Fig. 4. Nau-dependent col transcription during the DA3 muscle fusion process in Drosophila. (A-D) Double in situ hybridisation with intronic probes for col (green) and nau (red) nascent transcripts and Col immunostaining (blue) show that nau and col are co-expressed in (A) the DA3/DO5 progenitor cell, (B) the DA3 FC (outlined by a plain line) and (C), the DA3 muscle precursor when it contains two to three nuclei (outlined). nau remains transcribed in the DO5 FC (dashed outline in B), whereas col transcription is rapidly turned down. (E-H) col transcription (green dots) in (E,G) wt and (F,H) nau¹⁸⁸ mutant embryos (two segments are shown); the DA3 muscle is visualised by immunostaining for Col (red) and MHC (blue in E,F). In stage 15 nau¹⁸⁸ mutant embryos (F), the DA3 muscle is reduced, compared to wt (E) and most nuclei do not transcribe col. At stage 12, col expression in the DA3 muscle precursor (asterisk) when it contains two to three nuclei is similar in (H) nau¹⁸⁸ and (G) wt embryos, although only one nucleus, probably the FC nucleus, expresses high levels of col transcripts in nau¹⁸⁸ embryos. Arrowheads indicate col transcription in a dorsal multidendritic neuron. Scale bars: 5 µm.