Cell polarity, asynchronous nuclear divisions, and bidirectional cytokinesis in male meiosis in Magnolia denudata

Magnolia, a basal angiosperm genus important for evolutionary and phylogenetic studies, is known to have male meiotic features not seen in the vast majority of angiosperms. However, knowledge about male meiosis in Magnolia is still fragmentary. Here, we report findings from an extensive investigation into male meiosis in Magnolia denudata using a combination of light and electron microscopy methods. Male meiosis in M. denudata was synchronous in prophase I but asynchronous in subsequent nuclear divisions. The polarized microspore mother cells from late prophase I onward had an elongated cell shape and thickened callose wall areas at the two smaller ends of the cell. The first nuclear division occurred along the long axis of the cell and the first callose furrow formed at the equatorial plane of the first nuclear division at the late telophase I stage. The second equatorial callose furrow formed after telophase II in a plane perpendicular to the first callose furrow. While cytokinesis occurred centripetally from the two furrows, a central callose wall island (CWI) appeared in the center of the cell and dense assemblies of vesicles and short tubules decorated the cytoplasmic regions between the furrows and the CWI. This cytokinesis mode differs from either the centripetal or the centrifugal mode of cytokinesis in microsporogenesis in the vast majority of angiosperms. As a result of this unusual cytokinesis, a large central callose mass remains in the mature tetrads. These observations may be useful to studies of cytokinetic mechanisms, evolution of microsporogenesis, and phylogenetics of angiosperms.