Ultrastructural and cytochemical characterization of aphid invasion by the hyphomycete Verticillium lecanii

Chronological events of the interaction between the hyphomycete Verticillium 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) adherence of conidia to the host cuticle through a thin mucilagenous matrix; (ii) germination of the conidia and production of mycelium that colonizes the surface of the cuticle; (iii) penetration of germ tubes into the aphid cuticle 24 h after application of the pathogen; (iv) extensive lateral development of hyphae accompanied by pronounced degradation of the cuticular layers by 72 h. Labeling with the WGA/ovomucoid-gold complex showed that penetration and colonization of the cuticle by the fungus resulted from localized enzymatic hydrolysis, likely through the synergistic action of chitinases and 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 release of the fungus from aphid cadavers. These observations bring further insights into the mechanisms by which V; lecanii parasitizes M. euphorbiae. They also provide a basis for comparing the modes of action of V. lecanii against hosts from fungal and animal origins. (C) 1999 Academic Press.