Y-Linked Copy Number Polymorphism of Target of Rapamycin Is Associated with Sexual Size Dimorphism in Seed Beetles

The Y chromosome is theorized to facilitate evolution of sexual dimorphism by accumulating sexually antagonistic loci, but empirical support is scarce. Due to the lack of recombination, Y chromosomes are prone to degenerative processes, which poses a constraint on their adaptive potential. Yet, in the seed beetle, Callosobruchus maculatus segregating Y linked variation affects male body size and thereby sexual size dimorphism (SSD). Here, we assemble C. maculatus sex chromosome sequences and identify molecular differences associated with Y-linked SSD variation. The assembled Y chromosome is largely euchromatic and contains over 400 genes, many of which are ampliconic with a mixed autosomal and X chromosome ancestry. Functional annotation suggests that the Y chromosome plays important roles in males beyond primary reproductive functions. Crucially, we find that, besides an autosomal copy of the gene target of rapamycin (TOR), males carry an additional TOR copy on the Y chromosome. TOR is a conserved regulator of growth across taxa, and our results suggest that a Y-linked TOR provides a male specific opportunity to alter body size. A comparison of Y haplotypes associated with male size difference uncovers a copy number variation for TOR, where the haplotype associated with decreased male size, and thereby increased sexual dimorphism, has two additional TOR copies. This suggests that sexual conflict over growth has been mitigated by autosome to Y translocation of TOR followed by gene duplications. Our results reveal that despite of suppressed recombination, the Y chromosome can harbor adaptive potential as a male-limited supergene.

Weak sex-specific evolution of locomotor activity of Sepsis punctum (Diptera: Sepsidae) thermal experimental evolution lines

Elevated temperatures are expected to rise beyond what the physiology of many organisms can tolerate. Behavioural responses facilitating microhabitat shifts may mitigate some of this increased thermal selection on physiology, but behaviours are themselves mediated by physiology, and any behavioural response may trade-off against other fitness-related activities. We investigated whether experimental evolution in different thermal regimes (Cold: 15 °C; Hot: 31 °C; Intergenerational fluctuation 15/31 °C; Control: 23 °C) resulted in genetic differentiation of standard locomotor activity in the dung fly Sepsis punctum. We assessed individual locomotor performance, an integral part of most behavioral repertoires, across eight warm temperatures from 24 °C to 45 °C using an automated device. We found no evidence for generalist-specialist trade-offs (i.e. changes in the breadth of the performance curve) for this trait. Instead, at the warmest assay temperatures hot-selected flies showed somewhat higher maximal performance than all other, especially cold-selected flies, overall more so in males than females. Yet, the flies' temperature optimum was not higher than that of the cold-selected flies, as expected under the 'hotter-is-better' hypothesis. Maximal locomotor performance merely weakly increased with body size. These results suggest that thermal performance curves are unlikely to evolve as an entity according to theory, and that locomotor activity is a trait of limited use in revealing thermal adaptation.

Stressful environments favor deceptive alternative mating tactics to become dominant

BACKGROUND

Deceptive alternative mating tactics are commonly maintained at low frequencies within populations because males using them are less competitive and acquire lower fitness than those using dominant tactics. However, the successful invasion of a male deceptive tactic is plausible if deception carries no fitness cost to females. Among populations of the gift-giving spider Paratrechalea ornata, males very often offer females a deceptive worthless gift, rather than a nutritive gift. We tested the degree to which deceptive worthless gifts can occur in natural populations living under divergent environmental conditions (moderate and stressful). We examined the plasticity of morphological and behavioral traits and analyzed the fitness of females in relation to the gift type, also examining the paternity acquired by males offering either gift type.

RESULTS

We demonstrated that worthless gifts can become dominant under highly stressful environmental conditions (84-100%). Individuals in such environment reach smaller sizes than those in moderate conditions. We suggest that the size reduction probably favors low metabolic demands in both sexes and may reduce the costs associated with receiving deceptive worthless gifts for females. In contrast, males living under moderate conditions varied the use of the deceptive tactic (0-95%), and worthless gifts negatively influenced female fecundity. Furthermore, male size, rather than gift content, positively impacted paternity success in the moderate but not in the stressful environment.

CONCLUSIONS

Overall, this is the first empirical evidence that a reversible deceptive tactic can become dominant when the environment becomes harsh and mate choice becomes limited.

Is personality and its association with energetics sex-specific in yellow-necked mice Apodemus flavicollis?

For the last two decades, behavioral physiologists aimed to explain a plausible covariation between energetics and personality, predicted by the "pace-of-life syndrome" (POLS) hypothesis. However, the results of these attempts are mixed with no definitive answer as to which of the two most acknowledged models "performance" or "allocation" predicts covariation between consistent among-individual variation in metabolism and repeatable behavior (animal personality). The general conclusion is that the association between personality and energetics is rather context-dependent. Life-history, behavior, and physiology as well as its plausible covariation can be considered a part of sexual dimorphism. However, up to now, only a few studies demonstrated a sex-specific correlation between metabolism and personality. Therefore, we tested the relationships between physiological and personality traits in a single population of yellow-necked mice Apodemus flavicollis in the context of a plausible between-sexes difference in this covariation. We hypothesized that the performance model will explain proactive behavior in males and the allocation model will apply to females. Behavioral traits were determined using the latency of risk-taking and the open field tests, whereas the basal metabolic rates (BMR) was measured using indirect calorimetry. We have found a positive correlation between body mass-adjusted BMR and repeatable proactive behavior in male mice, which can support the performance model. However, the females were rather consistent mainly in avoidance of risk-taking that did not correlate with BMR, suggesting essential differences in personality between sexes. Most likely, the lack of convincing association between energetics and personality traits at the population level is caused by a different selection acting on the life histories of males and females. This may only result in weak support for the predictions of the POLS hypothesis when assuming that only a single model explaining the link between physiology and behavior operates in males and females. Thus, there is a need to consider the differences between sexes in behavioral studies to evaluate this hypothesis.

A possible genomic footprint of polygenic adaptation on population divergence in seed beetles?

