Host specificity and mechanism of resistance in a mycoparasitic system

Evidence obtained in this study implicates chitosan in the cell wall and γ-linolenic acid in the cellular lipids of the mucoraceous fungi as determinants of host specificity of the mycoparasite, Piptocephalis virginiana . The cell wall of the susceptible host, young (1-day-old) culture of Choanephora cucurbitarum , is composed of a single layer of randomly arranged microfibrils. Mechanical penetration of this host by the parasite resulted in a compatible interaction. Resistant hosts, Phascolomyces arliculosis and 7-day-old Choanephora cucurbitarum , invariably possessed 2-layered cell walls with characteristic macromolecular architecture; an outer layer with random microfibril orientation and an inner layer with parallel microfibril arrangement. The penetration of these resistant hosts was through enzymatic dissolution of the outer layer. The failure of the parasite to establish nutritional contact with the resistant host was either due to the papilla formation by the resistant host in response to invasion by the parasite or due to the development of an extensive sheath around the invading parasite, thereby preventing its direct contact with host protoplast. The parasite did not attempt penetration of the non-host species, Linderina permispora and Chaetomium cochlioides . A hypothesis concerning the induced development of papilla is presented and the function of the latter in resistance to a mycoparasite is discussed.