Sex differences in the effects of juvenile and adult diet on age-dependent reproductive effort.

Sexual selection should cause sex differences in patterns of resource allocation. When current and future reproductive effort trade off, variation in resource acquisition might further cause sex differences in age-dependent investment, or in sensitivity to changes in resource availability over time. However, the nature and prevalence of sex differences in age-dependent investment remain unclear. We manipulated resource acquisition at juvenile and adult stages in decorated crickets, Gryllodes sigillatus, and assessed effects on sex-specific allocation to age-dependent reproductive effort (calling in males, fecundity in females) and longevity. We predicted that the resource and time demands of egg production would result in relatively consistent female strategies across treatments, whereas male investment should depend sharply on diet. Contrary to expectations, female age-dependent reproductive effort diverged substantially across treatments, with resource-limited females showing much lower and later investment in reproduction; the highest fecundity was associated with intermediate lifespans. In contrast, long-lived males always signalled more than short-lived males, and male age-dependent reproductive effort did not depend on diet. We found consistently positive covariance between male reproductive effort and lifespan, whereas diet altered this covariance in females, revealing sex differences in the benefits of allocation to longevity. Our results support sex-specific selection on allocation patterns, but also suggest a simpler alternative: males may use social feedback to make allocation decisions and preferentially store resources as energetic reserves in its absence. Increased calling effort with age therefore could be caused by gradual resource accumulation, heightened mortality risk over time, and a lack of feedback from available mates.

Stabilizing sexual selection for female ornaments in a dance fly.

Ornamental traits function by improving attractiveness and are generally presumed to experience directional selection for mating success. However, given the greater investment of females in offspring than males, female-specific ornaments can in theory signal fecundity yet be constrained by fecundity costs. Theoretical work predicts that such constraints can lead to stabilizing selection via male choice for intermediately ornamented females. Female dance flies Rhamphomyia longicauda (Diptera: Empididae) display two female-specific ornaments in mating swarms - inflatable abdominal sacs and pinnate tibial scales. We investigated the intensity and form of sexual selection on female traits including ornaments and found no evidence for directional sexual selection. Instead, we found marginally nonsignificant quadratic selection for all three measures of ornament expression. Canonical analysis confirmed that the strongest vectors of nonlinear selection were associated with ornamental traits, although the significance of the quadratic coefficients associated with these vectors depended on the statistical approach. Direct Mitchell-Olds and Shaw tests for the location of the maximum fitted fitness value for both raw morphological traits and canonical axes revealed only one marginally nonsignificant result for the multivariate axis loading most heavily on pinnate leg scales. Together, these results provide the first tentative support for stabilizing selection on female-specific ornaments.

Genetic and environmental sources of covariance among internal reproductive traits in the yellow dung fly.

Substantial inter- and intraspecific variation is found in reproductive traits, but the evolutionary implications of this variation remain unclear. One hypothesis is that natural selection favours female reproductive morphology that allows females to control mating and fertilization and that diverse male reproductive traits arise as counter adaptations to subvert this control. Such co-evolution predicts the establishment of genetic correlations between male and female reproductive traits that closely interact during mating. Therefore, we measured phenotypic and genetic correlations between male and female reproductive tract characteristics in the yellow dung fly, Scathophaga stercoraria (Diptera: Scathophagidae), using a nested half-sib breeding experiment. We found significant heritabilities for the size of most reproductive tract traits investigated in both females (spermathecae and their ducts, accessory glands and their ducts) and males (testis size but not sperm length). Within the sexes, phenotypic and genetic correlations were mostly nil or positive, suggesting functional integration of or condition-dependent investment in internal reproductive traits. Negative intrasexual genetic correlations, potentially suggestive of resource allocation trade-offs, were not evident. Intersexual genetic correlations were mostly positive, reflecting expected allometries between male and female morphologies. Most interestingly, testis size correlated positively with female accessory gland size and duct length, potentially indicative of a co-evolutionary arms race. We discuss these and alternative explanations for these patterns of genetic covariance.

