Sexual conflict and sexual selection: lost in the chase

Fertility signalling games: should males obey the signal?

Game theory is frequently used to study conflicting interests between the two sexes. Males often benefit from a higher mating rate than females do. A temporal component of this conflict has rarely been modelled: females' interest in mating may depend on when females become fertile. This sets conditions for male-female coevolution, where females may develop fertility signals, and males may obey the signal, such that they only target signalling females. Modelling this temporal aspect to sexual conflict yields two equilibria: (i) a trivial equilibrium without signals and with males targeting all females, and (ii) a signalling equilibrium where all females signal before ovulation, and where either some, or all, males obey the signal. The 'all males obey the signal' equilibrium is more likely if we assume that discriminating males have an advantage in postcopulatory sperm competition, while in the absence of this benefit, we find the 'some males obey the signal' equilibrium. The history of game-theoretic models of sex differences often portrays one sex as the 'winner' and the opposite sex as the 'loser'. From early models emphasizing 'battle of the sexes'-style terminology, we recommend moving on to describe the situation as non-signalling equilibria having stronger unresolved sexual conflict than signalling equilibria. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

Cognition and Its Shaping Effect on Sexual Conflict: Integrating Biology and Psychology

While genetic variation is of crucial importance for organisms to be able to adapt to their ever-changing environments over generations, cognitive processes can serve the same purpose by acting at shorter time scales. Cognition, and its resulting behaviour, allows animals to display flexible, fast and reversible responses that, without implying a genetic change, are crucial for adaptation and survival. In the research field on sexual conflict, where studies focus on male and female mating strategies that increase the individual's reproductive fitness while forcing a cost on the partner, the role that cognition may play in how such strategies can be optimised has been widely overlooked. However, a careful analysis of behavioural studies shows that animals can develop and change their responses depending on what they perceive as well as on what they can predict from their experience, which can be of prime importance for optimising their reproductive fitness. As will be reviewed here, largely psychological processes, such as perception, memory, learning and decision-making, can not only modulate sexual conflict, but can also have a big impact on the reproductive success of a given individual. This review highlights the need for a more integrative view of sexual conflict where cognitive processes are also considered as a fundamental part of an animal's adaptive mating response.

Research gaps and new insights in the evolution of Drosophila seminal fluid proteins

While the striking effects of seminal fluid proteins (SFPs) on females are fairly conserved among Diptera, most SFPs lack detectable homologues among the SFP repertoires of phylogenetically distant species. How such a rapidly changing proteome conserves functions across taxa is a fascinating question. However, this and other pivotal aspects of SFPs' evolution remain elusive because discoveries on these proteins have been mainly restricted to the model Drosophila melanogaster. Here, we provide an overview of the current knowledge on the inter-specific divergence of the SFP repertoire in Drosophila and compile the increasing amount of relevant genomic information from multiple species. Capitalizing on the accumulated knowledge in D. melanogaster, we present novel sets of high-confidence SFP candidates and transcription factors presumptively involved in regulating the expression of SFPs. We also address open questions by performing comparative genomic analyses that failed to support the existence of many conserved SFPs shared by most dipterans and indicated that gene co-option is the most frequent mechanism accounting for the origin of Drosophila SFP-coding genes. We hope our update establishes a starting point to integrate further data and thus widen the understanding of the intricate evolution of these proteins.

The evolution of sex peptide: sexual conflict, cooperation, and coevolution

A central paradigm in evolutionary biology is that the fundamental divergence in the fitness interests of the sexes (‘sexual conflict’) can lead to both the evolution of sex‐specific traits that reduce fitness for individuals of the opposite sex, and sexually antagonistic coevolution between the sexes. However, clear examples of traits that evolved in this way – where a single trait in one sex demonstrably depresses the fitness of members of the opposite sex, resulting in antagonistic coevolution – are rare. The Drosophila seminal protein ‘sex peptide’ (SP) is perhaps the most widely cited example of a trait that appears to harm females while benefitting males. Transferred in the ejaculate by males during mating, SP triggers profound and wide‐ranging changes in female behaviour and physiology. Early studies reported that the transfer of SP enhances male fitness while depressing female fitness, providing the foundations for the widespread view that SP has evolved to manipulate females for male benefit. Here, we argue that this view is (i) a simplification of a wider body of contradictory empirical research, (ii) narrow with respect to theory describing the origin and maintenance of sexually selected traits, and (iii) hard to reconcile with what we know of the evolutionary history of SP's effects on females. We begin by charting the history of thought regarding SP, both at proximate (its production, function, and mechanism of action) and ultimate (its fitness consequences and evolutionary history) levels, reviewing how studies of SP were central to the development of the field of sexual conflict. We describe a prevailing paradigm for SP's evolution: that SP originated and continues to evolve to manipulate females for male benefit. In contrast to this view, we argue on three grounds that the weight of evidence does not support the view that receipt of SP decreases female fitness: (i) results from studies of SP's impact on female fitness are mixed and more often neutral or positive, with fitness costs emerging only under nutritional extremes; (ii) whether costs from SP are appreciable in wild‐living populations remains untested; and (iii) recently described confounds in genetic manipulations of SP raise the possibility that measures of the costs and benefits of SP have been distorted. Beyond SP's fitness effects, comparative and genetic data are also difficult to square with the idea that females suffer fitness costs from SP. Instead, these data – from functional and evolutionary genetics and the neural circuitry of female responses to SP – suggest an evolutionary history involving the evolution of a dedicated SP‐sensing apparatus in the female reproductive tract that is likely to have evolved because it benefits females, rather than harms them. We end by exploring theory and evidence that SP benefits females by functioning as a signal of male quality or of sperm receipt and storage (or both). The expanded view of the evolution of SP that we outline recognises the context‐dependent and fluctuating roles played by both cooperative and antagonistic selection in the origin and maintenance of reproductive traits.

