SEXUAL CONFLICT AND ANTAGONISTIC COEVOLUTION ACROSS WATER STRIDER POPULATIONS

Sexually antagonistic coevolution of the male nuptial gift and female feeding behaviour in decorated crickets

The evolution of nuptial gifts has traditionally been considered a harmonious affair, providing benefits to both mating partners. There is growing evidence, however, that receiving a nuptial gift can be actively detrimental to the female. In decorated crickets (Gryllodes sigillatus), males produce a gelatinous spermatophylax that enhances sperm transfer but provides little nutritional benefit and hinders female post-copulatory mate choice. Here, we examine the sexually antagonistic coevolution of the spermatophylax and the female feeding response to this gift in G. sigillatus maintained in experimental populations with either a male-biased or female-biased adult sex ratio. After 25 generations, males evolving in male-biased populations produced heavier spermatophylaxes with a more manipulative combination of free amino acids than those evolving in female-biased populations. Moreover, when the spermatophylax originated from the same selection regime, females evolving in male-biased populations always had shorter feeding durations than those evolving in female-biased populations, indicating the evolution of greater resistance. Across populations, female feeding duration increased with the mass and manipulative combination of free amino acids in the spermatophylax, suggesting sexually antagonistic coevolution. Collectively, our work demonstrates a key role for interlocus sexual conflict and sexually antagonistic coevolution in the mating system of G. sigillatus.

Female genital concealment and a corresponding male clasping apparatus in Australian ripple bugs (Hemiptera: Veliidae)

Conflicts of interest over reproduction between males and females are widespread in sexually reproducing species. This is exemplified in water striders (Gerridae), where females vigorously resist costly mating attempts, and males and females often exhibit elaborate grasping and anti-grasping morphological traits. Like water striders, their sister-group, the ripple bugs (Veliidae), share similar life histories and are expected to face similar conflicts over mating. Veliids in the genus Nesidovelia exhibit elaborate sexual dimorphism, which is predicted to function in intersexual antagonistic struggles. This includes concealed genitalia in females, and elaborate pregenital abdominal modifications in males. By documenting mating behaviour in Nesidovelia peramoena and freezing pairs in copula, we show that males and females struggle prior to mating, and male abdominal modifications function to gain access to the female's concealed genitalia. This is consistent with, though not limited to, sexual conflict.

Intraspecific evolution of sexually dimorphic characters in a female diving beetle can be promoted by demographic history and temperature

Previous studies have predicted that antagonistic intraspecific evolution of sexually dimorphic characters causing rapid speciation can be driven by demographic history and environmental variations. However, researchers have rarely examined this issue in the wild. Here, we examined intraspecific evolution of sexually dimorphic characters and its driving force by using a diving beetle, Acilius japonicus, which has very marked sexually dimorphic characters. Males with wider big suction cups could copulate with females with a higher success rate, whereas the mating durations of females with more hairs on their pronota were shorter. Females in a region with greater interpopulation genetic differentiation had more pronotal hairs. Considering that a previous study showed that less continuity among populations leads to a higher female cost of mating, this result suggests a greater female cost of mating in this region. Females at warmer sites also had more pronotal hairs. In light of the increase in O₂ consumption in warmer water, our result suggests that more pronotal hairs in females at warmer sites have been maintained to prevent prolonged underwater mating at higher O₂ demand. These findings suggest that demographic history and temperature can direct the evolution of sexually dimorphic characters related to sexual conflict in females.

Sex-biased demography modulates male harm across the genome

Recent years have seen an explosion of theoretical and empirical interest in the role that kin selection plays in shaping patterns of sexual conflict, with a particular focus on male harming traits. However, this work has focused solely on autosomal genes, and as such it remains unclear how demography modulates the evolution of male harm loci occurring in other portions of the genome, such as sex chromosomes and cytoplasmic elements. To investigate this, we extend existing models of sexual conflict for application to these different modes of inheritance. We first analyse the general case, revealing how sex-specific relatedness, reproductive value and the intensity of local competition combine to determine the potential for male harm. We then analyse a series of demographically explicit models, to assess how dispersal, overlapping generations, reproductive skew and the mechanism of population regulation affect sexual conflict across the genome, and drive conflict between nuclear and cytoplasmic genes. We then explore the effects of sex biases in these demographic parameters, showing how they may drive further conflicts between autosomes and sex chromosomes. Finally, we outline how different crossing schemes may be used to identify signatures of these intragenomic conflicts.

