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PhD position available under the joint supervision of the ESIM and Symbiosis teams - Funding by the International SIGNALIFE PhD Programme (http://signalife.unice.fr/).

Project title : "Cross-talk between aphid facultative symbiosis and plant nitrogen fixation symbiosis in the Acyrthosiphon pisum - Medicago truncatula interaction" Aim : question how the plant nitrogen fixation symbiosis and the aphid facultative symbiosis interfere in terms of induced phenotypic effects, and modulate the interaction between the pea aphid and its legume host plant.

Programme opened to all students with a Master diploma equivalent.Appliance on *the web site* for a specific project (http://signalife.unice.fr/17.php).Dead-line : 1st of AprilCandidates, contact  M. Poirié (poirie@sophia.inra.fr) and/or P. Frendo (frendo@sophia.inra.fr) as soon as possible

Breakthroughs :

First demonstration of secretion of a superoxide dismutase in insects and possible role in parasitoids

Following identification of intracellular (SOD1) and extracellular (SOD3) Cu,Zn superoxide dismutase transcripts in the venom apparatus of twoLeptopilinaspecies, parasitoids ofDrosophila, we could demonstrate thatL. boulardiSOD3 (LbSOD3) is indeed secreted and active in venom. No SOD protein nor SOD activity were detected inL. heterotomavenom evidencing quantitative interspecific variation.Leptopilinarecombinant SOD3s as well as a mammalian SODin vitroinhibited theDrosophilaphenoloxidase activity in a dose-dependent manner, demonstrating that SODs may interfere with theDrosophilamelanization process and therefore with production of the host cytotoxic compounds. Phylogenetic analyses ofLeptopilinaSODs indicated that the extracellular SODs are more related to cytoplasmic vertebrate SODs than to extracellular ones. This work provided the first demonstration that insect extracellular SODs are indeed secreted and active in an insect fluid and can be used as virulence factors to counteract the host immune response, a strategy largely used by bacterial and fungal pathogens but also protozoan parasites during infection.

Parasitoid main venom components : similar function, yet different tools

A striking feature that emerges from the recent accumulation of data regarding parasitoid wasp venom content is the lack of predictable observation of common major components. Our recent results on Leptopilina figitid wasps perfectly illustrate this point. Thanks to a combined transcriptomic and proteomic approach, we have identified the main secreted proteins in the venom ofL. heterotomaand of two well-characterized strains ofL. boulardi, ISm and ISy.Results revealed significant quantitative differences in venom components between theL. boulardistrains, in agreement with their different virulence properties. Strikingly, the two relatedLeptopilinaspecies did not share any abundant venom protein. The main identified proteins inL. boulardiwere RhoGAPs and serpins while an aspartylglucosaminidase was found to be abundant inL. heterotoma, as it is in some Asobara species (braconid wasps). Altogether, our data suggest that parasitoid venom can quickly evolve, mainly through rapid changes in regulation of gene expression. They also evidence venom evolutionary processes largely described in other venomous animals, i.e. the convergent recruitment of venom proteins between phylogenetically unrelated organisms, and the role of duplications in the emergence of multigenic families of virulence factors.

First evidence of inter-individual variation in parasitoid venom and evolutionary consequences

Intriguingly, the question of the level of venom variability inside species has been largely neglected, although it may partly determine the potential for parasitoid adaptation. To estimate this parameter, we have developed an approach combining detection of protein electrophoretic patterns on a single venom reservoir content and analysis of these patterns with a dedicated software. This has allowed demonstrating occurrence of inter-individual variability, mostly quantitative, in figitid and braconid species, using both laboratory strains and field populations (publication in J. Insect Physiol.). Whether occurrence of such variability may permit a selection of parasitoid venom components is currently tested in collaboration with the BPI team using Psyttalia biological control auxiliaries. The aim is to assess whether rearing conditions, including the raise of parasitoids on a substitute host, may lead to changes in female venom components and possibly in parasitism success against the targeted host.

The presence of secondary symbionts affects the immune aphid phenotype

Aphids can host several secondary symbionts (SS) in addition to the primary endosymbiontBuchnera aphidicola. These SS have been previously shown to alter various aphid phenotypes, including body color, heat resistance, or resistance to pathogens or parasites. Having characterized the pea aphid hemocytes, and thanks to the availability of lines harboring each SS separately in the same genetic background, we were able to demonstrate that i) plasmatocytes and granulocytes can phagocyte all primary and secondary symbionts but some of these SS seem able to survive inside hemocytes,  ii) that the presence of some SS alters the number of these hemocytes as well as the phenoloxidase activity in the aphid hemolymph, iii) that this effect can be symbiont-strain dependant. Part of these results have been published in PLoS One, others will be submitted soon.

Sex-determination : a genetic basis common to bees, bumblebees and ants

Little is known regarding sex determination mechanisms in Hymenoptera. However, sl-CSD,  one of the main proposed mechanism could drive small populations into a vortex of extinction. Indeed, this mechanism relies on a single locus (namedcsd), only individuals heterozygous at this locus developing into females while homozygous individuals develop into diploid males of reduced fitness. Thecsdgene has been identified in the honeybee and was considered to be unique to the Apis lineage. Our results have demonstrated that i)csdis not restricted to the Apidae but is also present in bumblebees and ants genome, ii) csd originates from an ancestral duplication of thefemgene (key gene in sex determination) in the ancestor of the sub-orderAculeata,  iii) thefemandcsdgenes are one of the rare examples of concerted evolution based on gene conversion. These results, published inNature Communications,raise the question of the occurrence and possible role of thecsdgene in other Hymenoptera, notably in parasitoid wasps, largely used in biological control.