Interpopulation variation in female remating is attributable to female and male effects in <Emphasis Type="Italic">Callosobruchus chinensis</Emphasis>

The evolution of female multiple mating is best understood by consideration of male and female reproductive perspectives. Females should usually be selected to remate at their optimum frequencies whereas males should be selected to manipulate female remating to their advantage. Female remating behavior may therefore be changed by variation of male and female traits. In this study, our objective was to separate the effects of female and male strains on female remating for the adzuki bean beetle, Callosobruchus chinensis, for which there is interstrain variation in female remating frequency. We found that interstrain variation in female remating is primarily attributable to female traits, suggesting genetic variation in female receptivity to remating in C. chinensis. Some interstrain variation in female remating propensity was attributable to an interaction between female and male strains, however, with the males of some strains being good at inducing nonreceptivity in females from one high-remating strain whereas others were good at inducing copulation in nonvirgin females from the high-remating strain. There is, therefore, interstrain variation in male ability to deter females from remating and in male ability to mate successfully with nonvirgin females. These results suggest that mating traits have evolved along different trajectories in different strains of C. chinensis.     

Female remating in butterflies: interaction between female genotype and nonfertile sperm

Female mating rate is fundamental to evolutionary biology as it determines the pattern of sexual selection and sexual conflict. Despite its importance, the genetic basis for female remating rate is largely unknown and has only been demonstrated in one species. In paternally investing species there is often a conflict between the sexes over female mating rate, as females remate to obtain male nutrient donations and males try to prevent female remating to ensure high fertilization success. Butterflies produce two types of sperm: fertilizing, eupyrene sperm, and large numbers of nonfertile, apyrene sperm. The function of apyrene sperm in the polyandrous, paternally investing green‐veined white butterfly, Pieris napi, is to fill the female’s sperm storage organ thereby reducing her receptivity. However, there is large variation in number of apyrene sperm stored. Here, I examine the genetic basis to this variation, and if variation in number of apyrene sperm stored is related to females’ remating rate. The number of apyrene sperm stored at the time of remating has a genetic component and is correlated with female remating tendency, whereas no such relationship is found for fertilizing sperm. The duration of the nonreceptivity period in P. napi also has a genetic component and is inversely related to the degree of polyandry. Sexual conflict over female remating rate appears to be present in this species, with males using their apyrene sperm to exploit a female system designed to monitor sperm in storage. Ejaculates with a high proportion of nonfertile sperm may have evolved to induce females to store more of these sperm, thereby reducing remating. As a counter‐adaptation, females have evolved a better detection system to regain control over their remating rate. Sexually antagonistic co‐evolution of apyrene sperm number and female sperm storage may be responsible for ejaculates with predominantly nonfertile sperm in this butterfly.     

Water Availability Affects Female Remating in the Seed Beetle,Callosobruchus chinensis

Benefits of multiple mating to females may come from the acquisition of water in male ejaculates. This hypothesis seems plausible in species in which males provide females with large ejaculates and has been tested with the prediction that females mate more frequently when an external source of water is unavailable. My study observed that females deprived of water were more likely to remate than females given water in the adzuki bean beetle, Callosobruchus chinensis. This result suggests that females may absorb the water in male ejaculates and thus change their remating receptivity according to the need for additional water. However, compared with related species, the ejaculate size is smaller, so ejaculatory hydration benefits are expected to be small in this species. There were no significant differences in lifetime fecundity and longevity between females that were allowed to receive one ejaculate from remating and females that were not allowed to do so when water was unavailable. This provides no evidence that receiving an additional ejaculate enhances female fitness. Thus, obtaining water from male ejaculates may partly compensate the costs of remating to females, although it alone would be insufficient to explain polyandry in C. chinensis. Increased mating frequency in water‐deprived females would not necessarily support the hypothesis that females remate for ejaculatory hydration benefits.     

No genetic correlation between the sexes in mating frequency in the bean beetle, Callosobruchus chinensis

Female multiple mating, which is common in animals, may have evolved not in response to fitness advantages to females but as a genetic corollary to selection on males to mate frequently. This nonadaptive hypothesis assumes a genetic correlation between females and males in mating frequency, which has received a few empirical investigations. We tested this hypothesis by observing the correlated response in male mating frequency in the adzuki bean beetle, Callosobruchus chinensis to artificial selection on female propensity to remate. Compared to control females, females from lines selected for increased or decreased female propensity to remate had, respectively, higher or lower mating frequency measured by the number of mating within a given period. This indicates that female receptivity to remating is genetically correlated with female mating frequency, and thus the artificial selection for female propensity to remate influenced female mating frequency. In contrast, males from the selected lines that diverged in female mating frequency did not vary significantly in their mating frequency. These results indicate that there is no genetic correlation between the sexes in mating frequency in C. chinensis. This study shows that the reason why females in C. chinensis remate despite suffering fitness costs cannot be explained by indirect selection resulting from selection on males to mate multiple times.     

