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.
