Fig. 9

Summary model for transcriptional (re)programming of individual syncytial nuclei in a developing muscle. Before fusion, stage 12, all founder cells (FCs) transcribe generic FC genes (duf, blue dot), and fusion competent myoblasts (FCMs) transcribe FCM-specific genes (sns, green dot). Each FC expresses specific identity transcription factors (iTFs; e.g. Col, grey) and transcribes iTF genes (e.g. col, red dot). In a growing muscle syncytium, stage 14, FCM gene transcription is transitorily maintained in newly fused FCM nuclei, before complete switch off and progressive activation of FC-generic transcription. Nuclear uptake of iTFs by fused FCMs is followed by iTF activation. Transcription of identity-realisation genes (e.g. Pax, mspo, kon, orange dot) is linked to iTF activation, while transcription of general muscle differentiation genes (Mhc), is activated and maintained in all myoblast nuclei, independently of iTF expression. Transcription of a subset of genes is restricted to the FC nucleus (e.g. Con, orange dot circled by a red line)