Why Do Female Callosobruchus maculatus Kick Their Mates?

Sexual conflict is now recognised as an important driver of sexual trait evolution. However, due to their variable outcomes and effects on other fitness components, the detection of sexual conflicts on individual traits can be complicated. This difficulty is exemplified in the beetle Callosobruchus maculatus, where longer matings increase the size of nutritious ejaculates but simultaneously reduce female future receptivity. While previous studies show that females gain direct benefits from extended mating duration, females show conspicuous copulatory kicking behaviour, apparently to dislodge mating males prematurely. We explore the potential for sexual conflict by comparing several fitness components and remating propensity in pairs of full sibling females where each female mated with a male from an unrelated pair of full sibling males. For one female, matings were terminated at the onset of kicking, whereas the other’s matings remained uninterrupted. While fecundity (number of eggs) was similar between treatments, uninterrupted matings enhanced adult offspring numbers and fractionally also longevity. However, females whose matings were interrupted at the onset of kicking exhibited an increased propensity to remate. Since polyandry can benefit female fitness in this species, we argue that kicking, rather than being maladaptive, may indicate that females prefer remating over increased ejaculate size. It may thus be difficult to assess the presence of sexual conflict over contested traits such as mating duration when females face a trade off between direct benefits gained from one mating and indirect benefits from additional matings.     

Female influence on pre- and post-copulatory sexual selection and its genetic basis in Drosophila melanogaster

Genetic variation among females is likely to influence the outcome of both pre- and post-copulatory sexual selection in Drosophila melanogaster. Here we use association testing to survey natural variation in 10 candidate female genes for their effects on female reproduction. Females from 91 chromosome two substitution lines were scored for phenotypes affecting pre- and post-copulatory sexual selection such as mating and remating rate, propensity to use sperm from the second male to mate, and measures of fertility. There were significant genetic contributions to phenotypic variation for all the traits measured. Resequencing of the 10 candidate genes in the 91 lines yielded 68 non-synonymous polymorphisms which were tested for associations with the measured phenotypes. Twelve significant associations (markerwise P<0.01) were identified. Polymorphisms in the putative serine protease homolog CG9897 and the putative odorant binding protein CG11797 associated with female propensity to remate and met an experimentwise significance of P<0.05. Several other associations, including those impacting both fertility and female remating rate suggest that sperm storage might be an important factor mitigating female influence on sexual selection.     

REMATING IN DROSOPHILA MELANOGASTER: ARE INDIRECT BENEFITS CONDITION DEPENDENT?

By measuring the direct and indirect fitness costs and benefits of sexual interactions, the feasibility of alternate explanations for polyandry can be experimentally assessed. This approach becomes more complicated when the relative magnitude of the costs and/or benefits associated with multiple mating (i.e., remating with different males) vary with female condition, as this may influence the strength and direction of sexual selection. Here, using the model organism Drosophila melanogaster, we test whether the indirect benefits that a nonvirgin female gains by remating (“trading‐up”) are influenced by her condition (body size). We found that remating by small‐bodied, low‐fecundity females resulted in the production of daughters of relatively higher fecundity, whereas the opposite pattern was observed for large‐bodied females. In contrast, remating had no measurable effect on the relative reproductive success of sons from dams of either body size. These results are consistent with a hypothesis based on sexually antagonistic genetic variation. The implications of these results to our understanding of the evolution and consequences of polyandry are discussed.     

Remating responses are consistent with male postcopulatory manipulation but not reinforcement in D. pseudoobscura

Reinforcement occurs when hybridization between closely related lineages produces low‐fitness offspring, prompting selection for elevated reproductive isolation specifically in areas of sympatry. Both premating and postmating prezygotic behaviors have been shown to be the target of reinforcing selection, but it remains unclear whether remating behaviors experience reinforcement, although they can also influence offspring identity and limit formation of hybrids. Here, we evaluated evidence for reinforcing selection on remating behaviors in Drosophila pseudoobscura, by comparing remating traits in females from populations historically allopatric and sympatric with Drosophila persimilis. We found that the propensity to remate was not higher in sympatric females, compared to allopatric females, regardless of whether the first mated male was heterospecific or conspecific. Moreover, remating behavior did not contribute to interspecific reproductive isolation among any population; that is, females showed no higher propensity to remate following a heterospecific first mating than following a conspecific first mating. Instead, we found that females are less likely to remate after initial matings with unfamiliar males, regardless of species identity. This is consistent with one scenario of postmating sexual conflict in which females are poorly defended against postcopulatory manipulation by males with whom they have not coevolved. Our results are generally inconsistent with reinforcement on remating traits and suggest that this behavior might be more strongly shaped by the consequences of local antagonistic male–female interactions than interactions with heterospecifics.     

