Fine structure of the fungus Metarrhizium anisopliae infecting three species of larval Elateridae (Coleoptera) : III. Penetration of the host integument

Abstract

Ultrastructural evidence is presented for four stages of fungal development in a typical penetration of the integument of larval Elateridae by Metarrhizium anisopliae: (1) penetration of the epicuticle by a peg produced by an appressorium, and the formation of a subepicuticular penetration plate; (2) production of lateral penetration hyphae and penetration hyphal bodies of Type I from the penetration plate; (3) formation of Type II penetration hyphal bodies from the Type I hyphal bodies, and their proliferation; and (4) stepwise penetration of the procuticle to the hypodermis by vertical penetration hyphae produced by Type II hyphal bodies. The fine structure of the organelles of the various fungal cells formed during penetration of the integument, and of the changes in the integumental layers during fungal penetration, is described. The fine structure suggests that penetration of the epicuticle of the host by the fungus, while facilitated initially by mechanical pressure, is primarily enzymatic, and that penetration of the procuticle is by mechanical separation of the cuticular laminae and fibrils, facilitated by enzymatic action.