The front legs of Sepsis flies (Diptera: Sepsidae) are stimulation devices.

Female-producing parthenogenesis is widespread in stick insects. It can be either rare and spontaneous in sexual species, or range from facultative to obligate, the latter sometimes in interspecific hybrids. This review synthesizes current knowledge on its origins, mechanisms, and evolutionary consequences, highlighting the distinction between hybrid and intra-specific origins. Hybrid-derived parthenogens are rare, obligate, and frequently polyploid, producing eggs via endoduplication which maintains heterozygosity. Intra-specific parthenogens are more frequent, typically diploid, and often homozygous due to gamete duplication.Facultative parthenogenesis allows both sexual and asexual reproduction, yet natural populations are usually either sexually reproducing or fully female, with intermediate sex ratios being rare. The mosaic distribution of mixed-sex and female-only populations without clear ecological differences suggests other factors drive the observed patterns. Sexual conflict has been proposed as a driver, but empirical data suggest mating confers fitness benefits to females rather than male-inflicted harm. In the Timema genus, parthenogenesis is linked to reduced selection efficiency and slower adaptation. Multiple obligate parthenogens evolved independently via gamete duplication, paralleling rare spontaneous parthenogenesis in sexual species. This suggests repeated selection for increased parthenogenesis frequencies in different genomic backgrounds. Overall, by providing an update on the current understanding of the phylogenetic distribution, mechanistic diversity, and transitions to parthenogenesis in stick insects, this review establishes Phasmatodea as a model to study the evolutionary significance of asexual reproduction.

ABSTRACT In sexually promiscuous species, sperm removal behavior (SRB) is a male strategy to increase reproductive success by removing rival sperm from the female prior to insemination. This behavior may, however, impose costs on both sexes, generating sexual conflict. We investigated the sex‐specific control over the duration of SRB in Metaplastes ornatus , a bush cricket species exhibiting this behavior. We used a double mating design experiment and recorded morphometric measurements from a wild population in Greece to examine whether male or female traits can predict the duration of SRB or whether repeatabilities differ between the sexes. We found a trend that lighter females mated more frequently, while the body mass of either sex did not affect SRB duration. Repeatability analyses suggested a potential, albeit weak, female influence on SRB duration. We also identified high rates of matings terminated before spermatophore transfer after the initiation of SRB. These events were associated with shorter SRB durations and fewer behavioral breaks. A pilot analysis of male genital morphology revealed that males with narrower subgenital plates may have greater copulation success. Our results suggest that female traits and behavior could play a critical role in shaping the outcome of SRB, indicating potential female counter‐adaptations in this sexually antagonistic system.

ABSTRACT Experiencing rejection from a romantic partner can be painful, and sexual rejection may have a lasting impact on satisfaction. However, some ways of communicating sexual rejection may elicit more constructive responses from the rejected partner, fostering continued connection in the face of rejection. Across three studies—a cross-sectional study of individuals in romantic relationships (Study 1; N = 315) and two dyadic daily diary studies (Studies 2–3; total N = 219 couples contributing 9,287 daily reports)—we tested whether specific ways of communicating sexual rejection are associated with the rejected partner’s emotional and behavioral responses. Across studies, perceived hostile rejection (e.g. the partner displayed criticism) was associated with more negative responses (i.e. resentment) from the rejected partner. In contrast, perceived reassuring rejection (e.g. the partner expressed warmth or future interest in sex) elicited more positive responses to rejection (i.e. understanding) and maintained the rejected partner’s sexual desire. The findings extend relationship theories to understand sexual conflict, and provide a framework of specific, observable behaviors that researchers and clinicians can incorporate to help couples navigate the challenges of sexual rejection.

