Drosophila immune response to the endoparasitoid wasp Leptopilina boulardi : characterization of a resistance reaction

Drosophila melanogaster is a main model in biology, notably immunity and evolution. Although the Drosophila innate immune processes to fight bacteria and fungi have largely been explored, less is known of the defence against endoparasitoid wasps whose successful development inside the insect host leads to its death. One of the most studied Drosophila – parasitoid wasp interaction involves Leptopilina boulardi that lays eggs inside host larvae and develop at their expense. Once the parasitoid egg has been recognized as a foreign invader, the Drosophila larva can mount a successful immune response, the encapsulation: the egg is surrounded by several layers of hemocytes and there is an increase of a specific types of hemocytes, the lamellocytes. The so-formed capsule is melanised and there is formation of reactive oxygen species, which kills the parasitoid. Alternatively, the immune response can be circumvented thanks to the venom components injected by the female wasp together with the egg. Using two Drosophila strains, resistant and susceptible to L. boulardi, which differ only in a region of chromosome 2R containing a major resistance gene. The resistance was found to be monogenic, with two alleles, the resistance allele being dominant (Rlb+>Rlb). The team previously identified edl/mae (R and S alleles) as a candidate gene. Mae (Modulator of the Activity of ETS) or edl (ETS-domain lacking) was described as a mediator of specific transcription factors of the ETS (E26 transformation-specific) family in Drosophila. Mae interacts with transcription factors trough a SAM (Steril Alpha Domain), a protein – protein interaction domain. Mae is known to regulate yan and pnt P2 transcription factors during the eye development and yan and pnt P2 appear to have a role during haematopoiesis. The objectives of my thesis were to decipher the possible role of edl/mae and identify the molecular and cellular events leading to success or failure of encapsulation. I used various approaches from fly genetics to flow cytometry. The involvement of edl/mae in Drosophila resistance was confirmed by using overexpression and interference of mae expression. The overexpression of the resistant allele in a susceptible background leads to a resistant phenotype. The interference of the expression of the susceptible allele results in an increased rate of parasitoid encapsulation. At the cellular level, an increased in the number of hemocytes after parasitism occurred earlier in the resistant strain than in the susceptible strain. It was also observed that the hematopoietic lymph gland of the resistant larvae busted before the one of the susceptible larvae. At the molecular level, potential interactants of mae were identified in silico and 2 were tested using interference of their expression which led to observing an increase of encapsulation. Overall, a key player in the resistance mechanism of the Drosophila to the parasitic wasp have been identify during this work and it lays the path for future work on regulation mechanism of the response at the molecular level.