中心体付着部位の変化によって起こるタイワンシジミの雄性発生 : セタシジミとの比較

抄録

To reveal the mechanism of androgenesis, we observed chromosomes, centrosomes and microtubules in fertilized C. fluminea eggs from meiosis to first mitosis. We also observed fertilized eggs of C. sandai, in which development is normal. In C. sandai, one of the centrosomes attached to the egg cortex and the other remained in the center of eggs at metaphase of meiosis. The spindle axis at metaphase of meiosis was perpendicular to the egg cortex. On the other hand, in androgenetic eggs of C. fluminea, two centrosomes attached to the egg cortex. The spindle axis was parallel to the egg cortex. As a result, all egg chromosomes were extruded with two polar bodies at first meiosis. Only the male pronucleus remained in eggs after the polar body formation. C. fluminea has two centrosome attachment sites, while C. sandai has only one attachment site at meiosis. We deduced that the change of attachment site may cause the androgenesis in C. fluminea. We also measured the size of male pronucleus in C. sandai and C. fluminea fertilized eggs during meiosis. At 20 min after fertilization, all egg chromosomes were extruded with two polar bodies in C. fluminea. Meiosis of C. fluminea was apparently completed at 20 min, but the male pronucleus didn't enlarge until 40 min had elapsed. At 40 min, the male pronucleus started to enlarge, which coincides with the period of second polar body formation in C. sandai. We suggest that the cell cycle of second meiosis is still functional in oocytes of C. fluminea.