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 Le ptopilina boulardi are encapsulated neither in resistant nor in suscep tible strains of Drosophila melanogaster but are encapsulated in Droso phila yakuba. Eggs of a nonimmune suppressive strain (= avirulent) of the same parasitoid are encapsulated in a resistant strain of D. melan ogaster and in D. yakuba but are not encapsulated in a susceptible str ain of D. melanogaster. Egg chorion in the 2 parasitoid strains showed the same morphology and the same modifications after egg laying whate ver the host strain. When a capsule is built, a small dotted dense lay er was first spread on the chorion, followed by accumulation layers of cells (plasmatocytes and lamellocytes) and lastly necrosis of the inn er haemocytes. The encapsulated eggs darken only at the time of necros is of haemocytes. In susceptible hosts, neither the tiny dense layer n or haemocyte accumulation occured. We concluded that (1) this tiny den se layer was present before the deposition of the first haemocytes, (2 ) inhibition of deposition of this dense layer was the initial event o f the induced immunosupression, (3) haemocytes other than lamellocytes were engaged in caspsule formation, (4) the immunosupressive factors did not target only the lamellocytes but also the plasmatocytes, (5) d arkening of the encapsulated eggs was due to cell necrosis rather than to extracellular melanin deposition.