Analysis of Factors Generating Inequality in Early Cleavages in Tubifex

我们研究了淡水少毛藻（Tubifex hattai）中决定前两次裂解不平等的因素。第一次卵裂的有丝分裂纺锤体位于卵子的中心，在一端有一个aster，而在另一端没有。第一次卵裂的不均匀性是由纺锤极的不对称组织决定的，而不是由纺锤在卵子中的位置决定的。在纺锤体的一极（在aster的中心）发现一个带有抗-微管蛋白抗体的点状中心体，而在另一极则没有。这个中心体似乎起源于母系。如果来自第一个减数分裂纺锤体的两个中心体在第一个细胞周期中都保留在卵子中，它们就会形成一个对称组织的纺锤体，从而导致卵子的均匀分裂。这表明管卵缺乏蛤卵中所见的使有丝分裂纺锤体偏心移位的机制。与第一次卵裂相比，第二次卵裂纺锤体的两极在形成紫菀的能力和γ -微管蛋白分布方面没有什么不同。然而，由于其中一个纺锤体极点与细胞皮层的相互作用，在细胞CD中形成了不对称的纺锤体，导致细胞分裂不均匀。因此，纺锤体组织中产生不对称的因素是第一次分裂纺锤体所固有的，而不是第二次分裂纺锤体所固有的。

We have investigated factors which determine inequality of the first two cleavages in the freshwater oligochaete Tubifex hattai. A mitotic spindle for the first cleavage, which is located at the center of the egg, prossesses an aster at one pole, but not at the other pole. Inequality of the first cleavage is determined by the asymmetric organization of the spindle poles, rather than by the spindle position in the egg. A centrosome which appears as a dot stained with an anti-gamma-tubulin antibody is found at one pole (at the center of the aster) of the spindle, but not at the other pole. This centrosome appears to be maternal in origin. If two centrosomes originating from the first meiotic spindle are both retained in an egg at the first cell cycle, they form a symmetrically organized spindle, which brings about the equal division of the egg. This suggests that Tubifex eggs lack such mechanisms as are seen in surf clam eggs to translocate the mitotic spindle eccentrically. In contrast to the first cleavage, the poles of the second cleavage spindle are not different from each other either in their ability to form asters or in gamma-tubulin distribution. As a result of an interaction of one of the spindle poles with the cell cortex, however, an asymmetric spindle is formed in the cell CD,giving rice to unequal division in this cell. Thus, factors generating asymmetry in spindle organization are intrinsic to the mitotic spindle in the first cleavage, but not in the second cleavage.