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.

The ultrastructure of sperm and female sperm storage organs in the water strider Gerris lacustris L. (Heteroptera) and a possible example of genital coevolution

The fine structural organization of the male and the female inner reproductive apparatuses of the water-strider Gerris lacustris was studied. The sperm of the species shows a long helicoidal acrosome provided with longitudinal tubules, and a short nucleus. The flagellum is characterized by crescent mitochondrial derivatives and a 9 + 9 + 2 axoneme, as occurs in all Heteroptera. The female reproductive apparatus is characterized by an extremely long spermathecal duct, filled with sperm, which plays the role of the main sperm storage organ. The duct has a thin epithelium surrounded by a complex of secretory and duct-forming cells. The spermathecal duct flows into the gynatrial sac. This region, together with the fertilization chamber, exhibits a simple epithelium with deep apical plasma membrane invaginations, and it does not show conspicuous secretions. The basal cell region shows plasma membrane infoldings forming thin cytoplasmic bands hosting mitochondria and large intercellular spaces. This organization is typical of epithelia active in fluid reabsorption. Two lateral large gynatrial glands open into the gynatrial sac. Such glands also exhibit secretory and duct forming cells. The same structure of these glands is also present along the proximal region of the fecundation canal. The duct forming cells of these regions have very wide ducts with peculiar cuticular finger-like structures at their opening into the gland duct lumen. The results of the present study suggest the occurrence of a coevolution between the sperm and the spermathecal duct lengths.

Genomic Determination of Reproductive Mode in Facultatively Parthenogenetic Opiliones

Sexual reproduction may pose myriad short-term costs to females. Despite these costs, sexual reproduction is near ubiquitous. Facultative parthenogenesis is theorized to mitigate some of the costs of sex, as individuals can participate in occasional sex to limit costs while obtaining many benefits. However, most theoretical models assume sexual reproduction is fixed following mating, with no possibility of clutches of mixed reproductive ontogeny. Therefore, we asked: if coercive males are present at high frequency in a population of facultative parthenogens, will their clutches be solely sexually produced, or will there be evidence of sexually and asexually-produced offspring? How will their offspring production compare to conspecifics in low-frequency male populations? We addressed our questions by collecting females and egg clutches of the facultatively parthenogenetic Opiliones species Leiobunum manubriatum and L. globosum. In L. manubriatum, females from populations with few males were not significantly more fecund than females from populations with higher male relative frequency, despite the potential release of the former from sexual conflict. We used 3 genotyping methods along with a custom set of DNA capture probes to reveal that offspring of L. manubriatum from these high male populations were primarily produced via asexual reproduction. This is surprising because sex ratios in these southern populations approach equality, increasing the probability for females to encounter mates and produce offspring sexually. We additionally found evidence for reproductive polymorphisms within populations. Rapid and accurate SNP genotyping data will continue to allow us to address broader evolutionary questions regarding the role of facultative reproductive modes in the maintenance of sex.

Morphology of genitalia and non-genitalic contact structures inTrouessartiafeather mites (Astigmata: Analgoidea: Trouessartiidae): is there evidence of correlated evolution between the sexes?

