ORGANIZATION OF THE MITOTIC APPARATUS DURING GENERATIVE CELL-DIVISIONIN NICOTIANA-TABACUM

In order to pin a more complete understanding of the organization of t he mitotic apparatus (MA) in the generative cells (GCs) of flowering p lants, pollen tubes of Nicotiana tabacum were examined using tubulin i mmunocytochemistry and Hoechst fluorescence. The observations were the n compared with previously published information on Tradescantia GCs a nd the MA of somatic cells. At the onset of division, the prominent mi crotubule (Mt) bundles characteristic of GCs are reorganized into a mo re random Mt network. At late prophase/prometaphase, kinetochores appe ar to interact with this network, resulting in the formation of K-fibe rs that frequently link in tree-like aggregates. The GC MA takes the f orm of a distinct spindle and often has pointed, focused poles; the me taphase 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 f ormed in association with the proximal face of the sperm nuclei. The p hragmoplast remains prominent for some time, so that its Mts as well a s another population generated from the distal face of the sperm nucle i constitute the initial sperm cytoskeleton. Comparisons indicate that the spindle in tobacco GCs falls on a continuum of organization betwe en that of somatic cells and the MA of Tradescantia GCs.