The microtubular cytoskeleton during megasporogenesis in the Nun orchid, Phaius tankervilliae

This study examines the microtubular cytoskeleton during megasporogenesis i n the Nun orchid, Phaius tankervilliae. The subepidermal cell located at th e terminal end of the nucellar filament differentiates first into an arches porial cell and then enlarges to become the megasporocyte. The megasporocyt e undergoes the first meiotic division, giving rise to two dyad cells of un equal size. Immunostaining reveals that microtubules become more abundant a s the megasporocyte increases in size. Microtubules congregate around the n ucleus forming a distinct perinuclear array and many microtubules radiate d irectly from the nuclear envelope. In the megasporocyte, prominent microtub ules are readily detected at the chalazal end of the cell cytoplasm. After meiosis I, the chalazal dyad cell expands in size at the expense of the mic ropylar dyad cell. At this stage, new microtubule organizing centres can be found at the corners of the cells. The appearance of these structures is s tage-specific and they are not found at any other stages of megasporogenesi s. The functional dyad cell undergoes the second meiotic division, resultin g in the formation of two megaspores of unequal size. The chalazal megaspor e enlarges and eventually gives rise to the embryo sac. As the functional m egaspore expands, the microtubules again form a distinct perinuclear array with many microtubules radiating from the nuclear envelope. A defined corti cal array of microtubules has not been found in P. tankervilliae during the course of megasporogenesis.