Efforts to unravel the genomic basis of incipient speciation are hampered by a mismatch between our toolkit and our understanding of the ecology and genetics of adaptation. While the former is focused on detecting selective sweeps involving few independently acting or linked speciation genes, the latter states that divergence typically occurs in polygenic traits under stabilizing selection. Here, we ask whether a role of stabilizing selection on polygenic traits in population divergence may be unveiled by using a phenotypically informed integrative approach, based on genome‐wide variation segregating in divergent populations. We compare three divergent populations of seed beetles (Callosobruchus maculatus) where previous work has demonstrated a prominent role for stabilizing selection on, and population divergence in, key life history traits that reflect rate‐dependent metabolic processes. We derive and assess predictions regarding the expected pattern of covariation between genetic variation segregating within populations and genetic differentiation between populations. Population differentiation was considerable (mean F_ST = 0.23–0.26) and was primarily built by genes showing high selective constraints and an imbalance in inferred selection in different populations (positive Tajima's D_NS in one and negative in one), and this set of genes was enriched with genes with a metabolic function. Repeatability of relative population differentiation was low at the level of individual genes but higher at the level of broad functional classes, again spotlighting metabolic genes. Absolute differentiation (d_XY) showed a very different general pattern at this scale of divergence, more consistent with an important role for genetic drift. Although our exploration is consistent with stabilizing selection on polygenic metabolic phenotypes as an important engine of genome‐wide relative population divergence and incipient speciation in our study system, we note that it is exceedingly difficult to firmly exclude other scenarios.

Concerted evolution of metabolic rate, economics of mating, ecology, and pace of life across seed beetles

Coevolution between females and males has led to remarkable differences between the sexes but has taken very different routes, even in closely related animal species, for reasons that are not well understood. We studied the physiological processes that convert resources into offspring (metabolism) in males and females of several related beetle species. We found that ecological factors dictate metabolic rate, which, in turn, have predictable direct and indirect effects on male–female coevolution. Our findings suggest that a complete understanding of differences between the sexes requires an understanding of how ecology affects metabolic processes and how these differ in the sexes.

Male–female coevolution has taken different paths among closely related species, but our understanding of the factors that govern its direction is limited. While it is clear that ecological factors, life history, and the economics of reproduction are connected, the divergent links are often obscure. We propose that a complete understanding requires the conceptual integration of metabolic phenotypes. Metabolic rate, a nexus of life history evolution, is constrained by ecological factors and may exert important direct and indirect effects on the evolution of sexual dimorphism. We performed standardized experiments in 12 seed beetle species to gain a rich set of sex-specific measures of metabolic phenotypes, life history traits, and the economics of mating and analyzed our multivariate data using phylogenetic comparative methods. Resting metabolic rate (RMR) showed extensive evolution and evolved more rapidly in males than in females. The evolution of RMR was tightly coupled with a suite of life history traits, describing a pace-of-life syndrome (POLS), with indirect effects on the economics of mating. As predicted, high resource competition was associated with a low RMR and a slow POLS. The cost of mating showed sexually antagonistic coevolution, a hallmark of sexual conflict. The sex-specific costs and benefits of mating were predictably related to ecology, primarily through the evolution of male ejaculate size. Overall, our results support the tenet that resource competition affects metabolic processes that, in turn, have predictable effects on both life history evolution and reproduction, such that ecology shows both direct and indirect effects on male–female coevolution.

Sex-limited chromosomes and non-reproductive traits

Sex chromosomes are typically viewed as having originated from a pair of autosomes, and differentiated as the sex-limited chromosome (e.g. Y) has degenerated by losing most genes through cessation of recombination. While often thought that degenerated sex-limited chromosomes primarily affect traits involved in sex determination and sex cell production, accumulating evidence suggests they also influence traits not sex-limited or directly involved in reproduction. Here, we provide an overview of the effects of sex-limited chromosomes on non-reproductive traits in XY, ZW or UV sex determination systems, and discuss evolutionary processes maintaining variation at sex-limited chromosomes and molecular mechanisms affecting non-reproductive traits.

Natural variation at a single gene generates sexual antagonism across fitness components in Drosophila

Mutations with conflicting fitness effects in males and females accumulate in sexual populations, reducing their adaptive capacity.^1,2 Although quantitative genetic studies indicate that sexually antagonistic polymorphisms are common,^3-5 their molecular basis and population genetic properties remain poorly understood.^6,7 Here, we show in fruit flies how natural variation at a single gene generates sexual antagonism through phenotypic effects on cuticular hydrocarbon (CHC) traits that function as both mate signals and protectors against abiotic stress⁸ across a latitudinal gradient. Tropical populations of Drosophila serrata have polymorphic CHCs producing sexual antagonism through opposing but sex-limited effects on these two fitness-related functions. We dissected this polymorphism to a single fatty-acyl CoA reductase gene, DsFAR2-B, that is expressed in oenocyte cells where CHCs are synthesized. RNAi-mediated disruption of the DsFAR2-B ortholog in D. melanogaster oenocytes affected CHCs in a similar way to that seen in D. serrata. Population genomic analysis revealed that balancing selection likely operates at the DsFAR2-B locus in the wild. Our study provides insights into the genetic basis of sexual antagonism in nature and connects sexually varying antagonistic selection on phenotypes with balancing selection on genotypes that maintains molecular variation.

Experimental evolution reveals differential evolutionary trajectories in male and female activity levels in response to sexual selection and metapopulation structure

Behavior is central to interactions with the environment and thus has significant consequences for individual fitness. Sexual selection and demographic processes have been shown to independently shape behavioral evolution. Although some studies have tested the simultaneous effects of these forces, no studies have investigated their interplay in behavioral evolution. We applied experimental evolution in the seed beetle Callosobruchus maculatus to investigate, for the first time, the interactive effects of sexual selection intensity (high [polygamy] vs. minimal [enforced monogamy]) and metapopulation structure (yes/no) on the evolution of movement activity, a crucial behavior involved in multiples functions (e.g., dispersal, predator avoidance, or resource acquisition) and thus, closely related to fitness. We found that the interactive effects of the selection regimes did not affect individual activity, which was assayed under two different environments (absence vs. presence of conspecific cues from both sexes). However, contrasting selection regimes led to sex- and context-dependent divergence in activity. The relaxation of sexual selection favored an increase in female, but not male, movement activity that was consistent between environmental contexts. In contrast, selection associated with the presence/absence of metapopulation structure led to context-dependent responses only in male activity. In environments containing cues from conspecifics, males from selection lines under population subdivision showed increased levels of activity compared to those assayed in an environment devoid of conspecifics cues, whereas the opposite was true for males from panmictic lines. These results underscore that both the effects of sexual selection and population spatial structure may be crucial in shaping sex-specific behavioral evolution.

