Ultrastructure of conidial germ-tube development in vitro by the insect pathogen Entomophaga aulicae

Abstract

In an effort to understand the factors influencing the formation of infection structures by the insect pathogen Entomophaga aulicae, we examined the ultrastructural development of conidial germ tubes formed in vitro under conditions that resemble those producing appressoria and protoplasts during infection. Conidia germinated on formvar in a nonaqueous environment produced a single viable germ tube, which in turn produced appressorium-like structures or secondary conidia, structures similar to those formed on the host. The formation of appressorium-like structures indicates that stimuli for appressorium formation are relatively nonspecific, whereas development of the infection hypha requires different triggers. Conidia germinated in a liquid culture medium, which supports the growth of the protoplast stage of the fungus, produced a single viable germ tube that continued to grow apically. This apical growth was accompanied by the loss of the outer layer of the germ tube wall and the presence of electron-opaque granules in an extensive system of cytoplasmic membranes. Protoplasts did not form directly from these germ tubes and ultrastructural details of tip growth in this medium did not resemble those previously described in infection hyphae prior to protoplast formation in the host.