Fig. 4. DPY-21 localizes to the X chromosomes of XX but not XO embryos, as expected for a component of the dosage compensation complex. (A-F) False-color confocal images of wild-type XX embryos (A,B), dpy-21(e428) mutant XX embryos (C), wild-type hermaphrodite gut nuclei (D), him-8(e1489) XO embryos (E) and him-8(e1489); xol-1(y9) mutant XO embryos (F) stained with DPY-21 antibodies (green), the DNA-intercalating dye 4',6 diamidino-2-phenylindole (DAPI) (blue) and an X-chromosome-specific marker (red) (either an X-chromosome-specific FISH probe or antibodies to SDC-3 or GFP, which identifies the DPY-27::GFP fusion protein used in D). (A-C,E,F) DPY-21 antibodies to amino acids 467-1102; (D) antibodies to the DPY-21 N terminus. The third column shows a merged image of the first two columns, and yellow indicates overlap in staining of DPY-21 and the X-chromosome marker. The fourth column shows the superimposition of DAPI images with images from the first two columns. Insets show the enlargement of a single nucleus indicated by the arrow. (A) In young embryos (<40 cells) that have not yet recruited the dosage compensation complex to X, DPY-21 is distributed throughout the nuclei. (B) In 40-cell stage embryos, DPY-21 exhibits a punctate pattern that is coincident with the X-localized SDC-3 protein. (C) Specificity of the DPY-21 antibody was shown in part by the absence of DPY-21 staining in dpy-21(e428) and dpy-21(y59) (data not shown), both of which contain an early amber stop mutation. SDC-3 localized to the X chromosomes of a dpy-21 mutant, indicating that DPY-21 is not essential for the recruitment of the dosage compensation complex to X. (D) The X-chromosome localization of DPY-21 is maintained throughout hermaphrodite development, as shown by the X-chromosome localization of DPY-21 in adult gut nuclei, which carry a DPY-27::GFP fusion protein. (E) DPY-21 is expressed, but fails to localize to the X chromosome of XO animals. (F) In xol-1(y9) mutant XO embryos, which have inappropriately activated dosage compensation, both DPY-21 and SDC-3 co-localize with the single X chromosome, indicating that the X-chromosome localization of DPY-21 is under the same sex-specific control as SDC-3. Scale bars: 5 µm.