RELATIONSHIP BETWEEN THE PREPROPHASE BAND, NUCLEUS AND SPINDLE IN DIVIDING ALLIUM COTYLEDON CELLS

The relationship between the preprophase band (PPB) and the nucleus was assessed during symmetrical and asymmetrical divisions in Allium cotyledons. The appearance of an early, broad PPB is accompanied by Mts that radiate in all directions from the surface of the nucleus. Some of these Mts contact the PPB, but no preferential relationship between the two arrays is evident. Transformation of the perinuclear array into a bipolar spindle lags behind narrowing of the PPB. When a bipolar spindle does appear, the number of specific Mt linkages to the PPB increases. Moreover, these Mts now emanate exclusively from the poles. Treatment with cytochalasin D not only stops PPB narrowing, but also inhibits formation of a bipolar spindle. Mts emanate from the nucleus in the manner seen in early control cells, and no preferential association with the PPB is seen. When nuclei in some cells destined to divide asymmetrically are centrifuged basipetally, an extra cortical band appears around the displaced nucleus, in addition to an apical PPB. The circumnuclear band disappears by prometaphase/metaphase, but the apical PPB remains. Our results point to the formation of specific linkages between the PPB and the bipolar spindle, and influence exerted by the nucleus/spindle on PPB organization and/or disappearance. Our prior experiments indicated that PPB characteristics in turn affect nuclear and spindle position.