Experimental sexual selection affects the evolution of physiological and life-history traits

Sexual selection and sexual conflict are expected to affect all aspects of the phenotype, not only traits that are directly involved in reproduction. Here, we show coordinated evolution of multiple physiological and life-history traits in response to long-term experimental manipulation of the mating system in populations of Drosophila pseudoobscura. Development time was extended under polyandry relative to monogamy in both sexes, potentially due to higher investment in traits linked to sexual selection and sexual conflict. Individuals (especially males) evolving under polyandry had higher metabolic rates and locomotor activity than those evolving under monogamy. Polyandry individuals also invested more in metabolites associated with increased endurance capacity and efficient energy metabolism and regulation, namely lipids and glycogen. Finally, polyandry males were less desiccation- and starvation resistant than monogamy males, suggesting trade-offs between resistance and sexually selected traits. Our results provide experimental evidence that mating systems can impose selection that influences the evolution of non-sexual phenotypes such as development, activity, metabolism and nutrient homeostasis.

Investigating the interaction between inter-locus and intra-locus sexual conflict using hemiclonal analysis in Drosophila melanogaster

Background

Divergence in the evolutionary interests of males and females leads to sexual conflict. Traditionally, sexual conflict has been classified into two types: inter-locus sexual conflict (IeSC) and intra-locus sexual conflict (IaSC). IeSC is modeled as a conflict over outcomes of intersexual reproductive interactions mediated by loci that are sex-limited in their effects. IaSC is thought to be a product of selection acting in opposite directions in males and females on traits with a common underlying genetic basis. While in their canonical formalisms IaSC and IeSC are mutually exclusive, there is growing support for the idea that the two may interact. Empirical evidence for such interactions, however, is limited.

Results

Here, we investigated the interaction between IeSC and IaSC in Drosophila melanogaster. Using hemiclonal analysis, we sampled 39 hemigenomes from a laboratory-adapted population of D. melanogaster. We measured the contribution of each hemigenome to adult male and female fitness at three different intensities of IeSC, obtained by varying the operational sex ratio. Subsequently, we estimated the intensity of IaSC at each sex ratio by calculating the intersexual genetic correlation (r_w,g,mf) for fitness and the proportion of sexually antagonistic fitness-variation. We found that the intersexual genetic correlation for fitness was positive at all three sex ratios. Additionally, at male biased and equal sex ratios the r_w,g,mf was higher, and the proportion of sexually antagonistic fitness variation lower, relative to the female biased sex ratio, although this trend was not statistically significant.

Conclusion

Our results indicate a statistically non-significant trend suggesting that increasing the strength of IeSC ameliorates IaSC in the population.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01992-0.

Sexual dimorphism in phenotypic plasticity and persistence under environmental change: An extension of theory and meta-analysis of current data

Populations must adapt to environmental changes to remain viable. Both evolution and phenotypic plasticity contribute to adaptation, with plasticity possibly being more important for coping with rapid change. Adaptation is complex in species with separate sexes, as the sexes can differ in the strength or direction of natural selection, the genetic basis of trait variation, and phenotypic plasticity. Many species show sex differences in plasticity, yet how these differences influence extinction susceptibility remains unclear. We first extend theoretical models of population persistence in changing environments and show that persistence is affected by sexual dimorphism for phenotypic plasticity, trait genetic architecture, and sex-specific selection. Our models predict that female-biased adaptive plasticity-particularly in traits with modest-to-low cross-sex genetic correlations-typically promotes persistence, though we also identify conditions where sexually monomorphic or male-biased plasticity promotes persistence. We then perform a meta-analysis of sex-specific plasticity under manipulated thermal conditions. Although examples of sexually dimorphic plasticity are widely observed, systematic sex differences are rare. An exception-cold resistance-is systematically female-biased and represents a trait wherein sexually dimorphic plasticity might elevate population viability in changing environments. We discuss our results in light of debates about the roles of evolution and plasticity in extinction susceptibility.

An unbiased test reveals no enrichment of sexually antagonistic polymorphisms on the human X chromosome

Mutations with beneficial effects in one sex can have deleterious effects in the other. Such 'sexually antagonistic' (SA) variants contribute to variation in life-history traits and overall fitness, yet their genomic distribution is poorly resolved. Theory predicts that SA variants could be enriched on the X chromosome or autosomes, yet current empirical tests face two formidable challenges: (i) identifying SA selection in genomic data is difficult; and (ii) metrics of SA variation show persistent biases towards the X, even when SA variants are randomly distributed across the genome. Here, we present an unbiased test of the theory that SA variants are enriched on the X. We first develop models for reproductive F_ST-a metric for quantifying sex-differential (including SA) effects of genetic variants on lifetime reproductive success-that control for X-linked biases. Comparing data from approximately 250 000 UK Biobank individuals to our models, we find F_ST elevations consistent with both X-linked and autosomal SA polymorphisms affecting reproductive success in humans. However, the extent of F_ST elevations does not differ from a model in which SA polymorphisms are randomly distributed across the genome. We argue that the polygenic nature of SA variation, along with sex asymmetries in SA effects, might render X-linked enrichment of SA polymorphisms unlikely.

Rapid evolution of sexual size dimorphism facilitated by Y-linked genetic variance

Sexual dimorphism is ubiquitous in nature but its evolution is puzzling given that the mostly shared genome constrains independent evolution in the sexes. Sex differences should result from asymmetries between the sexes in selection or genetic variation but studies investigating both simultaneously are lacking. Here, we combine a quantitative genetic analysis of body size variation, partitioned into autosomal and sex chromosome contributions and ten generations of experimental evolution to dissect the evolution of sexual body size dimorphism in seed beetles (Callosobruchus maculatus) subjected to sexually antagonistic or sex-limited selection. Female additive genetic variance (V_A) was primarily linked to autosomes, exhibiting a strong intersexual genetic correlation with males ([Formula: see text] = 0.926), while X- and Y-linked genes further contributed to the male V_A and X-linked genes contributed to female dominance variance. Consistent with these estimates, sexual body size dimorphism did not evolve in response to female-limited selection but evolved by 30-50% under male-limited and sexually antagonistic selection. Remarkably, Y-linked variance alone could change dimorphism by 30%, despite the C. maculatus Y chromosome being small and heterochromatic. Our results demonstrate how the potential for sexual dimorphism to evolve depends on both its underlying genetic basis and the nature of sex-specific selection.

