Environment-dependent intralocus sexual conflict in a dioecious plant

Dominance reversals: the resolution of genetic conflict and maintenance of genetic variation

Beneficial reversals of dominance reduce the costs of genetic trade-offs and can enable selection to maintain genetic variation for fitness. Beneficial dominance reversals are characterized by the beneficial allele for a given context (e.g. habitat, developmental stage, trait or sex) being dominant in that context but recessive where deleterious. This context dependence at least partially mitigates the fitness consequence of heterozygotes carrying one non-beneficial allele for their context and can result in balancing selection that maintains alternative alleles. Dominance reversals are theoretically plausible and are supported by mounting empirical evidence. Here, we highlight the importance of beneficial dominance reversals as a mechanism for the mitigation of genetic conflict and review the theory and empirical evidence for them. We identify some areas in need of further research and development and outline three methods that could facilitate the identification of antagonistic genetic variation (dominance ordination, allele-specific expression and allele-specific ATAC-Seq (assay for transposase-accessible chromatin with sequencing)). There is ample scope for the development of new empirical methods as well as reanalysis of existing data through the lens of dominance reversals. A greater focus on this topic will expand our understanding of the mechanisms that resolve genetic conflict and whether they maintain genetic variation.

Allelochemicals and soil microorganisms jointly mediate sex-specific belowground interactions in dioecious Populus cathayana

Little is known about how sex differences in root zone characteristics, such as contents of allelochemicals and soil microbial composition, mediate intra- and intersexual interactions in dioecious plants. We examined the processes and mechanisms of sex-specific belowground interactions mediated by allelochemicals and soil microorganisms in Populus cathayana females and males in replicated 30-yr-old experimental stands in situ and in a series of controlled experiments. Female roots released a greater amount and more diverse phenolic allelochemicals into the soil environment, resulting in growth inhibition of the same sex neighbors and deterioration of the community of soil microorganisms. When grown with males, the growth of females was consistently enhanced, especially the root growth. Compared with female monocultures, the presence of males reduced the total phenolic accumulation in the soil, resulting in a shift from allelopathic inhibition to chemical facilitation. This association was enhanced by a favorable soil bacterial community and increased bacterial diversity, and it induced changes in the orientation of female roots. Our study highlighted a novel mechanism that enhances female performance by males through alterations in the allelochemical content and soil microbial composition. The possibility to improve productivity by chemical mediation provides novel opportunities for managing plantations of dioecious plants.

The roles of sexual selection and sexual conflict in shaping patterns of genome and transcriptome variation

Sexual dimorphism is one of the most prevalent, and often the most extreme, examples of phenotypic variation within species, and arises primarily from genomic variation that is shared between females and males. Many sexual dimorphisms arise through sex differences in gene expression, and sex-biased expression is one way that a single, shared genome can generate multiple, distinct phenotypes. Although many sexual dimorphisms are expected to result from sexual selection, and many studies have invoked the possible role of sexual selection to explain sex-specific traits, the role of sexual selection in the evolution of sexually dimorphic gene expression remains difficult to differentiate from other forms of sex-specific selection. In this Review, we propose a holistic framework for the study of sex-specific selection and transcriptome evolution. We advocate for a comparative approach, across tissues, developmental stages and species, which incorporates an understanding of the molecular mechanisms, including genomic variation and structure, governing gene expression. Such an approach is expected to yield substantial insights into the evolution of genetic variation and have important applications in a variety of fields, including ecology, evolution and behaviour.

Polygenic signals of sex differences in selection in humans from the UK Biobank

Sex differences in the fitness effects of genetic variants can influence the rate of adaptation and the maintenance of genetic variation. For example, "sexually antagonistic" (SA) variants, which are beneficial for one sex and harmful for the other, can both constrain adaptation and increase genetic variability for fitness components such as survival, fertility, and disease susceptibility. However, detecting variants with sex-differential fitness effects is difficult, requiring genome sequences and fitness measurements from large numbers of individuals. Here, we develop new theory for studying sex-differential selection across a complete life cycle and test our models with genotypic and reproductive success data from approximately 250,000 UK Biobank individuals. We uncover polygenic signals of sex-differential selection affecting survival, reproductive success, and overall fitness, with signals of sex-differential reproductive selection reflecting a combination of SA polymorphisms and sexually concordant polymorphisms in which the strength of selection differs between the sexes. Moreover, these signals hold up to rigorous controls that minimise the contributions of potential confounders, including sequence mapping errors, population structure, and ascertainment bias. Functional analyses reveal that sex-differentiated sites are enriched in phenotype-altering genomic regions, including coding regions and loci affecting a range of quantitative traits. Population genetic analyses show that sex-differentiated sites exhibit evolutionary histories dominated by genetic drift and/or transient balancing selection, but not long-term balancing selection, which is consistent with theoretical predictions of effectively weak SA balancing selection in historically small populations. Overall, our results are consistent with polygenic sex-differential-including SA-selection in humans. Evidence for sex-differential selection is particularly strong for variants affecting reproductive success, in which the potential contributions of nonrandom sampling to signals of sex differentiation can be excluded.

