Abstract
Fertilized Zhikong scallop (Chlamys farreri) eggs were treated with cytochalasin B (CB 0.5 mg/L) at 14–15 min postfertilization to inhibit first polar body formation. The eggs were then stained with fluorescein isothiocyanate (FITC) -anti-α-tubulin and propidium iodide (PI) to examine their microtubule patterns and chromosome, respectively. Fluorescent microscope observations of treated eggs sampled every 2–3 min during meiotic maturation revealed meiotic apparatus assembly and correlated chromosome segregation. In CB-treated groups, meiosis I proceeded normally and produced two groups of dyads, with 19 in each group. Both dyad groups were retained in the eggs as they entered meiosis II. Two, three, or four asters (centrosome with microtubules around it) in meiosis II rearranged the spindle in several patterns: bipolar [24.0 ± 4.1 μm (long axis) × 18.3 ± 4.1 μm (diameter: metaphase plate)], tripolar (18.6 ± 3.9 μm × 9.9 ± 1.3 μm), separated bipolar (18.3 ± 2.8 μm × 11.2 ± 1.8 μm), and other unclassified spindle patterns. Corresponding chromosome segregation, including bipolar (18.9%), tripolar (38.9%), double bipolar (16.5%), and unclassified (25.6%), was observed during meiosis II in CB-treated eggs. The data indicated that chromosome segregation patterns determined by spindle patterns were critically influenced by the number of centrosomes in meiosis II eggs following inhibition of polar body 1 (PB1) formation with CB.
Highlights
Chromosome manipulation of marine bivalves has received considerable research interest
Following inhibition of polar body 1 (PB1) formation, tetraploids have been reported in Manila clam Ruditapes philippenarum (Li et al, 2017), Pacific abalone Haliotis discus hannai (Arai et al, 1986), American oyster Crassostrea virginica (Stanly et al, 1981), Pacific oyster Crassostrea gigas (Stephens and Dowing, 1988; McCombie et al, 2005), Hong Kong oyster Crassostrea hongkongensis (Qin et al, 2018), pearl oyster Pinctada martensii (He et al, 2000), and other mollusk species (Yamamoto and Sugawara, 1988; Guo and Allen, 1994; Yang et al, 2000a; Tan et al, 2017)
In the cytochalasin B (CB)-treated group, meiotic apparatuses and chromosome segregation during meiosis I were similar to those in the control group, except PB1 formation was blocked in the majority of fertilized eggs
Summary
Chromosome manipulation of marine bivalves has received considerable research interest. Yang et al (2000b) used a hematoxylin staining method to analyze chromosome segregation in fertilized Zhikong scallop (Chlamys farreri) eggs following blocking of PB1 formation by cytochalasin B (CB). They hypothesized that centrosomes and spindles might play an important role in meiosis. The probable cytological explanation for the production of tetraploids was documented, and the hypothesis that centrosomes and spindles are the primary factors controlling patterns of chromosome segregation was tested Fertilized eggs from both the control and CB-treated groups were sampled every 2–3 min during development until 40 min PF.
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