Receptive females mitigate costs of sexual conflict

Males typically gain fitness from multiple mating, whereas females often lose fitness from numerous mating, potentially leading to sexual conflict over mating. This conflict is expected to favour the evolution of female resistance to mating. However, females may incur male harassment if they refuse to copulate; thus, greater female resistance may increase costs imposed by males. Here, I show that the evolution of resistance to mating raises fitness disadvantages of interacting with males when mating is harmful in female adzuki bean beetles, Callosobruchus chinensis. Females that were artificially selected for higher and lower remating propensity evolved to accept and resist remating, respectively. Compared with females that evolved to accept remating, females that evolved to resist it suffered higher fitness costs from continuous exposure to males. The costs of a single mating measured by the effect on longevity did not differ among selection line females. This study indicates that receptive rather than resistant females mitigate the fitness loss resulting from sexual conflict, suggesting that even though mating is harmful, females can evolve to accept additional mating.     

Male irradiation affects female remating behavior in Anastrepha serpentina (Diptera: Tephritidae)

Female remating in target pest species can affect the efficacy of control methods such as the Sterile Insect Technique (SIT) but very little is known about the postcopulatory mating behavior of these pests. In this study, we investigated the remating behavior of female Anastrepha serpentina (Diptera: Tephritidae), an oligophagous pest of Sapotaceae. First, we tested how long the sexual refractory period of females lasted after an initial mating. Second, we tested the effect of male and female sterility, female ovipositing opportunities and male density on female propensity to remate. Lastly, we tested if the amount of sperm stored by females was correlated to the likelihood of females to remate. We found that receptivity of mass-reared A. serpentina females had a bimodal response, with up to 16% of mass-reared A. serpentina females remating five days after the initial copulation, decreasing to 2% at 10 and 15 days and increasing to 13% after 20 days. Compared to fertile males, sterile males were less likely to mate and less likely to inhibit females from remating. Copula duration of sterile males was shorter compared to fertile males. Remating females were less likely to mate with a sterile male as a second mate. Sterile females were less likely to mate or remate compared to fertile females. Opportunity to oviposit and male density had no effect on female remating probability. Sperm numbers were not correlated with female likelihood to remate. Information on the post-copulatory behavior of mass-reared A. serpentina will aid fruit fly managers in improving the quality of sterile males. We discuss our results in terms of the differences this species presents in female remating behavior compared to other tephritids.     

Female Remating Behavior in a Lekking Moth

In polyandrous species, male reproductive success will at least partly be determined by males' success in sperm competition. To understand the potential for post‐mating sexual selection, it is therefore important to assess the extent of female remating. In the lekking moth Achroia grisella, male mating success is strongly determined by female choice based on the attractiveness of male ultrasonic songs. Although observations have indicated that some females will remate, only little is known about the level of sperm competition. In many species, females are more likely to remate if their first mating involved an already mated male than if the first male was virgin. Potentially, this is because mated males are less well able to provide an adequate sperm supply, nutrients, or substances inhibiting female remating. This phenomenon will effectively reduce the strength of pre‐copulatory sexual selection because attractive males with high mating success will be more susceptible to sperm competition. We therefore performed an experiment designed both to provide a more precise estimate of female remating probability and simultaneously to test the hypothesis that female remating is influenced by male mating history. Overall, approximately one of five females remated with a second male. Yet, although females mated to non‐virgin males were somewhat more prone to remate, the effect of male mating history was not significant. The results revealed, however, that heavier females were more likely to remate. Furthermore, we found that females' second copulations were longer, suggesting that, in accordance with theory, males may invest more sperm in situations with an elevated risk of sperm competition.     