The Role of Phenotypic Plasticity in Moderating Evolutionary Conflict

Evolutionary conflicts arise when the fitness interests of interacting individuals differ. Well-known examples include sexual conflict between males and females and antagonistic coevolution between hosts and parasites. A common feature of such conflicts is that compensating evolutionary change in each of the parties can lead to little overt change in the interaction itself. As a result, evolutionary conflict is expected to persist even if the evolutionary dynamic between the parties reaches an equilibrium. In these cases, it is of interest to know whether certain kinds of interactions are expected to lead to greater or lesser evolutionary conflict at such evolutionary stalemates. Here we present a theoretical analysis showing that when one of the interacting parties can respond to the other through adaptive phenotypic plasticity, evolutionary conflict is reduced. Paradoxically, however, it is the party that does not express adaptive plasticity that experiences less conflict. Conflict for the party displaying adaptive plasticity can increase or decrease, depending on the situation.

Sperm removal during copulation confirmed in the oldest extant damselfly, Hemiphlebia mirabilis

Postcopulatory sexual selection may favour mechanisms to reduce sperm competition, like physical sperm removal by males. To investigate the origin of sperm removal, I studied the reproductive behaviour and mechanisms of sperm competition in the only living member of the oldest damselfly family, Hemiphlebia mirabilis, one species that was considered extinct in the 1980s. This species displays scramble competition behaviour. Males search for females with short flights and both sexes exhibit a conspicuous “abdominal flicking”. This behaviour is used by males during an elaborate precopulatory courtship, unique among Odonata. Females use a similar display to reject male attempts to form tandem, but eventually signal receptivity by a particular body position. Males immobilise females during courtship using their legs, which, contrarily to other damselflies, never autotomise. Copulation is short (range 4.1–18.7 min), and occurs in two sequential stages. In the first stage, males remove part of the stored sperm, and inseminate during the second stage, at the end of mating. The male genital ligula matches the size and form of female genitalia, and ends by two horns covered by back-oriented spines. The volume of sperm in females before copulation was 2.7 times larger than the volume stored in females whose copulation was interrupted at the end of stage I, indicative of a significant sperm removal. These results point out that sperm removal is an old character in the evolution of odonates, possibly dating back to the Permian.

Biomechanical Diversity of Mating Structures among Harvestmen Species Is Consistent with a Spectrum of Precopulatory Strategies

Diversity in reproductive structures is frequently explained by selection acting at individual to generational timescales, but interspecific differences predicted by such models (e.g., female choice or sexual conflict) are often untestable in a phylogenetic framework. An alternative approach focuses on clade- or function-specific hypotheses that predict evolutionary patterns in terms neutral to specific modes of sexual selection. Here we test a hypothesis that diversity of reproductive structures in leiobunine harvestmen (daddy longlegs) of eastern North America reflects two sexually coevolved but non-overlapping precopulatory strategies, a primitive solicitous strategy (females enticed by penis-associated nuptial gifts), and a multiply derived antagonistic strategy (penis exerts mechanical force against armature of the female pregenital opening). Predictions of sexual coevolution and fidelity to precopulatory categories were tested using 10 continuously varying functional traits from 28 species. Multivariate analyses corroborated sexual coevolution but failed to partition species by precopulatory strategy, with multiple methods placing species along a spectrum of mechanical antagonistic potential. These findings suggest that precopulatory features within species reflect different co-occurring levels of solicitation and antagonism, and that gradualistic evolutionary pathways exist between extreme strategies. The ability to quantify antagonistic potential of precopulatory structures invites comparison with ecological variables that may promote evolutionary shifts in precopulatory strategies.

