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The origin of intraspecific variation of virulence in an eukaryotic immune suppressive parasite.

Colinet D, Schmitz A, Cazes D, Gatti JL, Poirié M - PLoS Pathog. (2010)

Bottom Line: In contrast, a much higher level of both mRNA and protein is found in venom-producing tissues of virulent parasitoids.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.Understanding this variation would improve our knowledge of the mechanisms of transcriptional evolution currently under active investigation.

View Article: PubMed Central - PubMed

Affiliation: Institut National de la Recherche Agronomique, Sophia Antipolis, France. dominique.colinet@sophia.inra.fr

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.

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The RacGAP protein is necessary for virulence of ISm females.(A) D. melanogaster resistant L2 larvae were injected with ISm venom incubated either with the pre-immune serum as a control or a specific polyclonal antibody against LbGAP, then parasitized with the ISy avirulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae. (B) D. melanogaster resistant L2 larvae were injected with the pre-immune serum as a control or the specific antibody against LbGAP, and then parasitized with the ISm virulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae.
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ppat-1001206-g001: The RacGAP protein is necessary for virulence of ISm females.(A) D. melanogaster resistant L2 larvae were injected with ISm venom incubated either with the pre-immune serum as a control or a specific polyclonal antibody against LbGAP, then parasitized with the ISy avirulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae. (B) D. melanogaster resistant L2 larvae were injected with the pre-immune serum as a control or the specific antibody against LbGAP, and then parasitized with the ISm virulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae.

Mentions: Previous work had shown that LbGAP is sufficient for successful parasitism of resistant D. melanogaster hosts by virulent parasitoid females. In order to determine whether this factor is also necessary for virulence, we performed experiments of injection of ISm venom in resistant D. melanogaster larvae, which is known to protect avirulent ISy eggs from encapsulation [24],[29]. In the present work, ISm venom was incubated before injection either with a specific polyclonal antibody against LbGAP or with the preimmune serum as a control, and larvae were then submitted to parasitism by ISy females. Venom incubated with the preimmune serum conferred active protection to avirulent eggs, with only 18.6% of encapsulation (Figure 1A). By contrast, incubation of ISm venom with the antibody against LbGAP led to 75.9% of avirulent eggs being encapsulated (Chi2 = 38.43 ; ddl = 1; p<0.001). A second experiment was performed in which the LbGAP antibody or the preimmune serum were injected alone into resistant host larvae that were subsequently parasitized by virulent ISm parasitoids. The encapsulation rates were 0% with the preimmune serum and 34.8% following injection of the LbGAP antibody, respectively (Chi2 = 24.49 ; ddl = 1 ; p<0.001; Figure 1B). These results demonstrate that LbGAP is a venom protein needed for L. boulardi virulence against resistant D. melanogaster flies.


The origin of intraspecific variation of virulence in an eukaryotic immune suppressive parasite.

Colinet D, Schmitz A, Cazes D, Gatti JL, Poirié M - PLoS Pathog. (2010)

The RacGAP protein is necessary for virulence of ISm females.(A) D. melanogaster resistant L2 larvae were injected with ISm venom incubated either with the pre-immune serum as a control or a specific polyclonal antibody against LbGAP, then parasitized with the ISy avirulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae. (B) D. melanogaster resistant L2 larvae were injected with the pre-immune serum as a control or the specific antibody against LbGAP, and then parasitized with the ISm virulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2991256&req=5

ppat-1001206-g001: The RacGAP protein is necessary for virulence of ISm females.(A) D. melanogaster resistant L2 larvae were injected with ISm venom incubated either with the pre-immune serum as a control or a specific polyclonal antibody against LbGAP, then parasitized with the ISy avirulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae. (B) D. melanogaster resistant L2 larvae were injected with the pre-immune serum as a control or the specific antibody against LbGAP, and then parasitized with the ISm virulent line. The encapsulation rate was recorded after 48 h. In brackets are the numbers of injected larvae.
Mentions: Previous work had shown that LbGAP is sufficient for successful parasitism of resistant D. melanogaster hosts by virulent parasitoid females. In order to determine whether this factor is also necessary for virulence, we performed experiments of injection of ISm venom in resistant D. melanogaster larvae, which is known to protect avirulent ISy eggs from encapsulation [24],[29]. In the present work, ISm venom was incubated before injection either with a specific polyclonal antibody against LbGAP or with the preimmune serum as a control, and larvae were then submitted to parasitism by ISy females. Venom incubated with the preimmune serum conferred active protection to avirulent eggs, with only 18.6% of encapsulation (Figure 1A). By contrast, incubation of ISm venom with the antibody against LbGAP led to 75.9% of avirulent eggs being encapsulated (Chi2 = 38.43 ; ddl = 1; p<0.001). A second experiment was performed in which the LbGAP antibody or the preimmune serum were injected alone into resistant host larvae that were subsequently parasitized by virulent ISm parasitoids. The encapsulation rates were 0% with the preimmune serum and 34.8% following injection of the LbGAP antibody, respectively (Chi2 = 24.49 ; ddl = 1 ; p<0.001; Figure 1B). These results demonstrate that LbGAP is a venom protein needed for L. boulardi virulence against resistant D. melanogaster flies.

Bottom Line: In contrast, a much higher level of both mRNA and protein is found in venom-producing tissues of virulent parasitoids.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.Understanding this variation would improve our knowledge of the mechanisms of transcriptional evolution currently under active investigation.

View Article: PubMed Central - PubMed

Affiliation: Institut National de la Recherche Agronomique, Sophia Antipolis, France. dominique.colinet@sophia.inra.fr

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.

Show MeSH
Related in: MedlinePlus