Ultrastructural study of promycelial development and basidiospore initiation in Ustilago maydis

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Abstract
Cytoplasm and walls of developing promycelia and basidiospores of U. maydis were examined. The promycelial wall was derived from an inner wall layer of the teliospore. Small vesicles and probable Golgi cisternae apparently are associated with promycelial extension. Some promycelial septa contained small pores; spherical vesicles, formed centripetally in the electron-transparent lamella at the center of the septa, was apparently involved in promycelial fragmentation. An inner layer of the basidial wall gave rise to the wall of the developing basidiospore, and a collar of torn outer wall material surrounded the spore base. Spores were formed on short sterigmata. No significant vacuolation occurred in the promycelium during initial basidiospore formation, a feature unique among phragmobasidiomycetes examined thus far. The distribution of cytoplasmic organelles in the promycelium is like that seen in vegetative structures of other fungi. The placement of Ustilaginales in the Basidiomycetes is supported by the layered walls and small pores in the promycelium. Derivation of the basidiospore wall and the migration of the nucleus into the basidiospore before mitosis resembled aspects of budding in basidiomycetous yeasts.

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