Evolutionary Consequences of Male Driven Sexual Selection and Sex-Biased Fitness Modifications in Drosophila melanogaster and Members of the simulans Clade

Males have evolved a variety of behavioral, morphological, and physiological traits to manipulate their mates in order to maximize their chances of success. These traits are bound to influence how females respond to male behaviors and influence the nature of sexual selection/conflict. A common consequence of aggressive male mating strategies in Drosophila melanogaster is the reduction of female lifespan. Our study shows that this is common across members of the simulans clade. Reduced life expectancy of females implies that female contribution to a population is less than that of males per generation. Fitness differences between the sexes in every generation will invariably affect overall population fitness. How natural selection responds to the female deaths and thereby the unequal fitness of the sexes has rarely been addressed. We shed light on this issue and provide evidence, which suggests that additional gains of fitness by males due to their longevity and continued mating may provide one explanation as to why the loss of female fitness may be "invisible" (effectively neutral) to natural selection. Male driven sexual selection and additional, transgenerational gains of male fitness can be an important force of evolutionary change and need to be tested with other organisms.

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.

Demography can favour female-advantageous alleles

When female fecundity is relatively independent of male abundance, while male reproduction is proportional to female abundance, females have a larger effect on population dynamics than males (i.e. female demographic dominance). This population dynamic phenomenon might not appear to influence evolution, because male and female genomes still contribute equally much to the next generation. However, here we examine two evolutionary scenarios to provide a proof of principle that spatial structure can make female demographic dominance matter. Our two simulation models combine dispersal evolution with local adaptation subjected to intralocus sexual conflict and environmentally driven sex ratio biases, respectively. Both models have equilibria where one environment (without being intrinsically poorer) has so few reproductive females that trait evolution becomes disproportionately determined by those environments where females survive better (intralocus sexual conflict model), or where daughters are overproduced (environmental sex determination model). Surprisingly, however, the two facts that selection favours alleles that benefit females, and population growth is improved when female fitness is high, together do not imply that all measures of population performance are improved. The sex-specificity of the source-sink dynamics predicts that populations can evolve to fail to persist in habitats where alleles do poorly when expressed in females.

The relationship between sexual selection and sexual conflict

Evolutionary conflicts of interest arise whenever genetically different individuals interact and their routes to fitness maximization differ. Sexual selection favors traits that increase an individual's competitiveness to acquire mates and fertilizations. Sexual conflict occurs if an individual of sex A's relative fitness would increase if it had a "tool" that could alter what an individual of sex B does (including the parental genes transferred), at a cost to B's fitness. This definition clarifies several issues: Conflict is very common and, although it extends outside traits under sexual selection, sexual selection is a ready source of sexual conflict. Sexual conflict and sexual selection should not be presented as alternative explanations for trait evolution. Conflict is closely linked to the concept of a lag load, which is context-dependent and sex-specific. This makes it possible to ask if one sex can "win." We expect higher population fitness if females win.

Sex-biased gene expression and evolution of the x chromosome in nematodes

Studies of X chromosome evolution in various organisms have indicated that sex-biased genes are nonrandomly distributed between the X and autosomes. Here, to extend these studies to nematodes, we annotated and analyzed X chromosome gene content in four Caenorhabditis species and in Pristionchus pacificus. Our gene expression analyses comparing young adult male and female mRNA-seq data indicate that, in general, nematode X chromosomes are enriched for genes with high female-biased expression and depleted of genes with high male-biased expression. Genes with low sex-biased expression do not show the same trend of X chromosome enrichment and depletion. Combined with the observation that highly sex-biased genes are primarily expressed in the gonad, differential distribution of sex-biased genes reflects differences in evolutionary pressures linked to tissue-specific regulation of X chromosome transcription. Our data also indicate that X dosage imbalance between males (XO) and females (XX) is influential in shaping both expression and gene content of the X chromosome. Predicted upregulation of the single male X to match autosomal transcription (Ohno's hypothesis) is supported by our observation that overall transcript levels from the X and autosomes are similar for highly expressed genes. However, comparison of differentially located one-to-one orthologs between C. elegans and P. pacificus indicates lower expression of X-linked orthologs, arguing against X upregulation. These contradicting observations may be reconciled if X upregulation is not a global mechanism but instead acts locally on a subset of tissues and X-linked genes that are dosage sensitive.

Sex and speciation: Drosophila reproductive tract proteins- twenty five years later

The protein electrophoresis revolution, nearly fifty years ago, provided the first glimpse into the nature of molecular genetic variation within and between species and showed that the amount of genetic differences between newly arisen species was minimal. Twenty years later, 2D electrophoresis showed that, in contrast to general gene-enzyme variation, reproductive tract proteins were less polymorphic within species but highly diverged between species. The 2D results were interesting and revolutionary, but somewhat uninterpretable because, at the time, rapid evolution and selective sweeps were not yet part of the common vocabulary of evolutionary biologists. Since then, genomic studies of sex and reproduction-related (SRR) genes have grown rapidly into a large area of research in evolutionary biology and are shedding light on a number of phenomena. Here we review some of the major and current fields of research that have greatly contributed to our understanding of the evolutionary dynamics and importance of SRR genes and genetic systems in understanding reproductive biology and speciation.

