CELL DIVISION AND WALL STRUCTURE IN MICROSPORA

Abstract

SUMMARYCell division and wall structure are described for Microspora. Persistent centrioles replicate at prophase and establish the poles of metaphase spindles. Microtubules penetrate the nucleus at prometaphase, and during metaphase and anaphase, the nuclear envelope although vesiculated, remains around the spindle. Kinetochores are present. The orientation of the metaphase spindle is highly variable; anaphase elongation of the skew spindle causes it to become bent and twisted. After telophase, daughter nuclei come together in the middle of the cell. The centrioles migrate around the nuclei to the plane of cytokinesis established by a typical phycoplast of microtubules. Both imagination of the outer membrane and a cell plate probably contribute to cross wall formation. These results establish an affinity of Microspora with certain Ulotrichalean algae (e.g. Ulothrix and Stigeoclonium), but not others (Klebsormidium).The characteristic H‐shaped wall segments arise from two separate phases of wall secretion. During interphase, cell expansion is accommodated by the interlocking H‐shaped segments moving apart; concurrently, a new cylindrical wall is secreted inside these segments. Then during cytokinesis, the newly formed cross wall turns this cylinder into the typical wall segment. This pattern of wall secretion may be common in the Ulotrichales (and other algae) and it may have been derived by the acquisition of the filamentous habit from primitive unicellular ancestors. A scheme showing how this evolution could have occurred is proposed.