptic preference for MHC-dissimilar females in male red junglefowl,Gallus gallus

An increasing number of studies test the idea that females increase offspring fitness by biasing fertilization in favour of genetically compatible partners; however, few have investigated or controlled for corresponding preferences in males. Here, we experimentally test whether male red junglefowl, Gallus gallus, prefer genetically compatible females, measured by similarity at the major histocompatibility complex (MHC), a key gene complex in vertebrate immune function. Theory predicts that because some degree of MHC heterozygosity favours viability, individuals should prefer partners that carry MHC alleles different from their own. While male fowl showed no preference when simultaneously presented with an MHC-similar and an MHC-dissimilar female, they showed a ‘cryptic’ preference, by allocating more sperm to the most MHC-dissimilar of two sequentially presented females. These results provide the first experimental evidence that males might respond to the MHC similarity of a female through differential ejaculate expenditure. By revealing that cryptic male behaviours may bias fertilization success in favour of genetically compatible partners, this study demonstrates the need to experimentally disentangle male and female effects when studying preferences for genetically compatible partners.     

Fitness benefits of polyandry for experienced females

Females often mate with several different males, which may promote sperm competition and increase offspring viability. However, the potential benefits of polyandry remain controversial, particularly in birds where recent reviews have suggested that females gain few genetic benefits from extra‐pair mating. In tree swallows (Tachycineta bicolor), we found that females with prior breeding experience had more sires per brood when paired to genetically similar social mates, and, among experienced females, broods with more sires had higher hatching success. Individual females breeding in two consecutive years also produced broods with more sires when they were more genetically similar to their mate. Thus, experienced females were able to avoid the costs of mating with a genetically similar social mate and realize fitness benefits from mating with a relatively large number of males. This is one of the first studies to show that female breeding experience influences polyandry and female fitness in a natural population of vertebrates. Our results suggest that the benefits of polyandry may only be clear when considering both the number of mates females acquire and their ability to modify the outcome of sexual conflict.     

Do female black field crickets Teleogryllus commodus benefit from polyandry?

Female insects that mate multiply tend to have increased lifetime fitness, apparently because of greater access to male-derived resources (e.g. sperm, nuptial gifts) that elevate fertility/fecundity. Experiments that standardize the number of matings per female also show that polyandry can improve aspects of offspring performance, most notably early embryo survival (egg hatching success). This improvement is widely attributed to genetic benefits which would arise if polyandrous females skew paternity to produce fitter offspring. In two separate experiments with field crickets (Teleogryllus commodus) polyandrous females (two, three or four mates) did not have higher egg hatching success than monandrous females (effect sizes: r = 0.03 and 0.08 for the respective experiments), which is consistent with our finding of no sire effect on hatching success. Polyandry also had no effect on post-hatching offspring survival. Polyandrous females' offspring took significantly longer to mature but their sons were not heavier and their daughters were actually significantly smaller than those of monandrous females. Finally, after controlling for relative male size, monandrous females' sons were more successful when directly competing for a mate.     

