Parasitism-linked block of host plasma melanization

When parasitized by the Ichneumonid parasitoid Campoletis sonorensis, larva e of the Noctuid moth, Heliothis virescens, are unable to mount an effectiv e immune response against parasitoid eggs. Defensive melanization of plasma and cellular encapsulation of parasite eggs are dramatically inhibited by infection with the symbiotic immunosuppressive C. sonorensis ichnovirus (Cs IV), This study demonstrates that the CsIV-mediated inhibition of melanizat ion is associated with reduction in the enzymatic activity and protein tite r of key enzymes in the melanization pathway, phenoloxidase, dopachrome iso merase, and DOPA decarboxylase, Inhibition of the synthesis of key melaniza tion enzymes leads to reductions in the melanization substrates L-dihydroxy phenylalanine, N-acetyldopamine, and N-beta-alanyl dopamine from millimolar to nanomolar levels in parasitized larvae. By contrast, concentration of a precursor catecholamine, dopamine, rises fourfold in these larvae. Thus in CsIV-infected larvae, enzymatic deficiencies in the melanization pathway l ead to reduced concentrations of specific enzyme substrates, causing failur e of melanization in parasitized insects. (C) 2000 Academic Press.