Ecology and the evolution of sex chromosomes

Sex chromosomes are common features of animal genomes, often carrying a sex determination gene responsible for initiating the development of sexually dimorphic traits. The specific chromosome that serves as the sex chromosome differs across taxa as a result of fusions between sex chromosomes and autosomes, along with sex chromosome turnover-autosomes becoming sex chromosomes and sex chromosomes 'reverting' back to autosomes. In addition, the types of genes on sex chromosomes frequently differ from the autosomes, and genes on sex chromosomes often evolve faster than autosomal genes. Sex-specific selection pressures, such as sexual antagonism and sexual selection, are hypothesized to be responsible for sex chromosome turnovers, the unique gene content of sex chromosomes and the accelerated evolutionary rates of genes on sex chromosomes. Sex-specific selection has pronounced effects on sex chromosomes because their sex-biased inheritance can tilt the balance of selection in favour of one sex. Despite the general consensus that sex-specific selection affects sex chromosome evolution, most population genetic models are agnostic as to the specific sources of these sex-specific selection pressures, and many of the details about the effects of sex-specific selection remain unresolved. Here, I review the evidence that ecological factors, including variable selection across heterogeneous environments and conflicts between sexual and natural selection, can be important determinants of sex-specific selection pressures that shape sex chromosome evolution. I also explain how studying the ecology of sex chromosome evolution can help us understand important and unresolved aspects of both sex chromosome evolution and sex-specific selection.

Evolutionary genetics: Dissecting a sexually antagonistic polymorphism

Males and females experience divergent selection on many shared traits, which can lead to 'sexual antagonism' - opposing fitness effects of genetic variants in each sex. A new study in the fly Drosophila serrata links sexually antagonistic selection on cuticular hydrocarbons to a single major-effect gene.

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.

Are plant and animal sex chromosomes really all that different?

Sex chromosomes in plants have often been contrasted with those in animals with the goal of identifying key differences that can be used to elucidate fundamental evolutionary properties. For example, the often homomorphic sex chromosomes in plants have been compared to the highly divergent systems in some animal model systems, such as birds, Drosophila and therian mammals, with many hypotheses offered to explain the apparent dissimilarities, including the younger age of plant sex chromosomes, the lesser prevalence of sexual dimorphism, or the greater extent of haploid selection. Furthermore, many plant sex chromosomes lack complete sex chromosome dosage compensation observed in some animals, including therian mammals, Drosophila, some poeciliids, and Anolis, and plant dosage compensation, where it exists, appears to be incomplete. Even the canonical theoretical models of sex chromosome formation differ somewhat between plants and animals. However, the highly divergent sex chromosomes observed in some animal groups are actually the exception, not the norm, and many animal clades are far more similar to plants in their sex chromosome patterns. This begs the question of how different are plant and animal sex chromosomes, and which of the many unique properties of plants would be expected to affect sex chromosome evolution differently than animals? In fact, plant and animal sex chromosomes exhibit more similarities than differences, and it is not at all clear that they differ in terms of sexual conflict, dosage compensation, or even degree of divergence. Overall, the largest difference between these two groups is the greater potential for haploid selection in plants compared to animals. This may act to accelerate the expansion of the non-recombining region at the same time that it maintains gene function within it. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

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.

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 FST-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 FST elevations consistent with both X-linked and autosomal SA polymorphisms affecting reproductive success in humans. However, the extent of FST 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.

Thermal tolerance and preference are both consistent with the clinal distribution of house fly proto-Y chromosomes

Selection pressures can vary within localized areas and across massive geographical scales. Temperature is one of the best studied ecologically variable abiotic factors that can affect selection pressures across multiple spatial scales. Organisms rely on physiological (thermal tolerance) and behavioral (thermal preference) mechanisms to thermoregulate in response to environmental temperature. In addition, spatial heterogeneity in temperatures can select for local adaptation in thermal tolerance, thermal preference, or both. However, the concordance between thermal tolerance and preference across genotypes and sexes within species and across populations is greatly understudied. The house fly, Musca domestica, is a well-suited system to examine how genotype and environment interact to affect thermal tolerance and preference. Across multiple continents, house fly males from higher latitudes tend to carry the male-determining gene on the Y chromosome, whereas those from lower latitudes usually have the male determiner on the third chromosome. We tested whether these two male-determining chromosomes differentially affect thermal tolerance and preference as predicted by their geographical distributions. We identify effects of genotype and developmental temperature on male thermal tolerance and preference that are concordant with the natural distributions of the chromosomes, suggesting that temperature variation across the species range contributes to the maintenance of the polymorphism. In contrast, female thermal preference is bimodal and largely independent of congener male genotypes. These sexually dimorphic thermal preferences suggest that temperature-dependent mating dynamics within populations could further affect the distribution of the two chromosomes. Together, the differences in thermal tolerance and preference across sexes and male genotypes suggest that different selection pressures may affect the frequencies of the male-determining chromosomes across different spatial scales.

