INTRACELLULAR MOTILITY AND THE EVOLUTION OF THE ACTIN CYTOSKELETON DURING DEVELOPMENT OF THE MALE GAMETOPHYTE OF WHEAT (TRITICUM-AESTIVUM L.)

The uniaperturate pollen of wheat is dispersed in a partially hydrated condition. Amyloplasts are concentrated in the apertural hemisphere w here they surround the two sperms, while vigorously moving polysacchar ide-containing wall precursor bodies (P-particles) together with the v egetative nucleus occupy the other. This disposition is the product of a post-meiotic developmental sequence apparently peculiar to the gras ses. During vacuolation of the spore after release from the tetrad, th e nucleus is displaced to the pole of the cell opposite the site of th e germination aperture, already defined in the tetrad. Following polle n mitosis, the vegetative nucleus migrates along the wall of the veget ative cell towards the aperture, leaving the generative cell at the op posite pole isolated by a callose wall. As the vacuole is resorbed, th e generative cell rounds up, loses its wall and follows the vegetative nucleus, passing along the wall of the vegetative cell towards the ap erture where it eventually divides to produce the two sperms. Througho ut this period of nucleus and cell manoeuvrings, minor inclusions of t he vegetative cell cytoplasm-including mitochondria, lipid globuli and developing amyloplasts-move randomly. Coordinated vectorial movement begins after the main period of starch accumulation, when the amylopla sts migrate individually into the apertural hemisphere of the grain, a final redistribution betokening the attainment of germinability. In t he present paper we correlate aspects of the evolution of the actin cy toskeleton with these events in the developing grain, and relate the o bservations to published evidence from another monocotyledonous specie s concerning the timing of the expression of actin genes during male g ametophyte development, as revealed in the synthesis of actin mRNA.