CHANGES IN THE PATTERN OF ORGANIZATION OF THE MICROTUBULAR CYTOSKELETON DURING MEGASPOROGENESIS IN CYMBIDIUM SINENSE

In Cymbidium sinense embryo sac development follows the bisporic patte rn. This pattern of development is rare in orchids. Changes in the str ucture and organization of the microtubular cytoskeleton during megasp orogenesis in this orchid were followed using an immunofluorescence te chnique and confocal microscopy. At the initial stage of development t he microtubules in the archespore are randomly oriented throughout the cortex and cytoplasm. Later, microtubules undergo reorganization with more microtubules distributed in the cortex and around the nucleus. M egasporocyte formation is marked by the elongation of the archesporial cell. The cytoskeleton in the elongated megasporocyte is radially org anized with microtubules extending from the nucleus to the peripheral region of the cell. In the megasporocyte some of the microtubules also show a polarized pattern of distribution with more microtubules at th e chalazal end than the micropylar end. After meiosis I, a dyad is for med. Each dyad member possesses a complement of randomly oriented micr otubules. But some microtubules in the dyad also show radial orientati on. Soon afterwards microtubules in the degenerating micropylar dyad c ell disappear. The functional (chalazal) dyad cell proceeds to meiosis II giving rise to a two-nucleate embryo sac. The pattern of microtubu le organization in the two-nucleate embryo sac is quite complex. Each nucleus is surrounded by a set of randomly distributed microtubules. B ut the vacuolar region between the two nuclei is surrounded by a paral lel array of cytoplasmic microtubules. No preprophase bands were obser ved during the development of the megasporocyte. This is the first des cription of microtubules in bisporic embryo sac development.