Direct and indirect effects of male genital elaboration in female seed beetles

Our understanding of coevolution between male genitalia and female traits remains incomplete. This is perhaps especially true for genital traits that cause internal injuries in females, such as the spiny genitalia of seed beetles where males with relatively long spines enjoy a high relative fertilization success. We report on a new set of experiments, based on extant selection lines, aimed at assessing the effects of long male spines on females in Callosobruchus maculatus. We first draw on an earlier study using microscale laser surgery, and demonstrate that genital spines have a direct negative (sexually antagonistic) effect on female fecundity. We then ask whether artificial selection for long versus short spines resulted in direct or indirect effects on female lifetime offspring production. Reference females mating with males from long-spine lines had higher offspring production, presumably due to an elevated allocation in males to those ejaculate components that are beneficial to females. Remarkably, selection for long male genital spines also resulted in an evolutionary increase in female offspring production as a correlated response. Our findings thus suggest that female traits that affect their response to male spines are both under direct selection to minimize harm but are also under indirect selection (a good genes effect), consistent with the evolution of mating and fertilization biases being affected by several simultaneous processes.

The genetic architecture of sexual dimorphism in the moss Ceratodon purpureus

A central problem in evolutionary biology is to identify the forces that maintain genetic variation for fitness in natural populations. Sexual antagonism, in which selection favours different variants in males and females, can slow the transit of a polymorphism through a population or can actively maintain fitness variation. The amount of sexually antagonistic variation to be expected depends in part on the genetic architecture of sexual dimorphism, about which we know relatively little. Here, we used a multivariate quantitative genetic approach to examine the genetic architecture of sexual dimorphism in a scent-based fertilization syndrome of the moss Ceratodon purpureus. We found sexual dimorphism in numerous traits, consistent with a history of sexually antagonistic selection. The cross-sex genetic correlations (rmf) were generally heterogeneous with many values indistinguishable from zero, which typically suggests that genetic constraints do not limit the response to sexually antagonistic selection. However, we detected no differentiation between the female- and male-specific trait (co)variance matrices (Gf and Gm, respectively), meaning the evolution of sexual dimorphism may be constrained. The cross-sex cross-trait covariance matrix B contained both symmetric and asymmetric elements, indicating that the response to sexually antagonistic or sexually concordant selection, and the constraint to sexual dimorphism, are highly dependent on the traits experiencing selection. The patterns of genetic variances and covariances among these fitness components is consistent with partly sex-specific genetic architectures having evolved in order to partially resolve multivariate genetic constraints (i.e. sexual conflict), enabling the sexes to evolve towards their sex-specific multivariate trait optima.

Experimentally constrained early reproduction shapes life history trajectories and behaviour

The trade-off between current and future reproduction is a cornerstone of life history theory, but the role of within-individual plasticity on life history decisions and its connections with overall fitness and behaviour remains largely unknown. By manipulating available resources for oviposition at the beginning of the reproductive period, we experimentally constrained individual life history trajectories to take different routes in a laboratory study system, the beetle Callosobruchus maculatus, and investigated its causal effects on fecundity, survival and behaviour. Compared to females without resource limitations, females experiencing restricted conditions for oviposition had reduced fecundity early in life but increased fecundity when resources became plentiful (relative to both the previous phase and the control group) at the expense of longevity. Constrained reproduction in early life also affected behaviour, as movement activity changed differently in the two experimental groups. Experiencing reproductive constraints has, therefore, consequences for future reproduction investments and behaviour, which may lead individuals to follow different life history strategies.

Sexual selection increased offspring production via evolution of male and female traits

Phenotypic evolution driven by sexual selection can impact the fitness of individuals and thus population performance through multiple mechanisms, but it is unresolved how and when sexual selection affects offspring production by females. We examined the effects of sexual selection on offspring production by females using replicated experimental evolutionary lines of Callosobruchus chinensis that were kept under polygamy (with sexual selection) or monogamy (without sexual selection) for 21 generations. We found that polygamous-line pairs produced more offspring than monogamous-line pairs, because polygamous-line beetles evolved to be larger than monogamous-line beetles, and larger females were more fecund. Egg hatchability did not differ between polygamous- and monogamous-line pairs, as a result of the positive and negative effects of sexual selection cancelling out. When mated with an individual from a common tester line, both polygamous-line females and males showed higher hatchability in resultant eggs than monogamous ones. Further, cohabitation with a male reduced egg hatchability, and this effect was more pronounced in polygamous-line than in monogamous-line males. These results demonstrate multiple mechanisms by which sexual selection affects female fitness, with the net effect being positive. Analyses of how development time, body size and male genital morphology were influenced by selection regime suggest that these results arose from both evolution via good-gene processes and sexually antagonistic selection. Our results are also consistent with the hypothesis that the fitness consequences of sexual selection for females are dependent on the evolutionary history of the population.

Experimentally induced intrasexual mating competition and sex-specific evolution in female and male nematodes

Sexual dimorphism in life history traits and their trade-offs is widespread among sexually reproducing animals and is strongly influenced by the differences in reproductive strategies between the sexes. We investigated how intrasexual competition influenced specific life history traits, important to fitness and their trade-offs in the outcrossing nematode Caenorhabditis remanei. Here, we altered the strength of sex-specific selection through experimental evolution with increased potential for intrasexual competition by skewing the adult sex ratio towards either females or males (1:10 or 10:1) over 30 generations and subsequently measured the phenotypic response to selection in three traits related to fitness: body size, fecundity and tolerance to heat stress. We observed a greater evolutionary change in females than males for body size and peak fitness, suggesting that females may experience stronger net selection and potentially harbour higher amounts of standing genetic variance compared to males. Our study highlights the importance of investigating direct and indirect effects of intrasexual competition in both sexes in order to capture sex-specific responses and understand the evolution of sexual dimorphism in traits expressed by both sexes.