Sexual conflict is pervasive and can favor the evolution of differential gene expression patterns between males and females. The evolution of such sex-biased expression patterns is constrained by pleiotropic functions of differentially expressed genes, such as widespread expression across tissues. We investigated sex-biased gene expression and its relationship to tissue specificity in reproductive and somatic organs in the Northern pipefish, Syngnathus fuscus, a polygynandrous species with extreme paternal care and no evidence of sex chromosomes - conditions ripe for intra-locus sexual conflict. We found patterns of sex-biased expression in the gonads, liver, and gills of the Northern pipefish, with the largest number of sex-biased genes identified in the gonads. In general, sex-biased genes were only more tissue-specific in the reproductive tissues (gonads), but not in either of the somatic tissues (liver or gills). Sex-biased genes with evidence of branch-specific selection were also more tissue specific. We highlight the potential for different sex-specific selection pressures to be acting on each tissue type as there were widespread differences in the protein classes represented by sex-biased genes across both organs and sexes, although sex-biased genes did not experience stronger episodic selection than unbiased genes. Furthermore, our results support the hypothesis that broad expression can constrain the molecular evolution of a gene. The work presented here supports the potential for sex-biased gene expression to act as a mechanism for phenotypic differentiation between the sexes and increases our knowledge of patterns of gene expression in an unusual group of fishes.

Abstract Sexual conflict affects the amount and duration of offspring care each parent invests, resulting in multiple parental care strategies sometimes coexisting within a single population. Understanding the persistence of multiple parental care strategies requires a precise estimate of the benefits and costs associated with parental decisions. Even though the benefits of brood desertion are well known, the reproductive costs of desertion ( i.e., nestlings' physiological conditions and survival), are less explored. We use rock sparrows, Petronia petronia , a species in which both uniparental and biparental care occur in the same population, to investigate the costs of brood desertion. Specifically, we continuously monitored breeding attempts to explore the behavioural mechanisms (desertion decision and compensatory responses) and the reproductive and physiological consequences (offspring corticosterone concentrations, oxidative stress, telomere attrition) of parental care strategies. We show that male desertion was not related to the initial value of the brood (clutch size, brood size) but was associated with a reduction in the survival probability of the nestlings. Females caring alone increased their per capita feeding rate, partially compensating for the lack of male care. Nestlings deserted earlier also experienced higher oxidative stress and had higher corticosterone concentrations during the early stages of development, but these effects did not persist to fledging, and there were no differences in telomere attrition. Our findings indicate combined reproductive and physiological costs associated with brood desertion. Considering these costs is essential to understand the evolution and persistence of polymorphic patterns of care. Read the free Plain Language Summary for this article on the Journal blog.

Evidence for deceptive fertility in a wild primate.

ABSTRACTKin discrimination, that is, the differential treatment of kin and nonkin has been studied in a wide range of social contexts because kin selection theory predicts that genetic relatedness among interacting individuals can reduce conflicts and promote cooperation. However, the impact of kin discrimination on parental cooperation in biparental systems is neglected so far. Here, we compared the individual contributions of males and females to brood care of kin and nonkin pairs in Pelvicachromis taeniatus, a biparental cichlid fish with a pronounced kin‐mating preference. We conducted a cross‐fostering experiment to control for offspring effects on parental care and an experiment without brood manipulation. In both experiments, the relative contribution of the sexes differed depending on the degree of relatedness within a pair. In unrelated pairs, females cared more than males, whereas the sexes contributed equally in related pairs. Our results indicate that relatedness affects the resolution of sexual conflict. More equal contribution may suggest higher cooperation between sexes within related pairs. Enhanced parental cooperation may be an important benefit of inbreeding, which might have contributed to the evolution of inbreeding preference in P. taeniatus, but it may also explain inbreeding tolerance found in other biparental systems.

Male harm occurs when traits in males that increase their reproductive success incidentally reduce female fitness. In Drosophila melanogaster, many lab studies have revealed the presence of male harm, but recent work has shown that its expression can be dramatically reduced, even eliminated, when sexual interactions and mating occur in an environment that differs from traditional lab rearing vials. Here we follow up on this to separately test the effect of fly density and structural complexity of the mating environment in mediating the expression of male harm. We performed separate two-way factorial assays that measured the fitness of females while manipulating their exposure to males and the density of flies or the structural complexity of the environment during exposure. Male harm, quantified as the relative reduction in female fitness under increased male exposure, was not affected by density, but was significantly reduced -essentially eliminated- by increased structural complexity. Our results demonstrate that seemingly simple choices, like the environment used in a laboratory model system, can have profound impacts on the expression of harm and hence views on the prevalence of sexual conflict. This is noteworthy because conflict can shape other fundamental evolutionary processes including adaptation, purging, and speciation.

