Haemocyte changes in resistant and susceptible strains of D. melanogaster caused by virulent and avirulent strains of the parasitic wasp Leptopilina boulardi

Two strains of Drosophila melanogaster (resistant and susceptible) were parasitized by a virulent or avirulent strain of the parasitoid wasp Leptopilina boulardi. The success of encapsulation depends on both the genetic status of the host strain and the genetic status of the parasitoid strain: the immune cellular reaction (capsule) is observed only with the resistant strain–avirulent strain combination. The total numbers of host haemocytes increased in all 4 combinations, suggesting that an immune reaction was triggered in all hosts. Resistant host larvae infected with the virulent or avirulent strains of parasitoid wasp had slightly more haemocytes per mm 3 than did susceptible host larvae at the beginning of the reaction (less than 15 h post-parasitization). This difference disappeared later. Only the virulent parasitoid strain caused the production of a high percentage of altered lamellocytes (from a discoid shape to a bipolar shape), half the total number of lamellocytes are altered. This suggests that the alteration of lamellocyte shape alone is not sufficient to explain the lack of capsule formation seen in resistant hosts parasitized by the virulent strain. Lastly, there were very few altered lamellocytes in resistant or susceptible hosts parasitized by the avirulent parasitoid strain, two combinations in which no capsule was formed. As is now established for Drosophila-parasitoid interactions, virus-like particles contained in the long gland of the female wasp affect the morphology of the lamellocytes. The results presented here are further proof of the action (direct or indirect) of virus like particles of the virulent strain on lamellocytes.