Ontogeny of conidia in Fusarium sulphureum

Abstract

Development of the first conidium from a phialide in Fusarium sulphureum was observed by light microscopy. The conidiogenous cell, at first indistinguishable from a hyphal branch, develops an ellipsoidal apical initial which elongates to form a conidium. Electron microscope observations showed the cell wall of the conidiogenous cell and the conidium initial to be continuous with that of the hypha and is composed of two layers, a discrete, electron-dense outer layer and an electron transparent inner one. The outer layer, first at the apex of the conidium initial then more distally, increases in thickness, becomes fibrillar in appearance and expands away from the apex. The expanded fibrillar layer ruptures to form the collarette. During expansion of the fibrillar layer, elongation of the conidium initial is arrested and a new discrete outer wall layer forms at its apex; also the inner wall at the subapical constriction increases in thickness. After formation of the collarette, the conidium initial elongates and a basal septum forms at the base. The basal septum splits abscissionally to form the inner wall layers of both the conidium and a new conidium initial. New electron dense outer wall layers form on both of these inner wall layers before the aseptate conidium is released.