Sexual conflict in a changing environment

Sexual conflict has extremely important consequences for various evolutionary processes including its effect on local adaptation and extinction probability during environmental change. The awareness that the intensity and dynamics of sexual conflict is highly dependent on the ecological setting of a population has grown in recent years, but much work is yet to be done. Here, we review progress in our understanding of the ecology of sexual conflict and how the environmental sensitivity of such conflict feeds back into population adaptivity and demography, which, in turn, determine a population's chances of surviving a sudden environmental change. We link two possible forms of sexual conflict - intralocus and interlocus sexual conflict - in an environmental context and identify major gaps in our knowledge. These include sexual conflict responses to fluctuating and oscillating environmental changes and its influence on the interplay between interlocus and intralocus sexual conflict, among others. We also highlight the need to move our investigations into more natural settings and to investigate sexual conflict dynamics in wild populations.

Adaptive and nonadaptive causes of heterogeneity in genetic differentiation across the Mimulus guttatus genome

Genetic diversity becomes structured among populations over time due to genetic drift and divergent selection. Although population structure is often treated as a uniform underlying factor, recent resequencing studies of wild populations have demonstrated that diversity in many regions of the genome may be structured quite dissimilar to the genome-wide pattern. Here, we explored the adaptive and nonadaptive causes of such genomic heterogeneity using population-level, whole genome resequencing data obtained from annual Mimulus guttatus individuals collected across a rugged environment landscape. We found substantial variation in how genetic differentiation is structured both within and between chromosomes, although, in contrast to other studies, known inversion polymorphisms appear to serve only minor roles in this heterogeneity. In addition, much of the genome can be clustered into eight among-population genetic differentiation patterns, but only two of these clusters are particularly consistent with patterns of isolation by distance. By performing genotype-environment association analysis, we also identified genomic intervals where local adaptation to specific climate factors has accentuated genetic differentiation among populations, and candidate genes in these windows indicate climate adaptation may proceed through changes affecting specialized metabolism, drought resistance, and development. Finally, by integrating our findings with previous studies, we show that multiple aspects of plant reproductive biology may be common targets of balancing selection and that variants historically involved in climate adaptation among populations have probably also fuelled rapid adaptation to microgeographic environmental variation within sites.

Sex-specific spatial variation in fitness in the highly dimorphic Leucadendron rubrum

Sexual dimorphism in plants may emerge as a result of sex-specific selection on traits enhancing access to nutritive resources and/or to sexual partners. Here we investigated sex-specific differences in selection of sexually dimorphic traits and in the spatial distribution of effective fecundity (our fitness proxy) in a highly dimorphic dioecious wind-pollinated shrub, Leucadendron rubrum. In particular, we tested for the effect of density on male and female effective fecundity. We used spatial and genotypic data of parent and offspring cohorts to jointly estimate individual male and female effective fecundity on the one hand and pollen and seed dispersal kernels on the other hand. This methodology was adapted to the case of dioecious species. Explicitly modelling dispersal avoids the confounding effects of heterogeneous spatial distribution of mates and sampled seedlings on the estimation of effective fecundity. We also estimated selection gradients on plant traits while modelling sex-specific spatial autocorrelation in fecundity. Males exhibited spatial autocorrelation in effective fecundity at a smaller scale than females. A higher local density of plants was associated with lower effective fecundity in males but was not related to female effective fecundity. These results suggest sex-specific sensitivities to environmental heterogeneity in L. rubrum. Despite these sexual differences, we found directional selection for wider canopies and smaller leaves in both sexes, and no sexually antagonistic selection on strongly dimorphic traits in L. rubrum. Many empirical studies in animals similarly failed to detect sexually antagonistic selection in species expressing strong sexual dimorphism, and we discuss reasons explaining this common pattern.

The search for sexually antagonistic genes: Practical insights from studies of local adaptation and statistical genomics

Sexually antagonistic (SA) genetic variation-in which alleles favored in one sex are disfavored in the other-is predicted to be common and has been documented in several animal and plant populations, yet we currently know little about its pervasiveness among species or its population genetic basis. Recent applications of genomics in studies of SA genetic variation have highlighted considerable methodological challenges to the identification and characterization of SA genes, raising questions about the feasibility of genomic approaches for inferring SA selection. The related fields of local adaptation and statistical genomics have previously dealt with similar challenges, and lessons from these disciplines can therefore help overcome current difficulties in applying genomics to study SA genetic variation. Here, we integrate theoretical and analytical concepts from local adaptation and statistical genomics research-including F ST and F IS statistics, genome-wide association studies, pedigree analyses, reciprocal transplant studies, and evolve-and-resequence experiments-to evaluate methods for identifying SA genes and genome-wide signals of SA genetic variation. We begin by developing theoretical models for between-sex F ST and F IS, including explicit null distributions for each statistic, and using them to critically evaluate putative multilocus signals of sex-specific selection in previously published datasets. We then highlight new statistics that address some of the limitations of F ST and F IS, along with applications of more direct approaches for characterizing SA genetic variation, which incorporate explicit fitness measurements. We finish by presenting practical guidelines for the validation and evolutionary analysis of candidate SA genes and discussing promising empirical systems for future work.

Rapid reversal of a potentially constraining genetic covariance between leaf and flower traits in Silene latifolia

Genetic covariance between two traits generates correlated responses to selection, and may either enhance or constrain adaptation. Silene latifolia exhibits potentially constraining genetic covariance between specific leaf area (SLA) and flower number in males. Flower number is likely to increase via fecundity selection but the correlated increase in SLA increases mortality, and SLA is under selection to decrease in dry habitats. We selected on trait combinations in two selection lines for four generations to test whether genetic covariance could be reduced without significantly altering trait means. In one selection line, the genetic covariance changed sign and eigenstructure changed significantly, while in the other selection line eigenstructure remained similar to the control line. Changes in genetic variance-covariance structure are therefore possible without the introduction of new alleles, and the responses we observed suggest that founder effects and changes in frequency of alleles of major effect may be acting to produce the changes.