Why do females mate multiply? A review of the genetic benefits

The aim of this review is to consider the potential benefits that females may gain from mating more than once in a single reproductive cycle. The relationship between non-genetic and genetic benefits is briefly explored. We suggest that multiple mating for purely non-genetic benefits is unlikely as it invariably leads to the possibility of genetic benefits as well. We begin by briefly reviewing the main models for genetic benefits to mate choice, and the supporting evidence that choice can increase offspring performance and the sexual attractiveness of sons. We then explain how multiple mating can elevate offspring fitness by increasing the number of potential sires that compete, when this occurs in conjunction with mechanisms of paternity biasing that function in copula or post-copulation. We begin by identifying cases where females use pre-copulatory cues to identify mates prior to remating. In the simplest case, females remate because they identify a superior mate and 'trade up' genetically. The main evidence for this process comes from extra-pair copulation in birds. Second, we note other cases where pre-copulatory cues may be less reliable and females mate with several males to promote post-copulatory mechanisms that bias paternity. Although a distinction is drawn between sperm competition and cryptic female choice, we point out that the genetic benefits to polyandry in terms of producing more viable or sexually attractive offspring do not depend on the exact mechanism that leads to biased paternity. Post-copulatory mechanisms of paternity biasing may: (1) reduce genetic incompatibility between male and female genetic contributions to offspring; (2) increase offspring viability if there is a positive correlation between traits favoured post-copulation and those that improve performance under natural selection; (3) increase the ability of sons to gain paternity when they mate with polyandrous females. A third possibility is that genetic diversity among offspring is directly favoured. This can be due to bet-hedging (due to mate assessment errors or temporal fluctuations in the environment), beneficial interactions between less related siblings or the opportunity to preferentially fertilise eggs with sperm of a specific genotype drawn from a range of stored sperm depending on prevailing environmental conditions. We use case studies from the social insects to provide some concrete examples of the role of genetic diversity among progeny in elevating fitness. We conclude that post-copulatory mechanisms provide a more reliable way of selecting a genetically compatible mate than pre-copulatory mate choice. Some of the best evidence for cryptic female choice by sperm selection is due to selection of more compatible sperm. Two future areas of research seem likely to be profitable. First, more experimental evidence is needed demonstrating that multiple mating increases offspring fitness via genetic gains. Second, the role of multiple mating in promoting assortative fertilization and increasing reproductive isolation between populations may help us to understand sympatric speciation.     

Cryptic female preference for genetically unrelated males is mediated by ovarian fluid in the guppy

As inbreeding is costly, it has been suggested that polyandry may evolve as a means to reduce the negative fitness consequences of mating with genetically related males. While several studies provide support for this hypothesis, evidence of pure post-copulatory mechanisms capable of biasing paternity towards genetically unrelated males is still lacking; yet these are necessary to support inbreeding avoidance models of polyandry evolution. Here we showed, by artificially inseminating a group of female guppies with an equal number of sperm from related (full-sib) and unrelated males, that sperm competition success of the former was 10 per cent lower, on average, than that of the unrelated male. The paternity bias towards unrelated males was not due to differential embryo survival, as the size of the brood produced by control females, which were artificially inseminated with the sperm of a single male, was not influenced by their relatedness with the male. Finally, we collected ovarian fluid (OF) from virgin females. Using computer-assisted sperm analysis, we found that sperm velocity, a predictor of sperm competition success in the guppy, was significantly lower when measured in a solution containing the OF from a sister as compared with that from an unrelated female. Our results suggest that sperm-OF interaction mediates sperm competition bias towards unrelated mates and highlight the role of post-copulatory mechanisms in reducing the cost of mating with relatives in polyandrous females.     

Female control of sperm ejection and retention in the cornsilk fly Euxesta eluta (Diptera: Ulidiidae)

Promiscuous mating systems provide the opportunity for females to bias fertilization toward particular males. However, distinguishing between male sperm competition and active female sperm choice is difficult for species with internal fertilization. Nevertheless, species that store and use sperm of different males in different storing structures and species where females are able to expel all or part of the ejaculates after copulation may be able to bias fertilization. We report a series of experiments aimed at providing evidence of female sperm choice in Euxesta eluta (Hendel), a species of ulidiid fly that expels and consumes ejaculates after copulation. We found no evidence of greater reproductive success for females mated singly, multiply with the same male, or mated multiply with different males. Female E. eluta possesses two spherical spermathecae and a bursa copulatrix for sperm storage, with a ventral receptacle. There was no significant difference in storing more sperm in spermathecae 24 h after copulation than immediately after copulation. Females mated with protein-fed males had greater reproductive success than similar females mated to protein-deprived males. Protein-fed females prevented to consume the ejaculate, retained more sperm when mated to protein-fed males than when mated to protein-deprived males. Our results suggest that female E. eluta can exert control of sperm retention of higher quality males through ejaculate ejection.     