Sexual conflict in primates

Sexual conflict is increasingly recognized as a major force for evolutionary change and holds great potential for delineating variation in primate behavior and morphology. The goals of this review are to highlight the rapidly rising field of sexual conflict and the ongoing shift in our understanding of interactions between the sexes. We discuss the evidence for sexual conflict within the Order Primates, and assess how studies of primates have illuminated and can continue to increase our understanding of sexual conflict and sexual selection. Finally, we introduce a framework for understanding the behavioral, anatomical, and genetic expression of sexual conflict across primate mating systems and suggest directions for future research.

Evolution of genitalia: theories, evidence, and new directions

Many hypotheses have been proposed to explain why male intromittent genitalia consistently tend to diverge more rapidly than other body traits of the same individuals in a wide range of animal taxa. Currently the two most popular involve sexual selection: sexually antagonistic coevolution (SAC) and cryptic female choice (CFC). A review of the most extensive attempts to discriminate between these two hypotheses indicates that SAC is not likely to have played a major role in explaining this pattern of genital evolution. Promising lines for future, more direct tests of CFC include experimental modification of male genital form and female sensory abilities, analysis of possible male-female dialogues during copulation, and direct observations of genital behavior.

Intersexual arms race? Genital coevolution in nephilid spiders (Araneae, Nephilidae)

Genital morphology is informative phylogenetically and strongly selected sexually. We use a recent species-level phylogeny of nephilid spiders to synthesize phylogenetic patterns in nephilid genital evolution that document generalized conflict between male and female interests. Specifically, we test the intersexual coevolution hypothesis by defining gender-specific indices of genital complexity that summarize all relevant and phylogenetically informative traits. We then use independent contrasts to show that male and female genital complexity indices correlate significantly and positively across the phylogeny rather than among sympatric sister species, as predicted by reproductive character displacement. In effect, as females respond to selection for fecundity-driven fitness via giantism and polyandry (perhaps responding to male-biased effective sex ratios), male mechanisms evolve to monopolize females (male monogamy) via opportunistic mating, pre- and postcopulatory mate guarding, and/or plugging of female genitalia to exclude subsequent suitors. In males morphological symptoms of these phenomena range from self-mutilated genitalia to total castration. Although the results are compatible with both recently favored sexual selection hypotheses, sexually antagonistic coevolution, and cryptic female choice, the evidence of strong intersexual conflict and genitalic damage in both sexes is more easily explained as sexually antagonistic coevolution due to an evolutionary arms race.

Postcopulatory sexual selection: Darwin's omission and its consequences

In one of his few major oversights, Darwin failed to appreciate that male-male competition and sexual selection can continue even after copulation has begun. The postcopulatory equivalents of both direct male-male battles (sperm competition) and female choice (cryptic female choice) occur within the female's body. Recognition of this hidden, but intense, sexual competition provides new insights into a variety of fields. These include the hyperdiverse and paradoxically elaborate morphology of both sperm and male genitalia, the equally puzzling and elaborate morphology of nongenitalic male structures that are specialized to grasp and stimulate females, powerful manipulative effects of substances in male semen on female reproductive physiology, paradoxical male courtship behavior that occurs after copulation has already begun, variability in parental investments, and the puzzlingly complex and diverse interactions between sperm and female products that surround animal eggs and between male gametophytes and female tissues in flowering plants. Many bizarre traits are involved, including male genitalia that are designed to explode or fall apart during copulation leaving behind parts within the female, male genitalia that "sing" during copulation, potent seminal products that invade the female's body cavity and her nervous system to influence her behavior, and a virtual Kama Sutra of courtship behavior performed after rather than before genital coupling, including male-female dialogues during copulation.

Sexual conflict and sexually antagonistic coevolution in an annual plant

Background

Sexual conflict theory predicts sexually antagonistic coevolution of reproductive traits driven by conflicting evolutionary interests of two reproducing individuals. Most studies of the evolutionary consequences of sexual conflicts have, however, to date collectively investigated only a few species. In this study we used the annual herb Collinsia heterophylla to experimentally test the existence and evolutionary consequences of a potential sexual conflict over onset of stigma receptivity.