Sexually antagonistic selection promotes genetic divergence between males and females in an ant

Genetic diversity acts as a reservoir for potential adaptations, yet selection tends to reduce this diversity over generations. However, sexually antagonistic selection (SAS) may promote diversity by selecting different alleles in each sex. SAS arises when an allele is beneficial to one sex but harmful to the other. Usually, the evolution of sex chromosomes allows each sex to independently reach different optima, thereby circumventing the constraint of a shared autosomal genome. Because the X chromosome is found twice as often in females than males, it represents a hot spot for SAS, offering a refuge for recessive male-beneficial but female-costly alleles. Hymenopteran species do not have sex chromosomes; females are diploid and males are haploid, with sex usually determined by heterozygosity at the complementary sex-determining locus. For this reason, their entire genomes display an X-linked pattern, as every chromosome is found twice as often in females than in males, which theoretically predisposes them to SAS in large parts of their genome. Here we report an instance of sexual divergence in the Hymenoptera, a sexually reproducing group that lacks sex chromosomes. In the invasive ant Nylanderia fulva, a postzygotic SAS leads daughters to preferentially carry alleles from their mothers and sons to preferentially carry alleles from their grandfathers for a substantial region (∼3%) of the genome. This mechanism results in nearly all females being heterozygous at these regions and maintains diversity throughout the population, which may mitigate the effects of a genetic bottleneck following introduction to an exotic area and enhance the invasion success of this ant.

Variation in sexual dimorphism in a wind-pollinated plant: the influence of geographical context and life-cycle dynamics

Understanding the mechanisms causing phenotypic differences between females and males has long fascinated evolutionary biologists. An extensive literature exists on animal sexual dimorphism but less information is known about sex differences in plants, particularly the extent of geographical variation in sexual dimorphism and its life-cycle dynamics. Here, we investigated patterns of genetically based sexual dimorphism in vegetative and reproductive traits of a wind-pollinated dioecious plant, Rumex hastatulus, across three life-cycle stages using open-pollinated families from 30 populations spanning the geographic range and chromosomal variation (XY and XY1 Y2 ) of the species. The direction and degree of sexual dimorphism was highly variable among populations and life-cycle stages. Sex-specific differences in reproductive function explained a significant amount of temporal change in sexual dimorphism. For several traits, geographical variation in sexual dimorphism was associated with bioclimatic parameters, likely due to the differential responses of the sexes to climate. We found no systematic differences in sexual dimorphism between chromosome races. Sex-specific trait differences in dioecious plants largely result from a balance between sexual and natural selection on resource allocation. Our results indicate that abiotic factors associated with geographical context also play a role in modifying sexual dimorphism during the plant life-cycle.

Water availability drives population divergence and sex-specific responses in a dioecious plant

PREMISE

Water availability is an important abiotic factor, resulting in differences between plant species growing in xeric and mesic habitats. Species with populations occurring in both habitat types allow examination of whether water availability has acted as a selective force at the intraspecific level. Investigating responses to water availability with a dioecious species allows determination of whether males and females, which often have different physiologies and life histories, respond differently.

METHODS

An experiment varying water availability was performed under an outdoor rain-out shelter using plants from two mesic and two xeric populations of the dioecious plant Silene latifolia. Early growth rate, flowering propensity, flower size, and specific leaf area were measured. At the end of the season, the plants were harvested, aboveground and root biomass were measured, and the total number of flowers and fruit produced were counted.

RESULTS

Compared to the two mesic populations, plants from the two xeric populations grew more slowly, were less likely to flower, took longer to flower, had thicker leaves, invested less in aboveground biomass and more in root biomass, produced fewer flowers and fruit, but were more likely to live. Many traits exhibited significant habitat type × treatment interactions. Compared to the xeric populations, males-but not females-from mesic populations had less root biomass and greatly reduced their flower production in response to low water availability.

CONCLUSIONS

Mesic and xeric populations responded in ways congruent with water availability being a selective force for among-population divergence, especially for males.

Climatic factors and species range position predict sexually antagonistic selection across taxa

Sex differences in selection are ubiquitous in sexually reproducing organisms. When the genetic basis of traits is shared between the sexes, such sexually antagonistic selection (SAS) creates a potential constraint on adaptive evolution. Theory and laboratory experiments suggest that environmental variation and the degree of local adaptation may all affect the frequency and intensity of SAS. Here, we capitalize on a large database of over 700 spatially or temporally replicated estimates of sex-specific phenotypic selection from wild populations, combined with data on microclimates and geographical range information. We performed a meta-analysis to test three predictions from SAS theory, that selection becomes more concordant between males and females: (1) in more stressful environments, (2) in more variable environments and (3) closer to the edge of the species' range. We find partial empirical support for all three predictions. Within-study analyses indicate SAS decreases in extreme environments, as indicated by a relationship with maximum temperature, minimum precipitation and evaporative potential (PET). Across studies, we found that the average level of SAS at high latitudes was lower, where environmental conditions are typically less stable. Finally, we found evidence for reduced SAS in populations that are far from the centre of their geographical range. However, and notably, we also found some evidence of reduced average strength of selection in these populations, which is in contrast to predictions from classical theoretical models on range limit evolution. Our results suggest that environmental lability and species range position predictably influence sex-specific selection and sexual antagonism in the wild.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

