Fig. 1. H3K36 trimethylation is enriched on the fourth chromosome and reduced on the X chromosome.

(A) Immunostaining of a male third instar larvae polytene chromosome shows an accumulation of H3K36me3 (yellow) on chromosome 4 (red arrowhead) and a reduction on the X chromosome (green arrowhead) as compared to autosomal signals. DAPI staining of DNA in blue indicates banding pattern. (B) Immunostaining of female third instar larvae polytene chromosome detects no differences between any chromosome arms. Average exon H3K36me3 enrichment scores per chromosome in 2- to 4-hour mixed-sex embryos (C and D), 14- to 16-hour mixed-sex embryos (E and F), and mixed-sex third instar larvae (G). Enrichment scores for the ChIP-chip experiments (C, E, and G) are the ChIP over input log₂ ratio of the top 50% of the exon regions per gene. For ChIP-seq (D and F), the enrichment scores are reads adjusted for difference in position between ChIP and input. In ChIP-seq, only peaks in exons were used. Error bars (C to G) indicate the 95% confidence intervals. The statistical significances were determined by unpaired two-sample Wilcoxon tests comparing the X chromosome and chromosome 4 to the most similar autosome arms.