Positive correlations between the shapes of male and female sexual structures can be interpreted as cooperative or as combative. In the feather mite genus Trouessartia Canestrini, 1899, the spermaducts of females range from entirely internal to extending externally for varying lengths, whereas male primary genitalia range from gracile to massive. Males also possess a pair of adanal suckers used to hold onto the dorsal surface of the female during copulation. In the area of this attachment, females exhibit ornamentation and have strongly developed dorsal setae (setae h1), which we hypothesized serve to weaken the male’s hold during copulation. In male and female Trouessartia from 51 bird species, we compared female external spermaduct length and male genitalic “massiveness” and explored whether patterns of female dorsal ornamentation and (or) h1 seta size correlate with male adanal sucker size. Our results indicate that females with longer external spermaducts are associated with males with relatively massive genitalia. However, we found no significant relationship between male adanal sucker size and female ornamentation or h1 seta size. Further information regarding how the genitalia interact during sperm transfer is necessary to interpret correlations in genitalia size and strong intersexual differences in dorsal ornamentation and seta size in Trouessartia.

Divergence of seminal fluid gene expression and function among natural snail populations

Seminal fluid proteins (SFPs) can trigger drastic changes in mating partners, mediating post-mating sexual selection and associated sexual conflict. Also, cross-species comparisons have demonstrated that SFPs evolve rapidly and hint that post-mating sexual selection drives their rapid evolution. In principle, this pattern should be detectable within species as rapid among-population divergence in SFP expression and function. However, given the multiple other factors that could vary among populations, isolating divergence in SFP-mediated effects is not straightforward. Here, we attempted to address this gap by combining the power of a common garden design with functional assays involving artificial injection of SFPs in the simultaneously hermaphroditic freshwater snail, Lymnaea stagnalis. We detected among-population divergence in SFP gene expression, suggesting that seminal fluid composition differs among four populations collected in Western Europe. Furthermore, by artificially injecting seminal fluid extracted from these field-derived snails into standardized mating partners, we also detected among-population divergence in the strength of post-mating effects induced by seminal fluid. Both egg production and subsequent sperm transfer of partners differed depending on the population origin of seminal fluid, with the response in egg production seemingly closely corresponding to among-population divergence in SFP gene expression. Our results thus lend strong intraspecific support to the notion that SFP expression and function evolve rapidly, and confirm L. stagnalis as an amenable system for studying processes driving SFP evolution.

Artificial selection on sexual aggression: Correlated traits and possible trade-offs

Forced copulation is an extreme form of sexual aggression that can affect the evolution of sex-specific anatomy, morphology, and behavior. To characterize mechanistic and evolutionary aspects of forced copulation, we artificially selected male fruit flies based on their ability to succeed in the naturally prevalent behavior of forced matings with newly eclosed (teneral) females. The low and high forced copulation lineages showed rapid divergence, with the high lineages ultimately showing twice the rates of forced copulation as the low lineages. While males from the high lineages spent more time aggressively pursuing and mounting teneral females, their behavior toward non-teneral and heterospecific females was similar to that of males from the low lineages. Males from the low and high lineages also showed similar levels of male-male aggression. This suggests little or no genetic correlations between sexual aggression and non-aggressive pursuit of females, and between male aggression toward females and males. Surprisingly however, males from the high lineages had twice as high mating success than males from the low lineages when allowed to compete for consensual mating with mature females. In further experiments, we found no evidence for trade-offs associated with high forced mating rates: males from the high lineages did not have lower longevity than males from the low lineages when housed with females, and four generations of relaxed selection did not lead to convergence in forced mating rates. Our data indicate complex interactions among forced copulation success and consensual mating behavior, which we hope to clarify in future genomic work.

Sexual conflict and intrasexual polymorphism promote assortative mating and halt population differentiation

Sexual conflict is thought to be an important evolutionary force in driving phenotypic diversification, population divergence, and speciation. However, empirical evidence is inconsistent with the generality that sexual conflict enhances population divergence. Here, we demonstrate an alternative evolutionary outcome in which sexual conflict plays a conservative role in maintaining male and female polymorphisms locally, rather than promoting population divergence. In diving beetles, female polymorphisms have evolved in response to male mating harassment and sexual conflict. We present the first empirical evidence that this female polymorphism is associated with (i) two distinct and sympatric male morphological mating clusters (morphs) and (ii) assortative mating between male and female morphs. Changes in mating traits in one sex led to a predictable change in the other sex which leads to predictable within-population evolutionary dynamics in male and female morph frequencies. Our results reveal that sexual conflict can lead to assortative mating between male offence and female defence traits, if a stable male and female mating polymorphisms are maintained. Stable male and female mating polymorphisms are an alternative outcome to an accelerating coevolutionary arms race driven by sexual conflict. Such stable polymorphisms challenge the common view of sexual conflict as an engine of rapid speciation via exaggerated coevolution between sexes.