Long-term social monogamy, a prevalent mating system in avian species, is often associated with increased cooperation and coordination as well as reduced sexual conflict. Although many studies have highlighted the benefits of long-term partnerships for individuals, there remains a lack of insight into how closely partners associate with one another behaviorally. To date, studies investigating pair cohesion in seasonal and long-term partnerships are typically restricted to arrivals at the nest or feeding sites during the breeding season. Using fine-scale automated tracking data on chirruping wedgebills (Psopodes cristatus), a territorial socially monogamous species, we characterized how partners coordinate their movement during and after the breeding season. We used 12 pair-bonded individuals with consistently high localization rates that were tracked for a period between 32 and 69 days, with an average of 260,000 localizations per individual. We demonstrate that pairs (1) had extremely similar home ranges with a similarity index of 0.93 versus 0.18 for non-pairs, (2) maintained consistently closer proximity than expected from movement without paying attention to a partner, and (3) followed each other as they moved, with individuals following their moving partner in 42% of cases during and in 47% of cases after breeding. Our findings show that pair cohesion in socially monogamous territorial species can be very high in both a breeding and non-breeding context, illustrating that strong coordination among partners has important functions beyond reproduction and parental care.

Abstract In organisms with separate sexes, the expected evolutionary change in a trait due to selection can be expressed using sex-specific Robertson covariances (RCs), that is, the additive genetic covariance between the trait and female relative fitness and the additive genetic covariance between the trait and male relative fitness. Sex-specific RCs capture the effects of (1) direct and indirect selection acting on the trait in the sex it is measured in ("within-sex selection") and (2) direct and indirect selection experienced by the underlying loci when expressed in the opposite sex ("cross-sex selection"). Using hemiclonal analysis in Drosophila melanogaster, we investigated the expected response to within-sex and cross-sex selection for a suite of traits involved in interlocus sexual conflict (IeSC) at male-biased, equal, and female-biased adult sex ratios. Our results are consistent with the idea that IeSC and sexual selection become stronger with the degree of male bias in adult sex ratio. The expected responses to cross-sex selection were small and typically concordant relative to the expected response to within-sex selection, with no evidence of intralocus sexual conflict for the traits we investigated. On the contrary, our findings imply that cross-sex selection may substantially boost the rate of adaptation in females.

Amplexus by male cane toads (Rhinella marina) impairs a female’s mobility and may impose a risk of drowning. Near the arid-zone edge of the toads’ Australian invasion, artificial ponds provide the only permanent open water. Cane toads must access water to hydrate every few nights, creating a potential for sexual conflict. Our field-based experiments show that a female toad that approaches one of these steep-sided dams encounters numerous reproductively active males, most of which are facing the shore. When amplexed by these males, she may find herself in deep water even close to the shore and is vulnerable to drowning. In trials with tethered females, toads amplexed in deep water could not hold their heads above the water’s surface. Demographic effects of this sexual conflict are evident from population surveys: toad populations around dams are strongly male-biased whereas females are concentrated at mesic refuges around buildings that provide less dangerous conditions. Even around the same dam, female toads are often found on land whereas most males are found in the water. If sexual conflict around scarce waterbodies is lethal for female toads, we might reduce recruitment by allowing dense populations of male toads to persist.

Bipedalism, childhood, and ritualisation of human sexual behaviour: A hominin model scenario of ontogenetic selection.

