Insect immunity: early events in the encapsulation process of parasitoid (Leptopilina boulardi) eggs in resistant and susceptible strains of Drosophila

Eggs of an immune suppressive strain (= virulent) of the parasitoid Leptopilina boulardi are encapsulated neither in resistant nor in susceptible strains of Drosophila melanogaster but are encapsulated in Drosophila yakuba. Eggs of a nonimmune suppressive strain (= avirulent) of the same parasitoid are encapsulated in a resistant strain of D. melanogaster and in D. yakuba but are not encapsulated in a susceptible strain of D. melanogaster. Egg chorion in the 2 parasitoid strains showed the same morphology and the same modifications after egg laying whatever the host strain. When a capsule is built, a small dotted dense layer was first spread on the chorion, followed by accumulation layers of cells (plasmatocytes and lamellocytes) and lastly necrosis of the inner haemocytes. The encapsulated eggs darken only at the time of necrosis of haemocytes. In susceptible hosts, neither the tiny dense layer nor haemocyte accumulation occurred. We concluded that (1) this tiny dense layer was present before the deposition of the first haemocytes, (2) inhibition of deposition of this dense layer was the initial event of the induced immunosuppression, (3) haemocytes other than lamellocytes were engaged in capsule formation, (4) the immunosuppressive factors did not target only the lamellocytes but also the plasmatocytes, (5) darkening of the encapsulated eggs was due to cell necrosis rather than to extracellular melanin deposition.