Linking local adaptation with the evolution of sex differences

Many conspicuous forms of evolutionary diversity occur within species. Two prominent examples include evolutionary divergence between populations differentially adapted to their local environments (local adaptation), and divergence between females and males in response to sex differences in selection (sexual dimorphism sensu lato). These two forms of diversity have inspired vibrant research programmes, yet these fields have largely developed in isolation from one another. Nevertheless, conceptual parallels between these research traditions are striking. Opportunities for local adaptation strike a balance between local selection, which promotes divergence, and gene flow-via dispersal and interbreeding between populations-which constrains it. Sex differences are similarly constrained by fundamental features of inheritance that mimic gene flow. Offspring of each sex inherit genes from same-sex and opposite-sex parents, leading to gene flow between each differentially selected half of the population, and raising the question of how sex differences arise and are maintained. This special issue synthesizes and extends emerging research at the interface between the research traditions of local adaptation and sex differences. Each field can promote understanding of the other, and interactions between local adaptation and sex differences can generate new empirical predictions about the evolutionary consequences of selection that varies across space, time, and between the sexes.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

Do metrics of sexual selection conform to Bateman's principles in a wind-pollinated plant?

Bateman's principles posit that male fitness varies more, and relies more on mate acquisition, than female fitness. While Bateman's principles should apply to any organism producing gametes of variable sizes, their application to plants is potentially complicated by the high levels of polyandry suspected for plants, and by variation in the spatial distribution of prospective mates. Here we quantify the intensity of sexual selection by classical Bateman metrics using two common gardens of the wind-pollinated dioecious plant Mercurialis annua. Consistent with Bateman's principles, males displayed significantly positive Bateman gradients (a regression of fitness on mate number), whereas the reproductive success of females was independent of their ability to access mates. A large part of male fitness was explained by their mate number, which in turn was associated with males' abilities to disperse pollen. Our results suggest that sexual selection can act in plant species in much the same way as in many animals, increasing the number of mates through traits that promote pollen dispersal.

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.

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.

Wolbachia infection can bias estimates of intralocus sexual conflict

Males and females share most of their genome and develop many of the same traits. However, each sex frequently has different optimal values for these shared traits, creating intralocus sexual conflict. This conflict has been observed in wild and laboratory populations of insects and affects important evolutionary processes such as sexual selection, the maintenance of genetic variation, and possibly even speciation. Given the broad impacts of intralocus conflict, accurately detecting and measuring it is important. A common way to detect intralocus sexual conflict is to calculate the intersexual genetic correlation for fitness, with negative values suggesting conflict. Here, we highlight a potential confounder of this measure-cytoplasmic incompatibility caused by the intracellular parasite Wolbachia. Infection with Wolbachia can generate negative intersexual genetic correlations for fitness in insects, suggestive of intralocus sexual conflict. This is because cytoplasmic incompatibility reduces the fitness of uninfected females mated to infected males, while uninfected males will not suffer reductions in fitness if they mate with infected females and may even be fitter than infected males. This can lead to strong negative intersexual genetic correlations for fitness, mimicking intralocus conflict. We illustrate this issue using simulations and then present Drosophila simulans data that show how reproductive incompatibilities caused by Wolbachia infection can generate signals of intralocus sexual conflict. Given that Wolbachia infection in insect populations is pervasive, but populations usually contain both infected and uninfected individuals providing scope for cytoplasmic incompatibility, this is an important consideration for sexual conflict research but one which, to date, has been largely underappreciated.

Sex-specific dominance reversal of genetic variation for fitness

The maintenance of genetic variance in fitness represents one of the most longstanding enigmas in evolutionary biology. Sexually antagonistic (SA) selection may contribute substantially to maintaining genetic variance in fitness by maintaining alternative alleles with opposite fitness effects in the two sexes. This is especially likely if such SA loci exhibit sex-specific dominance reversal (SSDR)-wherein the allele that benefits a given sex is also dominant in that sex-which would generate balancing selection and maintain stable SA polymorphisms for fitness. However, direct empirical tests of SSDR for fitness are currently lacking. Here, we performed a full diallel cross among isogenic strains derived from a natural population of the seed beetle Callosobruchus maculatus that is known to exhibit SA genetic variance in fitness. We measured sex-specific competitive lifetime reproductive success (i.e., fitness) in >500 sex-by-genotype F1 combinations and found that segregating genetic variation in fitness exhibited pronounced contributions from dominance variance and sex-specific dominance variance. A closer inspection of the nature of dominance variance revealed that the fixed allelic variation captured within each strain tended to be dominant in one sex but recessive in the other, revealing genome-wide SSDR for SA polymorphisms underlying fitness. Our findings suggest that SA balancing selection could play an underappreciated role in maintaining fitness variance in natural populations.