How Does the Male Penisfilum Enter the Female Copulatory Pore in Hangingflies?

Hangingflies are characterized by the interesting nuptial feeding behavior and unusual belly-to-belly hanging mating position. However, the mating behavior and the copulatory mechanism remain poorly known for Bittacidae, especially how the elongated male penisfilum enters the copulatory pore of the female. In this study, the mating behavior and copulatory mechanism of Terrobittacus implicatus (Huang and Hua, 2006) were investigated to reveal the functional morphology of hangingfly genitalia. The results show that the male provides a prey as a nuptial gift to the female and twists his abdomen about 180° to form a belly-to-belly hanging mating position. During the penisfilum-entering process, the male epandrial lobes clamp the female subgenital plate with the aid of the female abdomen swelling. Then the male locates the female copulatory pore through his upper branch of the proctiger and inserts his penisfilum into the female spermathecal duct in cooperation with the short setae on the groove of the proctiger. The female subgenital plate where the epandrial lobes clamp is strongly sclerotized and melanized. The copulatory mechanism of Terrobittacus is briefly discussed.

The evolution of female genitalia

Female genitalia have been largely neglected in studies of genital evolution, perhaps due to the long-standing belief that they are relatively invariable and therefore taxonomically and evolutionarily uninformative in comparison with male genitalia. Contemporary studies of genital evolution have begun to dispute this view, and to demonstrate that female genitalia can be highly diverse and covary with the genitalia of males. Here, we examine evidence for three mechanisms of genital evolution in females: species isolating 'lock-and-key' evolution, cryptic female choice and sexual conflict. Lock-and-key genital evolution has been thought to be relatively unimportant; however, we present cases that show how species isolation may well play a role in the evolution of female genitalia. Much support for female genital evolution via sexual conflict comes from studies of both invertebrate and vertebrate species; however, the effects of sexual conflict can be difficult to distinguish from models of cryptic female choice that focus on putative benefits of choice for females. We offer potential solutions to alleviate this issue. Finally, we offer directions for future studies in order to expand and refine our knowledge surrounding female genital evolution.

Latitudinal variation and coevolutionary diversification of sexually dimorphic traits in the false blister beetle Oedemera sexualis

Sexual traits are subject to evolutionary forces that maximize reproductive benefits and minimize survival costs, both of which can depend on environmental conditions. Latitude explains substantial variation in environmental conditions. However, little is known about the relationship between sexual trait variation and latitude, although body size often correlates with latitude. We examined latitudinal variation in male and female sexual traits in 22 populations of the false blister beetle Oedemera sexualis in the Japanese Archipelago. Males possess massive hind legs that function as a female-grasping apparatus, while females possess slender hind legs that are used to dislodge mounting males. Morphometric analyses revealed that male and female body size (elytron length), length and width of the hind femur and tibia, and allometric slopes of these four hind leg dimensions differed significantly among populations. Of these, three traits showed latitudinal variation, namely, male hind femur was stouter; female hind tibia was slenderer, and female body was smaller at lower latitudes than at higher latitudes. Hind leg sizes and shapes, as measured by principal component analysis of these four hind leg dimensions in each sex, covaried significantly between sexes, suggesting coevolutionary diversification in sexual traits. Covariation between sexes was weaker when variation in these traits with latitude was removed. These results suggest that coevolutionary diversification between male and female sexual traits is mediated by environmental conditions that vary with latitude.

