Do longer genital spines in male seed beetles function as better anchors during mating?

Copulatory mechanism and genital coupling of the longhorn beetle Moechotypadiphysis (Coleoptera, Cerambycidae)

The function of insect external genitalia has played a significant role in exploring insect mating mechanisms and male fertilization strategies. However, due to the privacy of genital coupling, insect copulatory mechanisms have only been investigated in a few insect groups. In this study, we observed the mating behavior using freeze-fixated pairs in copula to reveal the copulatory mechanism of the longhorn beetle Moechotypadiphysis (Pascoe, 1871). At the beginning stage of mating, the male M.diphysis usually takes 30 min to control the female and then extends its median lobe and endophallus. Approximately 80% of males (19/24) of M.diphysis exhibit multiple expansions (the membranous endophallus expands and enters into the female genital tract), ranging from two to five times. There are two types of expansions: short ones lasting for 1.4 to 49 s and long ones ranging from 1.03 to 7.23 min. During copulation, male tarsi continuously grasped the female elytra, thorax, and abdomen to help the male to initiate and maintain copulation. Male genital structures are closely connected to female genital structures: the apical phallomere and flagellum on the male endophallus contacting the bursa copulatrix duct and the spermathecal duct of the female, and the abundant microstructures on the surface of the everted male endophallus directly anchoring the female genital tract. Finally, we discuss the possible reasons for the evolution of their complex mating-related structures. Our research will help to explore the evolutionary mechanisms of insect genital structures.

Anatomy of male and female genitalia of Acanthoscelidesobtectus (Say, 1831) (Coleoptera, Chrysomelidae, Bruchinae) in interaction

Armatures of the male intromittent copulatory structures have been surmised to increase male fitness by imposing physiological costs on female re-mating. Female kicking could, consequently, be a counterstrategy to avoid wounding or to prevent males from mating. The membranous endophallus of male Acanthoscelidesobtectus (Say, 1831) is armed with denticles. Checking if these denticles penetrate the wall of the female genital tract during copulation revealed that only the tip of the median lobe of the aedeagus is intromitted into the female genital opening during copulation. The everted endophallus extends over the full length of the ovipositor, and the spermatophore is placed in the bursa. Identification by means of light microscopy and Micro-CT of the exact relative position of male and female copulatory organs while mated confirmed that the denticles do not cause wounds in the vagina wall. Parts of the inner wall of the bursa copulatrix are covered with inward pointing denticles. Already mated females kick mounting males by vehement movements of their hind legs, thereby preventing mating. In contrast, virgin females usually accept the first male they encounter and terminate copulation by slower movements of their hind legs. The same applied to females who accepted re-mating the second day after the first copulation. Acanthoscelidesobtectus females kick males off to prevent rather than to terminate copulation. Copulatory structures as well as behaviour may have different functional roles in different beetle species, even within the Bruchinae.

Quantifying variation in female internal genitalia: no evidence for plasticity in response to sexual conflict risk in a seed beetle

Sexually antagonistic coevolution can drive the evolution of male traits that harm females, and female resistance to those traits. While males have been found to vary their harmfulness to females in response to social cues, plasticity in female resistance traits remains to be examined. Here, we ask whether female seed beetles Callosobruchus maculatus are capable of adjusting their resistance to male harm in response to the social environment. Among seed beetles, male genital spines harm females during copulation and females might resist male harm via thickening of the reproductive tract walls. We develop a novel micro computed tomography imaging technique to quantify female reproductive tract thickness in three-dimensional space, and compared the reproductive tracts of females from populations that had evolved under high and low levels of sexual conflict, and for females reared under a social environment that predicted either high or low levels of sexual conflict. We find little evidence to suggest that females can adjust the thickness of their reproductive tracts in response to the social environment. Neither did evolutionary history affect reproductive tract thickness. Nevertheless, our novel methodology was capable of quantifying fine-scale differences in the internal reproductive tracts of individual females, and will allow future investigations into the internal organs of insects and other animals.