Antagonistic pleiotropy in species with separate sexes, and the maintenance of genetic variation in life-history traits and fitness

Antagonistic pleiotropy (AP)-where alleles of a gene increase some components of fitness at a cost to others-can generate balancing selection, and contribute to the maintenance of genetic variation in fitness traits, such as survival, fecundity, fertility, and mate competition. Previous theory suggests that AP is unlikely to maintain variation unless antagonistic selection is strong, or AP alleles exhibit pronounced differences in genetic dominance between the affected traits. We show that conditions for balancing selection under AP expand under the likely scenario that the strength of selection on each fitness component differs between the sexes. Our model also predicts that the vast majority of balanced polymorphisms have sexually antagonistic effects on total fitness, despite the absence of sexual antagonism for individual fitness components. We conclude that AP polymorphisms are less difficult to maintain than predicted by prior theory, even under our conservative assumption that selection on components of fitness is universally sexually concordant. We discuss implications for the maintenance of genetic variation, and for inferences of sexual antagonism that are based on sex-specific phenotypic selection estimates-many of which are based on single fitness components.

Contribution of gene flow to the evolution of recombination suppression in sex chromosomes

Polymorphism of alleles that benefit one sex but harm the other (sexually antagonistic alleles) generates selective pressures for reduced recombination between themselves and sex-determination loci. Such polymorphism can be maintained within a population when selection coefficients are sufficiently balanced between males and females. However, if regulatory mutations restrict gene expression only to one sex, these alleles become neutral in the other sex and easily fixed within a population, removing the selective pressures for recombination suppression in sex chromosomes. When there is spatial variation in selection regimes, however, alleles that are deleterious in one sex and neutral in the other can be maintained in other neighboring populations and gene flow may continuously supply deleterious alleles. We hypothesized that this maintenance of genetic variation may promote the establishment of recombination suppression in sex chromosomes even in cases where selection is limited to one sex. Using individual-based simulations, we show that spatial variation in male-limited selection and gene flow can promote the establishment of Y-autosome fusions, a special case of recombination suppression in sex chromosomes. This can be explained by the fact that fused Y-chromosomes that capture alleles that are beneficial for local males have a higher mean fitness compared to unfused Y chromosomes in the presence of deleterious gene flow. We also simulated the case of sex-concordant selection and found that gene flow of alleles that are deleterious in both sexes did not substantially increase the establishment rates of Y-autosome fusions across the parameter space examined. This can be accounted for by the fact that foreign alleles that are deleterious in both sexes can be efficiently removed from the population compared to alleles that are neutral in females. These results indicate that how gene flow affects the establishment rates of Y-autosome fusions depends largely on selection regimes. Spatial variation in sex-specific selection and gene flow should be appreciated as a factor affecting sex chromosome evolution.

Field estimates of parentage reveal sexually antagonistic selection on body size in a population of Anolis lizards

Sexual dimorphism evolves when selection favors different phenotypic optima between the sexes. Such sexually antagonistic selection creates intralocus sexual conflict when traits are genetically correlated between the sexes and have sex‐specific optima. Brown anoles are highly sexually dimorphic: Males are on average 30% longer than females and 150% heavier in our study population. Viability selection on body size is known to be sexually antagonistic, and directional selection favors large male size whereas stabilizing selection constrains females to remain small. We build on previous studies of viability selection by measuring sexually antagonistic selection using reproductive components of fitness over three generations in a natural population of brown anoles. We estimated the number of offspring produced by an individual that survived to sexual maturity (termed RS^V), a measure of individual fitness that includes aspects of both individual reproductive success and offspring survival. We found directional selection on male body size, consistent with previous studies of viability selection. However, selection on female body size varied among years, and included periods of positive directional selection, quadratic stabilizing selection, and no selection. Selection acts differently in the sexes based on both survival and reproduction and sexual conflict appears to be a persistent force in this species.

Social effects for locomotion vary between environments in Drosophila melanogaster females

Despite strong purifying or directional selection, variation is ubiquitous in populations. One mechanism for the maintenance of variation is indirect genetic effects (IGEs), as the fitness of a given genotype will depend somewhat on the genes of its social partners. IGEs describe the effect of genes in social partners on the expression of the phenotype of a focal individual. Here, we ask what effect IGEs, and variation in IGEs between abiotic environments, has on locomotion in Drosophila. This trait is known to be subject to intralocus sexually antagonistic selection. We estimate the coefficient of interaction, Ψ, using six inbred lines of Drosophila. We found that Ψ varied between abiotic environments, and that it may vary across among male genotypes in an abiotic environment specific manner. We also found evidence that social effects of males alter the value of a sexually dimorphic trait in females, highlighting an interesting avenue for future research into sexual antagonism. We conclude that IGEs are an important component of social and sexual interactions and that they vary between individuals and abiotic environments in complex ways, with the potential to promote the maintenance of phenotypic variation.

Is Multifactorial Sex Determination in the House Fly, Musca domestica (L.), Stable Over Time?

