Control of division plane in normal and griseofulvin-treated microsporocytes of Magnolia

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Abstract
Meiotic cytokinesis in microsporocytes of Magnolia is an unusual form of the simultaneous type; phragmoplast expansion is not accompanied by a cell plate, wall deposition is centripetal, and infurrowing of the cytoplasm after first meiosis results in semicells connected by an isthmus. Dyad domains are further defined by interaction of extensive radial systems of microtubules emanating from the daughter nuclei and by a band of organelles polarized in the equatorial region. After second meiosis, phragmoplasts are organized in the interzonal regions between the sister nuclei in each semicell and also at the interfaces of microtubules forming secondary interzonals between non-sister nuclei. Wall deposition is not initiated until after phragmoplasts expand to the cell periphery and fuse in the isthmus. Centripetal wall deposition in boundaries of spore domains marked by radial arrays of microtubules results in simultaneous quadripartitioning of the microsporocyte into a tetrad of microspores. Treatment of microsporocytes with griseofulvin resulted in atypically placed nuclei and supernumerary nuclei. Abnormalities could be traced to displaced spindles and to spindles with multiple poles. Drug-induced multinucleate coenocytes were able to organize microtubules and initiate cytokinesis in altered patterns. The data suggest that spindle alignment and aggregation of spindle poles are two components of spatial control that are operative in determining the normal arrangement of nuclei, and that the final placement of walls is a function of the postmeiotic nuclear-based radial arrays of microtubules which define spore domains.