Aster-forming abilities of the egg, polar body, and sperm centrosomes in early starfish development.

Using ionophore-activated starfish eggs, the maternal centrosome accompanied by the female pronucleus (the egg centrosome) was confirmed to possess the ability for aster formation but generally not the capacity for reproduction. Inhibition of pronuclear fusion and removal of the male pronucleus and/or the sperm centrosome revealed that the egg centrosome lost its ability when it was only in the vicinity of the sperm centrosome. Hence, the entity that inactivated the egg centrosome was determined to be the adjacent sperm centrosome. When the second polar body was transplanted into a fertilized egg, its maternal centrosome (the polar body centrosome) organized an aster during cleavage; moreover, it possessed reproductive capacity. Furthermore, three different patterns of aster formation characterizing the sperm, egg, and polar body centrosomes occurred at each corresponding nucleus in the absence of nuclear fusion. When various kinds of centrosomes approached each other through nuclear fusion, there was observed only a pattern of aster formation at the fusion site that characterized a certain centrosome. Consequently, the sperm, polar body, and egg centrosomes were superior, intermediate, and inferior, respectively, in their ability for aster formation.