Methodology/Principal Findings

We conducted crosses within and between four greenhouse-grown populations originating from two regions. Our experimental setup allowed us to investigate male-female interactions at three levels of geographic distances between interacting individuals. Both recipient and pollen donor identity affected onset of stigma receptivity within populations, confirming previous results that some pollen donors can induce stigma receptivity. We also found that donors were generally better at inducing stigma receptivity following pollen deposition on stigmas of recipients from another population than their own, especially within a region. On the other hand, we found that donors did worse at inducing stigma receptivity in crosses between regions. Interestingly, recipient costs in terms of lowered seed number after early fertilisation followed the same pattern: the cost was apparent only if the pollen donor belonged to the same region as the recipient.

Conclusion/Significance

Our results indicate that recipients are released from the cost of interacting with local pollen donors when crossed with donors from a more distant location, a pattern consistent with a history of sexually antagonistic coevolution within populations. Accordingly, sexual conflicts may have important evolutionary consequences also in plants.

Monogamy and the battle of the sexes

Sexual conflict has been suggested to be important in the evolution of reproductive traits, with much recent theoretical and empirical evidence emphasizing its role in generating sexually antagonistic coevolution in the context of promiscuous mating. Here we shift attention to the role of sexual conflict in a monogamous mating context. Conflicts can arise, for example, when males are successful in imposing monandry at a cost to female fitness, or when females impose monogyny on males. Conflict over remating can also generate monogamy. For example, when males invest heavily in attempting to impose female monandry, the cost of their investment may prevent them from securing additional mates. We emphasize that sexual conflicts need not always generate sexually antagonistic coevolution, and that it is important to consider whether mating decisions are controlled primarily by males or females. Finally, we briefly discuss approaches to distinguish between conflict and classical modes of sexual selection, as this highlights difficulties associated with deciding whether monogamy is enforced by one sex or the other. We suggest that documenting the current fitness consequences of mate choice and mating patterns provides insight into the relative importance of classic and conflict modes of selection.

The dynamics of sexual conflict over mating rate with endosymbiont infection that affects reproductive phenotypes

Maternally inherited endosymbionts have been implicated as significant drivers of sexual conflict within their hosts, typically through sex-ratio manipulation. Empirical studies show that some of these endosymbionts have the potential to influence sexual conflict not by sex-ratio distortion, but by altering reproductive traits within their hosts. Research has already shown that reproductive traits involved in mating/fertilization process are integral 'players' in sexual conflict, thus suggesting the novel hypothesis that endosymbiont-induced changes in reproductive phenotypes can impact the dynamics of sexual conflict. Here, we use a standard quantitative genetic approach to model the effects of endosymbiont-induced changes in a female reproductive trait on the dynamics of sexual conflict over mating/fertilization rate. Our model shows that an endosymbiont-induced alteration of a host female reproductive trait that affects mating rate can maintain the endosymbiont infection within the host population, and does so in the absence of sex-ratio distortion and cytoplasmic incompatibility.

What determines sex roles in mate searching?

In a seminal paper, Hammerstein and Parker (1987) described how sex roles in mate searching can be frequency dependent: the need for one sex to perform mate searching is diminished when the opposite sex takes on the greater searching effort. Intriguingly, this predicts that females are just as likely to search as males, despite a higher potential reproductive rate by the latter sex. This prediction, however, is not supported by data: male mate searching prevails in nature. Counterexamples also exist in the empirical literature. Depending on the taxon studied, female mate searching can arise in either low- or high-density conditions, and suggested explanations differ accordingly. We examine these puzzling observations by building two models (with and without sperm competition). When sperm competition is explicitly included, male mate searching becomes the dominant pattern; when it is excluded, male mate searching predominates only if we assume that costs of searching are higher for females. Consequently, two hypotheses emerge from our models. The multiple-mating hypothesis explains male searching on the basis of the ubiquity of sperm competition, and predicts that female searching can arise in low-density situations in which sperm can become limiting. It can also explain cases of female pheromone production, where males pay the majority of search costs. The sex-specific cost hypothesis predicts the opposite pattern of female searching in high-density conditions, and it potentially applies to some species in which sperm limitation is unlikely.

Coevolution between harmful male genitalia and female resistance in seed beetles

Reproductive conflict between the sexes is thought to be a key force in the evolution of many reproductive characters, but persuasive evidence for its significance is still scarce. The spectacular evolution of male genitalia that impose physical injury on females during mating has often been suggested to be a product of sexually antagonistic coevolution, but our understanding of these extraordinary adaptations is very limited, and there are no direct data addressing their evolutionary elaboration. We show that more spiny male genitalia causes more harm to females during copulation and provide comparative evidence for the correlated evolution between these antagonistic adaptations in males and a female counteradaptation (the amount of connective tissue in the copulatory duct) in a group of insects. By combining comparative and experimental methods, we demonstrate that imbalance of relative armament of the sexes affects evolution of the economics of reproduction: as males evolve genitalia that are more harmful relative to the level of female counteradaptation, costs associated with mating for females increase and population fitness is depressed. Our results unveil a coevolutionary arms race between the sexes and are consistent with a proposed link between sexual conflict, species' viability, and the risk of extinction.

