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

Abstract

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.

Extracellular Superoxide Dismutase in Insects

“…This suggests that SOD3 is not used as a virulence factor by L. heterotoma as it is by L. boulardi, thus evidencing interspecific variation in venom toxins in Leptopilina parasitoids despite phylogenetic proximity and the use of similar hosts. We recently showed that the quantitative difference in the LbGAP venom toxin was at the origin of the intraspecific variation of virulence between L. boulardi ISm and ISy lines (12). No such variation was found for SOD3 proteins neither in sequence or in venom quantity and activity, which seems to preclude a role of SODs in intraspecific variation of L. boulardi virulence.…”

“…11). The venom of ISm females, a highly virulent strain against Drosophila melanogaster, contains a RhoGAP protein that is required for parasitism success (12). This factor targets Drosophila lamellocytes, the main capsule-forming hemocytes, and induces changes in their morphology by inactivating the RhoGTPases Rac1 and Rac2, both required for successful encapsulation (13)(14)(15)(16)(17).…”

Harnessing the natural Drosophila-parasitoid model for integrating insect immunity with functional venomics

“…While L. boulardi -17 infection affects the expression of more than 400 genes, only a small subset are differentially regulated after L. heterotoma -14 attack. This analysis and other inter-specific comparisons suggest that host-parasite resistance-virulence success interdependencies correlate to wasp venom compositions [11*,12*,53]. Intra-specific comparisons indicate that L. boulardi ( Lb ) strains differ in the expression of LbGAP alleles.…”

“…Intra-specific comparisons indicate that L. boulardi ( Lb ) strains differ in the expression of LbGAP alleles. LbGAP encodes a RhoGAP domain-containing venom protein that significantly affects host hemocyte shapes and function [11*,53,54]. Lbm , the more widely successful strain of Lb , expresses higher levels of LbGAP RNA than the less successful strain Lby , suggesting allelic differences in cis-regulation as the basis for variation in virulence [53].…”

Genetic analyses of resistance against Leptopilina victoriae in Drosophila bipectinata

“…However, the loss and gain of host resistance or parasitoid counter-resistance do not necessarily require extensive genetic changes but occur with simple genetic changes. For example, the cellular encapsulation responses of Drosophila melanogaster against Leptopilina clavipes eggs is controlled by some proteins in the N-glycosylation pathway (Mortimer et al 2012), and virulence/avirulence of a parasitoid L. boulardi against D. melanogaster was dependent on the expression level of a immune suppressive protein in venom (Colinet et al 2007(Colinet et al , 2010. These proteins would play key roles in the resistance and virulence mechanisms.…”