Gamma-tubulin and microtubule organization during microsporogenesis in Ginkgo biloba.

This is the first report on gamma-tubulin and microtubule arrays during microsporogenesis in a gymnosperm. Meiosis in Ginkgo biloba is polyplastidic, as is typical of the spermatophyte clade, and microtubule arrays are organized at various sites during meiosis and cytokinesis. In early prophase, a cluster of gamma-tubulin globules occurs in the central cytoplasm adjacent to the off-center nucleus. These globules diminish in size and spread over the surface of the nucleus. A system of microtubules focused on the gamma-tubulin forms a reticulate pattern in the cytoplasm. As the nucleus migrates to the center of the microsporocyte, gamma-tubulin becomes concentrated at several sites adjacent to the nuclear envelope. Microtubules organized at these foci of gamma-tubulin give rise to a multipolar prophase spindle. By metaphase I, the spindle has matured into a distinctly bipolar structure with pointed poles. In both first and second meiosis, gamma-tubulin becomes distributed throughout the metaphase spindles, but becomes distinctly polar again in anaphase. In telophase I, gamma-tubulin moves from polar regions to the proximal surface of chromosome groups/nuclei where interzonal microtubules are organized. No cell wall is deposited and the interzonal microtubules embrace a plate of organelles between the two nuclear cytoplasmic domains (NCDs) of the dyad. Following second meiosis, phragmoplasts that form between sister and non-sister nuclei fuse to form a complex six-sided structure that directs simultaneous cytokinesis. Gamma-tubulin becomes associated with nuclei after both meiotic divisions and is especially conspicuous in the distal hemisphere of each young microspore where an unusual encircling system of cortical microtubules develops.