Quantitative genetic models of sexual conflict based on interacting phenotypes

Evolutionary conflict arises between reproductive partners when alternative reproductive opportunities are available. Sexual conflict can generate sexually antagonistic selection, which mediates sexual selection and intersexual coevolution. However, despite intense interest, the evolutionary implications of sexual conflict remain unresolved. We propose a novel theoretical approach to study the evolution of sexually antagonistic phenotypes based on quantitative genetics and the measure of social selection arising from male-female interactions. We consider the phenotype of one sex as both a genetically influenced evolving trait as well as the (evolving) social environment in which the phenotype of the opposite sex evolves. Several important points emerge from our analysis, including the relationship between direct selection on one sex and indirect effects through selection on the opposite sex. We suggest that the proposed approach may be a valuable tool to complement other theoretical approaches currently used to study sexual conflict. Most importantly, our approach highlights areas where additional empirical data can help clarify the role of sexual conflict in the evolutionary process.

Evolutionary conflicts of interest between males and females

Sexual conflict arises from differences in the evolutionary interests of males and females and can occur over traits related to courtship, mating and fertilisation through to parental investment. Theory shows that sexual conflict can lead to sexually antagonistic coevolution (SAC), where adaptation in one sex can lead to counter-adaptation in the other. Thus, sexual conflict can lead to evolutionary change within species. In addition, SAC can--through its effects on traits related to the probability of mating and of zygote formation--potentially lead to reproductive isolation. In this review, I discuss that, although sexual conflict is ubiquitous, the actual expression of sexual conflict leading to SAC is less frequent. The balance between the benefits and costs of the manipulation of one sex by the other, and the availability of mechanisms by which conflict is expressed, determine whether actual sexual conflict is likely to occur. New insights address the relationship between sexual conflict and conflict resolution, adaptation, sexual selection and fitness. I suggest that it will be useful to examine systematically the parallels and contrasts between sexual and other evolutionary conflicts. Understanding why some traits, but not others, are subject to evolutionary change by SAC will require data on the mechanisms of the traits involved and on the relative benefits and costs of manipulation and resistance to manipulation.

A time-sequence functional analysis of mating behaviour and genital coupling in Drosophila: role of cryptic female choice and male sex-drive in the evolution of male genitalia

Male genitalia in Drosophila exemplify strikingly rapid and divergent evolution, whereas female genitalia are relatively invariable. Whereas precopulatory and post-copulatory sexual selection has been invoked to explain this trend, the functional significance of genital structures during copulation remains obscure. We used time-sequence analysis to study the functional significance of external genitalic structures during the course of copulation, between D. melanogaster and D. simulans. This functional analysis has provided new information that reveals the importance of male-driven copulatory mechanics and strategies in the rapid diversification of genitalia. The posterior process, which is a recently evolved sexual character and present only in males of the melanogaster clade, plays a crucial role in mounting as well as in genital coupling. Whereas there is ample evidence for precopulatory and/or post-copulatory female choice, we show here that during copulation there is little or no physical female choice, consequently, males determine copulation duration. We also found subtle differences in copulatory mechanics between very closely related species. We propose that variation in male usage of novel genitalic structures and shifts in copulatory behaviour have played an important role in the diversification of genitalia in species of the Drosophila subgroup.

Sexually antagonistic coevolution in insects is associated with only limited morphological diversity

Morphological traits involved in male-female sexual interactions, such as male genitalia, often show rapid divergent evolution. This widespread evolutionary pattern could result from sustained sexually antagonistic coevolution, or from other types of selection such as female choice or selection for species isolation. I reviewed the extensive but under-utilized taxonomic literature on a selected subset of insects, in which male-female conflict has apparently resulted in antagonistic coevolution in males and females. I checked the sexual morphology of groups comprising 500-1000 species in six orders for three evolutionary trends predicted by the sexually antagonistic coevolution hypothesis: males with species-specific differences and elaborate morphology in structures that grasp or perforate females in sexual contexts; corresponding female structures with apparently coevolved species-specific morphology; and potentially defensive designs of female morphology. The expectation was that the predictions were especially likely to be fulfilled in these groups. A largely qualitative overview revealed several surprising patterns: sexually antagonistic coevolution is associated with frequent, relatively weak species-specific differences in males, but male designs are usually relatively simple and conservative (in contrast to the diverse and elaborate designs common in male structures specialized to contact and hold females in other species, and also in weapons such as horns and pincers used in intra-specific battles); coevolutionary divergence of females is not common; and defensive female divergence is very uncommon. No cases were found of female defensive devices that can be facultatively deployed. Coevolutionary morphological races may have occurred between males and females of some bugs with traumatic insemination, but apparently as a result of female attempts to control fertilization, rather than to reduce the physical damage and infections resulting from insertion of the male's hypodermic genitalia. In sum, the sexually antagonistic coevolution that probably occurs in these groups has generally not resulted in rapid, sustained evolutionary divergence in male and female external sexual morphology. Several limitations of this study, and directions for further analyses are discussed.