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

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

Release from intralocus sexual conflict? Evolved loss of a male sexual trait demasculinizes female gene expression

The loss of sexual ornaments is observed across taxa, and pleiotropic effects of such losses provide an opportunity to gain insight into underlying dynamics of sex-biased gene expression and intralocus sexual conflict (IASC). We investigated this in a Hawaiian field cricket, Teleogryllus oceanicus, in which an X-linked genotype ( flatwing) feminizes males' wings and eliminates their ability to produce sexually selected songs. We profiled adult gene expression across somatic and reproductive tissues of both sexes. Despite the feminizing effect of flatwing on male wings, we found no evidence of feminized gene expression in males. Instead, female transcriptomes were more strongly affected by flatwing than males', and exhibited demasculinized gene expression. These findings are consistent with a relaxation of IASC constraining female gene expression through loss of a male sexual ornament. In a follow-up experiment, we found reduced testes mass in flatwing males, whereas female carriers showed no reduction in egg production. By contrast, female carriers exhibited greater measures of body condition. Our results suggest sex-limited phenotypic expression offers only partial resolution to IASC, owing to pleiotropic effects of the loci involved. Benefits conferred by release from intralocus conflict could help explain widespread loss of sexual ornaments across taxa.

Male harassment leads to fitness costs for females by disrupting oviposition site preferences

In many species, a difference in the optimal number of copulations for males and females leads to sexual conflict. This is well documented in the bean beetle Callosobruchus maculatus, where both sexes mate multiply and females incur fitness costs from injuries caused by the male genitalia. Here, we demonstrate that sexual conflict also decreases female fitness due to male harassment. We hypothesized that harassment costs would come as 1) decreased clutch size, egg size, or both and by 2) disruption of female preference for higher-quality oviposition substrate. Mated females were housed with two bean types—cowpeas, their preferred natal hosts, and toxic pinto beans. They were then submitted to either no, moderate, or high male harassment in the oviposition site. Females under harassment produced smaller clutch sizes but not smaller eggs, resulting in the absence of an egg-size/clutch-size trade-off. Additionally, females did not exhibit a preference for their natal cowpeas hosts over toxic pinto beans when males were present at the oviposition site, although they do so when harassing males are not present. Harassment disrupted female responses to variation in oviposition substrate quality, resulting in considerable fitness consequences in the form of lower offspring production and survival.

The genomic footprint of sexual conflict

Genes with sex-biased expression show a number of unique properties and this has been seen as evidence for conflicting selection pressures in males and females, forming a genetic ‘tug-of-war’ between the sexes. However, we lack studies of taxa where an understanding of conflicting phenotypic selection in the sexes has been linked with studies of genomic signatures of sexual conflict. Here, we provide such a link. We used an insect where sexual conflict is unusually well understood, the seed beetle Callosobruchus maculatus, to test for molecular genetic signals of sexual conflict across genes with varying degrees of sex-bias in expression. We sequenced, assembled and annotated its genome and performed population resequencing of three divergent populations. Sex-biased genes showed increased levels of genetic diversity and bore a remarkably clear footprint of relaxed purifying selection. Yet, segregating genetic variation was also affected by balancing selection in weakly female-biased genes, while male-biased genes showed signs of overall purifying selection. Female-biased genes contributed disproportionally to shared polymorphism across populations, while male-biased genes, male seminal fluid protein genes and sex-linked genes did not. Genes showing genomic signatures consistent with sexual conflict generally matched life-history phenotypes known to experience sexually antagonistic selection in this species. Our results highlight metabolic and reproductive processes, confirming the key role of general life-history traits in sexual conflict.

Gustatory cues to kinship among males moderate the productivity of females

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

Kin but less than kind: within-group male relatedness does not increase female fitness in seed beetles

Theory maintains within-group male relatedness can mediate sexual conflict by reducing male-male competition and collateral harm to females. We tested whether male relatedness can lessen female harm in the seed beetle Callosobruchus maculatus. Male relatedness did not influence female lifetime reproductive success or individual fitness across two different ecologically relevant scenarios of mating competition. However, male relatedness marginally improved female survival. Because male relatedness improved female survival in late life when C. maculatus females are no longer producing offspring, our results do not provide support for the role of within-group male relatedness in mediating sexual conflict. The fact that male relatedness improves the post-reproductive part of the female life cycle strongly suggests that the effect is non-adaptive. We discuss adaptive and non-adaptive mechanisms that could result in reduced female harm in this and previous studies, and suggest that cognitive error is a likely explanation.

Sexual selection and the evolution of male and female cognition: A test using experimental evolution in seed beetles

The mating system is thought to be important in shaping animal intelligence and sexual selection has been depicted as a driver of cognitive evolution, either directly by promoting superior cognitive ability during mate competition, or indirectly via genic capture of sexually selected traits. However, it remains unclear if intensified sexual selection leads to general improvements in cognitive abilities. Here, we evaluated this hypothesis by applying experimental evolution in seed beetles. Replicate lines, maintained for 35 generations of either enforced monogamy (eliminating sexual selection) or polygamy, were challenged to locate and discriminate among mates (male assays) or host seeds (female assays) in a spatial chemosensory learning task. All lines displayed learning between trials. Moreover, polygamous males outperformed monogamous males, providing evidence that sexual selection can lead to the evolution of improved male cognition. However, there were no differences between regimes in rates of male learning, and polygamous females showed no improvement in host search and even signs of reduced learning. Hence, sexual selection increased performance in cognitively demanding mate search, but it did not lead to general increases in cognitive abilities. We discuss the possibility that sexually antagonistic selection is an important factor maintaining abundant genetic variation in cognitive traits.

Sexual antagonism drives the displacement of polymorphism across gene regulatory cascades

Males and females have different reproductive roles and are often subject to contrasting selection pressures. This sexual antagonism can lead, at a given locus, to different alleles being favoured in each sex and, consequently, to genetic variation being maintained in a population. Although the presence of sexually antagonistic (SA) polymorphisms has been documented across a range of species, their evolutionary dynamics remain poorly understood. Here, we study SA selection on gene expression, which is fundamental to sexual dimorphism, via the evolution of regulatory binding sites. We show that for sites longer than 1 nucleotide, expression polymorphism is maintained only when intermediate expression levels are deleterious to both sexes. We then show that, in a regulatory cascade, expression polymorphism tends to become displaced over evolutionary time from the target of SA selection to upstream regulators. Our results have consequences for understanding the evolution of sexual dimorphism, and provide specific empirical predictions for the regulatory architecture of genes under SA selection.