Sperm competition risk and male genital anatomy: comparative evidence for reduced duration of female sexual receptivity in primates with penile spines

Selection pressures influencing the way in which males stimulate females during copulation are not well understood. In mammals, copulatory stimulation can influence female remating behaviour, both via neuroendocrine mechanisms mediating control of sexual behaviour, and potentially also via effects of minor injury to the female genital tract. Male adaptations to increase copulatory stimulation may therefore function to reduce sperm competition risk by reducing the probability that females will remate. This hypothesis was tested using data for primates to explore relationships between male penile anatomy and the duration of female sexual receptivity. It was predicted that penile spines or relatively large bacula might function to increase copulatory stimulation and hence to reduce the duration of female sexual receptivity. Results of the comparative analyses presented show that, after control for phylogenetic effects, relatively high penile spinosity of male primates is associated with a relatively short duration of female sexual receptivity within the ovarian cycle, although no evidence was found for a similar relationship between baculum length and duration of female sexual receptivity. The findings presented suggest a new potential function for mammalian penile spines in the context of sexual selection, and add to growing evidence that sperm competition and associated sexual conflict are important selection pressures in the evolution of animal genitalia.     

Does polyandry control population sex ratio via regulation of a selfish gene?

The extent of female multiple mating (polyandry) can strongly impact on the intensity of sexual selection, sexual conflict, and the evolution of cooperation and sociality. More subtly, polyandry may protect populations against intragenomic conflicts that result from the invasion of deleterious selfish genetic elements (SGEs). SGEs commonly impair sperm production, and so are likely to be unsuccessful in sperm competition, potentially reducing their transmission in polyandrous populations. Here, we test this prediction in nature. We demonstrate a heritable latitudinal cline in the degree of polyandry in the fruitfly Drosophila pseudoobscura across the USA, with northern population females remating more frequently in both the field and the laboratory. High remating was associated with low frequency of a sex-ratio-distorting meiotic driver in natural populations. In the laboratory, polyandry directly controls the frequency of the driver by undermining its transmission. Hence we suggest that the cline in polyandry represents an important contributor to the cline in sex ratio in nature. Furthermore, as the meiotic driver causes sex ratio bias, variation in polyandry may ultimately determine population sex ratio across the USA, a dramatic impact of female mating decisions. As SGEs are ubiquitous it is likely that the reduction of intragenomic conflict by polyandry is widespread.     

Convenience polyandry or convenience polygyny? Costly sex under female control in a promiscuous primate

Classic sex roles depict females as choosy, but polyandry is widespread. Empirical attempts to understand the evolution of polyandry have often focused on its adaptive value to females, whereas 'convenience polyandry' might simply decrease the costs of sexual harassment. We tested whether constraint-free female strategies favour promiscuity over mating selectivity through an original experimental design. We investigated variation in mating behaviour in response to a reversible alteration of sexual dimorphism in body mass in the grey mouse lemur, a small primate where female brief sexual receptivity allows quantifying polyandry. We manipulated body condition in captive females, predicting that convenience polyandry would increase when females are weaker than males, thus less likely to resist their solicitations. Our results rather support the alternative hypothesis of 'adaptive polyandry': females in better condition are more polyandrous. Furthermore, we reveal that multiple mating incurs significant energetic costs, which are strikingly symmetrical between the sexes. Our study shows that mouse lemur females exert tight control over mating and actively seek multiple mates. The benefits of remating are nevertheless not offset by its costs in low-condition females, suggesting that polyandry is a flexible strategy yielding moderate fitness benefits in this small mammal.     

Ageing and the evolution of female resistance to remating in seed beetles

Female remating behaviour is a key mating system parameter that is predicted to evolve according to the net effect of remating on female fitness. In many taxa, females commonly resist male remating attempts because of the costs of mating. Here, we use replicated populations of the seed beetle Acanthoscelides obtectus selected for either early or late life reproduction and show that 'Early' and 'Late' females evolved different age-specific rates of remating. Early females were more likely to remate with control males as they aged, while Late females were more resistant to remating later in life. Thus, female remating rate decreases with age when direct selection on late-life fitness is operating and increases when such selection is relaxed. Our findings not only demonstrate that female resistance to remating can evolve rapidly, but also that such evolution is in accordance with the genetic interests of females.     

No selection for change in polyandry under experimental evolution

What drives mating system variation is a major question in evolutionary biology. Female multiple mating (polyandry) has diverse evolutionary consequences, and there are many potential benefits and costs of polyandry. However, our understanding of its evolution is biased towards studies enforcing monandry in polyandrous species. What drives and maintains variation in polyandry between individuals, genotypes, populations and species remains poorly understood. Genetic variation in polyandry may be actively maintained by selection, or arise by chance if polyandry is selectively neutral. In Drosophila pseudoobscura, there is genetic variation in polyandry between and within populations. We used isofemale lines to found replicate populations with high or low initial levels of polyandry and tracked polyandry under experimental evolution over seven generations. Polyandry remained relatively stable, reflecting the starting frequencies of the experimental populations. There were no clear fitness differences between high versus low polyandry genotypes, and there was no signature of balancing selection. We confirmed these patterns in direct comparisons between evolved and ancestral females and found no consequences of polyandry for female fecundity. The absence of differential selection even when initiating populations with major differences in polyandry casts some doubt on the importance of polyandry for female fitness.     