Assessing sexual conflict in the Drosophila melanogaster laboratory model system

We describe a graphical model of interlocus coevolution used to distinguish between the interlocus sexual conflict that leads to sexually antagonistic coevolution, and the intrinsic conflict over mating rate that is an integral part of traditional models of sexual selection. We next distinguish the 'laboratory island' approach from the study of both inbred lines and laboratory populations that are newly derived from nature, discuss why we consider it to be one of the most fitting forms of laboratory analysis to study interlocus sexual conflict, and then describe four experiments using this approach with Drosophila melanogaster. The first experiment evaluates the efficacy of the laboratory model system to study interlocus sexual conflict by comparing remating rates of females when they are, or are not, provided with a spatial refuge from persistent male courtship. The second experiment tests for a lag-load in males that is due to adaptations that have accumulated in females, which diminish male-induced harm while simultaneously interfering with a male's ability to compete in the context of sexual selection. The third and fourth experiments test for a lag-load in females owing to direct costs from their interactions with males, and for the capacity for indirect benefits to compensate for these direct costs.

Lonely hearts or sex in the city? Density-dependent effects in mating systems

Two very basic ideas in sexual selection are heavily influenced by numbers of potential mates: the evolution of anisogamy, leading to sex role differentiation, and the frequency dependence of reproductive success that tends to equalize primary sex ratios. However, being explicit about the numbers of potential mates is not typical to most evolutionary theory of sexual selection. Here, we argue that this may prevent us from finding the appropriate ecological equilibria that determine the evolutionary endpoints of selection. We review both theoretical and empirical advances on how population density may influence aspects of mating systems such as intrasexual competition, female choice or resistance, and parental care. Density can have strong effects on selective pressures, whether or not there is phenotypic plasticity in individual strategies with respect to density. Mating skew may either increase or decrease with density, which may be aided or counteracted by changes in female behaviour. Switchpoints between alternative mating strategies can be density dependent, and mate encounter rates may influence mate choice (including mutual mate choice), multiple mating, female resistance to male mating attempts, mate searching, mate guarding, parental care, and the probability of divorce. Considering density-dependent selection may be essential for understanding how populations can persist at all despite sexual conflict, but simple models seem to fail to predict the diversity of observed responses in nature. This highlights the importance of considering the interaction between mating systems and population dynamics, and we strongly encourage further work in this area.

Natural variation in male-induced 'cost-of-mating' and allele-specific association with male reproductive genes in Drosophila melanogaster

One of the most sharply defined sexual conflicts arises when the act of mating is accompanied by an inflated risk of death. Several reports have documented an increased death rate of female Drosophila as a result of recurrent mating. Transgenic and mutation experiments have further identified components of seminal fluid that are at least in part responsible for this toxicity. Variation among males in their tendency for matings to be toxic to their partners has also been documented, but here for the first time we identify polymorphism within particular genes conferring differential post-mating female mortality. Such polymorphism is important, as it raises the challenge of whether sexual conflict models can provide means for maintenance of polymorphism. Using a set of second chromosome extraction lines, we scored differences in post-mating female fecundity and longevity subsequent to mating, and identified significant among-line differences. Seventy polymorphisms in ten male reproductive genes were scored and permutation tests were used to identify significant associations between genotype and phenotype. One polymorphism upstream of PEBII and an amino acid substitution in CG17331 were both associated with male-induced female mortality. The same allele of CG17331 that is toxic to females also induces greater refractoriness to remating in the females, providing an example of an allele-specific sexual conflict. Postcopulatory sexual selection could lead to sexual conflict by favouring males that prevent their mates from mating, even when there is a viability cost to those females.

Ecological divergence promotes the evolution of cryptic reproductive isolation

Speciation can involve the evolution of 'cryptic' reproductive isolation that occurs after copulation but before hybrid offspring are produced. Because such cryptic barriers to gene exchange involve post-mating sexual interactions, analyses of their evolution have focused on sexual conflict or traditional sexual selection. Here, we show that ecological divergence between populations of herbivorous walking sticks is integral to the evolution of cryptic reproductive isolation. Low female fitness following between-population mating can reduce gene exchange between populations, thus acting as a form of cryptic isolation. Female walking sticks show reduced oviposition rate and lower lifetime fecundity following between-population versus within-population mating, but only for mating between populations using different host-plant species. Our results indicate that even inherently sexual forms of reproductive isolation can evolve as a by-product of ecological divergence and that post-mating sexual interactions do not necessarily evolve independently of the ecological environment.

