Ultrastructure and chemical composition of the ballistospore wall ofConidiobolus obscurus

The ballistospores of the entomopathogenConidiobolus obscurus are spheroidal cells with a papilla corresponding to the zone of attachment on the sporophore. They are covered by a mucus responsible for the attachment of the spore to the insect. Chemical and cytochemical investigations of the nature of the wall components have been undertaken to better understand fungus-insect interactions in entomopathology. β(1→3)-Glucans and chitin together represented the main components of the wall which did not contain chitosan and uronic acids. Transmission electron microscopy revealed that the spore wall was composed of a thick electron-lucent inner layer and a thin outer electron-dense layer, the latter being absent at the papilla region. The spore is covered by a mucilaginous layer that upon spore impact on a substratum, is dispersed and forms a halo around the spore. Shadow replicas of the discharged spores showed that they are covered by rodlets except on the papilla which displayed a chitinous, microfibrillar structure. The ontogeny of the rodlets deposited on the surface of young spores was characterized by a progressive organization of separate rodlets and then a clustering of the rodlets in fascicles. Shadow replicas and chemical and enzymatic investigations of the halo surrounding discharged spores showed that the mucus was composed of long β(1→3)-glucan microfibrils embedded in amorphous proteins partly covered by rodlets discharged from the spore surface.