Intra-specific variation in female remating in Callosobruchus chinensis and C. maculatus

The effects of mating duration on female remating (exp. 1) and under different male densities (exp. 2) were examined in two strains of the adzuki bean beetle, Callosobruchus chinensis and in one strain of the bruchid beetle, C. maculatus. In experiment 1, the frequency of female remating was markedly different between the two strains of C. chinensis. Females of the jC strain, reared long-term in the laboratory, did not remate after being allowed to mate freely (=monogamy), whereas females of the isC strain, recently established from the field, showed high remating frequencies (=polyandry). In both strains, the frequency of female remating increased after the duration of the first mating was deliberately shortened. The relation between mating duration and remating frequency was significantly different, however, between the two strains. In a closely related species, C. maculatus, which manifests polyandry, this relation was more similar to that of the field-derived (=isC) than to that of the laboratory-derived (=jC) strain of C. chinensis. The reasons for the inter-strain variation observed in the remating frequencies of C. chinensis are also discussed. In experiment 2, the mating duration of the three strains was compared under different male densities. Only the lab-derived strain demonstrated a significantly shorter mating duration when one female was placed together with five males than when paired with one male. The shorter mating duration (approximately 26 s) was similar to that of females allowed to remate in the monogamous strain in experiment 1.     

Standing genetic variance for female resistance to harm from males and its relationship to intralocus sexual conflict

Interlocus sexual conflict theory predicts that some male adaptations are harmful to their mates. Females are therefore expected to evolve resistance to this harm. Using cytogenetic cloning techniques, we tested for heritable genetic variation among females for resistance to harm from males and determined whether propensity to remate, female body size, and intralocus conflict contributes to this variation. We found low but significant heritability for female resistance, but this variation accounted for more than half of the standing genetic variation for net fitness among females. We found no association between female resistance and female body size or level of intralocus sexual conflict. Reluctance to remate was found to be an important factor contributing to the female resistance phenotype, and we found a positive selection gradient on this trait. However, we observed only a nonsignificant positive correlation between a female's resistance and her net fitness. One factor contributing to the observed nominal level of selection on female resistance was that males cause the greatest amount of harm to females with the highest intrinsic fecundity.     

Strategic ejaculation and level of polyandry in <Emphasis Type="Italic">Callosobruchus chinensis</Emphasis> (Coleoptera: Bruchidae)

Variation in the level of polyandry of females produces a difference in the risk of sperm competition among males. As a consequence, investment in ejaculate expenditure by males should vary. We compared the number of sperm ejaculated by males into the female reproductive organ of six strains of the adzuki bean beetle, Callosobruchus chinensis (Coleoptera: Bruchidae), when males were reared at different larval densities in a bean. A significant positive correlation was found between the remating frequency of females and the ratio of the ejaculate sizes of high-density and low-density males as a measure of the response to the risk of sperm competition among males. The measure was estimated by dividing the number of sperm ejaculated by males reared at high larval density in a bean with the number of sperm ejaculated by males reared alone. The number of sperm transferred by a male to a female was not correlated with the duration of copulation. The results suggest an evolutionary relationship between ejaculatory expenditure and the level of polyandry in C. chinensis.     

Nuptial gift consumption influences female remating in a scorpionfly: male or female control of mating rate?

In the scorpionfly Panorpa cognata, males provide females with saliva secretions as nuptial food gifts. Consequently, females derive material benefits and possibly also genetic benefits from multiple matings. Females therefore generally should have a high motivation to remate. Males, on the other hand, do not share this interest, which will generate a sexual conflict over remating interval, possibly leading to male adaptations that prevent females from remating with other males. In this study, I found that mated females were less prone to copulate than virgin females, despite female benefits of multiple matings. Further, I found that the remating interval was significantly longer if the first copulation was long compared to shorter matings. This effect does not entirely depend on copulation duration per se, but on the amount of saliva, that a female is consuming during copulation. These results suggest a mating-induced refractory period and can be interpreted as male manipulation of female remating behaviour mediated through substances in the nuptial gift. Alternatively, receiving large nuptial gifts may decrease the prospective direct fitness benefits from further copulations, and thus change optimal female remating rate. Furthermore, gift size has been shown to correlate with male nutritional condition, which may be an indicator of male genetic quality. Females may therefore benefit indirectly by not remating following copulations involving large saliva gifts. In this scenario, female remating interval would be an effect of cryptic female choice.