Ultrastructure of teliospore formation in the cedar-apple rust fungus Gymnosporangium juniperi-virginianae

Abstract

A telium of G. jttniperi-virginianae consists of a palisade-like layer of hyphae located beneath the host epidermis. The terminal cell of each hypha within the telium disintegrates before the onset of teliospore formation. The cell immediately beneath the terminal cell functions as a sporogenous cell giving rise to teliospore initials. Each sporogenous cell contains two nuclei which divide mitotically as each initial forms. Two daughter nuclei move into the teliospore initial and two remain in the sporogenous cell. The initial elongates and is delimited from the sporogenous cell by a basal septum. The nuclei within the initial then divide and a centripetally developing septum separates the initial into two binucleate cells. The lower cell dies and becomes the pedicel of the spore while the upper cell or teliospore mother cell continues to develop. The nuclei of the mother cell divide and a centripetally developing central septum divides the cell into two binucleate cells. At this stage, the young teliospore is delimited. Its wall thickens and the spore enlarges, becoming more ellipsoid. Eventually, the nuclei within each cell of the teliospore fuse. After karyogamy, synaptonemal complexes appear in the nuclei of spores still within the telium.