Fig. 2. Distribution of acetylated histone H4 at lysine 8 in S. ocellaris and S. coprophila embryonic germ nuclei at the resting stage. (A-D) Chromosome DAPI staining, (A'-D') Indirect immunolabeling with H4Ac8 antibody and (A''-D'') superimposed images. where the antibody staining is in red. (A) S. ocellaris germ nuclei prior to, (B) during and (C) after the occurrence of X_p chromosome elimination. (D) A S. coprophila nucleus before the elimination of X_p and L chromosomes. In all cases only four chromosomes of the complement are highly H4Ac8-labelled (A'-D' and A''-D''). (B-B'') a S. ocellaris nucleus undergoing elimination where it is seen that the X_p chromosome expelled from the nucleus (asterisk in B) is not stained (asterisks in B' and B''); a labeled somatic nucleus is also shown for comparison (arrows). (C'-C'') A post-elimination S. ocellaris nucleus where it is possible to observe that only one of the chromosomes corresponding to the longest autosomes pair is labeled (arrows). (D-D'') The four S. coprophila chromosomes that are H4Ac8-labelled (short arrows in D') correspond to the ones that are slightly less stained with DAPI (short arrows in D); the H4Ac8-unstained eight chromosomes contain three L chromosomes (possibly the ones indicated by long arrows in D''). Bar, 10 µm.

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