Sex determination pathways evolve rapidly, usually because of turnover of master regulatory genes at the top of the developmental pathway. Polygenic sex determination is expected to be a transient state between ancestral and derived conditions. However, polygenic sex determination has been observed in numerous animal species, including the house fly, Musca domestica House fly males carry a male-determining factor (M) that can be located on any chromosome, and an individual male may have multiple M factors. Females lack M and/or have a dominant allele of the Md-tra gene (Md-tra ^D ) that acts as a female-determining locus even in the presence of multiple copies of M. We found the frequency and linkage of M in house flies collected in Chino, CA (USA) was relatively unchanged between 1982 and 2014. The frequency of females with Md-tra ^D in the 2014 collection was 33.6% (n = 140). Analysis of these results, plus previously published data, revealed a strong correlation between the frequencies of Md-tra ^D and multiple M males, and we find that these populations are expected to have balanced sex ratios. We also find that fitness values that allow for the invasion and maintenance of multiple sex determining loci suggest that sexually antagonistic selection could be responsible for maintaining polygenic sex determination in house fly populations. The stability over time and equilibrium frequencies within populations suggest the house fly polygenic sex determination system is not in transition, and provide guidance for future investigations on the factors responsible for the polymorphism.

Rapid evolution of the intersexual genetic correlation for fitness in Drosophila melanogaster

Sexual antagonism (SA) arises when male and female phenotypes are under opposing selection, yet genetically correlated. Until resolved, antagonism limits evolution toward optimal sex‐specific phenotypes. Despite its importance for sex‐specific adaptation and existing theory, the dynamics of SA resolution are not well understood empirically. Here, we present data from Drosophila melanogaster, compatible with a resolution of SA. We compared two independent replicates of the “LH_M” population in which SA had previously been described. Both had been maintained under identical, controlled conditions, and separated for around 200 generations. Although heritabilities of male and female fitness were similar, the intersexual genetic correlation differed significantly, being negative in one replicate (indicating SA) but close to zero in the other. Using population sequencing, we show that phenotypic differences were associated with population divergence in allele frequencies at nonrandom loci across the genome. Large frequency changes were more prevalent in the population without SA and were enriched at loci mapping to genes previously shown to have sexually antagonistic relationships between expression and fitness. Our data suggest that rapid evolution toward SA resolution has occurred in one of the populations and open avenues toward studying the genetics of SA and its resolution.

The large-X effect in plants: increased species divergence and reduced gene flow on the Silene X-chromosome

The disproportionately large involvement of the X-chromosome in the isolation of closely related species (the large-X effect) has been reported for many animals, where X-linked genes are mostly hemizygous in the heterogametic sex. The expression of deleterious recessive mutations is thought to drive the frequent involvement of the X-chromosome in hybrid sterility, as well as to reduce interspecific gene flow for X-linked genes. Here, we evaluate the role of the X-chromosome in the speciation of two closely related plant species - the white and red campions (Silene latifolia and S. dioica) - that hybridize widely across Europe. The two species evolved separate sexes and sex chromosomes relatively recently (~10(7)  years), and unlike most animal species, most X-linked genes have intact Y-linked homologs. We demonstrate that the X-linked genes show a very small and insignificant amount of interspecific gene flow, while gene flow involving autosomal loci is significant and sufficient to homogenize the gene pools of the two species. These findings are consistent with the hypothesis of the large-X effect in Silene and comprise the first report of this effect in plants. Nonhemizygosity of many X-linked genes in Silene males indicates that exposure of recessive mutations to selection may not be essential for the occurrence of the large-X effect. Several possible causes of the large-X effect in Silene are discussed.

Plant contributions to our understanding of sex chromosome evolution

A minority of angiosperms have male and female flowers separated in distinct individuals (dioecy), and most dioecious plants do not have cytologically different (heteromorphic) sex chromosomes. Plants nevertheless have several advantages for the study of sex chromosome evolution, as genetic sex determination has evolved repeatedly and is often absent in close relatives. I review sex-determining regions in non-model plant species, which may help us to understand when and how (and, potentially, test hypotheses about why) recombination suppression evolves within young sex chromosomes. I emphasize high-throughput sequencing approaches that are increasingly being applied to plants to test for non-recombining regions. These data are particularly illuminating when combined with sequence data that allow phylogenetic analyses, and estimates of when these regions evolved. Together with comparative genetic mapping, this has revealed that sex-determining loci and sex-linked regions evolved independently in many plant lineages, sometimes in closely related dioecious species, and often within the past few million years. In reviewing recent progress, I suggest areas for future work, such as the use of phylogenies to allow the informed choice of outgroup species suitable for inferring the directions of changes, including testing whether Y chromosome-like regions are undergoing genetic degeneration, a predicted consequence of losing recombination.

Sex-specific phenotypic selection and geographic variation in gender divergence in a gynodioecious shrub

In sexually polymorphic plant species the extent of gender divergence in floral morphology and phenology may be influenced by gender-specific selection patterns imposed by pollinators, which may change geographically. Distribution margins are areas where changes in the pollinator fauna, and thus variation in gender divergence of floral traits, are expected. We tested for pollination-driven geographic variation in the gender divergence in floral and phenological traits in the gynodioecious shrub Daphne laureola, in core and marginal areas differing in the identity of the main pollinator. Pollinators selected for longer corolla tubes in hermaphrodite individuals only in core populations, which in turn recorded higher fruit set. Consistent with these phenotypic selection patterns, gender divergence in flower corolla length was higher in core populations. Moreover, pollinators selected towards delayed flowering on hermaphrodite individuals only in marginal populations, where the two sexes differed more in flowering time. Our results support that a shift in main pollinators is able to contribute to geographic variation in the gender divergence of sexually polymorphic plant species.

