Effects of reproductive state and host resource experience on mating decisions in a walnut fly

Prior experience with conspecifics or essential resources, aswell as physiological condition, can have important influenceson an animal's reproductive behavior. While effects of experienceand physiological state (such as reproductive condition) aregenerally treated separately in theoretical discussions, theyoften interact. No previous study has attempted to distinguisheffects of experience on physiological state from other effectsof experience in the context of mating behavior. In a studyof a walnut-infesting tephritid fly (Rhagoletis juglandis),we examined the effects of host fruit experience on mating behavior.We manipulated physiological state in terms of egg load (definedas the number of mature oocytes in a female's ovaries) independentlyof fruit experience to distinguish the effects of these variables.We found that females with high egg loads were significantlymore likely to copulate than low–egg load females; thelevel of fruit experience had no effect on propensity to copulate,except via effects on egg load. In contrast, females with priorexposure to fruit copulated for a significantly shorter durationthan control females, while egg load had no effect on copulationduration. These results suggest that female reproductive conditionand exposure to essential resources can have important, albeitdiverse effects on mating behavior. We discuss how distinguishingdifferent types of variables may provide insight into sexualconflict over mating decisions, as well as which sex controlsspecific aspects of behavior.     

Bark beetle-mediated fungal infections of susceptible trees induce resistance to subsequent infections in a dose dependent manner

1 Experiments were conducted to determine whether propagule loads on the twig beetles Pityophthorus setosus and Pityophthorus carmeli (Coleoptera: Scolytidae) influence the pathogen infection of the host tree in the Monterey pine- Fusarium circinatum system.
2 On an average, F. circinatum was isolated from 2.6% and 3.3% of trapped P. setosus and P. carmeli , respectively, although the isolation percentages varied over the season, being highest in the spring and lowest in late summer and fall for both species. Mean pathogen load was 13.4 and 22.6 propagules per beetle, on P. setosus and P. carmeli , respectively, and decreased from May to November for both species. The pathogen was also isolated from approximately 55% of both beetle species that emerged from infected branches. Mean propagule load on emerged P. setosus and P. carmeli was 39 and 66.5, respectively.
3 On the basis of these data, beetle species were treated with one of three propagule loads (low, medium, high) and caged onto live branches to determine whether they could transmit the pathogen. At all propagule loads, both species transmitted the pathogen, and transmission percentage and lesion length, a measure of tree susceptibility, were positively correlated with propagule load.
4 To investigate further whether the previous transmission by beetles could affect response of the same trees to subsequent infection with F. circinatum , different branches were inoculated on the same trees used in the transmission study, and lesion lengths were measured. Lesion lengths were lower on trees that had been previously exposed to beetles treated with high or medium propagule loads than on trees that had previously been exposed to beetles treated with low propagule loads. This suggests that the initial infection by beetles carrying high or medium propagule loads induced resistance to subsequent infections of the host, whereas infections caused by beetles with low propagule loads did not.     

Surfing downhill: when should population range expansion be characterized by reductions in fitness?

Deleterious mutations are found in all populations. Their existence at low frequencies is easily understood, but explaining how they reach high frequencies has long been a challenging problem for population geneticists and evolutionary biologists. Some cases of apparently deleterious alleles are explained by pleiotropy or environmental context dependence, but for universally deleterious alleles, two mechanisms are generally invoked to explain how they can reach high frequencies: (i) genetic drift in small populations and (ii) ‘hitchhiking’ (sensu Maynard Smith J, Haigh J, Genetical Research, 1974, 23, 23–35) involving tight linkage to beneficial mutations. However, these oft‐cited explanations do not immediately resolve the problem because many real populations of interest have population sizes and recombination rates that are large enough to render it nearly impossible for all but the most weakly deleterious (i.e. nearly neutral) mutations to establish and persist. Furthermore, both mechanisms are usually silent about patterns of intraspecific variation in mutation load. In this issue, Peischl S, Dupanloup I, Kirkpatrick M, Excoffier L (Molecular Ecology, 2013) develop and explore a mechanism that puts drift and hitchhiking of deleterious mutations into a specific spatial and demographic context: range expansions. Importantly, their findings provide a plausible explanation for puzzling empirical patterns, such as the paradoxical observation that genotypes at the leading edge of a range expansion are sometimes less fit than those in the ancestral range (when fitness is assessed in a common environment).     

Effect of host deprivation on egg quality,egg load,and oviposition in a solitary parasitoid,Chetogena edwardsii (Diptera: Tachinidae)

Solitary parasitoids are limited to laying one egg per host because larvae compete within hosts. If host encounter rate is low, females should not increase the number of eggs/host in response. The tachinid fly, Chetogena edwardsii,was used to evaluate the effect of host deprivation on egg accumulation, oviposition behavior, and egg quality in a solitary parasitoid. Females deprived of hosts for 2– 7 days accumulate about 1 day's supply of eggs. Egg output of deprived females once hosts are restored does not differ from that of control females. Deprived females retain one egg in the uterus where it undergoes embryogenesis. Maggots emerging from retained eggs are more likely to survive in hosts molting in 40 h or less after receipt of an egg than are maggots emerging from eggs fertilized shortly before oviposition. Egg retention is a consequence of host deprivation that permits females to broaden the range of hosts they can exploit to include soon-to-molt hosts and possibly multiply parasitized hosts.     

