Details of mitosis are described by serial-section electron microscopy of synchronized yeast-phase cultures of five species ofTaphrina. Based primarily on the morphology and behaviour of the nucleus-associated organelles (NAOs) and modes of spindle formation we conclude that three species are typical ascomycetes which have some significant differences in their mitotic systems. Each of two cultures of a fourth species,T. deformans, was apparently a mixed culture. An ascomycete type of mitosis was shared by cells of both cultures, but one culture also had cells with another ascomycete mitotic system characterized by elaborate and unique NAOs, whereas other cells of the second culture contained a basidiomycete mitotic apparatus. The fifth species contained a typical basidiomycetous mitotic system, which indicates that it is misidentified. The detailed differences among the mitotic systems of the trueTaphrinaspecies suggest that the genus as a whole is polyphyletic but undoubtedly ascomycetous. However, a previously unreported pattern of spindle formation found in three species is only otherwise known in a red alga, which may support a common ancestry for the red algae and the ascomycetes. New observations pertinent to the mechanisms of mitosis include, in various species, (a) evidence for splitting of NAOs, and their attendant spindle microtubule arrays, during prophase, (b) spindle microtubule polymerization adjacent to the nuclear envelope prior to NAO insertion into the envelope, and (c) presence of only two to four nonkinetochore microtubules during anaphasendash;telophase.
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