DROP: Molecular voucher database for identification of Drosophila parasitoids.

Molecular identification is increasingly used to speed up biodiversity surveys and laboratory experiments. However, many groups of organisms cannot be reliably identified using standard databases such as GenBank or BOLD due to lack of sequenced voucher specimens identified by experts. Sometimes a large number of sequences are available, but with too many errors to allow identification. Here, we address this problem for parasitoids of Drosophila by introducing a curated open-access molecular reference database, DROP (Drosophila parasitoids). Identifying Drosophila parasitoids is challenging and poses a major impediment to realize the full potential of this model system in studies ranging from molecular mechanisms to food webs, and in biological control of Drosophila suzukii. In DROP, genetic data are linked to voucher specimens and, where possible, the voucher specimens are identified by taxonomists and vetted through direct comparison with primary type material. To initiate DROP, we curated 154 laboratory strains, 856 vouchers, 554 DNA sequences, 16 genomes, 14 transcriptomes, and six proteomes drawn from a total of 183 operational taxonomic units (OTUs): 114 described Drosophila parasitoid species and 69 provisional species. We found species richness of Drosophila parasitoids to be heavily underestimated and provide an updated taxonomic catalogue for the community. DROP offers accurate molecular identification and improves cross-referencing between individual studies that we hope will catalyse research on this diverse and fascinating model system. Our effort should also serve as an example for researchers facing similar molecular identification problems in other groups of organisms.

Variation of success of Leptopilina boulardi in Drosophila yakuba: the mechanisms explored.

Despite an increasing knowledge of insect immune defences and virulence strategies used by parasitoids to escape them, the mechanisms underlying variation of success between parasitoid strains are still poorly understood. We have investigated this point using two lines of the parasitoid wasp Leptopilina boulardi that differ in virulence towards Drosophila yakuba. By injecting oil drops in D. yakuba larvae parasitized by virulent IS(y) females and then dissecting the larvae at different times following injection, we demonstrate that the IS(y) line alters host encapsulation ability but only during the early parasitism period. This effect is mimicked by injecting venom gland extracts, indicating that venom proteins are likely involved in immunosuppression. By contrast, the IS(m) line, unsuccessful on D. yakuba, has no immunosuppressive effect. This variation in virulence may be explained by the striking difference we report in haemocytic profiles between IS(m)- and IS(y)-parasitized larvae. We discuss our results in the light of our knowledge of the strategies evolved by Leptopilina species to counteract the D. melanogaster immune system as well as the role of parasitoid venoms in intra-specific variation of parasitoid virulence.

Dexamethasone inhibition of the cellular immune response of Drosophila melanogaster against a parasitoid.

Host larvae of Drosophila melanogaster injected with the eicosanoid biosynthesis inhibitor, dexamethasone, prior to parasitization by the wasp Leptopilina boulardi, exhibited significantly reduced rates of melanotic encapsulation in comparison with control and saline-injected larvae. The results of this investigation suggest that prostaglandins and other eicosanoids are involved as cell-signaling molecules in the hemocytic encapsulation reaction of D. melanogaster larvae.

Virulence strategies in parasitoid Hymenoptera as an example of adaptive diversity.

Parasitoids are mostly insects that develop at the expense of other arthropods, which will die as a result of the interaction. Their reproductive success thus totally depends on their ability to successfully infest their host whose reproductive success relies on its own ability to avoid or overcome parasitism. Such intense selective pressures have resulted in extremely diverse adaptations in parasitoid strategies that ensure parasitism success. For instance, wasp-specific viruses (polydnaviruses) are injected into the host by parasitoid females to modulate its physiology and immunity. This article synthesizes available physiological and molecular data on parasitoid virulence strategies and discusses the evolutionary processes at work.

Parasite suppression of the oxidations of eumelanin precursors in Drosophila melanogaster.

In insects, eukaryotic endoparasites encounter a series of innate immune effector responses mediated in large part by circulating blood cells (hemocytes) that rapidly form multilayer capsules around foreign organisms. Critical components of the encapsulation response are chemical and enzyme-catalyzed oxidations involving phenolic and catecholic substrates that lead to synthesis of eumelanin. These responses are initiated immediately upon infection and are very site-specific, provoking no undesirable systemic responses in the host. In this study, we were interested to learn if the principal oxidation pathways leading to the synthesis of eumelanin in larvae of Drosophila melanogaster were targets for inhibition by immune suppressive factors (ISF) derived from a virulent strain of the endoparasitic wasp Leptopilina boulardi. Comparative in vitro assays monitored by sensitive electrochemical detection methods showed that ISF derived from female reproductive tissues significantly diminished the oxidations of the two diphenol eumelanin precursors, dopamine and 5,6-dihydroxyindole (DHI). The oxidations of the monophenol tyrosine, and two other related diphenols, dopa and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), were not significantly inhibited by ISF. The data suggest that melanogenesis represents at least one of the host responses suppressed by L. boulardi ISF, and that the oxidation pathways selectively targeted for inhibition are those synthesizing decarboxylated pigment precursors derived from DHI. These observations, together with previous reports of adverse effects of ISF on the ability of hemocytes to adhere to foreign surfaces, suggest a multifaceted approach by the parasitoid to circumvent the innate immune response of D. melanogaster.

Cellular immune response to parasite infection in the Drosophila lymph gland is developmentally regulated.

The mechanisms by which an organism becomes immune competent during its development are largely unknown. When infected by eggs of parasitic wasps, Drosophila larvae mount a complex cellular immune reaction in which specialized host blood cells, lamellocytes and crystal cells, are activated and recruited to build a capsule around the parasite egg to block its development. Here, we report that parasitization by the wasp Leptopilina boulardi leads to a dramatic increase in the number of both lamellocytes and crystal cells in the Drosophila larval lymph gland. Furthermore, a limited burst of mitosis follows shortly after infection, suggesting that both cell division and differentiation of lymph gland hemocytes are required for encapsulation. These changes, observed in the lymph glands of third-instar, but never of second-instar hosts, are almost always accompanied by dispersal of the anterior lobes themselves. To confirm a link between host development and immune competence, we infected mutant hosts in which development is blocked during larval or late larval stages. We found that, in genetic backgrounds where ecdysone levels are low (ecdysoneless) or ecdysone signaling is blocked (nonpupariating allele of the transcription factor broad), the encapsulation response is severely compromised. In the third-instar ecdysoneless hosts, postinfection mitotic amplification in the lymph glands is absent and there is a reduction in crystal cell maturation and postinfection circulating lamellocyte concentration. These results suggest that an ecdysone-activated pathway potentiates precursors of effector cell types to respond to parasitization by proliferation and differentiation. We propose that, by affecting a specific pool of hematopoietic precursors, this pathway thus confers immune capacity to third-instar larvae.