Impact of parasitoid-associated polydnaviruses on plant-mediated herbivore interactions

A symbiotic gene stimulates aggressive behavior favoring the survival of parasitized caterpillars

Animals often exhibit increased aggression in response to starvation, while parasites often manipulate host behavior. In contrast, underlying molecular mechanisms for these behavioral changes are mostly unknown. The diamondback moth, Plutella xylostella, is an agricultural pest that feeds on cruciferous plants as larvae, while Cotesia vestalis is a parasitoid wasp that parasitizes diamondback moth larvae. In this study, we determined that unparasitized diamondback moth larvae exhibit increased aggression and cannibalism when starved, while starved larvae parasitized by C. vestalis were more aggressive than unparasitized larvae. C. vestalis harbors a domesticated endogenized virus named Cotesia vestalis bracovirus (CvBV) that wasps inject into parasitized hosts. Starvation increased octopamine (OA) levels in the central nervous system (CNS) of diamondback moth larvae while a series of experiments identified a CvBV-encoded gene product named Assailant that further increased aggression in starved diamondback moth larvae. We determined that Assailant increases OA levels by activating tyramine beta-hydroxylase (PxTβh), which is a key enzyme in the OA biosynthesis pathway. Ectopic expression of assailant in Drosophila melanogaster likewise upregulated expression of DmTβh and OA, which increased aggressive behavior in male flies as measured by a well-established assay. While parasitized hosts are often thought to be at a competitive disadvantage to nonparasitized individuals, our results uncover how a parasitoid uses an endogenized virus to increase host aggression and enhance survival of offspring when competing against unparasitized hosts.

Parasitoid Calyx Fluid and Venom Affect Bacterial Communities in Their Lepidopteran Host Labial Salivary Glands

The influence of gut and gonad bacterial communities on insect physiology, behaviour, and ecology is increasingly recognised. Parasitism by parasitoid wasps alters many physiological processes in their hosts, including gut bacterial communities. However, it remains unclear whether these changes are restricted to the gut or also occur in other tissues and fluids, and the mechanisms underlying such changes are unknown. We hypothesise that host microbiome changes result from the injection of calyx fluid (that contain symbiotic viruses known as polydnaviruses) and venom during parasitoid oviposition and that these effects vary by host tissue. To test this, we microinjected Pieris brassicae caterpillars with calyx fluid and venom from Cotesia glomerata, using saline solution and natural parasitism by C. glomerata as controls. We analysed changes in the bacterial community composition in the gut, regurgitate, haemolymph, and labial salivary glands of the host insects. Multivariate analysis revealed distinct bacterial communities across tissues and fluids, with high diversity in the salivary glands and haemolymph. Parasitism and injection of calyx fluid and venom significantly altered bacterial communities in the salivary glands. Differential abundance analysis showed that parasitism affected bacterial relative abundance in the haemolymph, and that Wolbachia was only found in the haemolymph of parasitized caterpillars. Altogether, our findings reveal that parasitism influences the host haemolymph microbiome, and both parasitism and injection of calyx fluid and venom drive changes in the bacterial community composition within the host salivary glands. Given that the composition of salivary glands can influence plant response to herbivory, we discuss these results in the broader context of plant-parasitoid interactions.

Functional analysis of (E)-β-farnesene synthases involved in accumulation of (E)-β-farnesene in German chamomile (Matricaria chamomilla L.)

German chamomile (Matricaria chamomilla L.) is a traditional medicinal aromatic plant, and the sesquiterpenoids in its flowers have important medicinal value. The (E)-β-farnesene (EβF) is one of the active sesquiterpenoid components and is also a major component of aphid alarm pheromones. In this study, two EβF synthase (βFS) genes (McβFS1 and McβFS2), were cloned from German chamomile. Subcellular localization analysis showed that both McβFS1 and McβFS2 were localized in the cytoplasm and nucleus. Tissue-specific expression analysis revealed that McβFS1 and McβFS2 were expressed in all flower stages, with the highest levels observed during the tubular flower extension stage. Prokaryotic expression and enzyme activity results showed that McβFS1 and McβFS2 possess catalytic activity. Overexpression of McβFS1 and McβFS2 in the hairy roots of German chamomile led to the accumulation of EβF, demonstrating enzyme activity in vivo. The promoters of McβFS1 and McβFS2 were cloned and analyzed. After treating German chamomile with methyl jasmonate (MeJA) and methyl salicylate (MeSA), the transcription levels of McβFS1 and McβFS2 were found to be regulated by both hormones. In addition, feeding experiments showed that aphid infestation upregulated the expression levels of McβFS1 and McβFS2. Our study provides valuable insights into the biosynthesis of EβF, laying a foundation for further research into its metabolic pathways.

