The Process of First Polar Body Formation in Eggs of the Androgenetic ClamCorbicula fluminea

Abstract

The hermaphorodic clam Corbicula fluminea has the unusual reproduction mode of androgenesis. In fertilized eggs, all maternal chromosomes and centrosomes are simultaneously extruded as two polar bodies. To reveal the process of this unusual polar body formation, the process was observed by confocal microscopy. In particular, actin organization of oocytes labeled with anti-α- and γ-tubulin antibodies and rhodamine phalloidin were observed. The meiotic spindle was organized near the animal pole and was parallel to the oocyte surface at metaphase of the first meiosis. A thin actin layer was uniformly distributed across the oocyte membrane at metaphase. At anaphase of the first meiosis, two circular, actin-poor regions appeared where the meiotic half spindles docked. All chromosomes and centrosomes were located within the two bulges formed at anaphase in the actin-poor regions. No overlapping of microtubules was observed from either centrosome at anaphase. The centrosomes were located at the apical region of the bulges. Microtubules radiated for the bulge base. At late anaphase, around the actin-poor regions, actin accumulated and two actin rings became distinct. The diameter of these rings gradually decreased and cytokinesis occurred at the base of the bulges. Thus, all centrosomes and chromosomes were extruded together with the polar body. The present study suggests that centrosomes at the first meiosis could have the equal ability to approach and attach to the membrane and induce polar body formation.