The Nucleotide Sequence And Genome Organization Of The Whitefly Transmitted Sweetpotato Mild Mottle Virus: A Close Relationship With Members Of The Family Potyviridae

peer reviewe

Convergent Use of RhoGAP Toxins by Eukaryotic Parasites and Bacterial Pathogens

Inactivation of host Rho GTPases is a widespread strategy employed by bacterial pathogens to manipulate mammalian cellular functions and avoid immune defenses. Some bacterial toxins mimic eukaryotic Rho GTPase-activating proteins (GAPs) to inactivate mammalian GTPases, probably as a result of evolutionary convergence. An intriguing question remains whether eukaryotic pathogens or parasites may use endogenous GAPs as immune-suppressive toxins to target the same key genes as bacterial pathogens. Interestingly, a RhoGAP domain–containing protein, LbGAP, was recently characterized from the parasitoid wasp Leptopilina boulardi, and shown to protect parasitoid eggs from the immune response of Drosophila host larvae. We demonstrate here that LbGAP has structural characteristics of eukaryotic RhoGAPs but that it acts similarly to bacterial RhoGAP toxins in mammals. First, we show by immunocytochemistry that LbGAP enters Drosophila immune cells, plasmatocytes and lamellocytes, and that morphological changes in lamellocytes are correlated with the quantity of LbGAP they contain. Demonstration that LbGAP displays a GAP activity and specifically interacts with the active, GTP-bound form of the two Drosophila Rho GTPases Rac1 and Rac2, both required for successful encapsulation of Leptopilina eggs, was then achieved using biochemical tests, yeast two-hybrid analysis, and GST pull-down assays. In addition, we show that the overall structure of LbGAP is similar to that of eukaryotic RhoGAP domains, and we identify distinct residues involved in its interaction with Rac GTPases. Altogether, these results show that eukaryotic parasites can use endogenous RhoGAPs as virulence factors and that despite their differences in sequence and structure, eukaryotic and bacterial RhoGAP toxins are similarly used to target the same immune pathways in insects and mammals

Rapid and dfferential evolution of the venom composition of a parasitoid wasp depending on the host strain

Abstract: Parasitoid wasps rely primarily on venom to suppress the immune response and regulatethe physiology of their host. Intraspecific variability of venom protein composition has beendocumented in some species, but its evolutionary potential is poorly understood. We performed anexperimental evolution initiated with the crosses of two lines of Leptopilina boulardi of differentvenom composition to generate variability and create new combinations of venom factors. Theoffspring were maintained for 10 generations on two strains of Drosophila melanogaster differing inresistance/susceptibility to the parental parasitoid lines. The venom composition of individuals wascharacterized by a semi-automatic analysis of 1D SDS-PAGE electrophoresis protein profiles whoseaccuracy was checked by Western blot analysis of well-characterized venom proteins. Results madeevident a rapid and differential evolution of the venom composition on both hosts and showed thatthe proteins beneficial on one host can be costly on the other. Overall, we demonstrated the capacityof rapid evolution of the venom composition in parasitoid wasps, important regulators of arthropodpopulations, suggesting a potential for adaptation to new hosts. Our approach also proved relevantin identifying, among the diversity of venom proteins, those possibly involved in parasitism successand whose role deserves to be deepened

Variability of venom components in immune suppressive parasitoid wasps: From a phylogenetic to a population approach

International audienceEndoparasitoid wasps develop at the expense of other insects, leading to their death. Eggs deposited inside the host body induce an immune response, which results in the formation of a melanized cellular capsule around the egg. To evade or counteract this response, endoparasitoids have evolved different strategies, the most often reported being injection into the host of immunosuppressive factors, notably venom proteins, along with the egg. The analysis of venom components has been performed independently in species of different taxa, but the present picture is far from complete. Intriguingly, the question of the level of venom variability inside species has been neglected, although it may partly determine the potential for parasitoid adaptation. Here, we present a short review of our present knowledge of venom components in endoparasitoids, as well as of the only well-known example of intraspecific variability in a venom immune suppressive protein being responsible for variation in parasitoid virulence. We then present data evidencing inter-individual variation of venom protein profiles, using a gel electrophoresis approach, both in laboratory strains and field populations of a figitid and a braconid species. Whether occurrence of such variability may permit a selection of parasitoid venom components driven by the host remains to be tested, notably in the context of the production and use of biological control auxiliaries

Variation in the Venom of Parasitic Wasps, Drift, or Selection? Insights From a Multivariate QST Analysis

