Figure 2.—

such-1(t1668) embryos lacking centrioles undergo monopolar mitosis: immunofluorescence of mitotic one-cell–stage embryos of the indicated parental genotypes. Centrioles (SAS-4, red), microtubules (α-tubulin, green), and Hoechst counterstain to visualize DNA (blue). Insets show sixfold-magnified view of the SAS-4 signal in the center of the asters. Bar, 10 μm. (A) Wild-type embryo with four centrioles; the two centrosomes nucleate two dense microtubule asters and direct bipolar spindle assembly. (B) such-1(t1668) class I or II embryo (the two classes cannot be distinguished in fixed specimens during mitosis) with four centrioles and a bipolar spindle. A total of 1/28 embryos with a monopolar spindle harbored centrioles and a dense array of microtubules, probably corresponding to the subset of embryos with a putative defect in the timing of centriole disengagement (described in the legend of Figure 1C). (C) such-1(t1668) class III embryo, which lack centrioles. Microtubules are nucleated around the chromatin, resulting in monopolar mitosis. (D) spd-5(or213) embryo, in which centrosome function is compromised despite the presence of centrioles, thus also resulting in monopolar mitosis.