Postmeiotic Mitosis, Basidiospore Development, and Septation in Ustilago maydis

Abstract

Postmeiotic mitosis, basidiospore development and metabasidial septation are analyzed ultrastructurally in Ustilago maydis (Basidiomycota, Ustilaginales). The spindle pole body at mitotic prophase is composed of two globular elements connected by a middle piece. During the transition from interphase II to mitotic prophase the globular elements enlarge, the middle piece elongates, and the spindle pole body typically moves to an apical position on the migratory nucleus. Cytoplasmic microtubules are associated with organeUe movement into the basidiospore. Sterigmata arise endogenously. Mitotic nuclei divide into a spindle- containing portion which moves into the basidiospore and a nucleolar portion which degenerates in the metabasidium. One postmitotic sibhng nucleus remains in the basidiospore while the other returns to the metabasidium. Primary metabasidial and sterigmal septa are traversed by a simple, eccentric to centric pore that may become walled-off. Complete, adventitious metabasidial septa result in the formation of fragmentation zones. The nuclear and spindle pole body cycles of U. maydis support the inclusion of the Ustilaginales in the Basidiomycetes, and these features are similar to those reported for the heterobasidiomycetous yeasts.