Male-male lethal combat in the quasi-gregarious parasitoid Anastatus disparis (Hymenoptera: Eupelmidae)

How frequently are insects wounded in the wild? A case study using Drosophila melanogaster

Wounding occurs across multicellular organisms. Wounds can affect host mobility and reproduction, with ecological consequences for competitive interactions and predator-prey dynamics. Wounds are also entry points for pathogens. An immune response is activated upon injury, resulting in the deposition of the brown-black pigment melanin in insects. Despite the abundance of immunity studies in the laboratory and the potential ecological and evolutionary implications of wounding, the prevalence of wounding in wild-collected insects is rarely systematically explored. We investigated the prevalence and potential causes of wounds in wild-collected Drosophilidae flies. We found that 31% of Drosophila melanogaster were wounded or damaged. The abdomen was the most frequently wounded body part, and females were more likely to have melanized patches on the ventral abdomen, compared with males. Encapsulated parasitoid egg frequency was approximately 10%, and just under 1% of Drosophilidae species had attached mites, which also caused wounds. Wounding is prevalent in D. melanogaster, likely exerting selection pressure on host immunity for two reasons: on a rapid and efficient wound repair and on responding efficiently to opportunistic infections. Wounding is thus expected to be an important driver of immune system evolution and to affect individual fitness and population dynamics.

Testing the local mate competition rule in a quasi-gregarious parasitoid with facultative superparasitism

As an important domain of evolutionary ecology, sex allocation theory well explains the evolution of investment into female versus male offspring. Local mate competition (LMC) is a good predictor of sex allocation, where the optimal sex ratio becomes less female-biased and asymptotically approaches 0.5 as the number of foundresses increases. Parasitoid wasps, with haplodiploid sex determination, offer excellent opportunities to test how organisms manipulate their offspring sex ratio in response to environmental variation, and many species have been proved to allocate sex according to predictions under LMC. When hosts are spatially clustered, as in gregarious species, the mating systems of quasi-gregarious parasitoids meet the essential assumptions (female mating before dispersal) of LMC. However, inconsistent with predictions, in the quasi-gregarious species Anastatus disparis (Hymenoptera: Eupelmidae), a strongly female-biased eclosion sex ratio (0.156 ± 0.018 to 0.185 ± 0.016) was observed as the number of females laying eggs in a patch increased. Superparasitism, in which 44.7% of parasitized hosts contained more than one egg but only one adult emerged from each host, was common in A. disparis. However, the egg sex ratio was determined by microsatellites and likely fit the predictions of LMC theory. Male-biased offspring mortality arising from superparasitism during development likely contributes to the shift from the primary sex ratio predicted under LMC to the observed female-biased eclosion sex ratio. Inconsistent with results in gregarious parasitoids, the role of superparasitism in driving sex ratio shifts in quasi-gregarious parasitoids should be incorporated into LMC-based predictions of sex ratios.

The Biology of Aging in Insects: From Drosophila to Other Insects and Back

An enormous amount of work has been done on aging in Drosophila melanogaster, a classical genetic and molecular model system, but also in numerous other insects. However, these two extensive bodies of work remain poorly integrated to date. Studies in Drosophila often explore genetic, developmental, physiological, and nutrition-related aspects of aging in the lab, while studies in other insects often explore ecological, social, and somatic aspects of aging in both lab and natural populations. Alongside exciting genomic and molecular research advances in aging in Drosophila, many new studies have also been published on aging in various other insects, including studies on aging in natural populations of diverse species. However, no broad synthesis of these largely separate bodies of work has been attempted. In this review, we endeavor to synthesize these two semi-independent literatures to facilitate collaboration and foster the exchange of ideas and research tools. While lab studies of Drosophila have illuminated many fundamental aspects of senescence, the stunning diversity of aging patterns among insects, especially in the context of their rich ecology, remains vastlyunderstudied. Coupled with field studies and novel, more easily applicable molecular methods, this represents a major opportunity for deepening our understanding of the biology of aging in insects and beyond.

Life history of aggression in Anastatus disparis (Hymenoptera: Eupelmidae) with extreme male-male combat

Aggressive behaviour is common in animals and typically has lifetime consequences. As younger males have higher residual reproductive value than older males and lose more from injuries than older males do, the propensity for fighting tends to increase with age in many empirical reports and species. However, fighting patterns in those empirical reports cannot confirm the hypothesis that individuals cannot readily inflict injuries on their opponents. To address this shortcoming, a parasitoid wasp species, Anastatus disparis (Hymenoptera: Eupelmidae), was used as an experimental model to explore the characteristics of aggression from a life-history perspective; this wasp exhibits extreme fighting, resulting in contestants experiencing injury and death. Results showed that the energetic costs of fighting to injury significantly shortened life and caused the loss of most mating ability. Inconsistent with general predictions, the frequency and intensity of fighting in A. disparis significantly decreased with male age. Further study results showed significantly more young males were received by and successfully mated with virgin females, and most genes related to energy metabolism were downregulated in aged males. Our study provided supporting evidence that young A. disparis males show more aggression likely because of their resource holding potential and sexual attractiveness decline with age.