Sexual conflict over mating and fertilization: an overview

Sexual conflict is a conflict between the evolutionary interests of individuals of the two sexes. The sexes can have different trait optima but this need not imply conflict if their optima can be attained simultaneously. Conflict requires an interaction between males and females (e.g. mating or parental care), such that the optimal outcomes for each sex cannot be achieved simultaneously. It is important to distinguish between battleground models, which define the parameter space for conflict and resolution models, which seek solutions for how conflicts are resolved. Overt behavioural conflict may or may not be manifest at resolution. Following Fisherian principles, an immediate (i.e. direct) benefit to a male that has a direct cost to his female partner can have an indirect benefit to the female via her male progeny. Female resistance to mating has been claimed to represent concurrence rather than conflict, due to female benefits via sons (males with low mating advantage are screened out by resistance). However, the weight of current evidence (both theoretical and empirical) supports sexual conflict for many cases. I review (i) conflicts over mate quality, encounters between males and females of genetically diverged subpopulations, mating rate and inbreeding, (ii) the special features of postcopulatory sexual conflict and (iii) some general features of importance for conflict resolution.

Detecting sexual conflict and sexually antagonistic coevolution

We begin by providing an operational definition of sexual conflict that applies to both inter- and intralocus conflict. Using this definition, we examine a series of simple coevolutionary models to elucidate fruitful approaches for detecting interlocus sexual conflict and resultant sexually antagonistic coevolution. We then use published empirical examples to illustrate the utility of these approaches. Three relevant attributes emerge. First, the dynamics of sexually antagonistic coevolution may obscure the conflict itself. Second, competing models of inter-sexual coevolution may yield similar population patterns near equilibria. Third, a variety of evolutionary forces underlying competing models may be acting simultaneously near equilibria. One main conclusion is that studies of emergent patterns in extant populations (e.g. studies of population and/or female fitness) are unlikely to allow us to distinguish among competing coevolutionary models. Instead, we need more research aimed at identifying the forces of selection acting on shared traits and sexually antagonistic traits. More specifically, we need a greater number of functional studies of female traits as well as studies of the consequences of both male and female traits for female fitness. A mix of selection and manipulative studies on these is likely the most promising route.

Cryptic sexual conflict in gift-giving insects: chasing the chase-away

The chase-away model of sexual selection posits that elaborate male sexual displays arise because they exploit preexisting biases in females' sensory systems and induce females to mate in a suboptimal manner. An essential element of this hypothesis is that such manipulation should quickly lead to female resistance to male displays. Nuptial food gifts may be a frequent conduit by which males attempt to influence the mating behavior of females against females' own reproductive interests. In decorated crickets Gryllodes sigillatus, such inducements come in the form of a spermatophylax, a gelatinous mass forming part of the male's spermatophore and consumed by the female after mating. We conducted experiments in which spermatophylaxes obtained from male G. sigillatus were offered as novel food gifts to females of a non-gift-giving species (Acheta domesticus) having no evolutionary history of spermatophylax consumption. Female A. domesticus that were allowed to consume the spermatophylax took significantly longer to remate than when given no such opportunity. In contrast, when female G. sigillatus were prevented from consuming their partners' nuptial gifts, there was no difference in their propensity to remate relative to females permitted to consume a food gift after mating. These results suggest that the spermatophylax synthesized by male G. sigillatus contains substances designed to inhibit the sexual receptivity of their mates but that female G. sigillatus have evolved reduced responsiveness to these substances.

Divergence in replicated phylogenies: the evolution of partial post-mating prezygotic isolation in bean weevils

By tradition, speciation research has been focused on processes leading to either premating or post-zygotic reproductive isolation. The processes which generate isolation after mating but before zygote formation are less well understood. Here, we study divergence in characters which contribute to post-mating prezygotic isolation, such as egg production and remating rate. We propose that 'replicated' laboratory phylogenies with known histories can be used to yield insights into the processes of divergence. We performed a series of cross-matings between populations within two strains of the bean weevil Callosobruchus maculatus. Each strain has a unique and independent origin and both have been kept in the same set of laboratories during the last few decades. Our results show that divergence has occurred between laboratory populations within strains with regards to the effects that mating has on female reproductive behaviour, showing that the evolution of partial post-mating prezygotic isolation can be rapid. More importantly, the pattern of divergence across populations was distinct in the two strains, suggesting that coevolutionary trajectories are not determined by environmental factors but are to some extent arbitrary. We discuss the limitations of the novel empirical strategy employed here, and conclude that our results lend support to the hypothesis that post-mating sexual selection is capable of rapidly generating post-mating prezygotic isolation.