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Abstract The role of sexual selection in mediating levels of sexual conflict has been demonstrated in many experimental evolution studies on Drosophila spp. where competition among males for mating was the target of selection. Sexual selection has also been shown to affect the evolution of life-histories. However, the influence of divergent life-histories on reproductive strategies and, therefore, sexual selection and possibly sexual conflict has been less well studied. We examined D. melanogaster populations selected for a short development time and early age at reproduction for changes in reproductive behavior and traits that are proxies of sexual selection. We report a large reduction in reproductive competition experienced by the males of these populations, compared to ancestral populations that are not consciously selected for rapid development or early reproduction, potentially leading to reduced sexual selection. We show that rapidly developing and early reproducing populations have very low levels of mating in their lifetime (females are more or less monandrous), low courtship levels, shorter copulation duration, and longer time from eclosion to first mating, compared to the controls. These results are discussed in the context of the previously demonstrated reduction of inter-locus sexual conflict in these populations. We show that life-history strategies might have a large and significant impact on sexual selection, with each influencing the other and contributing to the complexities of adaptation.Significance statementSexual conflict, often manifested as an arms-race between males and females trying to enhance their own reproductive success at some cost to the other, is of great evolutionary interest because it can maintain genetic variation in populations, prevent the independent optimization of male and female traits, and also promote speciation. Sexual selection, or variation in mating success, is well known to affect levels of sexual conflict. However, it is not so clear whether, and how, the regular evolution of life-histories also affects sexual selection. Here, we show that life-history evolution in fruit fly populations selected for traits not directly related to sexual conflict might, nevertheless, mediate the possible evolution of altered sexual conflict levels through effects on sexual selection. Populations that evolved to develop to adulthood fast, and reproduce relatively early in life, are shown to potentially experience less sexual selection, which can explain the low sexual conflict levels earlier observed in them.

Examples of genomic conflict were apparent soon after the inception of the field of modern genetics. Despite these early discoveries, the relevance of genomic conflict to the core principles of genetics has been largely unappreciated. In this synthetic review I will describe why knowledge of the logic and diverse forms of genomic conflict is essential to understanding all subfields of genetics. Because there are so many ways in which some parts of all prokaryotic and eukaryotic genomes can evolve to gain a reproductive advantage at the expense of other parts, the prevalence of genomic conflict is universal, and it influences all aspects of genetic form and function.

Males and females have conflicting interests on the frequency and outcomes of mating interactions. Males maximize their fitness by mating with as many females as possible, whereas choosy females often reduce receptivity following copulation. Alternative male mating tactics can be adaptive in their expression to a variety of mating contexts, including interactions with a relatively unreceptive mated female. Male Rabidosa punctulata wolf spiders can adopt distinctive mating tactics when interacting with a female, a complex courtship display, and/or a more coercive direct mount tactic that often involves grappling with females for copulation. In this study, we set up female mating treatments with initial trials and then paired mated and unmated females with males to observe both female remating frequencies and the male mating tactics used during the interactions. Males adopted different mating tactics depending on the mating status of the female they were paired with. Males were more likely to adopt a direct mount tactic with already-mated females and courtship with unmated females. Already-mated females were considerably less receptive to males during experimental trials, although they did remate 34% of the time, the majority of which were with males using a direct mount tactic. Whereas males adjusting to these contextual cues were able to gain more copulations, the observation of multiple mating in female R. punctulata introduces the potential for sperm competition. We discuss this sexual conflict in terms of the fitness consequences of these mating outcomes for both males and females.

Conspicuous male sexual traits (e.g. weapons for male-male competition and displays for courting females) may attract predators. Under conditions of high predation risk, females typically become less choosy with respect to mates to reduce the time spent on mate selection. However, post-copulatory sexual traits, such as sperm ejaculation for sperm competition and sperm removal for cryptic female choice (CFC), may increase with predation risk because they are more inconspicuous to predators. To examine this hypothesis, we observed the reproductive behaviour in the Japanese pygmy squid, Idiosepius paradoxus, in which the male attaches ejaculated spermatangia to the female's body and the female removes the spermatangia after copulation. Squid from two populations (Ohmura and Oki), with low and high predation levels, respectively, were copulated in tanks under controlled presence/absence of predator conditions. Among the Ohmura individuals, spermatangia removal was suppressed in the presence of a predator. Females may not be able to remove spermatangia effectively when facing a predator because they feel threatened by the predator; as a result, more spermatangia were retained during trials in which they were exposed to predators. In contrast, squid from the Oki (high predation) population, which is exposed to a higher predation risk, were not strongly affected by the predator presence. While the males ejaculated more spermatangia, the females removed more of them. The effect of sexual conflict may be greater than that of the predation risk in the pygmy squid. This suggests adaptive differences in post-copulatory sexual selection traits linked to predation.