Sexual conflict in its ecological setting

Sexual conflict can lead to rapid and continuous coevolution between females and males, without any inputs from varying ecology. Yet both the degree of conflict and selection on antagonistic traits are known to be sensitive to local ecological conditions. This leads to the longstanding question: to what extent does variation in ecological context drive sexually antagonistic coevolution? In water striders, there is much information about the impacts of ecological factors on conflict, and about patterns of antagonistic coevolution. However, the connection between the two is poorly understood. Here, we first review the multiple ways in which ecological context might affect the coevolutionary trajectory of the sexes. We then review ecological and coevolutionary patterns in water striders, and connections between them, in light of theory and new data. Our analysis suggests that ecological variation does impact observed patterns of antagonistic coevolution, but highlights significant uncertainty due to the multiple pathways by which ecological factors can influence conflict and its evolutionary outcome. To the extent that water striders are a reasonable reflection of other systems, this observation serves as both an opportunity and a warning: there is much to learn, but gaining insight may be a daunting process in many systems.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

The role of ecology, neutral processes and antagonistic coevolution in an apparent sexual arms race

Some of the strongest examples of a sexual 'arms race' come from observations of correlated evolution in sexually antagonistic traits among populations. However, it remains unclear whether these cases truly represent sexually antagonistic coevolution; alternatively, ecological or neutral processes might also drive correlated evolution. To investigate these alternatives, we evaluated the contributions of intersex genetic correlations, ecological context, neutral genetic divergence and sexual coevolution in the correlated evolution of antagonistic traits among populations of Gerris incognitus water striders. We could not detect intersex genetic correlations for these sexually antagonistic traits. Ecological variation was related to population variation in the key female antagonistic trait (spine length, a defence against males), as well as body size. Nevertheless, population covariation between sexually antagonistic traits remained substantial and significant even after accounting for all of these processes. Our results therefore provide strong evidence for a contemporary sexual arms race.

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.

Selection on an antagonistic behavioral trait can drive rapid genital coevolution in the burying beetle, Nicrophorus vespilloides

Male and female genital morphology varies widely across many taxa, and even among populations. Disentangling potential sources of selection on genital morphology is problematic because each sex is predicted to respond to adaptations in the other due to reproductive conflicts of interest. To test how variation in this sexual conflict trait relates to variation in genital morphology we used our previously developed artificial selection lines for high and low repeated mating rates. We selected for high and low repeated mating rates using monogamous pairings to eliminate contemporaneous female choice and male-male competition. Male and female genital shape responded rapidly to selection on repeated mating rate. High and low mating rate lines diverged from control lines after only 10 generations of selection. We also detected significant patterns of male and female genital shape coevolution among selection regimes. We argue that because our selection lines differ in sexual conflict, these results support the hypothesis that sexually antagonistic coevolution can drive the rapid divergence of genital morphology. The greatest divergence in morphology corresponded with lines in which the resolution of sexual conflict over mating rate was biased in favor of male interests.

The Genetic Basis of Baculum Size and Shape Variation in Mice

The rapid divergence of male genitalia is a preeminent evolutionary pattern. This rapid divergence is especially striking in the baculum, a bone that occurs in the penis of many mammalian species. Closely related species often display diverse baculum morphology where no other morphological differences can be discerned. While this fundamental pattern of evolution has been appreciated at the level of gross morphology, nearly nothing is known about the genetic basis of size and shape divergence. Quantifying the genetic basis of baculum size and shape variation has been difficult because these structures generally lack obvious landmarks, so comparing them in three dimensions is not straightforward. Here, we develop a novel morphometric approach to quantify size and shape variation from three-dimensional micro-CT scans taken from 369 bacula, representing 75 distinct strains of the BXD family of mice. We identify two quantitative trait loci (QTL) that explain ∼50% of the variance in baculum size, and a third QTL that explains more than 20% of the variance in shape. Together, our study demonstrates that baculum morphology may diverge relatively easily, with mutations at a few loci of large effect that independently modulate size and shape. Based on a combination of bioinformatic investigations and new data on RNA expression, we prioritized these QTL to 16 candidate genes, which have hypothesized roles in bone morphogenesis and may enable future genetic manipulation of baculum morphology.

Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection, Mate Choice, and Sexual Conflict

Genital coevolution between the sexes is expected to be common because of the direct interaction between male and female genitalia during copulation. Here we review the diverse mechanisms of genital coevolution that include natural selection, female mate choice, male-male competition, and how their interactions generate sexual conflict that can lead to sexually antagonistic coevolution. Natural selection on genital morphology will result in size coevolution to allow for copulation to be mechanically possible, even as other features of genitalia may reflect the action of other mechanisms of selection. Genital coevolution is explicitly predicted by at least three mechanisms of genital evolution: lock and key to prevent hybridization, female choice, and sexual conflict. Although some good examples exist in support of each of these mechanisms, more data on quantitative female genital variation and studies of functional morphology during copulation are needed to understand more general patterns. A combination of different approaches is required to continue to advance our understanding of genital coevolution. Knowledge of the ecology and behavior of the studied species combined with functional morphology, quantitative morphological tools, experimental manipulation, and experimental evolution have been provided in the best-studied species, all of which are invertebrates. Therefore, attention to vertebrates in any of these areas is badly needed.

The evolution of sexually antagonistic phenotypes

Sexual conflict occurs whenever there is sexually antagonistic selection on shared traits. When shared traits result from interactions (e.g., mating rate) and have a different genetic basis in each sex (i.e., interlocus conflict), then sex-specific traits that shift the value of these interaction traits toward the sex-specific optimum will be favored. Male traits can be favored that increase the fitness of their male bearers, but decrease the fitness of interacting females. Likewise, female traits that reduce the costs of interacting with harmful males may simultaneously impose costs on males. If the evolution of these antagonistic traits changes the nature of selection acting on the opposite sex, interesting coevolutionary dynamics will result. Here we examine three current issues in the study of sexually antagonistic interactions: the female side of sexual conflict, the ecological context of sexual conflict, and the strength of evidence for sexually antagonistic coevolution.

Is sexual conflict an "engine of speciation"?

At the end of the last century, sexual conflict was identified as a powerful engine of speciation, potentially even more important than ecological selection. Earlier work that followed--experimental, comparative, and mathematical--provided strong initial support for this assertion. However, as the field matures, both the power of sexual conflict and constraints on the evolution of reproductive isolation as driven by sexual conflict are becoming better understood. From theoretical studies, we now know that speciation is only one of several possible evolutionary outcomes of sexual conflict. In line with these predictions, both experimental evolution studies and comparative analyses of fertilization proteins and of species richness show that sexual conflict leads to, or is associated with, reproductive isolation and speciation in some cases but not in others. Increased genetic variation (especially in females) without reproductive isolation is an underappreciated consequence of sexually antagonistic selection.

Human-caused habitat fragmentation can drive rapid divergence of male genitalia

The aim of this study rests on three premises: (i) humans are altering ecosystems worldwide, (ii) environmental variation often influences the strength and nature of sexual selection, and (iii) sexual selection is largely responsible for rapid and divergent evolution of male genitalia. While each of these assertions has strong empirical support, no study has yet investigated their logical conclusion that human impacts on the environment might commonly drive rapid diversification of male genital morphology. We tested whether anthropogenic habitat fragmentation has resulted in rapid changes in the size, allometry, shape, and meristics of male genitalia in three native species of livebearing fishes (genus: Gambusia) inhabiting tidal creeks across six Bahamian islands. We found that genital shape and allometry consistently and repeatedly diverged in fragmented systems across all species and islands. Using a model selection framework, we identified three ecological consequences of fragmentation that apparently underlie observed morphological patterns: decreased predatory fish density, increased conspecific density, and reduced salinity. Our results demonstrate that human modifications to the environment can drive rapid and predictable divergence in male genitalia. Given the ubiquity of anthropogenic impacts on the environment, future research should evaluate the generality of our findings and potential consequences for reproductive isolation.