Meta-analytic evidence that sexual selection improves population fitness

Sexual selection has manifold ecological and evolutionary consequences, making its net effect on population fitness difficult to predict. A powerful empirical test is to experimentally manipulate sexual selection and then determine how population fitness evolves. Here, we synthesise 459 effect sizes from 65 experimental evolution studies using meta-analysis. We find that sexual selection on males tends to elevate the mean and reduce the variance for many fitness traits, especially in females and in populations evolving under stressful conditions. Sexual selection had weaker effects on direct measures of population fitness such as extinction rate and proportion of viable offspring, relative to traits that are less closely linked to population fitness. Overall, we conclude that the beneficial population-level consequences of sexual selection typically outweigh the harmful ones and that the effects of sexual selection can differ between sexes and environments. We discuss the implications of these results for conservation and evolutionary biology.

Sexual selection has the potential to either increase or decrease absolute fitness. Here, Cally et al. perform a meta-analysis of 65 experimental evolution studies and find that sexual selection on males tends to increase fitness, especially in females evolving under stressful conditions.

Genome-wide sexually antagonistic variants reveal long-standing constraints on sexual dimorphism in fruit flies

The evolution of sexual dimorphism is constrained by a shared genome, leading to ‘sexual antagonism’, in which different alleles at given loci are favoured by selection in males and females. Despite its wide taxonomic incidence, we know little about the identity, genomic location, and evolutionary dynamics of antagonistic genetic variants. To address these deficits, we use sex-specific fitness data from 202 fully sequenced hemiclonal Drosophila melanogaster fly lines to perform a genome-wide association study (GWAS) of sexual antagonism. We identify approximately 230 chromosomal clusters of candidate antagonistic single nucleotide polymorphisms (SNPs). In contradiction to classic theory, we find no clear evidence that the X chromosome is a hot spot for sexually antagonistic variation. Characterising antagonistic SNPs functionally, we find a large excess of missense variants but little enrichment in terms of gene function. We also assess the evolutionary persistence of antagonistic variants by examining extant polymorphism in wild D. melanogaster populations and closely related species. Remarkably, antagonistic variants are associated with multiple signatures of balancing selection across the D. melanogaster distribution range and in their sister species D. simulans, indicating widespread and evolutionarily persistent (about 1 million years) genomic constraints on the evolution of sexual dimorphism. Based on our results, we propose that antagonistic variation accumulates because of constraints on the resolution of sexual conflict over protein coding sequences, thus contributing to the long-term maintenance of heritable fitness variation.

This study characterises antagonistic loci across the genome of the fruit fly Drosophila melanogaster, finding them to be preferentially associated with variation in coding sequences and to be selectively maintained across worldwide populations of D. melanogaster, and even its sister species D. simulans.

Male responses to sperm competition when rivals vary in number and familiarity

Males of many species adjust their reproductive investment to the number of rivals present simultaneously. However, few studies have investigated whether males sum previous encounters with rivals, and the total level of competition has never been explicitly separated from social familiarity. Social familiarity can be an important component of kin recognition and has been suggested as a cue that males use to avoid harming females when competing with relatives. Previous work has succeeded in independently manipulating social familiarity and relatedness among rivals, but experimental manipulations of familiarity are confounded with manipulations of the total number of rivals that males encounter. Using the seed beetle Callosobruchus maculatus, we manipulated three factors: familiarity among rival males, the maximum number of rivals encountered simultaneously and the total number of rivals encountered over a 48 h period. Males produced smaller ejaculates when exposed to more rivals in total, regardless of the maximum number of rivals they encountered simultaneously. Males did not respond to familiarity. Our results demonstrate that males of this species can sum the number of rivals encountered over separate days, and therefore the confounding of familiarity with the total level of competition in previous studies should not be ignored.

Male-limited secondary sexual trait interacts with environment in determining female fitness

Selection for secondary sexual trait (SST) elaboration may increase intralocus sexual conflict over the optimal values of traits expressed from shared genomes. This conflict can reduce female fitness, and the resulting gender load can be exacerbated by environmental stress, with consequences for a population's ability to adapt to novel environments. However, how the evolution of SSTs interacts with environment in determining female fitness is not well understood. Here, we investigated this question using replicate lines of bulb mites selected for increased or decreased prevalence of a male SST-thickened legs used as weapons. The fitness of females from these lines was measured at a temperature to which the mites were adapted (24°C), as well as at two novel temperatures: 18°C and 28°C. We found the prevalence of the SST interacted with temperature in determining female fecundity. At 28°C, females from populations with high SST prevalence were less fecund than females from populations in which the SST was rare, but the reverse was true at 18°C. Thus, a novel environment does not universally depress female fitness more in populations with a high degree of sexually selected dimorphism. We discuss possible consequences of the interaction we detected for adaptation to novel environments.

Temperature effects on life-history trade-offs, germline maintenance and mutation rate under simulated climate warming

Mutation has a fundamental influence over evolutionary processes, but how evolutionary processes shape mutation rate remains less clear. In asexual unicellular organism, increased mutation rates have been observed in stressful environments and the reigning paradigm ascribes this increase to selection for evolvability. However, this explanation does not apply in sexually reproducing species, where little is known about how the environment affects mutation rate. Here we challenged experimental lines of seed beetle, evolved at ancestral temperature or under simulated climate warming, to repair induced mutations at ancestral and stressful temperature. Results show that temperature stress causes individuals to pass on a greater mutation load to their grand-offspring. This suggests that stress-induced mutation rates, in unicellular and multicellular organisms alike, can result from compromised germline DNA repair in low condition individuals. Moreover, lines adapted to simulated climate warming had evolved increased longevity at the cost of reproduction, and this allocation decision improved germline repair. These results suggest that mutation rates can be modulated by resource allocation trade-offs encompassing life-history traits and the germline and have important implications for rates of adaptation and extinction as well as our understanding of genetic diversity in multicellular organisms.

