Ultrastructure of meiosis in the hollyhock rust fungus,Puccinia malvacearum I. Prophase I — Prometaphase I

A thoroughly documented account of the ultrastructure of the meiotic spindle pole body (SPB) cycle in a rust (Basidiomycota, Uredinales) is presented for the first time. The three-dimensional structure of the SPB and spindle during meiosis in the hollyhock rust fungusPuccinia malvacearum is analyzed from serial sections of preselected stages. This paper covers prophase I to prometaphase I. At late prophase I, the nucleolus disperses and does not reappear until the end of meiosis. The SPB at late prophase I consists of two, 4-layered discs, 0.8–1.0 μm in diameter, connected by a middle piece (MP). The SPB is associated with a differentiated region of the nuclear envelope and nucleoplasm. At late diplotene to diakinesis, each disc generates a half spindle as it inserts into an otherwise intact nuclear envelope. The MP connecting the interdigitating half spindles elongates and eventually splits transversely during subsequent spindle elongation. Each half MP, which is attached to a SPB disc, becomes inserted in a sheath-like extension of the nuclear envelope. The intranuclear late prometaphase I spindle always becomes oriented perpendicularly to the longitudinal axis and sagittal plane of the metabasidium. There are 200–290 spindle microtubules (MTs) at each SPB at late prometaphase. The nonkinetochore MTs form a coherent central spindle around which the kinetochore MTs and bivalents are spread. A metaphase plate is absent. The results are compared with SPB behavior and spindle structure in early meiosis of other basidiomycetes and ascomycetes.