Devin Arbuthnott

SEXUAL SELECTION IS INEFFECTUAL OR INHIBITS THE PURGING OF DELETERIOUS MUTATIONS IN DROSOPHILA MELANOGASTER

The effects of sexual selection on population mean fitness are unclear and a subject of debate. Recent models propose that, because reproductive success may be condition dependent, much of the genome may be a target of sexual selection. Under this scenario, mutations that reduce health, and thus nonsexual fitness, may also be deleterious with respect to reproductive success, meaning that sexual selection may contribute to the purging of deleterious alleles. We tested this hypothesis directly by subjecting replicate Drosophila melanogaster populations to two treatments that altered the opportunity for sexual selection and then tracked changes in the frequency of six separate deleterious alleles with recessive and visible phenotypic effects. While natural selection acted to decrease the frequency of all six mutations, the addition of sexual selection did not aid in the purging of any of them, and for three of them appears to have hampered it. Courtship and mating have harmful effects in this species and mate choice assays showed that males directed more courtship and mating behavior toward wild‐type over mutant females, providing a likely explanation for sexual selections cost. Whether this cost extends to other mutations (e.g., those lacking visible phenotypic effects) is an important topic for future research.

The ecology of sexual conflict: ecologically dependent parallel evolution of male harm and female resistance in Drosophila melanogaster.

The prevalence of sexual conflict in nature, along with the potentially stochastic nature of the resulting coevolutionary trajectories, makes it an important driver of phenotypic divergence and speciation that can operate even in the absence of environmental differences. The majority of empirical work investigating sexual conflicts role in population divergence/speciation has therefore been done in uniform environments and any role of ecology has largely been ignored. However, theory suggests that natural selection can constrain phenotypes influenced by sexual conflict. We use replicate populations of Drosophila melanogaster adapted to alternative environments to test how ecology influences the evolution of male effects on female longevity. The extent to which males reduce female longevity, as well as female resistance to such harm, both evolved in association with adaptation to the different environments. Our results demonstrate that ecology plays a central role in shaping patterns of population divergence in traits under sexual conflict.

Courtship and mate discrimination within and between species of Timema walking-sticks

The evolution of premating isolation via divergence in mating behaviour has been strongly implicated in the process of speciation. Timema walking-sticks show weak to moderate reproductive isolation between populations of the same species on different host plants, and high levels of isolation between species. In this paper we conducted experimental studies of within-species and between-species mating behaviour in Timema to address two central issues pertaining to the influence of behaviour on speciation: (1) how divergence in courtship influences reproductive isolation within and between species, and (2) whether the same or different traits mediate premating isolation within and between species. Mating behaviour involves three phases: pairing (whereby the male climbs onto the females dorsal surface), courtship (which involves leg and antenna waving), and copulation. We found that courtship was qualitatively similar across the genus, but there were statistically significant quantitative differences in leg and antenna waving frequencies between Timema species. However, no-choice trials within and between species showed that discrimination within species is a function of whether males will court females after pairing, while discrimination between species is a function of whether males and females will pair. Because in both cases mate discrimination occurs before courtship is performed, we infer that courtship does not directly influence reproductive isolation in Timema. Moreover, because within-species and between-species discrimination take place at different stages in the mating sequence, intraspecific mate discrimination and species recognition appear to represent distinct processes. These findings suggest that, at least in Timema, speciation may involve the accumulation or replacement of mate discrimination mechanisms used within species.

The influence of a local temperature inversion on the foraging behaviour of big brown bats, Eptesicus fuscus

