The effect of blocking polar body I (PB 1) with cytochalasin B (CB) on subsequent chromosome segre- gation was studied in fertilized eggs of the Pacific oyster, Crassostrea gigas. To block the release of PB 1, fertilized eggs were treated with CB (1.O &ml) for 15 min beginning at 5 min post-fertilization at 25°C. Chromosome segre- gation in both control and CB-treated eggs was analyzed with an acetic orcein stain. In untreated eggs, ten maternal tetrad chromosomes went through meiosis I and II, and released two polar bodies, reaching a haploid number of 10 chromatids. In CB-treated eggs, meiosis I proceeded normally and pro- duced two groups of dyads, ten in each group. However, blocking PB 1 dramatically changed chromosome segre- gation in meiosis II. In the majority of the treated eggs (68%), the two groups of dyads from meiosis I entered meiosis II through a “tripolar segregation”, although two other types of segregation, namely “united bipolar” (7%) and “separated bipolar” (12%) were also observed. After anaphase II, chromatids at the peripheral pole were re- leased as polar body II (PB2). The release of two sets of chromatids as PB2 through either a united bipolar or a separated bipolar segregation resulted in the formation of meiosis I triploids (14%). The release of one set of chro- matids as PB2 from an unmixed tripolar or a separated bipolar segregation formed meiosis I tetraploids (20%). Aneuploids (56%) were produced, primarily when the two groups of dyads from meiosis I united or overlapped before entering the tripolar segregation.
All Science Journal Classification (ASJC) codes
- Agricultural and Biological Sciences(all)