Extra-pair copulations (EPCs) are the poorly known antecedents of extra-pair fertilizations (EPFs) in birds. EPFs occur in most bird species that have been examined, but sexual conflict will generally reward females hiding their EPCs from males attempting to protect their paternity. EPCs will be difficult for researchers to document, and necessarily underestimated, in that case. We measured EPC behaviors and EPF frequency in a colonial seabird, the Nazca booby Sula granti, in which all copulations occur in a visually open setting with numerous possible copulatory partners readily available. Females are larger and more physically powerful than males, and are the numerically limiting sex, perhaps limiting options for males to control females. We found that all copulations were voluntary, and females’ sexual activities were wholly unconstrained by male coercion. Most females had multiple copulatory partners in the weeks preceding egg-laying. Despite the commonness of EPC, EPFs did not occur. The different schedules of EPC and within-pair copulation (WPC) provided a sufficient explanation for this outcome: during the ovulation window days before laying, WPC rate increased and EPC rate approached zero. To our knowledge, this is the first robust evidence of complete sexual agency in a female bird aside from lek-mating species, contributing a valuable exemplar to the literature on sexual conflict over reproduction.

ABSTRACT Climate change is increasingly recognised as a critical threat to global biodiversity, yet its impacts on reproductive processes remain poorly understood in many marine taxa. Elasmobranchs (sharks, skates and rays) represent a particularly vulnerable group due to their peculiar life histories, low fecundity, and diverse reproductive modes. In this review, we synthesise current knowledge on how rising ocean temperatures may affect elasmobranch fertility across key stages of the reproductive cycle, including gametogenesis, mate searching, sperm storage, fertilisation, embryonic development, and offspring production. Evidence from 88 species suggests that thermal stress could impair sperm and oocyte quality, disrupt reproductive timing, alter embryonic growth and survival, and potentially exacerbate sexual conflict. While some reproductive strategies such as embryonic diapause, sperm storage, or behavioural thermoregulation may provide short‐term buffering capacity, their effectiveness under rapid and sustained warming remains uncertain. We further highlight the macroevolutionary significance of elasmobranch reproductive diversity, as well as the conservation implications of fertility constraints under global change. Addressing these knowledge gaps is essential to refining demographic models, improving extinction risk assessments, and guiding the design of climate‐resilient management strategies, including fisheries regulations and marine protected areas. By explicitly linking reproductive biology with conservation policy, we argue that understanding thermal fertility limits is key to predicting elasmobranch population trajectories in a warming ocean.

ABSTRACTCichlids are a textbook model system of adaptive radiation and a fascinating example of rapid sex chromosome evolution. Yet in these fish, as in most other taxa, the mechanisms causing sex chromosome turnover and the subsequent impact thereof are unknown. Sexual antagonism was long thought to be a driver of sex chromosome emergence, but experimental support remains scarce. Here, we show that sex‐biased genes, often used as indicators of sexual antagonism, are enriched in three different sex chromosome systems of Lake Tanganyika cichlid species that diverged less than 4 million years ago. Moreover, gene expression is feminised in species that transitioned from an XY to a ZW system on the same chromosome. This is achieved by gain of female‐biased genes, increase of female sex‐bias as well as decrease of male‐bias depending on the tissue investigated. We further show that XY sex chromosomes have more male‐biased genes but without higher intensity of sex‐biased expression. A large fraction of sex‐bias in gene expression evolved adaptively, with a stronger signature in females than males. While we find that sex‐bias in gene expression clearly depends on the heterogametic system, we find only weak support for sex‐biased expression priming chromosomes to become sex chromosomes. Overall, we conclude that there is little evidence that sexual antagonism drives sex chromosome emergence but that it likely plays a role during sex chromosome differentiation. We see rapid emergence of antagonistic expression in sex‐linked genes.

Abstract Sexual selection and sexual conflict often result in the evolution of morphological traits that function to improve reproductive success, often termed sexual weapons and ornaments. Sexual weapons serve to increase the reproductive success of the ardent sex (typically males in dioecious taxa) by force, whereas sexual ornaments are considered ‘desirable’ by the opposite sex, or may exploit pre-existing sensory bias. Cartilaginous fishes (Chondrichthyes: sharks, skates, rays, and chimaeras) exhibit a complex spectrum of reproductive modes and marked variation in the prevalence of genetic polyandry and multiple mating. For these reasons, Chondrichthyes represent an ideal group to study sexual selection, sexual conflict, and their evolutionary consequences. In this review, we summarise existing knowledge regarding the function of several putative ‘weapons of sexual conflict’ (sexual weaponry used to coerce or force females to mate) and ornaments possessed by cartilaginous fishes. Subsequently, we discuss what chondrichthyans and these traits can tell us about sexual selection more broadly, and we highlight major knowledge gaps in the field. A lack of observational data impedes our ability to make robust claims about the function of several traits. However, there is reason to suggest that weaponry resulting from sexually antagonistic selection is abundant in chondrichthyan taxa, whilst only one potential case of sexual ornamentation is known.

