Female Gametophyte Development in Maize: Microtubular Organization and Embryo Sac Polarity.

The developmental stages of the maize embryo sac were correlated with the corresponding silk lengths of ear florets in the female inflorescence. The development of embryo sacs in the ovules of spikes occurs in a gradient pattern with the initiation of the embryo sac beginning at the base of the ear and progressing to the top. At the beginning of meiosis, the presence of conspicuous cortical microtubules coincides with the extensive elongation of the megasporocyte. The spindles at metaphase I and II align along the long axis of the megasporocyte leading to the linear alignment of the dyad and tetrad of megaspores. During megagametogenesis, micropylar and chalazal nuclei of the embryo sac undergo synchronized divisions and migration at the second and third mitosis. Radiate perinuclear microtubules are present during the interphase of the second and third mitosis, and inter-sister nuclear microtubules occur at the late four-nucleate embryo sac. The configuration and orientation of the spindles, phragmoplasts, and pairs of nuclei result in precise positioning of the nuclei. The fusion of the polar nuclei and the formation of a microtubule organizing center-like structure in the filiform apparatus occur right after the first division of the antipodal cells. The different patterns of organization of microtubules in the cells of the mature embryo sac reflect their structural adaptations for their future function.