In zygotes of almost all animals, it has been believed that only the sperm centrosome acts as the mitotic spindle poles. As first proposed a century ago by Boveri (1887), this uniparental (paternal) inheritance of the centrosome must depend on the selective loss of the maternal centrosomes. To trace the fate and duplicating capacity of all the maternal centrosomes/centrioles, including those cast off into polar bodies, we used two kinds of procedures: (1) suppression of polar body (PB) extrusion and (2) transplantation of PB centro-somes into artificially activated eggs.

Gametes used in this study were from the starfish, Asterina pectinifera. Oocyte maturation was induced with 1-methyladenine (Kanatani, 1969). Suppression of PB extrusion and artificial activation were done according to Washitani-Nemoto et al. (1994). Micromanipulation was performed by the method of Saiki & Hamaguchi (1993). Behaviour of the centrosomes was examined by staining with an antibody against α-tubulin, polarisation and differential interference-contrast microscopy and transmission electron microscopy.

In starfish oocytes, no centriole duplication occurs in meiosis II, hence each pole of a meiosis II spindle is formed by the splitting of paired centrioles in the inner centrosome of a meiosis I spindle into singles. Eventually, each of a second PB (PB2) and a mature egg inherits only one centriole from a meiosis II spindle (a PB1 inherits a pair of centrioles). So, either PB2 and the mature egg inherit a single centriole (Fig. 1; cf. Sluder et al., 1989; Kato et al., 1990). When mature eggs were artificially activated with the Ca2+-ionophore A23187, a single monaster was formed.