Assessing the potential for egg chemoattractants to mediate sexual selection in a broadcast spawning marine invertebrate

In numerous species, egg chemoattractants play a critical role in guiding sperm towards unfertilized eggs (sperm chemotaxis). Until now, the known functions of sperm chemotaxis include increasing the effective target size of eggs, thereby promoting sperm-egg encounters, and facilitating species recognition. Here, we report that in the broadcast spawning mussel, Mytilus galloprovincialis, egg chemoattractants may play an unforeseen role in sexual selection by enabling sperm to effectively 'choose' between the eggs of different conspecific females. In an initial experiment, we confirmed that sperm chemotaxis occurs in M. galloprovincialis by showing that sperm are attracted towards unfertilized eggs when given the choice of eggs or no eggs in a dichotomous chamber. We then conducted two cross-classified mating experiments, each comprising the same individual males and females crossed in identical male × female combinations, but under experimental conditions that offered sperm 'no-choice' (each fertilization trial took place in a Petri dish and involved a single male and female) or a 'choice' of a female's eggs (sperm were placed in the centre of a dichotomous choice chamber and allowed to choose eggs from different females). We show that male-by-female interactions characterized fertilization rates in both experiments, and that there was remarkable consistency between patterns of sperm migration in the egg-choice experiment and fertilization rates in the no-choice experiment. Thus, sperm appear to exploit chemical cues to preferentially swim towards eggs with which they are most compatible during direct sperm-to-egg encounters. These results reveal that sperm differentially select eggs on the basis of chemical cues, thus exposing the potential for egg chemoattractants to mediate mate choice for genetically compatible partners. Given the prevalence of sperm chemotaxis across diverse taxa, our findings may have broad implications for sexual selection in other mating systems.     

Postcopulatory inbreeding avoidance in guppies

In many species, the negative fitness effects of inbreeding have facilitated the evolution of a wide range of inbreeding avoidance mechanisms. Although avoidance mechanisms operating prior to mating are well documented, evidence for postcopulatory mechanisms of inbreeding avoidance remain scarce. Here, we examine the potential for paternity biases to favour unrelated males when their sperm compete for fertilizations though postcopulatory inbreeding avoidance mechanisms in the guppy, Poecilia reticulata. To test this possibility, we used a series of artificial inseminations to deliver an equal number of sperm from a related (either full sibling or half sibling) and unrelated male to a female while statistically controlling for differences in sperm quality between rival ejaculates. In this way, we were able to focus exclusively on postcopulatory mechanisms of inbreeding avoidance and account for differences in sperm competitiveness between rival males. Under these carefully controlled conditions, we report a significant bias in paternity towards unrelated males, although this effect was only apparent when the related male was a full sibling. We also show that sperm competition generally favours males with highly viable sperm and thus that some variance in sperm competitiveness can be attributed to difference in sperm quality. Our findings for postcopulatory inbreeding avoidance are consistent with prior work on guppies, revealing that sperm competition success declines linearly with the level of relatedness, but also that such effects are only apparent at relatedness levels of full siblings or higher. These findings reveal that postcopulatory processes alone can facilitate inbreeding avoidance.     

Males with high genetic similarity to females sire more offspring in sperm competition in Peron's tree frog Litoria peronii

Recent work has confirmed that genetic compatibility among mates can be an important determinant of siring success in sperm competition experiments and in free-ranging populations. Most of this work points towards mate choice of less related mates. However, there may also be the potential for mate choice for intermediate or even genetically similar mates to prevent outbreeding depression or hybridization with closely related taxa. We studied relatedness effects on post-copulatory gametic choice and/or sperm competition in an external fertilizer, Peron's tree frog (Litoria peronii), since external fertilizers offer exceptional control in order to test gametic interaction effects on probability of paternity and zygote viability. Sperm competition experiments were done blindly with respect to genetic relatedness among males and females. Thereafter, paternity of offspring was assigned using eight microsatellite loci. Three hybridization trials between L. peronii and a closely related sympatric species Litoria tyleri were also carried out. In the sperm competition trials, males that are more genetically similar to the female achieved higher siring success compared with less genetically similar males. The hybridization trials confirmed that the two species can interbreed and we suggest that the risk of hybridization may contribute to selection benefits for genetically more similar males at fertilization. To our knowledge, this study is the first to show evidence for post-copulatory selection of sperm from genetically more similar individuals within a natural population.