Genetic Variation in Male and Female Reproductive Characters Associated with Sexual Conflict in Drosophila melanogaster

Evolution of mate harm resistance in females from Drosophila melanogaster populations selected for faster development and early reproduction

Interlocus sexual conflict is predicted to result in sexually antagonistic coevolution between male competitive traits, which are also female-detrimental, and mate harm resistance (MHR) in females. Little is known about the connection between life history evolution and sexually antagonistic coevolution. Here, we investigated the evolution of MHR in a set of experimentally evolved populations, where mate-harming ability has been shown to have substantially reduced in males as a correlated response to the selection for faster development and early reproduction. We measured mortality and fecundity in females of these populations and those in their matched controls under different male exposure conditions. We observed that the evolved females were more susceptible to mate harm-suffering from significantly higher mortality under continuous exposure to control males within the 20-day assay period. Though these evolved females are known to have shorter lifespan substantially higher mortality was not observed under virgin and single-mating conditions. We used fecundity data to show that this higher mortality in the experimentally evolved females was not due to the cost of egg production and hence can only be attributed to reduced MHR. Further analysis indicated that this decreased MHR is unlikely to be due purely to the smaller size of these females. Instead, it is more likely to be an indirect experimentally evolved response attributable to the changed breeding ecology and/or male trait evolution. Our results underline the implications of changes in life history traits, including lifespan, for the evolution of MHR in females.

A genome-wide test for paternal indirect genetic effects on lifespan in Drosophila melanogaster

Exposing sires to various environmental manipulations has demonstrated that paternal effects can be non-trivial also in species where male investment in offspring is almost exclusively limited to sperm. Whether paternal effects also have a genetic component (i.e. paternal indirect genetic effects (PIGEs)) in such species is however largely unknown, primarily because of methodological difficulties separating indirect from direct effects of genes. PIGEs may nevertheless be important since they have the capacity to contribute to evolutionary change. Here we use Drosophila genetics to construct a breeding design that allows testing nearly complete haploid genomes (more than 99%) for PIGEs. Using this technique, we estimate the variance in male lifespan due to PIGEs among four populations and compare this to the total paternal genetic variance (the sum of paternal indirect and direct genetic effects). Our results indicate that a substantial part of the total paternal genetic variance results from PIGEs. A screen of 38 haploid genomes, randomly sampled from a single population, suggests that PIGEs also influence variation in lifespan within populations. Collectively, our results demonstrate that PIGEs may constitute an underappreciated source of phenotypic variation.

Genetic and social contributions to sex differences in lifespan in Drosophila serrata

Sex differences in lifespan remain an intriguing puzzle in evolutionary biology. While explanations range from sex differences in selection to sex differences in the expression of recessive lifespan‐altering mutations (via X‐linkage), little consensus has been reached. One unresolved issue is the extent to which genetic influences on lifespan dimorphism are modulated by the environment. For example, studies have shown that sex differences in lifespan can either increase or decrease depending upon the social environment. Here, we took an experimental approach, manipulating multiple axes of the social environment across inbred long‐ and short‐lived genotypes and their reciprocal F1s in the fly Drosophila serrata. Our results reveal strong genetic effects and subtle yet significant genotype‐by‐environment interactions for male and female lifespan, specifically due to both population density and mating status. Further, our data do not support the idea that unconditional expression of deleterious X‐linked recessive alleles in heterogametic males accounts for lower male lifespan.