Remating and sperm competition in replicate populations of Drosophila melanogaster adapted to alternative environments

The prevalence of sexual conflict in nature, as well as the supposedly arbitrary direction of the resulting coevolutionary trajectories, suggests that it may be an important driver of phenotypic divergence even in a constant environment. However, natural selection has long been central to the operation of sexual conflict within populations and may therefore constrain or otherwise direct divergence among populations. Ecological context may therefore matter with respect to the diversification of traits involved in sexual conflict, and if natural selection is sufficiently strong, such traits may evolve in correlation with environment, generating a pattern of ecologically-dependent parallel evolution. In this study we assess among-population divergence both within and between environments for several traits involved in sexual conflict. Using eight replicate populations of Drosophila melanogaster from a long-term evolution experiment, we measured remating rates and subsequent offspring production of females when housed with two separate males in sequence. We found no evidence of any variation in male reproductive traits (offense or defense). However, the propensity of females to remate diverged significantly among the eight populations with no evidence of any environmental effect, consistent with sexual conflict promoting diversification even in the absence of ecological differences. On the other hand, females adapted to one environment (ethanol) tended to produce a higher proportion of offspring sired by their first mate as compared to those adapted to the other (cadmium) environment, suggesting ecologically-based divergence of this conflict phenotype. Because we find evidence for both stochastic population divergence operating outside of an ecological context and environment-dependent divergence of traits under sexual conflict, the interaction of these two processes is an important topic for future work.

Freshwater biodiversity and aquatic insect diversification

Inland waters cover less than 1% of Earth's surface but harbor more than 6% of all insect species: Nearly 100,000 species from 12 orders spend one or more life stages in freshwater. Little is known about how this remarkable diversity arose, although allopatric speciation and ecological adaptation are thought to be primary mechanisms. Freshwater habitats are highly susceptible to environmental change and exhibit marked ecological gradients. Standing waters appear to harbor more dispersive species than running waters, but there is little understanding of how this fundamental ecological difference has affected diversification. In contrast to the lack of evolutionary studies, the ecology and habitat preferences of aquatic insects have been intensively studied, in part because of their widespread use as bioindicators. The combination of phylogenetics with the extensive ecological data provides a promising avenue for future research, making aquatic insects highly suitable models for the study of ecological diversification.

Co-evolution of the mating position and male genitalia in insects: a case study of a hangingfly

Hangingflies are unique for the male providing a nuptial gift to the female during mating and taking a face-to-face hanging copulation with the female. Their male genitalia are peculiar for an extremely elongated penisfilum, a pair of well-developed epandrial lobes (9th tergum), and a pair of degenerated gonostyli. However, the co-evolution of their face-to-face copulation behavior and the male genitalia has rarely been studied hitherto. In this paper the mating behavior of the hangingfly Bittacus planus Cheng, 1949 was observed under laboratory conditions, and the morphology of the male and female external genitalia was investigated using light and scanning electron microscopy. The male provides an insect prey as a nuptial gift to the female in courtship and mating process, and commits a face-to-face copulation. During copulation, the male abdomen twists temporarily about 180° to accommodate their face-to-face mating position. The aedeagal complex has an extremely elongated penisfilum, corresponding to the elongated spermathecal duct of the female. The well-developed epandrial lobes serve as claspers to grasp the female subgenital plate during copulation, replacing the function of gonostyli, which are greatly reduced in Bittacidae. The modified proctiger assists the penisfilum to stretch and to enter into the female spermathecal duct. The possible reasons why this species might mate face-to-face are briefly discussed.

Quantitative genetic insights into the coevolutionary dynamics of male and female genitalia

The spectacular variability that typically characterizes male genital traits has largely been attributed to the role of sexual selection. Among the evolutionary mechanisms proposed to account for this diversity, two processes in particular have generated considerable interest. On the one hand, females may exploit postcopulatory mechanisms of selection to favour males with preferred genital traits (cryptic female choice; CFC), while on the other hand females may evolve structures or behaviours that mitigate the direct costs imposed by male genitalia (sexual conflict; SC). A critical but rarely explored assumption underlying both processes is that male and female reproductive traits coevolve, either via the classic Fisherian model of preference-trait coevolution (CFC) or through sexually antagonistic selection (SC). Here, we provide evidence for this prediction in the guppy (Poecilia reticulata), a polyandrous livebearing fish in which males transfer sperm internally to females via consensual and forced matings. Our results from a paternal half-sibling breeding design reveal substantial levels of additive genetic variation underlying male genital size and morphology-two traits known to predict mating success during non-consensual matings. Our subsequent finding that physically interacting female genital traits exhibit corresponding levels of genetic (co)variation reveals the potential intersexual coevolutionary dynamics of male and female genitalia, thereby fulfilling a fundamental assumption underlying CFC and SC theory.