Traumatic mating increases anchorage of mating male and reduces female remating duration and fecundity in a scorpionfly species

Traumatic mating is the male wounding his mate during mating using specialized anatomy. However, why males have evolved to injure their mates during mating remains poorly understood. We studied traumatic mating in Dicerapanorpa magna to determine its effects on male and female fitness. The sharp teeth on male gonostyli penetrate the female genitalia and cause copulatory wounds, and the number of scars on the female genitals is positively related to the number of times females mated. When the injurious teeth were encased with low-temperature wax, preventing their penetration of the female's genitalia during mating, male mating success and copulation duration were reduced significantly, indicating the importance of the teeth in allowing the male to secure copulation, remain in copula and effectively inseminate his mate. The remating experiments showed that traumatic mating had little effect on the female mating refractory period, but significantly reduced female remating duration with subsequent males, probably benefiting the first-mating male with longer copulation duration and transferring more sperm into the female's spermatheca. The copulatory wounds reduced female fecundity, but did not accelerate the timing of egg deposition. This is probably the first report that traumatic mating reduces female remating duration through successive remating experiments in animals. Overall, our results provide evidence that traumatic mating in the scorpionfly helps increase the male's anchoring control during mating and provides him advantage in sperm competition, but at the expense of lowering female fecundity.

Sexual conflict drives micro- and macroevolution of sexual dimorphism in immunity

BACKGROUND

Sexual dimorphism in immunity is believed to reflect sex differences in reproductive strategies and trade-offs between competing life history demands. Sexual selection can have major effects on mating rates and sex-specific costs of mating and may thereby influence sex differences in immunity as well as associated host-pathogen dynamics. Yet, experimental evidence linking the mating system to evolved sexual dimorphism in immunity are scarce and the direct effects of mating rate on immunity are not well established. Here, we use transcriptomic analyses, experimental evolution and phylogenetic comparative methods to study the association between the mating system and sexual dimorphism in immunity in seed beetles, where mating causes internal injuries in females.

RESULTS

We demonstrate that female phenoloxidase (PO) activity, involved in wound healing and defence against parasitic infections, is elevated relative to males. This difference is accompanied by concomitant sex differences in the expression of genes in the prophenoloxidase activating cascade. We document substantial phenotypic plasticity in female PO activity in response to mating and show that experimental evolution under enforced monogamy (resulting in low remating rates and reduced sexual conflict relative to natural polygamy) rapidly decreases female (but not male) PO activity. Moreover, monogamous females had evolved increased tolerance to bacterial infection unrelated to mating, implying that female responses to costly mating may trade off with other aspects of immune defence, an hypothesis which broadly accords with the documented sex differences in gene expression. Finally, female (but not male) PO activity shows correlated evolution with the perceived harmfulness of male genitalia across 12 species of seed beetles, suggesting that sexual conflict has a significant influence on sexual dimorphisms in immunity in this group of insects.

CONCLUSIONS

Our study provides insights into the links between sexual conflict and sexual dimorphism in immunity and suggests that selection pressures moulded by mating interactions can lead to a sex-specific mosaic of immune responses with important implications for host-pathogen dynamics in sexually reproducing organisms.

No evidence for divergence in male harmfulness or female resistance in response to changes in the opportunity for dispersal