Sexual dimorphism and sex-biased gene expression in an egg parasitoid species, Anastatus disparis

Background

Differences in the expression of genes present in both sexes are assumed to contribute to sex differences including behavioural, physiological and morphological dimorphisms. For enriching our knowledge of gender differences in an important egg parasitoid wasp, Anastatus disparis (Hymenoptera: Eupelmidae), sex-biased differences in gene expression were investigated using Illumina-based transcriptomic analysis.

Results

A total of 15,812 resulting unigenes were annotated, and a large set of genes accounting for 50.09% of the total showed sex-biased expression and included 630 sex-specific genes. Gene Ontology (GO) enrichment analyses showed that the functional categories associated with sex-biased genes were mainly related to reproduction. In addition, the transcriptome data provided evidence that sex pheromones in A. disparis are produced by the female, and activity of Δ12-desaturases appear to have been replaced by Δ9-desaturases playing roles in sex pheromone production. The large set of sex-biased genes identified in this study provide a molecular background for sexually dimorphic traits such as flyability, longevity, and aggression in this species and suggests candidate venom proteins expressed only in females that could be used for biological control.

Conclusions

This study provides comprehensive insight into sexually dimorphic traits of a parasitoid wasp and can inform future research into the molecular mechanisms underlying such traits and the application of parasitoids to the biological control of pest species.

Effect of Winning Experience on Aggression Involving Dangerous Fighting Behavior in Anastatus disparis (Hymenoptera: Eupelmidae)

Aggressive behavior is widely observed in animal species for acquiring important resources and usually includes both dangerous and nondangerous fighting patterns. Only a few species show dangerous fighting patterns that are defined by fights ending with contestants being severely injured or killed. Prior experience, an important factor in many species, has been demonstrated to affect a contestant's subsequent fighting behavior. Few studies have focused on the effect of experience on aggression involving dangerous fighting patterns. Here, an egg parasitoid wasp, Anastatus disparis, which shows extreme and dangerous fighting behavior to acquire mating opportunities, was used as an experimental model. Our results showed that the fighting intensity of the winning males significantly decreased subsequent fighting behavior, which was inconsistent with general predictions. Transcriptomic analyses showed that many genes related to energy metabolism were downregulated in winners, and winners increased their fighting intensity after dietary supplementation. Our study suggested that fighting in A. disparis is a tremendous drain on energy. Thus, although males won at combat, significant reductions in available energy constrained the intensity of subsequent fights and influenced strategic decisions. In addition, winners might improve their fighting skills and abilities from previous contests, and their fighting intensity after dietary supplementation was significantly higher than that of males without any fighting experience. Generally, in A. disparis, although winners increased their fighting ability with previous experience, the available energy in winners was likely to be a crucial factor affecting the intensity and strategic decisions in subsequent fights.

Effect of variation in objective resource value on extreme male combat in a quasi-gregarious species, Anastatus disparis

BACKGROUND

Aggressive behaviour is widely observed in animal kingdom, which compete for resources such as territory, food and mates. Resource value is the most important non-strategic factor influencing fighting behaviour, and may vary among contests and contestants. Usually, contestants adjust their fighting behaviour when the resource value changes, and as potentially damaging and energetically costly, individuals of most species usually avoid conflict escalation. However, in a quasi-gregarious egg parasitoid, Anastatus disparis (Hymenoptera: Eupelmidae), mates are valuable resources and females mate only once; thus, males engage in frequently extreme combat behaviour to acquire mating opportunities, even in the absence of females. In this study, we attempted to test whether males of this species have the ability to adjust their fighting behaviour in response to changes in the objective value of female.

RESULTS

Our results suggested that objective resource value in A. disparis is likely to be influenced by female mating status rather than by fecundity. Consistent with a number of empirical studies, A. disparis males adjusted their fighting behaviour according to the value of the contested resources: males significantly increased their fighting intensity to acquire mating opportunities with virgin females but decreased their fighting intensity for mated females. We also found that rather than chemical cues, visual cues and physical sexual contact appear to play a role in determining males' ability to detect variation in female mating status.

CONCLUSIONS

Our study suggested that although in this species, males have evolved extreme fighting behaviour and females are valuable resources, males do not always escalate fighting behaviour in competition for mating with a female. Valuable resources and variation in resource value were detected and estimated by A. disparis males, which then adjusted their fighting behaviour accordingly and to some extent avoided incoming fighting costs.