Polyandry as a mediator of sexual selection before and after mating

The Darwin–Bateman paradigm recognizes competition among males for access to multiple mates as the main driver of sexual selection. Increasingly, however, females are also being found to benefit from multiple mating so that polyandry can generate competition among females for access to multiple males, and impose sexual selection on female traits that influence their mating success. Polyandry can reduce a male's ability to monopolize females, and thus weaken male focused sexual selection. Perhaps the most important effect of polyandry on males arises because of sperm competition and cryptic female choice. Polyandry favours increased male ejaculate expenditure that can affect sexual selection on males by reducing their potential reproductive rate. Moreover, sexual selection after mating can ameliorate or exaggerate sexual selection before mating. Currently, estimates of sexual selection intensity rely heavily on measures of male mating success, but polyandry now raises serious questions over the validity of such approaches. Future work must take into account both pre- and post-copulatory episodes of selection. A change in focus from the products of sexual selection expected in males, to less obvious traits in females, such as sensory perception, is likely to reveal a greater role of sexual selection in female evolution.

The role of sexual selection and conflict in mediating among-population variation in mating strategies and sexually dimorphic traits in Sepsis punctum

The black scavenger fly Sepsis punctum exhibits striking among-population variation in the direction and magnitude of sexual size dimorphism, modification to the male forelimb and pre-copulatory behaviour. In some populations, male-biased sexual size dimorphism is observed; in other, less dimorphic, populations males court prior to mating. Such variation in reproductive traits is of interest to evolutionary biologists because it has the potential to limit gene flow among populations, contributing to speciation. Here, we investigate whether large male body size and modified forefemur are associated with higher male mating success within populations, whether these traits are associated with higher mating success among populations, and if these traits carry viability costs that could constrain their response to sexual selection. Flies from five distinct populations were reared at high or low food, generating high and low quality males. The expression of body size, forelimb morphology and courtship rate were each greater at high food, but high food males experienced higher mating success or reduced latency to first copulation in only one of the populations. Among populations, overall mating success increased with the degree of male-bias in overall body size and forelimb modification, suggesting that these traits have evolved as a means of increasing male mating rate. The increased mating success observed in large-male populations raises the question of why variation in magnitude of dimorphism persists among populations. One reason may be that costs of producing a large size constrain the evolution of ever-larger males. We found no evidence that juvenile mortality under food stress was greater for large-male populations, but development time was considerably longer and may represent an important constraint in an ephemeral and competitive growth environment.

Sexual conflict inGerris gillettei(Insecta: Hemiptera): influence of effective mating rate and morphology on reproductive success

In water striders, the interests of both sexes diverge over the decision to mate, leading to precopulatory sexual conflict. The influence of mating rate and key persistence and resistance traits on reproductive success has seldom been investigated in the context of multiple matings. We used amplified fragment length polymorphism (AFLP) based genetic parentage analyses to estimate mating and reproductive success in Gerris gillettei Lethierry and Severin, 1896, while allowing for free multiple matings. We tested the hypotheses that males should display stronger opportunity for sexual selection and steeper Bateman gradients. In each sex, persistence and resistance traits should also impact mating and reproductive success. Surprisingly, males and females had similarly high and variable effective mating rates (i.e., number of genetic partners), and both sexes produce more offspring when mating with more partners. As predicted, exaggerated persistence traits allowed males to mate with more partners and sire more offspring. However, we found no evidence for an impact of resistance traits for females. The mating environment may have favoured low resistance in females, but high promiscuity can be beneficial for females. This first description of the genetic mating system for a water strider species suggests that the determinants of fitness can be further deciphered using the sexual selection framework.