The Toll/IMD pathways mediate host protection against dipteran parasitoids

Parasitoids have utilized a variety of strategies to counteract host defense. They are in different taxonomic status and exhibit phenotypic and genetic diversity, and thus are thought to evolve distinct anti-defense mechanisms. In this study, we investigated the performance of two closely related parasitoids, Exorista japonica and Exorista sorbillans (Diptera: Tachinidae) that are biological control agents in agriculture and major insect pests in sericulture, on the host Bombyx mori. We show that the host is more susceptible to E. sorbillans infection while relatively resistant to E. japonica infection. Moreover, the expression levels of host antimicrobial peptides (AMPs) genes are repressed at early infection and induced at late infection of E. japonica, while AMPs are over-expressed at early infection and return to normal levels at late infection of E. sorbillans. In parallel, Toll and IMD pathway genes are generally induced at late infection of E. japonica, whereas these genes are up-regulated at early infection and down-regulated at late infection of E. sorbillans. Activating of host Toll/IMD pathways and AMPs expression by lipopolysaccharide (LPS) represses the larval growth of E. sorbillans. Conversely, inhibiting host Toll/IMD pathways by RNA interference significantly promotes E. japonica development. Therefore, the Toll/IMD pathways are required in the host for defense against infection of dipteran parasitoids. Overall, our study provides the new insight into the diversified host-parasitoid interactions, and offers a theoretical basis for further studies of the adaptive mechanism of dipteran parasitoids.

Indirect plant-mediated interactions between heterospecific parasitoids that develop in different caterpillar species

Parasitoids induce physiological changes in their herbivorous hosts that affect how plants respond to herbivory. The signature of parasitoids on induced plant responses to feeding by parasitized herbivores indirectly impacts insect communities interacting with the plant. The effect may extend to parasitoids and cause indirect interaction between parasitoids that develop inside different herbivore hosts sharing the food plant. However, this type of interactions among parasitoid larvae has received very little attention. In this study, we investigated sequential and simultaneous plant-mediated interactions among two host-parasitoid systems feeding on Brassica oleracea plants: Mamestra brassicae parasitized by Microplitis mediator and Pieris rapae parasitized by Cotesia rubecula. We measured the mortality, development time, and weight of unparasitized herbivores and performance of parasitoids that had developed inside the two herbivore species when sharing the food plant either simultaneously or sequentially. Plant induction by parasitized or unparasitized hosts had no significant effect on the performance of the two herbivore host species. In contrast, the two parasitoid species had asymmetrical indirect plant-mediated effects on each other's performance. Cotesia rubecula weight was 15% higher on plants induced by M. mediator-parasitized hosts, compared to control plants. In addition, M. mediator development time was reduced by 30% on plants induced by conspecific but not heterospecific parasitoids, compared to plants induced by its unparasitized host. Contrary to sequential feeding, parasitoids had no effect on each other's performance when feeding simultaneously. These results reveal that indirect plant-mediated interactions among parasitoid larvae could involve any parasitoid species whose hosts share a food plant.

The effects of plant domestication on the foraging and performance of parasitoids

Domestication-related changes in the chemical traits of crop plants affect parasitoid foraging success, development, and survival. For example, herbivore-induced changes in the production of volatiles by domesticated plants can enhance or reduce parasitoid attraction. While the trade-off between nutrient content and chemical defense in cultivated plants can increase the suitability of hosts for parasitoids, their increased health and size can positively affect their immune response against parasitoids. Overall, plant domestication is expected to significantly affect their relationship with parasitoids due to altered plant morphology, physical characteristics, chemical defenses, and new plant associations. This review highlights the need for research on the effects of plant domestication on host-parasitoid interactions in the interest of better controlling insect pests.

Evolution of koinobiont parasitoid host regulation and consequences for indirect plant defence

Tritrophic interactions among plants, herbivorous insects and their parasitoids have been well studied in the past four decades. Recently, a new angle has been uncovered: koinobiont parasitoids, that allow their host to keep feeding on the plant for a certain amount of time after parasitism, indirectly alter plant responses against herbivory via the many physiological changes induced in their herbivorous hosts. By affecting plant responses, parasitoids may indirectly affect the whole community of insects interacting with plants induced by parasitized herbivores and have extended effects on plant fitness. These important findings have renewed research interests on parasitoid manipulation of their host development. Parasitoids typically arrest their host development before the last instar, resulting in a lower final weight compared to unparasitized hosts. Yet, some parasitoids prolong their host development, leading to larger herbivores that consume more plant material than unparasitized ones. Furthermore, parasitoid host regulation is plastic and one parasitoid species may arrest or promote its host growth depending on the number of eggs laid, host developmental stage and species as well as environmental conditions. The consequences of plasticity in parasitoid host regulation for plant–insect interactions have received very little attention over the last two decades, particularly concerning parasitoids that promote their host growth. In this review, we first synthesize the mechanisms used by parasitoids to regulate host growth and food consumption. Then, we identify the evolutionary and environmental factors that influence the direction of parasitoid host regulation in terms of arrestment or promotion of host growth. In addition, we discuss the implication of different host regulation types for the parasitoid’s role as agent of plant indirect defence. Finally, we argue that the recent research interests about parasitoid plant-mediated interactions would strongly benefit from revival of research on the mechanisms, ecology and evolution of host regulation in parasitoids.