Bisgrove S R, Kropf D L
Department of Biology, University of Utah, Salt Lake City 84112-0840, USA.
Dev Biol. 1998 Feb 15;194(2):246-56. doi: 10.1006/dbio.1997.8832.
Zygotes and embryos of the fucoid brown alga Pelvetia compressa undergo a series of asymmetric cleavages. We are interested in the developmental role of these cleavages and the mechanism controlling their alignment. To assess the importance of division plane alignment, the orientation of the first asymmetric division was altered and the effects on subsequent embryo elongation rates were analyzed. Although this division is normally oriented transversely (90 degrees) to the growth axis, deviations up to 45 degrees had no significant effects on embryo elongation. However, division planes that were parallel with the growth axis (0-45 degrees) had drastic effects. Embryo elongation was severely inhibited and the wall often bifurcated and avoided the rhizoid tip. The orientation of the division plane is determined by the position of the centrosomes. We therefore investigated centrosomal position and function during the first cell cycle within the three-dimensional context of the cell. We found that, after karyogamy, microtubule organization changed from a radially symmetric circumnuclear array into a bipolar centrosomal array. The reorganization coincided with the migration of the centrosomes around the nucleus. The centrosomes separated slowly and asynchronously until they reached opposite sides of the nuclear envelope. At this time the centrosomal axis, defined by the position of the two centrosomes, was oriented randomly with respect to the cortical growth axis. The centrosomal axis then rotated into alignment parallel with the growth axis late in the first cell cycle. These results indicate that the growth axis and the centrosomal axis develop independently of each other and that the centrosomal axis does not align with the growth axis until just prior to mitosis.
岩藻状褐藻扁江蓠的合子和胚胎会经历一系列不对称分裂。我们对这些分裂的发育作用以及控制其排列的机制感兴趣。为了评估分裂平面排列的重要性，改变了第一次不对称分裂的方向，并分析了其对后续胚胎伸长率的影响。尽管这种分裂通常与生长轴横向（90度）取向，但高达45度的偏差对胚胎伸长没有显著影响。然而，与生长轴平行（0 - 45度）的分裂平面却产生了巨大影响。胚胎伸长受到严重抑制，细胞壁常常分叉并避开假根尖端。分裂平面的取向由中心体的位置决定。因此，我们在细胞的三维环境中研究了第一次细胞周期期间中心体的位置和功能。我们发现，在核融合后，微管组织从径向对称的核周阵列转变为双极中心体阵列。这种重组与中心体围绕细胞核的迁移同时发生。中心体缓慢且异步地分离，直到它们到达核膜的相对两侧。此时，由两个中心体的位置定义的中心体轴相对于皮层生长轴随机取向。然后，在第一个细胞周期后期，中心体轴旋转到与生长轴平行的方向。这些结果表明，生长轴和中心体轴彼此独立发育，并且中心体轴直到有丝分裂前不久才与生长轴对齐。
