ORGANIZATION OF THE MITOTIC APPARATUS DURING GENERATIVE CELL-DIVISION IN NICOTIANA-TABACUM

In order to pin a more complete understanding of the organization of the mitotic apparatus (MA) in the generative cells (GCs) of flowering plants, pollen tubes of Nicotiana tabacum were examined using tubulin immunocytochemistry and Hoechst fluorescence. The observations were then compared with previously published information on Tradescantia GCs and the MA of somatic cells. At the onset of division, the prominent microtubule (Mt) bundles characteristic of GCs are reorganized into a more random Mt network. At late prophase/prometaphase, kinetochores appear to interact with this network, resulting in the formation of K-fibers that frequently link in tree-like aggregates. The GC MA takes the form of a distinct spindle and often has pointed, focused poles; the metaphase plate is usually oblique. Karyokinesis involves both anaphase A and B; lengthening of interzonal Mts is accompanied by elongation of the spindle. In late anaphase/early telophase, phragmoplast Mts are formed in association with the proximal face of the sperm nuclei. The phragmoplast remains prominent for some time, so that its Mts as well as another population generated from the distal face of the sperm nuclei constitute the initial sperm cytoskeleton. Comparisons indicate that the spindle in tobacco GCs falls on a continuum of organization between that of somatic cells and the MA of Tradescantia GCs.