The outcome of sexual conflict can depend on the social environment, as males respond to changes in the inclusive fitness payoffs of harmfulness and harm females less when they compete with familiar relatives. Theoretical models also predict that if limited male dispersal predictably enhances local relatedness while maintaining global competition, kin selection can produce evolutionary divergences in male harmfulness among populations. Experimental tests of these predictions, however, are rare. We assessed rates of dispersal in female and male seed beetles Callosobruchus maculatus, a model species for studies of sexual conflict, in an experimental setting. Females dispersed significantly more often than males, but dispersing males travelled just as far as dispersing females. Next, we used experimental evolution to test whether limiting dispersal allowed the action of kin selection to affect divergence in male harmfulness and female resistance. Populations of C. maculatus were evolved for 20 and 25 generations under one of three dispersal regimens: completely free dispersal, limited dispersal and no dispersal. There was no divergence among treatments in female reproductive tract scarring, ejaculate size, mating behaviour, fitness of experimental females mated to stock males or fitness of stock females mated to experimental males. We suggest that this is likely due to insufficient strength of kin selection rather than a lack of genetic variation or time for selection. Limited dispersal alone is therefore not sufficient for kin selection to reduce male harmfulness in this species, consistent with general predictions that limited dispersal will only allow kin selection if local relatedness is independent of the intensity of competition among kin.

Genital coupling, morphology and evolution of male holding structures in Cicadinae (Hemiptera: Cicadidae)

Male and female genitalia include some of the most complex and morphologically diverse structures in Metazoa. Ornamentations in genitalia have been studied in several groups, and a variety of functional roles have been proposed. Although complex features of the genitalia have been observed in internal genitalia in cicadas, their functions have not yet been elucidated. These ornamentations, together with precopulatory sexual selection, make cicadas good models for evolutionary studies on genital coupling. We explore the structural interaction of male and female genitalia in Guyalna bonaerensis (Berg) (Cicadinae) and the morphology of male ornamentations in Cicadinae generally. We group these ornamentations into two traits according to their inferred function: anchoring or gripping. We analyse the theca and vesica of 24 species and perform ancestral trait reconstruction under maximum likelihood and stochastic mapping on a Bayesian tree. Ornamentations of the male vesica and the female seminal ampoule possibly ensure male attachment by working as an active lock to avoid the premature termination of intercourse. These ornamentations emerged independently in different lineages in Cicadinae, reinforcing the suggestion that they are important adaptations to achieve complete copulation. Our results foster questions for the field of sexual selection and associated mechanisms shaping the evolution of male and female genitalia.

Gustatory cues to kinship among males moderate the productivity of females

Males of many species harm females as a byproduct of intrasexual competition, but this harm can be reduced if males are less competitive in the presence of familiar relatives. We determined the cue males use to identify competitors in this context. We assessed genetic variance in a putative kin recognition trait (cuticular hydrocarbons) in male seed beetles Callosobruchus maculatus and found that five hydrocarbons had significant components of additive genetic variance and could serve as relatedness cues. Next, we tested whether hydrocarbons were the mechanism males use to distinguish the social identities of competitors when strategically adjusting their competitiveness/harmfulness. Pairs of female and male C. maculatus were mated in the presence of hydrocarbons extracted from males that differed in their relatedness and familiarity to the focal male. Females were more productive after mating in the presence of extracts from the focal male’s nonrelatives, if those extracts were also unfamiliar to the focal male. Relatedness had no effect on productivity when extracts were familiar to the focal male. These results may be reconciled with those of previous studies that manipulated the relatedness and familiarity of competing males if the difference between the effect of harmfulness on productivity following a single mating and the effect on lifetime reproductive fitness after multiple matings is accounted for. This study provides a novel demonstration of the mechanism of social recognition in the moderation of sexual conflict.

Males harm females less when competing with familiar relatives

Sexual conflict occurs when reproductive partners have different fitness optima, and can lead to the evolution of traits in one sex that inflict fitness costs on the opposite sex. Recently, it has been proposed that antagonism by males towards females should be reduced when they compete with relatives, because reducing the future productivity of a female would result in an indirect fitness cost for a harmful male. We tested this prediction in the seed beetle Callosobruchus maculatus, the males of which harm females with genital spines and pre-copulatory harassment. We compared lifespan, lifetime egg production and lifetime offspring production among females housed with groups of males that varied in their familiarity and relatedness. Females produced significantly more eggs and offspring when grouped with males who were both related and familiar to each other. There was no effect of male relatedness or familiarity on female lifespan. Our results suggest that males plastically adjust their harmfulness towards females in response to changes in inclusive fitness payoffs, and that in this species both genetic relatedness and social familiarity mediate this effect.