Sex-differential selection and the evolution of X inactivation strategies

X inactivation—the transcriptional silencing of one X chromosome copy per female somatic cell—is universal among therian mammals, yet the choice of which X to silence exhibits considerable variation among species. X inactivation strategies can range from strict paternally inherited X inactivation (PXI), which renders females haploid for all maternally inherited alleles, to unbiased random X inactivation (RXI), which equalizes expression of maternally and paternally inherited alleles in each female tissue. However, the underlying evolutionary processes that might account for this observed diversity of X inactivation strategies remain unclear. We present a theoretical population genetic analysis of X inactivation evolution and specifically consider how conditions of dominance, linkage, recombination, and sex-differential selection each influence evolutionary trajectories of X inactivation. The results indicate that a single, critical interaction between allelic dominance and sex-differential selection can select for a broad and continuous range of X inactivation strategies, including unequal rates of inactivation between maternally and paternally inherited X chromosomes. RXI is favored over complete PXI as long as alleles deleterious to female fitness are sufficiently recessive, and the criteria for RXI evolution is considerably more restrictive when fitness variation is sexually antagonistic (i.e., alleles deleterious to females are beneficial to males) relative to variation that is deleterious to both sexes. Evolutionary transitions from PXI to RXI also generally increase mean relative female fitness at the expense of decreased male fitness. These results provide a theoretical framework for predicting and interpreting the evolution of chromosome-wide expression of X-linked genes and lead to several useful predictions that could motivate future studies of allele-specific gene expression variation.

With the exception of its most primitive members, mammal species practice X inactivation, where one copy of each X chromosome pair is silenced in each cell of the female body. The particular copy of the X that is silenced nevertheless shows considerable variability among species, and the evolutionary causes for this variability remain unclear. Here, we show that X inactivation strategies are likely to evolve in response to the sex-differential fitness properties of X-linked genetic variation. Genetic variation with similar effects on male and female fitness will generally favor the evolution of random X inactivation, potentially including preferential inactivation of the maternally inherited X chromosome. Variation with opposing fitness effects in each sex (“sexually antagonistic” variation, which includes mutations that both decrease female fitness and enhance male fitness) selects for preferential or complete inactivation of the paternally inherited X. Paternally biased X inactivation patterns appear to be common in nature, which suggests that sexually antagonistic genetic variation might be an important factor underlying the evolution of X inactivation. The theory provides a conceptual framework for understanding the evolution of X inactivation strategies and generates several novel predictions that may soon be tested with modern genome sequencing technologies.

Genetic drift in antagonistic genes leads to divergence in sex-specific fitness between experimental populations of Drosophila melanogaster

Males and females differ in their reproductive roles and as a consequence are often under diverging selection pressures on shared phenotypic traits. Theory predicts that divergent selection can favor the invasion of sexually antagonistic alleles, which increase the fitness of one sex at the detriment of the other. Sexual antagonism can be subsequently resolved through the evolution of sex-specific gene expression, allowing the sexes to diverge phenotypically. Although sexual dimorphism is very common, recent evidence also shows that antagonistic genetic variation continues to segregate in populations of many organisms. Here we present empirical data on the interaction between sexual antagonism and genetic drift in populations that have independently evolved under standardized conditions. We demonstrate that small experimental populations of Drosophila melanogaster have diverged in male and female fitness, with some populations showing high male, but low female fitness while other populations show the reverse pattern. The between-population patterns are consistent with the differentiation in reproductive fitness being driven by genetic drift in sexually antagonistic alleles. We discuss the implications of our results with respect to the maintenance of antagonistic variation in subdivided populations and consider the wider implications of drift in fitness-related genes.

Antagonistic versus nonantagonistic models of balancing selection: characterizing the relative timescales and hitchhiking effects of partial selective sweeps

Antagonistically selected alleles--those with opposing fitness effects between sexes, environments, or fitness components--represent an important component of additive genetic variance in fitness-related traits, with stably balanced polymorphisms often hypothesized to contribute to observed quantitative genetic variation. Balancing selection hypotheses imply that intermediate-frequency alleles disproportionately contribute to genetic variance of life-history traits and fitness. Such alleles may also associate with population genetic footprints of recent selection, including reduced genetic diversity and inflated linkage disequilibrium at linked, neutral sites. Here, we compare the evolutionary dynamics of different balancing selection models, and characterize the evolutionary timescale and hitchhiking effects of partial selective sweeps generated under antagonistic versus nonantagonistic (e.g., overdominant and frequency-dependent selection) processes. We show that the evolutionary timescales of partial sweeps tend to be much longer, and hitchhiking effects are drastically weaker, under scenarios of antagonistic selection. These results predict an interesting mismatch between molecular population genetic and quantitative genetic patterns of variation. Balanced, antagonistically selected alleles are expected to contribute more to additive genetic variance for fitness than alleles maintained by classic, nonantagonistic mechanisms. Nevertheless, classical mechanisms of balancing selection are much more likely to generate strong population genetic signatures of recent balancing selection.