The first two cleavages in Tubifex involve distinct mechanisms to generate asymmetry in mitotic apparatus

We have investigated factors which determine inequality of the first two cleavages in Tubifex hattai. A mitotic spindle for the first cleavage, which is located at the center of the egg, possesses 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--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. 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 -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 rise 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.