Summary When females mate with multiple partners, the risk of sperm competition depends on female mating history. To maximize fitness, males should adjust their mating investment according to this risk. In polyandrous butterflies, males transfer a large, nutritious ejaculate at mating. Larger ejaculates delay female remating and confer an advantage in sperm competition. We test whether male ejaculate size in the butterfly Pieris napi (Lepidoptera) varies with female mating history and thus sperm competition, and whether males assess sperm competition using the male‐transferred anti‐aphrodisiac methyl salicylate (MeS) as a cue. Both sexes responded physiologically to MeS in a dose‐dependent manner. Males, however, were more sensitive to MeS than females. Ejaculates transferred by males mating with previously mated females were on average 26% larger than ejaculates transferred by males mating with virgin females, which conforms to sperm competition theory and indicates that males tailored their reproductive investment in response to sperm competition. Furthermore, ejaculates transferred by males mating with virgin females with artificially added MeS were also 26% larger than ejaculates transferred to control virgin females. Male‐transferred anti‐aphrodisiac pheromone not only functions as a male deterrent, but also carries information on female mating history and thus allows males to assess sperm competition. Lay Summary

Because selection is often sex-dependent, alleles can have positive effects on fitness in one sex and negative effects in the other, resulting in intralocus sexual conflict. Evolutionary theory predicts that intralocus sexual conflict can drive the evolution of sex limitation, sex-linkage, and sex chromosome differentiation. However, evidence that sex-dependent selection results in sex-linkage is limited. Here, we formally partition the contribution of Y-linked and non-Y-linked quantitative genetic variation in coloration, tail, and body size of male guppies (Poecilia reticulata)—traits previously implicated as sexually antagonistic. We show that these traits are strongly genetically correlated, both on and off the Y chromosome, but that these correlations differ in sign and magnitude between both parts of the genome. As predicted, variation in attractiveness was found to be associated with the Y-linked, rather than with the non-Y-linked component of genetic variation in male ornamentation. These findings show how the evolution of Y-linkage may be able to resolve sexual conflict. More generally, they provide unique insight into how sex-specific selection has the potential to differentially shape the genetic architecture of fitness traits across different parts of the genome.

Postcopulatory sexual selection (PCSS), comprised of sperm competition and cryptic female choice, has emerged as a widespread evolutionary force among polyandrous animals. There is abundant evidence that PCSS can shape the evolution of sperm. However, sperm are not the whole story: they are accompanied by seminal fluid substances that play many roles, including influencing PCSS. Foremost among seminal fluid models is Drosophila melanogaster , which displays ubiquitous polyandry, and exhibits intraspecific variation in a number of seminal fluid proteins (Sfps) that appear to modulate paternity share. Here, we first consolidate current information on the identities of D. melanogaster Sfps. Comparing between D. melanogaster and human seminal proteomes, we find evidence of similarities between many protein classes and individual proteins, including some D. melanogaster Sfp genes linked to PCSS, suggesting evolutionary conservation of broad-scale functions. We then review experimental evidence for the functions of D. melanogaster Sfps in PCSS and sexual conflict. We identify gaps in our current knowledge and areas for future research, including an enhanced identification of PCSS-related Sfps, their interactions with rival sperm and with females, the role of qualitative changes in Sfps and mechanisms of ejaculate tailoring. This article is part of the theme issue ‘Fifty years of sperm competition’.

The past half century has seen the development of the field of post-ejaculatory sexual selection, the sequel to sexual selection for mate-acquisition (pre-ejaculatory) described by Darwin. In richness and diversity of adaptations, post-ejaculatory selection rivals that of pre-ejaculatory sexual selection. Anisogamy—and hence two sexes—likely arose by primeval gamete competition, and sperm competition remains a major force maintaining high sperm numbers. The post-ejaculatory equivalent of male–male competition for matings, sperm competition was an intense ancestral form of sexual selection, typically weakening as mobility and internal fertilization developed in many taxa, when some expenditure became diverted into pre-ejaculatory competition. Sperm competition theory has been relatively successful in explaining variation in relative testes size and sperm numbers per ejaculate and is becoming more successful in explaining variation in sperm phenotype. Sperm competition has generated many other male adaptations such as seminal fluid proteins that variously modify female reproduction towards male interests, and copulatory plugs, prolonged copulations and post-ejaculatory guarding behaviour that reduce female remating probability, many of which result in sexual conflict. This short survey of conceptual developments is intended as a broad overview, mainly as a primer for new researchers. This article is part of the theme issue ‘Fifty years of sperm competition'.

Sperm production and allocation strategies have been a central concern of sperm competition research for the past 50 years. But during the ‘sexual cascade’ there may be strong selection for alternative routes to maximizing male fitness. Especially with the evolution of internal fertilization, a common and by now well-studied example is the accessory ejaculate investment represented by seminal fluid, the complex mixture of proteins, peptides and other components transferred to females together with sperm. How seminal fluid investment should covary with sperm investment probably depends on the mechanism of seminal fluid action. If seminal fluid components boost male paternity success by directly enhancing sperm function or use, we might often expect a positive correlation between the two forms of male investment, whereas trade-offs seem more likely if seminal fluid acts independently of sperm. This is largely borne out by a broad taxonomic survey to establish the prevailing patterns of seminal fluid production and allocation during animal evolution, in light of which I discuss the gaps that remain in our understanding of this key ejaculate component and its relationship to sperm investment, before outlining promising approaches for examining seminal fluid-mediated sperm competitiveness in the post-genomic era. This article is part of the theme issue ‘Fifty years of sperm competition’.

Scholars increasingly document different forms of conflict-related sexual violence, their distinct causes, and their sharply varying deployment by armed organizations. In this paper, I first summarize recent research on this variation, emphasizing findings that contradict or complicate popular beliefs. I then discuss distinct interpretations of the claim that such violence is part of a continuum of violence between peace and war. After analyzing recent research on the internal dynamics of armed organizations, I suggest that widespread rape often occurs as a practice rather than as a strategy. Finally, I advance some principles to guide policy in light of recent research.