Female mating experience and genetic background independently influence male mating success in fruit flies

When the reproductive interests of males and females conflict, males can evolve traits that are harmful to females, and females can coevolve traits to resist this harm. In the fruit fly, Drosophila melanogaster, there is genetic variation in female resistance traits, which can affect the pre- and post-mating success of males that try to mate with them. However, it is not clear to what extent the expression of these phenotypes can be modified by environmental factors such as sociosexual experience. Here, we tested how the genetic background of a female and her previous mating experience interact to affect the mating success of focal males. In the experience phase, we placed females from 28 distinct genetic backgrounds individually either with a single male (low conflict) or with three males (high conflict) for 48 hr. In the subsequent test phase, we measured the mating and post-mating fertilization success of focal males paired individually with each female. We found that focal males paired with females from the high-conflict treatment were less successful at mating, took longer to mate when they were successful, and had a lower proportion of paternity share. Furthermore, we identified significant female genetic variation associated with male mating success. These results indicate that female experience, along with intrinsic genetic factors, can independently influence different fitness components of her subsequent mates and has implications for our understanding of plastic female mating strategies and the evolution of sexually antagonistic traits in males and females.

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.

Genetic variation in male-induced harm in Drosophila melanogaster

In Drosophila melanogaster, prolonged exposure to males reduces the longevity and fecundity of females. This harm arises from the effects of male courtship behaviours and the toxic side effects of the accessory gland proteins (Acps) in their seminal fluids. Here, we examine the relationship between male exposure and its harmful effect on the lifetime fitness of his mates, and quantify the genetic basis for this variation. We found significant additive genetic variation in the magnitude of harm that males impose on females by exposing females to males from a variety of hemiclonal backgrounds for either a brief or prolonged period of time and measuring their fecundity, a meaningful fitness index. Furthermore, we discovered a strong negative correlation between the magnitude of harm and the short-term effects of male exposure on female fitness. We discuss the evolutionary significance of these results with regards to potential life-history trade-offs in females, and its relationship to male body size.

Phenotypic plasticity in female mate choice behavior is mediated by an interaction of direct and indirect genetic effects in Drosophila melanogaster

Female mate choice is a complex decision‐making process that involves many context‐dependent factors. In Drosophila melanogaster, a model species for the study of sexual selection, indirect genetic effects (IGEs) of general social interactions can influence female mate choice behaviors, but the potential impacts of IGEs associated with mating experiences are poorly understood. Here, we examined whether the IGEs associated with a previous mating experience had an effect on subsequent female mate choice behaviors and quantified the degree of additive genetic variation associated with this effect. Females from 21 different genetic backgrounds were housed with males from one of two distinct genetic backgrounds for either a short (3 hr) or long (48 hr) exposure period and their subsequent mate choice behaviors were scored. We found that the genetic identity of a previous mate significantly influenced a female's subsequent interest in males and preference of males. Additionally, a hemiclonal analysis revealed significant additive genetic variation associated with experience‐dependent mate choice behaviors, indicating a genotype‐by‐environment interaction for both of these parameters. We discuss the significance of these results with regard to the evolution of plasticity in female mate choice behaviors and the maintenance of variation in harmful male traits.

Hemiclonal analysis of interacting phenotypes in male and female Drosophila melanogaster

Background

Identifying the sources of variation in mating interactions between males and females is important because this variation influences the strength and/or the direction of sexual selection that populations experience. While the origins and effects of variation in male attractiveness and ornamentation have received much scrutiny, the causes and consequences of intraspecific variation in females have been relatively overlooked. We used cytogenetic cloning techniques developed for Drosophila melanogaster to create “hemiclonal” males and females with whom we directly observed sexual interaction between individuals of different known genetic backgrounds and measured subsequent reproductive outcomes. Using this approach, we were able to quantify the genetic contribution of each mate to the observed phenotypic variation in biologically important traits including mating speed, copulation duration, and subsequent offspring production, as well as measure the magnitude and direction of intersexual genetic correlation between female choosiness and male attractiveness.

Results

We found significant additive genetic variation contributing to mating speed that can be attributed to male genetic identity, female genetic identity, but not their interaction. Furthermore we found that phenotypic variation in copulation duration had a significant male-associated genetic component. Female genetic identity and the interaction between male and female genetic identity accounted for a substantial amount of the observed phenotypic variation in egg size. Although previous research predicts a trade-off between egg size and fecundity, this was not evident in our results. We found a strong negative genetic correlation between female choosiness and male attractiveness, a result that suggests a potentially important role for sexually antagonistic alleles in sexual selection processes in our population.

