Figure 3.
Meiotic targeting for methylation of H3-Lys9 and transcriptional repression of an unpaired X chromosome. (a–d) Staining with antibody to methylated H3-Lys9 (green) in adult gonads counterstained with DAPI (red in all panels). (a) Wild-type N2 (XX) late pachytene–diplotene oocytes. Arrow indicates an individual nucleus with normal enrichment for methylated H3-Lys9 at diplotene stage in oogenesis12. (b) N2 (XO) mid-pachytene spermatocytes; arrows indicate X chromosome stained with antibody to methylated H3-Lys9 present in all nuclei. (c) tra-2 (q276) XX late pachytene–diplotene spermatocytes have a pattern of staining with antibody to methylated H3-Lys9 similar to that of XX oocytes (a). Arrows indicate unstained chromosome in representative nuclei, identified by FISH costaining as the paired X homologs in the inset (arrow; blue shows DNA FISH). (d) her-1 (hv1y101) XO early- to mid-pachytene oocytes, with arrows indicating unpaired X chromosomes in several nuclei. (e–j) Staining with antibody to methylated H3-Lys4 (green) in N2 (XX) germ cells initially does not label the X chromosome in pachytene cells (arrow in e) but labels all six chromosome pairs in early (f) and late (g) diakinetic oocytes. (h–j) In her-1 XO oocytes, accumulation of methylated H3-Lys4 on the unpaired X chromosome (arrows) is markedly delayed in early (i) and understained in late (j) oocytes. (k–n) Strong mRNA in situ hybridization of a probe for an X-linked oocyte-enriched gene (F52D2.2; ref. 14) is detected in developing oocytes (arrows) of XX her-1 (hv1y101) worms (k), but little or no signal is observed in developing oocytes of XO worms (l). Other X-linked oocyte-enriched genes showed similar defects in transcriptional activity (data not shown). In contrast, mRNA of an autosome-linked oocyte-specific gene (emo-1) is easily detected in developing oocytes (arrows) of both XX (m) and XO (n) worms. Scale bars, 5 μm.