Sexual conflict and correlated evolution between male persistence and female resistance traits in the seed beetle Callosobruchus maculatus

Traumatic mating (or copulatory wounding) is an extreme form of sexual conflict whereby male genitalia physically harm females during mating. In such species females are expected to evolve counter-adaptations to reduce male-induced harm. Importantly, female counter-adaptations may include both genital and non-genital traits. In this study, we examine evolutionary associations between harmful male genital morphology and female reproductive tract morphology and immune function across 13 populations of the seed beetle Callosobruchus maculatus We detected positive correlated evolution between the injuriousness of male genitalia and putative female resistance adaptations across populations. Moreover, we found evidence for a negative relationship between female immunity and population productivity, which suggests that investment in female resistance may be costly due to the resource trade-offs that are predicted between immunity and reproduction. Finally, the degree of female tract scarring (harm to females) was greater in those populations with both longer aedeagal spines and a thinner female tract lining. Our results are thus consistent with a sexual arms race, which is only apparent when both male and female traits are taken into account. Importantly, our study provides rare evidence for sexually antagonistic coevolution of male and female traits at the within-species level.

Male-female interactions drive the (un)repeatability of copula duration in an insect

Across the animal kingdom the duration of copulation varies enormously from a few seconds to several days. Functional explanations for this variation are largely embedded within sperm competition theory in which males modulate the duration of copula in order to optimize their fitness. However, copulation is the union of two protagonists which are likely to have separate and often conflicting reproductive interests, yet few experimental designs specifically assess the effect of male-female interactions on the duration of copulation. This can result in inexact assertions over which sex controls copulatory behaviour. Here we analyse the repeatability of copulatory behaviour in the seed beetle Callosobruchus maculatus to determine which sex exerts primary influence over copulation duration. In C. maculatus, copulation follows two distinct phases: an initial quiescent phase followed by a period of vigorous female kicking behaviour that culminates in the termination of copulation. When males or females copulated with several novel mates, copulatory behaviour was not significantly repeatable. By contrast, when males or females mated repeatedly with the same mate, copula duration was repeatable. These data suggest copulatory behaviour in C. maculatus to be largely the product of male-female interactions rather than the consistent, sex-specific modulation of copula duration of one protagonist in response to the phenotypic variation presented by the other protagonist.

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.

Functions, diversity, and evolution of traumatic mating

Copulation can involve the wounding of the mating partner by specialised devices. This type of mating, which we term traumatic mating, has been regarded as exceptional. Its prevalence, however, has not been compared across taxa, nor have its functions and putative evolutionary pathways. A categorisation has been lacking to date. We here show that traumatic mating is a widespread and diverse phenomenon that likely evolved via several pathways. Its putative functions include: (i) anchorage during mating; (ii) stimulation of short-term female reproductive investment; (iii) male paternity advantages; and (iv) enhanced fertilisation efficiency in transitions to internal fertilisation. Both natural and sexual selection have likely contributed to the parallel evolution of traumatic intromittent organs in phylogenetically distant taxa. These organs are sometimes remarkably similar in shape and often, but not always, inject sperm. The target sites of trauma infliction and the nature of secretions delivered alongside sperm are thus far poorly studied, but data on both are needed to elucidate the function of traumatic mating. The few existing studies that explicitly quantify fitness impacts of traumatic mating indicate that this strategy may often be costly to the party being wounded. However, a comprehensive approach to assess overall investments and returns for both sexes is a major target for future work. Finally, for the first time, we corroborate quantitatively the hypothesis that traumatic mating evolved relatively more often among hermaphroditic than among gonochoric taxa.