Ultrastructure of basidium and basidiospore development in three species of the fungus Exobasidium

Abstract

The sequence of events occurring within basidia of Exobasidium was shown to be quite different from that previously described in one-celled holobasidia. Karyogamy was found to occur in thin-walled hyphal compartments or probasidia comprising part of a stroma-like system within the intercellular leaf spaces. Meiosis also began in these compartments but was not completed until after the nucleus had migrated into the large, prominent metabasidium that became exposed on the abaxial leaf surface. Following sterigmatal development and a postmeiotic mitotic division, usually four to seven basidiospores developed per basidium. Each spore possessed a hilar appendage on its abaxial side, but neither a hilar appendage body nor a hilar droplet was observed. The thin basidiospore wall was continuous with that of its sterigma. Development of a hilar septum delimited the spore from its sterigma. Detached basidiospores of E. vaccinii and E. japonicum became two celled following mitosis and development of a near-median transverse septum. Basidiospores of E. camelliae var. gracilis usually developed three transverse septa. Based on our observations Exobasidium should be placed in the Teliomycetes.