ABSTRACT To maximise foraging efficiency, it is reasonable to expect animals to forage in the highest quality patches. Insectivorous bats should therefore travel to and forage at sites with the highest insect abundance. Since insects are ectothermic, their levels of activity should be higher in warmer areas, making these high quality patches for bats. A nightly temperature inversion occurring in the Cypress Hills (Saskatchewan, Canada) presented an opportunity to test our hypothesis that big brown bats (Eptesicus fuscus) select foraging sites based on temperature as a proxy for insect abundance. If temperature is an important determinant of the foraging behaviour of E. fuscus, we expect bats to forage in the warmest site closest to local night roosts. We tracked 18 bats for a total of 111 nights over two years and found that individuals often spent at least some of each foraging bout in an area where the temperature inversion was small or non-existent. Bats sometimes travelled up to 11 km to reach this site. Foraging in areas where the temperature inversion was small provides indirect evidence that local temperature fluctuations are not a major influence on the selection of foraging area by E. fuscus. Also, since there was little difference in the temperature between the nearby predicted foraging sites and actual foraging sites, we argue that the effect of temperature on insect activity cannot be used to predict foraging habitat selection by these bats. We found that the insect community of the foraging area was different than that of the roosting area, and that beetles were more abundant in the foraging site. Our data suggests that insect community composition is potentially a stronger direct influence on bat foraging behaviour than is temperature.

Hydrocarbon divergence and reproductive isolation in Timema stick insects

BackgroundIndividuals commonly prefer certain trait values over others when choosing their mates. If such preferences diverge between populations, they can generate behavioral reproductive isolation and thereby contribute to speciation. Reproductive isolation in insects often involves chemical communication, and cuticular hydrocarbons, in particular, serve as mate recognition signals in many species. We combined data on female cuticular hydrocarbons, interspecific mating propensity, and phylogenetics to evaluate the role of cuticular hydrocarbons in diversification of Timema walking-sticks.ResultsHydrocarbon profiles differed substantially among the nine analyzed species, as well as between partially reproductively-isolated T. cristinae populations adapted to different host plants. In no-choice trials, mating was more likely between species with similar than divergent hydrocarbon profiles, even after correcting for genetic divergences. The macroevolution of hydrocarbon profiles, along a Timema species phylogeny, fits best with a punctuated model of phenotypic change concentrated around speciation events, consistent with change driven by selection during the evolution of reproductive isolation.ConclusionAltogether, our data indicate that cuticular hydrocarbon profiles vary among Timema species and populations, and that most evolutionary change in hydrocarbon profiles occurs in association with speciation events. Similarities in hydrocarbon profiles between species are correlated with interspecific mating propensities, suggesting a role for cuticular hydrocarbon profiles in mate choice and speciation in the genus Timema.

Differential effects of genetic vs. environmental quality in Drosophila melanogaster suggest multiple forms of condition dependence

Condition is a central concept in evolutionary ecology, but the roles of genetic and environmental quality in condition-dependent trait expression remain poorly understood. Theory suggests that condition integrates genetic, epigenetic and somatic factors, and therefore predicts alignment between the phenotypic effects of genetic and environmental quality. To test this key prediction, we manipulated both genetic (mutational) and environmental (dietary) quality in Drosophila melanogaster and examined responses in morphological and chemical (cuticular hydrocarbon, CHC) traits in both sexes. While the phenotypic effects of diet were consistent among genotypes, effects of mutation load varied in magnitude and direction. Average effects of diet and mutation were aligned for most morphological traits, but non-aligned for the male sexcombs and CHCs in both sexes. Our results suggest the existence of distinct forms of condition dependence, one integrating both genetic and environmental effects and the other purely environmental. We propose a model to account for these observations.

Misalignment of natural and sexual selection among divergently adapted Drosophila melanogaster populations

The effect of sexual selection on nonsexual fitness is a major unanswered question in evolutionary biology that may have important implications for adaptation, diversification and the evolution of mate preferences. If reproductive success is condition dependent, the resulting sexual selection will tend to align with natural selection, promoting adaptation. One prediction under such a scenario is that adaptation to a novel environment should increase male mating success and hence sexual fitness. Environmentally induced plasticity in mate preferences could also contribute to an alignment of natural and sexual selection if the changes cause females to prefer locally adapted males as mates. We tested for both forms of alignment using a set of 10 independent populations of Drosophila melanogaster that were adapted to one of two environments. Competitive mating trials were performed between pairs of populations adapted to these two environments, with the trials designed to separate the effects of local adaptation on male mating success from plasticity of female mate preferences in response to these environments. Contrary to expectations under an alignment of natural and sexual selection, males did not have higher mating success when competing in the environment to which they were adapted. Furthermore, there was no evidence that females altered their mate choice based on their rearing environment, indicating the absence of any adaptive plasticity in mate preferences. Overall, despite previous evidence of reciprocal adaptation to these different environments, increased nonsexual fitness did not translate into higher mating success, indicating a lack of any alignment with natural selection of this component of male sexual fitness.