Transgenerational effects of maternal sexual interactions in seed beetles

Mating often bears large costs to females, especially in species with high levels of sexual conflict over mating rates. Given the direct costs to females associated with multiple mating, which include reductions in lifespan and lifetime reproductive success, past research focused on identifying potential indirect benefits (through increases in offspring fitness) that females may accrue. Far less attention has, however, been devoted to understanding how costs of sexual interactions to females may extend across generations. Hence, little is known about the transgenerational implications of variation in mating rates, or the net consequences of maternal sexual activities across generations. Using the seed beetle, Callosobruchus maculatus, a model system for the study of sexual conflict, we investigate the effects of mating with multiple males versus a single male, and tease apart effects due to sexual harassment and those due to mating per se, over three generations. A multigenerational analysis indicated that females that were exposed to ongoing sexual harassment and who also were permitted to mate with multiple males showed no difference in net fitness compared to females that mated just once without ongoing harassment. Intriguingly, however, females that were continually harassed, but permitted to mate just once, suffered a severe decline in net fitness compared to females that were singly (not harassed) or multiply mated (harassed, but potentially gaining benefits via mating with multiple males). Overall, the enhanced fitness in multiply mated compared to harassed females may indicate that multiple mating confers transgenerational benefits. These benefits may counteract, but do not exceed (i.e., we found no difference between singly and multiply mated females), the large transgenerational costs of harassment. Our study highlights the importance of examining transgenerational effects from an inclusive (looking at both indirect benefits but also costs) perspective, and the need to investigate transgenerational effects across several generations if we are to fully understand the consequences of sexual interactions, sexual conflict evolution, and the interplay of sexual conflict and multi-generational costs and benefits.

Evolutionary genetics of personality in the Trinidadian guppy II: sexual dimorphism and genotype-by-sex interactions

Sexual dimorphism in behaviour and personality has been identified in a number of species, but few studies have assessed the extent of shared genetic architecture across the sexes. Under sexually antagonistic selection, mechanisms are expected to evolve that reduce evolutionary conflict, resulting in genotype-by-sex (GxS) interactions. Here we assess the extent of sexual dimorphism in four risk-taking behaviour traits in the Trinidadian guppy, Poecilia reticulata, and apply a multivariate approach to test for GxS interactions. We also quantify the among-individual and genetic covariances between personality and size and growth, which are known a priori to differ between the sexes. We found significant sexual dimorphism in three of the four behaviours, although r_mf between sex-specific homologous traits was significantly <+1 for only one behaviour. Using multivariate models, we then estimated sex-specific genetic (co)variance matrices (G_m and G_f) and tested for asymmetry of the cross-trait cross-sex genetic covariance structure (submatrix B). While G_m and G_f were not significantly different from each other overall, their respective leading eigenvectors were poorly aligned. Statistical support for asymmetry in B was found, but limited to a single trait pair for which the cross-sex covariances differed (i.e., COV_A(m,f) ≠ COV_A(f,m)). Thus, while single- and multi-trait perspectives evidence some GxS, the overall picture is one of similarity between the sexes in their genetic (co)variance structures. Our results suggest behavioural traits related to risk-taking may lack the sex-specific genetic architecture for further dimorphism to evolve under what is hypothesised to be antagonistic selection.

Evolution of sex-specific pace-of-life syndromes: genetic architecture and physiological mechanisms

Sex differences in life history, physiology, and behavior are nearly ubiquitous across taxa, owing to sex-specific selection that arises from different reproductive strategies of the sexes. The pace-of-life syndrome (POLS) hypothesis predicts that most variation in such traits among individuals, populations, and species falls along a slow-fast pace-of-life continuum. As a result of their different reproductive roles and environment, the sexes also commonly differ in pace-of-life, with important consequences for the evolution of POLS. Here, we outline mechanisms for how males and females can evolve differences in POLS traits and in how such traits can covary differently despite constraints resulting from a shared genome. We review the current knowledge of the genetic basis of POLS traits and suggest candidate genes and pathways for future studies. Pleiotropic effects may govern many of the genetic correlations, but little is still known about the mechanisms involved in trade-offs between current and future reproduction and their integration with behavioral variation. We highlight the importance of metabolic and hormonal pathways in mediating sex differences in POLS traits; however, there is still a shortage of studies that test for sex specificity in molecular effects and their evolutionary causes. Considering whether and how sexual dimorphism evolves in POLS traits provides a more holistic framework to understand how behavioral variation is integrated with life histories and physiology, and we call for studies that focus on examining the sex-specific genetic architecture of this integration.

Individual biological sensitivity to environmental influences: testing the differential susceptibility properties of the 5HTTLPR polymorphism in relation to depressive symptoms and delinquency in two adolescent general samples

The gene–environment interaction research field in psychiatry has traditionally been dominated by the diathesis–stress framework, where certain genotypes are assumed to confer increased risk for adverse outcomes in a stressful environment. In later years, theories of differential susceptibility, or biological sensitivity, suggest that candidate genes that interact with environmental events do not exclusively confer a risk for behavioural or psychiatric disorders but rather seem to alter the sensitivity to both positive and negative environmental influences. The present study investigates the susceptibility properties of the serotonin transporter-linked polymorphic region (5HTTLPR) in relation to depressive symptoms and delinquency in two separate adolescent community samples: n = 1457, collected in 2006; and n = 191, collected in 2001. Two-, three-, and four-way interactions between the 5HTTLPR, positive and negative family environment, and sex were found in relation to both depressive symptoms and delinquency. However, the susceptibility properties of the 5HTTLPR were distinctly less pronounced in relation to depressive symptoms. If the assumption that the 5HTTLPR induces differential susceptibility to both positive and negative environmental influences is correct, the previous failures to measure and control for positive environmental factors might be a possible explanation for former inconsistent findings within the research field.

Males harm females less when competing with familiar relatives

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

The Evolution of Dark Matter in the Mitogenome of Seed Beetles

Animal mitogenomes are generally thought of as being economic and optimized for rapid replication and transcription. We use long-read sequencing technology to assemble the remarkable mitogenomes of four species of seed beetles. These are the largest circular mitogenomes ever assembled in insects, ranging from 24,496 to 26,613 bp in total length, and are exceptional in that some 40% consists of non-coding DNA. The size expansion is due to two very long intergenic spacers (LIGSs), rich in tandem repeats. The two LIGSs are present in all species but vary greatly in length (114-10,408 bp), show very low sequence similarity, divergent tandem repeat motifs, a very high AT content and concerted length evolution. The LIGSs have been retained for at least some 45 my but must have undergone repeated reductions and expansions, despite strong purifying selection on protein coding mtDNA genes. The LIGSs are located in two intergenic sites where a few recent studies of insects have also reported shorter LIGSs (>200 bp). These sites may represent spaces that tolerate neutral repeat array expansions or, alternatively, the LIGSs may function to allow a more economic translational machinery. Mitochondrial respiration in adult seed beetles is based almost exclusively on fatty acids, which reduces the need for building complex I of the oxidative phosphorylation pathway (NADH dehydrogenase). One possibility is thus that the LIGSs may allow depressed transcription of NAD genes. RNA sequencing showed that LIGSs are partly transcribed and transcriptional profiling suggested that all seven mtDNA NAD genes indeed show low levels of transcription and co-regulation of transcription across sexes and tissues.