Stress and the costs of extra-territorial movement in a social carnivore

Costs associated with extra-territorial movement are believed to have favoured the evolution of delayed dispersal and sociality across a range of social vertebrates, but remain surprisingly poorly understood. Here we reveal a novel mechanism that may contribute substantially to the costs of extra-territorial movement: physiological stress. We show that subordinate male meerkats, Suricata suricatta, exhibit markedly elevated faecal glucocorticoid metabolite levels (a non-invasive measure of hypothalamic–pituitary–adrenal axis activity) while conducting extra-territorial prospecting forays. While brief increases in glucocorticoid levels are unlikely to be costly, chronic elevations, arising from prolonged and/or frequent forays, are expected to compromise fitness through their diverse negative effects on health. Our findings strongly suggest that prolonged extra-territorial movements do result in chronic stress, as the high glucocorticoid levels of prospectors do not diminish on longer forays and are no lower among males with greater prospecting experience. A generalized ‘stress’ of extra-territorial movement may therefore have strengthened selection for delayed dispersal and sociality in this and other species, and favoured the conduct of brief forays over extended periods of floating. Our findings have implications too for understanding the rank-related distribution of physiological stress in animal societies, as extra-territorial movements are often conducted solely by subordinates.     

TESTING THE EFFECTS OF MATING SYSTEM VARIATION ON RATES OF MOLECULAR EVOLUTION IN PRIMATES

Post‐copulatory sexual selection has been proposed to drive the rapid evolution of reproductive proteins, and, more recently, to increase genome‐wide mutation rates. Comparisons of rates of molecular evolution between lineages with different levels of female multiple mating represent a promising, but under‐utilized, approach for testing the effects of sperm competition on sequence evolution. Here, I use comparisons between primate species with divergent mating systems to examine the effects of sperm competition on reproductive protein evolution, as well as on sex‐averaged mutation rates. Rates of nonsynonymous substitution are higher for testis‐specific genes along the chimpanzee lineage in comparison to the human lineage, consistent with expectations. However, the data reported here do not allow firm conclusions concerning the effects of mating system on genome‐wide mutation rates, with different results obtained from different species pairs. Ultimately, comparative studies encompassing a range of mating systems and other life history traits will be required to make broad generalizations concerning the genomic effects of sperm competition.     

Skew‐normal/independent linear mixed models for censored responses with applications to HIV viral loads

Often in biomedical studies, the routine use of linear mixed‐effects models (based on Gaussian assumptions) can be questionable when the longitudinal responses are skewed in nature. Skew‐normal/elliptical models are widely used in those situations. Often, those skewed responses might also be subjected to some upper and lower quantification limits (QLs; viz., longitudinal viral‐load measures in HIV studies), beyond which they are not measurable. In this paper, we develop a Bayesian analysis of censored linear mixed models replacing the Gaussian assumptions with skew‐normal/independent (SNI) distributions. The SNI is an attractive class of asymmetric heavy‐tailed distributions that includes the skew‐normal, skew‐t, skew‐slash, and skew‐contaminated normal distributions as special cases. The proposed model provides flexibility in capturing the effects of skewness and heavy tail for responses that are either left‐ or right‐censored. For our analysis, we adopt a Bayesian framework and develop a Markov chain Monte Carlo algorithm to carry out the posterior analyses. The marginal likelihood is tractable, and utilized to compute not only some Bayesian model selection measures but also case‐deletion influence diagnostics based on the Kullback–Leibler divergence. The newly developed procedures are illustrated with a simulation study as well as an HIV case study involving analysis of longitudinal viral loads.     

Wolbachia infection and incompatibility dynamics in experimental selection lines

High and low levels of Wolbachia-induced cytoplasmic incompatibility (CI) were selected for in the parasitic wasp Nasonia vitripennis, in the single-infected strain Ti277. After nine generations of selection, males from lines selected for high incompatibility level (HI lines) were significantly more incompatible with uninfected females (AsymC) than the maternal strain. The reverse response, a full compatibility with AsymC, was observed in eight out of 12 lines selected for low incompatibility (LO lines), correlated with loss of Wolbachia infection. Bacterial density estimates in the eggs of some HI lines increased significantly. The procedure for line maintenance resulted in introgression of AsymC nuclear genome into the Ti277 background. Significant changes of CI level and bacterial density due to the introgression were also observed in the control lines, possibly reflecting an effect of host genotype on bacterial density and CI. After selection had been relaxed for six generations, bacterial density in the five high-infected HI lines declined back to a level comparable to the other lines. The data are consistent with the ‘bacterial dosage’ model, but with an upper threshold of bacterial infection above which there is no correlation between infection level and CI level. We further investigate the maternal transmission of bacterial density by a mother–daughter regression on bacterial density. The pattern observed is consistent with a density dependent regulation of bacterial numbers around an ‘equilibrium’ density, independent of any effects of CI. The equilibrium value is likely to be determined by both bacterial strain and host genotype.     

Influence of host deprivation on egg load and oviposition behaviour of Brachymeria intermedia, a parasitoid of gypsy moth

Abstract. Mated female Brachymeria intermedia (Hymenoptera: Chalcididae) deprived, since emergence, from pupae of their host Lymantria dispar (Lepidoptera: Lymantriidae), accumulated eggs but had a very low rate of hostacceptance. Parasitoids that were host-deprived after encountering pupae early in life also accumulated eggs, but maintained a high acceptance rate. Thus early exposure to hosts promoted active reproductive behaviour. Total egg production depended on the total number of pupae encountered, indicating that B.intermedia adjust their egg production to host availability. Hence, in B. intermedia both the physiological state of the parasitoid (age and egg load) and the informational state (in this case host-availability and experience) interact to shape oviposition behaviour.