Ultrastructural and cytochemical characterization of aphid invasion by thehyphomycete Verticillium lecanii

Chronological events of the interaction between the hyphomycete Verticilliu m lecanii and the potato aphid Macrosiphum euphorbiae were investigated by light, scanning, and transmission electron microscopy. The parasitism of M. euphorbiae by V. lecanii appears to involve the following events: (i) adhe rence of conidia to the host cuticle through a thin mucilagenous matrix; (i i) germination of the conidia and production of mycelium that colonizes the surface of the cuticle; (iii) penetration of germ tubes into the aphid cut icle 24 h after application of the pathogen; (iv) extensive lateral develop ment of hyphae accompanied by pronounced degradation of the cuticular layer s by 72 h. Labeling with the WGA/ovomucoid-gold complex showed that penetra tion and colonization of the cuticle by the fungus resulted from localized enzymatic hydrolysis, likely through the synergistic action of chitinases a nd mechanical pressure; (v) production of blastospores and massive invasion of aphid internal tissues; (vi) assimilation of nutrients and accumulation of lipids by fungal cells; and (vii) production of conidiophores and relea se of the fungus from aphid cadavers. These observations bring further insi ghts into the mechanisms by which V; lecanii parasitizes M. euphorbiae. The y also provide a basis for comparing the modes of action of V. lecanii agai nst hosts from fungal and animal origins. (C) 1999 Academic Press.