Analysis of Factors Generating Inequality in Early Cleavages in Tubifex

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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.