Chromosome segregation in fertilized eggs from zhikong scallop Chlamys farreri (Jones & Preston) following polar body 1 inhibition with cytochalasin B

Chromosome segregation in fertilized eggs of the zhikong scallop, Chlamys farreri, following polar body 1 (PB1) inhibition with cytochalasin B (CB) was studied. The fertilized eggs were treated with CB (0.75 mg/L) at 7-10 min postfertilization until polar body 2 (PB2) was released in control groups. The embryos were sampled every 5-10 min after fertilization and fixed in Carnoy fixative. Chromosome segregation in both control groups and treated groups were analyzed using a hematoxylin stain method. In fertilized eggs of control groups, the 19 tetrad chromosomes went through meiosis I and II, and released PB1 and PB2, finally reaching 19 chromatids. In CB treated groups, meiosis I proceeded normally and produced two groups of dyads, 19 in each group. With the CB treatment, both of the two dyad groups were retained in the eggs and entered meiosis II. The segregation in meiosis II had four patterns: bipolar, tripolar, tetrapolar, and unsynchronized segregation. When the two groups of dyads from meiosis I united, the treated eggs entered meiosis II through tripolar (40.9%) and bipolar (11.4%) segregation patterns. Otherwise the two groups of dyads segregated separately and formed tetrapolar segregation (15.7%). Also a small proportion of treated eggs (4.0%) underwent meiosis II in an "unsynchronized segregation" pattern, which means that the two groups of dyads from meiosis I did not segregate synchronously. There were 28.0% of treated eggs that could not be classified. The four segregation patterns produced different ploidies of embryos in CB treated groups, such as triploids, tetraploids, pentaploids, and aneuploids.