Resource limitation and responses to rivals in males of the fruit fly Drosophila melanogaster

Plastic male mating behavior evolves in response to the competitive environment

Male reproductive phenotypes can evolve in response to the social and sexual environment. The expression of many such phenotypes may also be plastic within an individual's lifetime. For example, male Drosophila melanogaster show significantly extended mating duration following a period of exposure to conspecific male rivals. The costs and benefits of reproductive investment, and plasticity itself, can be shaped by the prevailing sociosexual environment and by resource availability. We investigated these ideas using experimental evolution lines of D. melanogaster evolving under three fixed sex ratios (high, medium, and low male-male competition) on either rich or poor adult diets. We found that males evolving in high-competition environments evolved longer mating durations overall. In addition, these males expressed a novel type of plastic behavioral response following exposure to rival males: they both significantly reduced and showed altered courtship delivery, and exhibited significantly longer mating latencies. Plasticity in male mating duration in response to rivals was maintained in all of the lines, suggesting that the costs of plasticity were minimal. None of the evolutionary responses tested were consistently affected by dietary resource regimes. Collectively, the results show that fixed behavioral changes and new augmentations to the repertoire of reproductive behaviors can evolve rapidly.

Sperm allocation strategies in a sperm heteromorphic insect

Theories predict that in polyandrous species, the focal male should increase sperm allocation per mate in the presence of rivals to gain greater share of paternity, but in the presence of additional mates, he should reduce sperm allocation per mate to save sperm for insemination of more mates. However, empirical findings are often inconsistent and reasons behind are unclear. Furthermore, many studies use copulation duration as an estimate of the number of sperm transferred. Yet, empirical evidence for such assumption is largely lacking. Here, we used a sperm heteromorphic insect Ephestia kuehniella whose males produce two types of sperm, eupyrenes (fertile) and apyrenes (nonfertile), to test these postulations. We allowed focal males to detect chemical and acoustic but no tactile cues from rivals or additional mates both before and during mating and measured copulation duration and sperm allocation in successive copulations. We demonstrate that males transfer significantly more eupyrenes per mate in the presence of rivals and that the sperm allocation pattern persists in successive copulations under this condition. However, males do not adjust apyrene allocation in response to rivals probably because apyrenes play a relatively minor role in male reproductive success. Contrary to a previous study, focal males do not respond to additional mates most likely due to the lack of tactile cues in the present study. We reveal that sperm allocation is not a function of copulation duration in this insect for spermatophore formation and delivery occupy most of copulation duration and sperm transfer is complete near the end of copulation.

The role of complex cues in social and reproductive plasticity

Phenotypic plasticity can be a key determinant of fitness. The degree to which the expression of plasticity is adaptive relies upon the accuracy with which information about the state of the environment is integrated. This step might be particularly beneficial when environments, e.g. the social and sexual context, change rapidly. Fluctuating temporal dynamics could increase the difficulty of determining the appropriate level of expression of a plastic response. In this review, we suggest that new insights into plastic responses to the social and sexual environment (social and reproductive plasticity) may be gained by examining the role of complex cues (those comprising multiple, distinct sensory components). Such cues can enable individuals to more accurately monitor their environment in order to respond adaptively to it across the whole life course. We briefly review the hypotheses for the evolution of complex cues and then adapt these ideas to the context of social and sexual plasticity. We propose that the ability to perceive complex cues can facilitate plasticity, increase the associated fitness benefits and decrease the risk of costly 'mismatches' between phenotype and environment by (i) increasing the robustness of information gained from highly variable environments, (ii) fine-tuning responses by using multiple strands of information and (iii) reducing time lags in adaptive responses. We conclude by outlining areas for future research that will help to determine the interplay between complex cues and plasticity.

Sex-specific effects of social isolation on ageing in Drosophila melanogaster

Social environments can have a major impact on ageing profiles in many animals. However, such patterns in variation in ageing and their underlying mechanisms are not well understood, particularly because both social contact and isolation can be stressful. Here, we use Drosophila melanogaster fruitflies to examine sex-specific effects of social contact. We kept flies in isolation versus same-sex pairing throughout life, and measured actuarial (lifespan) and functional senescence (declines in climbing ability). To investigate underlying mechanisms, we determined whether an immune stress (wounding) interacted with effects of social contact, and assessed behaviours that could contribute to differences in ageing rates. Pairing reduced lifespan for both sexes, but the effect was greater for males. In contrast, pairing reduced the rate of decline in climbing ability for females, whereas for males, pairing caused more rapid declines with age. Wounding reduced lifespan for both sexes, but doubled the negative effect of pairing on male lifespan. We found no evidence that these effects are driven by behavioural interactions. These findings suggest that males and females are differentially sensitive to social contact, that environmental stressors can impact actuarial and functional senescence differently, and that these effects can interact with environmental stressors, such as immune challenges.

Interactions between the developmental and adult social environments mediate group dynamics and offspring traits in Drosophila melanogaster

Developmental conditions can strongly influence adult phenotypes and social interactions, which in turn affect key evolutionary processes such as sexual selection and sexual conflict. While the implications of social interactions in phenotypically mixed populations at the individual level are increasingly well known, how these effects influence the fate of groups remains poorly understood, which limits our understanding of the broader ecological implications. To address this problem we manipulated adult phenotypes and social composition in Drosophila melanogaster – by experimentally manipulating the larval density of the group-members – and measured a range of group-level outcomes across the lifespan of groups. Adult groups composed of exclusively low larval-density individuals showed high courtship levels, and low early reproductive rates, group growth rates, offspring mass and offspring eclosion success, relative to high larval-density or mixed larval-density groups. Furthermore, high larval-density groups had lower survival. Offspring mass increased with time, but at a reduced rate in groups when male group members (but not females) were from a mixture of larval-densities; peak reproductive rates were also earlier in these groups. Our results suggest that that variation in developmental conditions experienced by adult group members can modify the reproductive output of groups.