Determination of first cleavage plane: the relationships between the orientation of the mitotic apparatus for first cleavage and the position of meiotic division-related structures in starfish eggs.

In order to understand when the orientation of the first cleavage plane is fixed along the animal-vegetal axis in starfish eggs, the behavior of the sperm aster was examined by indirect immunofluorescence staining. After duplication, the sperm aster organizes the mitotic apparatus for first cleavage perpendicular to the cleavage plane. The sperm aster located in the egg periphery just after fertilization and moved to the site close to the animal pole rather than the egg center by meiosis II. At early metaphase II, duplication of the sperm aster was detected but the axis of the resultant sperm diaster randomly pointed. Subsequently, its axis had already turned perpendicular to the animal-vegetal axis before pronucleus fusion. These results indicate that the orientation processes of the sperm diaster consist of positioning before its duplication and successive determining its azimuth. Furthermore, the azimuth and position of the mitotic apparatus for first cleavage did not change by shifting or eliminating the meiotic division-related structures such as the germinal vesicle, meiotic spindle, and female pronucleus by micromanipulation. These results show that none of them determines the first cleavage plane. Therefore, we discuss the pointing mechanism of the first cleavage plane without the influence of these meiotic division-related structures.