Sexual conflict over shared traits-intralocus sexual conflict (IaSC)-may be common and consequential, but experimental tests of its relative magnitude are challenging and limited in number. We use a sex-limited selection experiment, designed to subject haplotypes of Drosophila melanogaster to selection for male fitness without opposing selection acting on female fitness. Importantly, we use three novel base populations to compare results with those from the LHM population, the sole population investigated using this technique. In contrast with previous studies, we find that male fitness of haplotypes subject to male-limited selection (ML populations) are not consistently better than their matched (MC) controls when tested in the 'wildtype' state. Males from ML lines did not outperform controls in competitive fitness assays, mate choice trials, fecundity induction or sperm offense tests. As predicted, genetic variation for male fitness was reduced, with low fitness haplotypes apparently removed by selection, but this was only surveyed in one replicate population pair and included a potential artefact in the protocol. Female fitness was markedly reduced by carriage of ML haplotypes, as predicted by sexual antagonism. Hence, our results are only partially consistent with the IaSC hypothesis, raising questions about the relative contribution of sexual conflict to the standing genetic variation in these populations and the potential role of artefacts in the protocol that may have obscured our ability to detect IaSC.

Uncovering the genetic basis of sexually selected traits and traits involved in sexual conflict is a key to understand the association between sexual and non-sexual fitness. 6-phosphogluconate dehydrogenase (6Pgdh) is a metabolic gene associated with pentose phosphate pathway. It was shown to be involved in sexual selection and conflict in bulb mite, Rhizoglyphus robini. Two previously identified 6Pgdh genotypes are associated with variation in male reproductive fitness: the "winning" 6Pgdh allele (S) confers advantage in male reproductive success compared to the alternative F allele, but mating with S-bearing males decreases female fecundity. Physiological mechanisms of these differences remain a puzzle. We compare gene expression patterns between males from the S- and F-allele homozygous lines to identify which molecular pathways are affected by 6Pgdh polymorphism. Moreover, we test for linkage disequilibrium in gene-coding regions associated with genetic polymorphism in 6Pgdh and show that polymorphism in 6Pgdh is in linkage disequilibrium with nonsynonymous single nucleotide polymorphisms in five genes, four of which are located within the same chromosome. We show that male genotype in 6Pgdh is associated with differential expression of genes distributed throughout the whole genome. Among differentially expressed genes, we found overrepresentation of several categories associated with fructose metabolism, including an enzyme associated with both pentose phosphate metabolism and glycolysis. Differential expression in genes associated with a number of other general categories highlight the connection between sexual fitness and gene expression in a number of important pathways, potentially affecting performance of a whole organism.

Crossing over during meiosis ensures proper chromosome segregation and promotes genetic diversity. In species with chromosomal sex determination, recombination between sex chromosomes is often reduced or eliminated; yet, the evolutionary forces driving this shift remain debated. One extreme outcome, achiasmatic meiosis, typically completely halts recombination in the heterogametic sex. Here, we use a population genetic model to compare 2 leading hypotheses for the evolution of achiasmy: (1) selection to reduce recombination load from sexually antagonistic alleles and (2) selection to avoid aneuploidy driven by heteromorphic sex chromosomes. We analyze how mutations promoting achiasmy can invade autosomes, X chromosomes, or Y chromosomes under each selective regime. Our results reveal that the Y chromosome provides the most permissive context for invasion due to male-limited expression and selection. Moreover, we predict a shift in the primary selective forces across the trajectory of sex-chromosome divergence: sexually antagonistic selection is more likely to drive achiasmy in young, homomorphic sex chromosomes, whereas heteromorphy-dependent aneuploidy becomes the primary force in highly diverged, heteromorphic sex chromosomes. These results provide a unified framework for understanding transitions to achiasmy across diverse taxa.