The evolution of between-species reproductive interference capability under different within-species mating regimes

Sexual selection sometimes favors male traits that benefit their bearers, but harm their mates. The harmful effects of male traits may also extend to females of other species via heterospecific mating interactions. This could affect the coexistence of closely related species during secondary contact. We examined the evolution of the interspecific interfering capability of a beetle (Callosobruchus chinensis) with a congener (C. maculatus) using C. chinensis males reared under conditions of monogamy and polygamy for 17 generations. After experimental evolution, C. chinensis males reared under polygamous conditions imposed greater impacts on offspring production by C. maculatus females than did C. chinensis males reared under monogamous conditions. However, the mechanism by which differential mating regimes altered the effect of C. chinensis males on C. maculatus females was unclear, because we did not find evidence for the expected genital evolution in C. chinensis, despite their body size divergence. Our findings suggest that traits that originally evolved through sexual selection in two allopatric species could influence the coexistence of these species or the likelihood of reinforcement during secondary contact.

The effects of stress and sex on selection, genetic covariance, and the evolutionary response

The capacity of a population to adapt to selection (evolvability) depends on whether the structure of genetic variation permits the evolution of fitter trait combinations. Selection, genetic variance and genetic covariance can change under environmental stress, and males and females are not genetically independent, yet the combined effects of stress and dioecy on evolvability are not well understood. Here, we estimate selection, genetic (co)variance and evolvability in both sexes of Tribolium castaneum flour beetles under stressful and benign conditions, using a half-sib breeding design. Although stress uncovered substantial latent heritability, stress also affected genetic covariance, such that evolvability remained low under stress. Sexual selection on males and natural selection on females favoured a similar phenotype, and there was positive intersex genetic covariance. Consequently, sexual selection on males augmented adaptation in females, and intralocus sexual conflict was weak or absent. This study highlights that increased heritability does not necessarily increase evolvability, suggests that selection can deplete genetic variance for multivariate trait combinations with strong effects on fitness, and tests the recent hypothesis that sexual conflict is weaker in stressful or novel environments.

Male-benefit sexually antagonistic genotypes show elevated vulnerability to inbreeding

Background

There is theoretical and empirical evidence for strong sexual selection in males having positive effects on population viability by serving to purify the genome of its mutation load at a low demographic cost. However, there is also theoretical and empirical evidence for negative effects of sexual selection on female fitness, and therefore population viability, known as the gender load. This can take the form of sexually antagonistic (SA) genetic variation where alleles with a selective advantage in males pose a detriment to female fitness, and vice versa. Here, using seed beetles, we shed light on a previously unexplored manifestation of the gender load: the effect of SA genetic variation on tolerance to inbreeding.

Results

We found that genotypes encoding high male, but low female fitness exhibited significantly greater rates of extinction upon enforced inbreeding relative to genotypes encoding high female but low male fitness. Also, genotypes encoding low fitness in both sexes exhibited greater rates of extinction relative to generally high-fitness genotypes (though marginally non-significant), an expected finding attributable to variation in mutation load across genotypes. Despite follow-up investigations aiming to identify the mechanism(s) underlying these findings, it remains unclear whether the gender load and the mutation load have independent consequences for tolerance to inbreeding, or whether these two types of genetic architecture interact epistatically to render male-benefit genetic variation relatively intolerant to inbreeding.

Conclusions

Regardless of the underlying mechanism(s), our results show that male-benefit/female-detriment SA genetic variation poses a previously unseen detriment to population viability due to its elevated vulnerability to inbreeding/homozygosity. This suggests that sexual selection in the context of SA genetic variance for fitness may enhance the gender load on population viability more than previously appreciated, due to selecting for male-benefit SA genetic variation that engenders lineages to extinction upon inbreeding. We note that our results imply that SA alleles that are sexually selected for in males may be underrepresented or even lacking in panels of inbred lines.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0981-4) contains supplementary material, which is available to authorized users.

Sex-specific lifespan and its evolution in nematodes

Differences between sexes of the same species in lifespan and aging rate are widespread. While the proximal and evolutionary causes of aging are well researched, the factors that contribute to sex differences in these traits have been less studied. The striking diversity of nematodes provides ample opportunity to study variation in sex-specific lifespan patterns associated with shifts in life history and mating strategy. Although the plasticity of these sex differences will make it challenging to generalize from invertebrate to vertebrate systems, studies in nematodes have enabled empirical evaluation of predictions regarding the evolution of lifespan. These studies have highlighted how natural and sexual selection can generate divergent patterns of lifespan if the sexes are subject to different rates or sources of mortality, or if trade-offs between complex traits and longevity are resolved differently in each sex. Here, we integrate evidence derived mainly from nematodes that addresses the molecular and evolutionary basis of sex-specific aging and lifespan. Ultimately, we hope to generate a clearer picture of current knowledge in this area, and also highlight the limitations of our understanding.

Life history evolution, reproduction, and the origins of sex-dependent aging and longevity

Males and females in many species differ in how they age and how long they live. These differences have motivated much research, concerning both their evolution and the underlying mechanisms that cause them. We review how differences in male and female life histories have evolved to shape patterns of aging and some of the mechanisms and pathways involved. We pay particular attention to three areas where considerable potential for synergy between mechanistic and evolutionary research exists: (1) the role of estrogens, androgens, the growth hormone/insulin-like growth factor 1 pathway, and the mechanistic target of rapamycin signaling pathway in sex-dependent growth and reproduction; (2) sexual conflict over mating rate and fertility, and how mate presence or mating can become an avenue for males and females to directly affect each other's life span; and (3) the link between dietary restriction and aging, and the emerging understanding that only the restriction of certain nutrients is involved and that this is linked to reproduction. We suggest that ideas about life histories, sex-dependent selection, and sexual conflict can inform and be informed by the ever more refined and complex understanding of the mechanisms that cause aging.