Remating and Sperm Competition in Replicate Populations of Drosophila melanogaster Adapted to Alternative Environments

The prevalence of sexual conflict in nature, as well as the supposedly arbitrary direction of the resulting coevolutionary trajectories, suggests that it may be an important driver of phenotypic divergence even in a constant environment. However, natural selection has long been central to the operation of sexual conflict within populations and may therefore constrain or otherwise direct divergence among populations. Ecological context may therefore matter with respect to the diversification of traits involved in sexual conflict, and if natural selection is sufficiently strong, such traits may evolve in correlation with environment, generating a pattern of ecologically-dependent parallel evolution. In this study we assess among-population divergence both within and between environments for several traits involved in sexual conflict. Using eight replicate populations of Drosophila melanogaster from a long-term evolution experiment, we measured remating rates and subsequent offspring production of females when housed with two separate males in sequence. We found no evidence of any variation in male reproductive traits (offense or defense). However, the propensity of females to remate diverged significantly among the eight populations with no evidence of any environmental effect, consistent with sexual conflict promoting diversification even in the absence of ecological differences. On the other hand, females adapted to one environment (ethanol) tended to produce a higher proportion of offspring sired by their first mate as compared to those adapted to the other (cadmium) environment, suggesting ecologically-based divergence of this conflict phenotype. Because we find evidence for both stochastic population divergence operating outside of an ecological context and environment-dependent divergence of traits under sexual conflict, the interaction of these two processes is an important topic for future work.

Female Stick Insects Mate Multiply to Find Compatible Mates.

Why females of many species mate multiply in the absence of direct benefits remains an open question in evolutionary ecology. Interacting and mating with multiple males can be costly to females in terms of time, resources, predation risk, and disease transmission. A number of indirect genetic benefits have been proposed to explain such behaviors, but the relative importance of these mechanisms in natural systems remains unclear. We tested for several direct and indirect benefits of polyandry in the walking stick Timema cristinae. We found no evidence of direct benefits with respect to longevity or fecundity. However, male × female genotypic interactions affected egg-hatching success and offspring production independent of relatedness, suggesting that mating with certain males benefits females and that the best male may differ for each female. Furthermore, multiply mated females biased paternity toward one or few males, and the extent of this bias was positively correlated to egg-hatching success. Our data, therefore, provide evidence for indirect benefits through compatibility effects in this species. By mating multiply, females may improve their chances of mating with a compatible male if compatibility cannot be assessed before mating. Such compatibility effects can explain the evolution and maintenance of polyandry in Timema and many other species.

Female Life-History Trade-Offs and the Maintenance of Genetic Variation in Drosophila melanogaster

Why do we observe substantial variation in fitness-related traits under strong natural or sexual selection? While there is support for several selective and neutral mechanisms acting in select systems, we lack a comprehensive analysis of the relative importance of various mechanisms within a single system. Furthermore, while sexually selected male traits have been a central focus of this paradox, female sexual traits have rarely been considered. In this study, I evaluate the contribution of various selective mechanisms to the maintenance of substantial variation in female attractiveness and offspring production observed among Drosophila melanogaster genotypes. I tested for contributions from antagonistic pleiotropy, frequency-dependent selection, changing environments, and sexual conflict. I found negative genetic correlations between some traits (male attractiveness vs. female resistance to male harm, early-life offspring production vs. reproductive senescence) and genotype-specific changes in fitness between environments. However, no measurement found strong trade-offs among the fitness components of these genotypes. Overall, I find little evidence that any one mechanism is strong enough to maintain genetic variation on its own. Instead, I suggest that many mechanisms may weaken the selection among genotypes, which would collectively allow neutral processes such as mutation-selection balance to maintain genetic variation within populations.