Fig. 6. CTCF, a zinc finger DNA binding protein, and RNA polymerase II are expressed when protamines start to accumulate at the late canoe stage. (A-C) Anti-CTCF (C) and anti-histone antibody (A) staining on testes squashes of flies expressing protamine-eGFP (B). (A) Core histones are detectable in young elongating nuclei (arrow) and in early canoe stage spermatids (arrowhead), but not in later stages. (B) Protamine-eGFP expression starts in late canoe stage spermatids (double arrow). (C) Anti-CTCF is detectable in young elongating nuclei (arrow) and early canoe stage spermatids (arrowhead). During the late canoe stage, expression of CTCF overlaps with that of protamine-eGFP (double arrow), whereas CTCF is no longer visible in individualising spermatids. (D) Active RNA polymerase II is detectable by antibody staining in late canoe stage nuclei (double arrow), but not in any other post-meiotic stage. (E) Hoechst staining of the same cells as in D. Bars, 5 µm.