Conclusion

These results further our understanding of sexual selection because they identify that genetic identity plays a significant role in phenotypic variation in female behaviour and fecundity. This variation may be potentially due to ongoing sexual conflict found between the sexes for interacting phenotypes. Our unexpected observation of a negative correlation between female choosiness and male attractiveness highlights the need for more explicit theoretical models of genetic covariance to investigate the coevolution of female choosiness and male attractiveness.

Bending for love: losses and gains of sexual dimorphisms are strictly correlated with changes in the mounting position of sepsid flies (Sepsidae: Diptera)

BACKGROUND

Sexually dimorphic structures contribute the largest number of morphological differences between closely related insect species thus implying that these structures evolve fast and are involved in speciation. The current literature focuses on the selective forces that drive these changes, be it 'sexual conflict' or 'female choice'. However, there are only few studies examining the function of sexual dimorphisms and even fewer that investigate how functional changes influence dimorphisms. This is largely due to the paucity of taxa for which the morphology, behavior, and phylogenetic relationships for multiple species are known. Here we present such data for sepsid flies. Sepsids have starkly dimorphic forelegs whose function can be documented under laboratory conditions. We use data from 10 genes to reconstruct the phylogenetic relationships for 33 species and test whether mounting positions are correlated with the presence and absence of sexual dimorphisms in the forelegs.

RESULTS

The phylogenetic tree fully resolves the relationship with 29 of the 31 nodes of the tree having a posterior probability of 1.0. Twenty-eight of the 31 sepsid species have sexually dimorphic forelegs. All 28 species with such forelegs have the same mounting technique whereby the male uses his modified forelegs to grasp the female wingbase. Mapping mounting behavior and foreleg morphology onto the tree reveals that the wing grasp evolved once and was reduced twice. All changes in the mounting behavior are strictly and statistically significantly correlated with the origin and losses of sexually dimorphic legs (concentrated changes test: P < 0.001); i.e., the two species that have independently lost the wing grasp have both also re-evolved monomorphic legs. The wing grasp in these species is replaced with a novel but very similar mounting technique not involving the forelegs: the males bend their abdomens forward and directly establish genital contact to the female. In addition, one of the secondarily monomorphic species, Sepsis secunda, has evolved a new sexual dimorphism, a 'bump' on the dorsal side of the 4th tergite, which is now touching the ventral side of the female abdomen.

CONCLUSION

Our study reveals that the evolution of sexually dimorphic legs in Sepsidae can only be understood once the function of the legs during mating is considered and the relationships of species with and without sexual dimorphisms are known. We demonstrate that homoplasy in sexually dimorphic structures can be due to homoplasy in mating behavior. We furthermore document that the two species with secondarily monomorphic legs have independently replaced the typical sepsid wing grasp with very similar, new mounting techniques. This suggests that convergent evolution may be common in mating behaviors.

Drosophila melanogaster virgins are more likely to mate with strangers than familiar flies

Recent evidence shows that females of many species can discriminate against males and/or male phenotypes they have mated with previously. However, these studies have not tested whether actual mating is necessary to induce the avoidance behaviour. A preference for strangers may have evolved because it avoids multiple matings with similar genotypes. Alternatively, there may be selection against mating with familiar individuals directly. By choosing its first mate among unfamiliar individuals (which are less likely close relatives than are those encountered early in life), a virgin might disentangle some of the potential benefits of avoiding genetic incompatibility and inbreeding in the offspring from the costs of remating. In this study, we test whether Drosophila melanogaster flies bias their mate choice towards strangers according to previous, non-copulatory, experience. Based on 173 trials over 12 weeks, virgin females presented with two virgin males were 59% more likely to mate with a novel male than the one which she had been housed with for 8 h the day before. Hence we present the first report showing that a dipteran can distinguish between previously encountered and not previously encountered conspecifics.