Ultrastructure of Sporogenesis in a Moss, Ditrichum pallidum II. Metaphase I through the Tetrad

Events of meiosis subsequent to prophase are reported for a moss, Ditrichum pallidum. Condensed metaphase I bivalents are deployed along the equator of an open spindle consisting of continuous microtubules (MTs) and chro- mosomal MTs attached at kinetochores. Structurally simple spindle poles are lo- cated in the cytoplasm within opposite cleavage furrows between pairs of cyto- plasmic lobes into which telophase II nuclei will be distributed. No discrete structure was observed associated with lobing or with spindle MT orientation. After a short inframeiotic interphase, meiosis II occurs in the undivided cytoplasm with spindles perpendicular to each other. Each of the poles is located near the plastid within each of the four lobes. Intersporal septa are formed by coalescence of vesicles simultaneously along cleavage planes predetermined by precocious lobing. Fused vesicle membranes contribute plasma membrane to the proximal surfaces of the spores, and the fibrillar contents of the vesicles, indistinguishable from the polysaccharide fibrils of the sporocyte wall, separate the young spores within the tetrad. Spore tetrads of Ditrichum pallidum (Hedw.) Hampe are arranged tetrahedrally within the sporocyte wall which persists throughout meiosis. The tetrahedral arrangement of the spores is correlated with the lobing of the sporocyte cytoplasm during prophase and results from an integration of spindle orientation, chromosome distribution and septum formation. Details of meiotic prophase in D. pallidum were reported in an earlier paper (Brown & Lemmon 1980). The present paper describes the nature of the meiotic spindles and the cleavage of cytoplasm into spores. A third paper in this series will describe spore wall development.