Fig. 10. The role of Twist homodimers and Twist/Da heterodimers during the subdivision of the mesoderm. (A) Lateral view of a stage 10 embryo showing modulated expression of Twist. Each segment has an anterior domain that expresses lower Twist levels and a posterior domain that expresses higher levels of Twist. (B) Cartoon of similar staged embryo. High Twist levels are indicated by dark blue, lower levels by light blue. Da is expressed uniformly in the mesoderm at this stage. (C) Close up of a mesodermal segment at stage 10. Cells that express high Twist levels favor formation of Twist homodimers relative to that of Twist/Da heterodimers. Twist homodimers in these cells promote the somatic muscle program. By increasing Twist/Da heterodimers levels either by reducing Twist levels or by overexpressing the linked Twist-Da dimer, the somatic muscle program is inhibited. Formation of Twist/Da heterodimers are favored relative to Twist/Twist homodimers in domains that express low Twist levels. Higher Twist/Da heterodimers levels lead to repression of somatic myogenesis in these cells, which are destined form tissues such as visceral muscle, fat body and gonadal mesoderm.