Differentiation of traits among populations can evolve by drift when gene flow is low relative to drift or selection when there are different local optima in each population (heterogeneous selection), whereas homogeneous selection tends to prevent evolution of such a differentiation. Analyses of geographical variations in venom composition have been done in several taxa such as wasps, spiders, scorpions, cone snails and snakes, but surprisingly never in parasitoid wasps, although their venom should constrain their ability to succeed on locally available hosts. Such a study is now facilitated by the development of an accurate method (quantitative digital analysis) that allows analyzing the quantitative variation of large sets of proteins from several individuals. This method was used here to analyse the venom-based differentiation of four samples of Leptopilina boulardi and five samples of L. heterotoma from populations along a 300 km long south-north gradient in the Rhône-Saône valley (South-East of France). A major result is that the composition of the venom allows to differentiate the populations studied even when separated by few kilometers. We further analyzed these differentiations on the populations (reared under similar conditions to exclude environmental variance) with a QST analysis which compared the variance of a quantitative trait (Q) among the subpopulations (S) to the total variance (T). We also used random forest clustering analyses to detect the venom components the most likely to be adapted locally. The signature of the natural selection was strong for L. heterotoma and L. boulardi. For the latter, the comparison with the differentiation observed at some neutral markers revealed that differentiation was partly due to some local adaptation. The combination of methods used here appears to be a powerful framework for population proteomics and for the study of eco-evolutionary feedbacks between proteomic level and population and ecosystem levels. This is of interest not only for studying field evolution at an intermediate level between the genome and phenotypes, or for understanding the role of evolution in chemical ecology, but also for more applied issues in biological control

The Origin of Intraspecific Variation of Virulence in an Eukaryotic Immune Suppressive Parasite

Occurrence of intraspecific variation in parasite virulence, a prerequisite for coevolution of hosts and parasites, has largely been reported. However, surprisingly little is known of the molecular bases of this variation in eukaryotic parasites, with the exception of the antigenic variation used by immune-evading parasites of mammals. The present work aims to address this question in immune suppressive eukaryotic parasites. In Leptopilina boulardi, a parasitic wasp of Drosophila melanogaster, well-defined virulent and avirulent strains have been characterized. The success of virulent females is due to a major immune suppressive factor, LbGAP, a RacGAP protein present in the venom and injected into the host at oviposition. Here, we show that an homologous protein, named LbGAPy, is present in the venom of the avirulent strain. We then question whether the difference in virulence between strains originates from qualitative or quantitative differences in LbGAP and LbGAPy proteins. Results show that the recombinant LbGAPy protein has an in vitro GAP activity equivalent to that of recombinant LbGAP and similarly targets Drosophila Rac1 and Rac2 GTPases. In contrast, a much higher level of both mRNA and protein is found in venom-producing tissues of virulent parasitoids. The F1 offspring between virulent and avirulent strains show an intermediate level of LbGAP in their venom but a full success of parasitism. Interestingly, they express almost exclusively the virulent LbGAP allele in venom-producing tissues. Altogether, our results demonstrate that the major virulence factor in the wasp L. boulardi differs only quantitatively between virulent and avirulent strains, and suggest the existence of a threshold effect of this molecule on parasitoid virulence. We propose that regulation of gene expression might be a major mechanism at the origin of intraspecific variation of virulence in immune suppressive eukaryotic parasites. Understanding this variation would improve our knowledge of the mechanisms of transcriptional evolution currently under active investigation

Involvement of the Cytokine MIF in the Snail Host Immune Response to the Parasite Schistosoma mansoni

We have identified and characterized a Macrophage Migration Inhibitory Factor (MIF) family member in the Lophotrochozoan invertebrate, Biomphalaria glabrata, the snail intermediate host of the human blood fluke Schistosoma mansoni. In mammals, MIF is a widely expressed pleiotropic cytokine with potent pro-inflammatory properties that controls cell functions such as gene expression, proliferation or apoptosis. Here we show that the MIF protein from B. glabrata (BgMIF) is expressed in circulating immune defense cells (hemocytes) of the snail as well as in the B. glabrata embryonic (Bge) cell line that has hemocyte-like features. Recombinant BgMIF (rBgMIF) induced cell proliferation and inhibited NO-dependent p53-mediated apoptosis in Bge cells. Moreover, knock-down of BgMIF expression in Bge cells interfered with the in vitro encapsulation of S. mansoni sporocysts. Furthermore, the in vivo knock-down of BgMIF prevented the changes in circulating hemocyte populations that occur in response to an infection by S. mansoni miracidia and led to a significant increase in the parasite burden of the snails. These results provide the first functional evidence that a MIF ortholog is involved in an invertebrate immune response towards a parasitic infection and highlight the importance of cytokines in invertebrate-parasite interactions