All meiotic chromosomes and both centrosomes at spindle pole in the zygotes discarded as two polar bodies in clam Corbicula leana: unusual polar body formation observed by antitubulin immunofluorescence.

To understand the unusual polar body formation in the androgenetic clam, Corbicula leana, whole-mount eggs stained with monoclonal antibodies against alpha-tubulin, gamma-tubulin, and 4'-6'-diamidino-2-phenylindole were examined. The meiotic spindle was located at the peripheral region of the egg at metaphase I, and its axis was parallel to the egg surface. After segregation of chromosomes at anaphase I, cytoplasmic bulges formed at both meiotic spindle pole sites. Centrosomes were located at the apical portion of the each bulge. From the apical portion of the bulge a bundle of astral microtubules radiated toward the bulge base in late anaphase resembling a half spindle. Maternal chromosomes and both centrosomes were all distributed in two "first polar bodies" and were eventually discarded. After the polar body formation only one male pronucleus existed in the egg cytoplasm. The present study showed that the anaphase microtubules originating from a single aster can induce the polar body formation without overlapping of microtubules from the opposing aster.