A review of the natural history and laboratory culture methods for the yellow dung fly, Scathophaga stercoraria.

The yellow dung fly Scathophaga stercoraria (L.) (Diptera: Scathophagidae) is a widespread and locally abundant fly associated with the dung of large mammals, especially farm animals. This species has recently become a standard test organism for evaluating toxic effects of veterinary pharmaceuticals in livestock dung. In this context, a review of its natural history and a general description of the field and laboratory rearing methods of this species are provided here to benefit the scientific community as well as government regulators and applicants of eco-toxicological studies. For guidance, means and ranges are included for all relevant standard life history traits stemming from previously published data on Swiss populations.

The assessment of insemination success in yellow dung flies using competitive PCR.

In spite of considerable interest in postcopulatory sexual selection, separating the effects of sperm competition from cryptic female choice remains difficult because mechanisms underlying postcopulatory processes are poorly understood. One methodological challenge is to quantify insemination success for individual males within the sperm stores of multiply mated females to discover how insemination translates into eventual paternity. Any proposed method must be applicable in organisms without extensive DNA sequence information (which include the majority of model species for sexual selection). Here, we describe the development and application of microsatellite competitive-multiplex-PCR for quantifying relative contributions to a small number of sperm in storage. We studied how DNA template characteristics affect PCR amplification of known concentrations of mixed DNA and generated regressions for correcting observations of allelic signal strength based on such characteristics. We used these methods to examine patterns of sperm storage in twice-mated female yellow dung flies, Scathophaga stercoraria. We confirm previous findings supporting sperm displacement and demonstrate that average paternity for the last mate accords with the mean proportion of sperm stored. We further find consistent skew in storage across spermathecae, with more last male sperm stored in the singlet spermatheca on one side of the body than in the doublet on the opposite side. We also show that the time between copulations may be important for effectively sorting sperm. Finally, we demonstrate that male size may influence the opportunity for sperm choice, suggesting future work to disentangle the roles of male competition and cryptic female choice.

Diet-dependent female evolution influences male lifespan in a nuptial feeding insect.

Theory predicts that lifespan will depend on the dietary intake of an individual, the allocation of resources towards reproduction and the costs imposed by the opposite sex. Although females typically bear the majority of the cost of offspring production, nuptial feeding invertebrates provide an ideal opportunity to examine the extent to which reproductive interactions through gift provisioning impose a cost on males. Here we use experimental evolution in an Australian ground cricket to assess how diet influences male lifespan and how the costs of mating evolve for males. Our findings show that males had significantly shorter lifespans in populations that adapted to a low-quality diet and that this divergence is driven by evolutionary change in how females interact with males over reproduction. This suggests that the extent of sexual conflict over nuptial feeding may be under-realized by focusing solely on the consequences of reproductive interactions from the female's perspective.

Contrasting sexual selection on males and females in a role-reversed swarming dance fly, Rhamphomyia longicauda Loew (Diptera: Empididae).

Although there are several hypotheses for sex-specific ornamentation, few studies have measured selection in both sexes. We compare sexual selection in male and female dance flies, Rhamphomyia longicauda (Diptera: Empididae). Swarming females display size-enhancing abdominal sacs, enlarged wings and decorated tibiae, and compete for nuptial gifts provided by males. Males preferentially approach large females, but the nature of selection and whether it is sex-specific are unknown. We found contrasting sexual selection for mating success on structures shared by males and females. In females, long wings and short tibiae were favoured, whereas males with short wings and long tibiae had a mating advantage. There was no assortative mating. Females occupying potentially advantageous swarm positions were large and, in contrast to selection for mating success, tended to have larger tibiae than those of rivals. We discuss our findings in the context of both the mating biology of dance flies, and the evolution of sexual dimorphism in general.