The strongest form of intralocus sexual conflict occurs when two conditions are met: (i) there is a positive intersexual genetic correlation for a trait and (ii) the selection gradients on the trait in the two sexes are in opposite directions. Intralocus sexual conflict can constrain the adaptive evolution of both sexes and thereby contribute to a species' ‘gender load’. Previous studies of adult lifetime fitness of the same sets of genes expressed in both males and females have established that there is substantial intralocus conflict in the LH M laboratory-adapted population of Drosophila melanogaster . Here, we investigated whether a highly dimorphic trait—adult locomotory activity—contributed substantially to the established intralocus sexual conflict. To measure the selection gradient on activity level, both this trait and adult lifetime fitness were measured under the same environmental conditions to which the flies were adapted. We found significant phenotypic variation in both sexes for adult locomotory activity, and that the selection gradients on this variation were large and in opposite directions in the two sexes. Using hemiclonal analysis to screen 99% of the entire genome, we found abundant genetic variation for adult locomotory activity and showed that this variation occurs on both the X and autosomes. We also established that there is a strong positive intersexual genetic correlation for locomotory activity. These assays revealed that, despite the strong, extant sexual dimorphism for the trait, locomotory activity continues to contribute strongly to intralocus sexual conflict in this population.
Invasive sea lamprey (Petromyzon marinus) are controlled in the Great Lakes using the lampricide 3-trifluoromethyl-4-nitrophenol (TFM), which is applied to streams infested with larval lamprey. However, lamprey that survive treatments (residuals) remain a challenge because they may subsequently undergo metamorphosis into parasitic juvenile animals that migrate downstream to the Great Lakes, where they feed on important sport and commercial fishes. The goal of this study was to determine if body size and life stage could potentially influence sea lamprey tolerance to TFM by influencing patterns of TFM uptake and elimination. Because mass specific rates of oxygen consumption (M˙O2) are lower in larger compared to smaller lamprey, we predicted that TFM uptake would be negatively correlated to body size, suggesting that large larvae would be more tolerant to TFM exposure. Accordingly, TFM uptake and M˙O2 were measured in larvae ranging in size from 0.2-4.2g using radio-labelled TFM (14C-TFM) and static respirometry. Both were inversely proportional to wet mass (M), and could be described usingthe allometric power relationship: Y=aMb, in which M˙O2=1.86M0.53 and TFM Uptake=7.24M0.34. We also predicted that body size would extend to rates of TFM elimination, which was measured following the administration of 14C-TFM (via intraperitoneal injection). However, there were no differences in the half-lives of elimination of TFM (T 1/2-TFM). There were also no differences in M˙O2 or TFM uptake amongst size-matched larval, metamorphosing (stages 6-7), or post-metamorphic (juvenile) sea lamprey. However, the T1/2-TFM was significantly lower in larval than post-metamorphic lamprey (juvenile), indicating the larval lamprey cleared TFM more efficiently than juvenile lamprey. We conclude that larger larval sea lamprey are more likely to survive TFM treatments suggesting that body size might be an important variable to consider when treating streams with TFM to control these invasive species.
Abstract In the fruit fly, Drosophila melanogaster , variation in body size is influenced by a number of different factors and may be strongly associated with individual condition, performance and success in reproductive competitions. Consequently, intra-sexual variation in size in this model species has been frequently explored in order to better understand how sexual selection and sexual conflict may operate and shape evolutionary trajectories. However, measuring individual flies can often be logistically complicated and inefficient, which can result in limited sample sizes. Instead, many experiments use large and/or small body sizes that are created by manipulating the developmental conditions experienced during the larval stages, resulting in “phenocopied” flies whose phenotypes resemble what is seen at the extremes of a population’s size distribution. While this practice is fairly common, there has been remarkedly few direct tests to empirically compare the behaviour or performance of phenocopied flies to similarly-sized individuals that grew up under typical developmental conditions. Contrary to assumptions that phenocopied flies are reasonable approximations, we found that both large and small-bodied phenocopied males frequently differed from their standard development equivalents in their mating frequencies, their lifetime reproductive successes, and in their effects on the fecundity of the females they interacted with. Our results highlight the complicated contributions of environment and genotype to the expression of body size phenotypes and lead us to strongly urge caution in the interpretation of studies solely replying upon phenocopied individuals.
In Drosophila melanogaster , males engage in both extensive pre- and post-copulatory competition for the opportunity to mate with females and subsequently sire offspring. The selection pressure for increased male reproductive success has resulted in the evolution of a wide diversity of sexual traits. However, despite strong selection, individuals often exhibit considerable phenotypic variation in the expression of these traits, and it is unclear if any of this variation is owing to underlying genetic trade-offs. Here, using hemiclonal flies, we examine how male reproductive success covaries with their ability to induce long-term stimulation of oogenesis and oviposition in their mates, and how this relationship may change over time. We found that males from hemiclone lines with phenotypes that were more successful in a short-term reproductive ‘scramble’ environment were less effective at stimulating long-term fecundity in females. Furthermore, we observed that males from hemiclone lines which showed the most improvement over a longer reproductive interaction period also tended to stimulate higher long-term fecundity in females. Together, these results indicate the presence of genetic trade-offs between different male reproductive traits and offer insights into the maintenance of their variation.
Modification of offspring sex ratios in response to parental quality is predicted when the long-term fitness returns of sons and daughters differ. One factor that may influence a mother's sex allocation decision is the quality (or attractiveness) of her mate. We investigated whether the sex ratios of offspring produced by female Drosophila melanogaster are biased with respect to the age of the males to which they are mated, and whether there is an adaptive basis for this phenomenon. We found that females mated to old males (13 d post-eclosion) initially produced a greater proportion of daughters than did females mated to young males (1 d post-eclosion). This pattern does not appear to be due to a systematic difference in the numbers or mortality of the X- and Y-bearing sperm originating from old and young fathers, as the overall sex ratios of all offspring produced from a single copulation did not differ between broods fathered by the two types of males. The sons of older males fared worse in competitive mating assays than did the sons of younger males, while daughters of old and young males were of comparable fitness. These results suggest that there is an adaptive basis for the observed sex ratio modification.
Abstract An evolutionary conflict of interest exists between parents and their offspring over the partitioning of parental investment (PI) among siblings. When the direct fitness benefits to offspring of increased PI, outweigh the inclusive fitness costs from lost future sibling fitness, selection should favour the evolution of offspring selfishness over altruism. In theory, this conflict is heightened when females are not strictly monogamous, as current offspring should be less altruistic towards future half‐siblings than they would be towards full‐siblings. Using data collected on foetal growth rate (representing prenatal PI) in primates, I test the prediction from theory that the resolution of the parent‐offspring conflict will be closer to the offspring's evolutionary optima in polyandrous species than in more monandrous species. Using phylogenetic comparative analysis, and controlling for allometry, I show that offspring are able to obtain more PI when the probability of future full‐siblings decreases, and that this is most pronounced in taxa where there is the opportunity for direct foetal access to the maternal bloodstream. These results support the hypothesis that the resolution of prenatal PI conflict is influenced by both a species’ mating system and by its placental structure.
