Fig. 2. Overexpression of Twist or Twist-Twist in vivo leads to ectopic muscle formation. (A-F) Wild-type embryos; (G-L) ectopic expression of UAS-twist; (M-R) ectopic expression of UAS-twi-twi. In this and the following figures, dorsal is up and anterior to the left, unless noted. Expression of Twist (Twi; G) or Twi-Twi (M) using one copy of da-GAL4 driver led to conversion of all ectoderm into somatic muscle (cf. Baylies and Bate, 1996). G and M show high magnification of multinucleated, Myosin heavy chain (Mhc)-positive external cells (arrows). Insets show a lateral view of whole embryos. Fused di- and tri-nucleated cells, which are characteristic of somatic myogenesis, were found. Since no ectodermal derivatives were detected, these muscles did not spread out to form a pattern owing to lack of epidermis. Expression of Twist (H) or Twi-Twi (N) in the mesoderm using twist-GAL4 driver led to ectopic formation of Mhc-positive muscle cells, shown here ventrally (arrowheads). Note that Mhc is found in cells where it is never usually expressed. Extra muscle formation was supported by the ectopic expression of founder cell markers, such as Kr (I and O; white arrow; asterisks are shown as a reference). Zfh1 expression in pericardial and cardial cells was lost in embryos overexpressing Twist (J) and Twi-Twi (P). Fas III expression in visceral muscle progenitors was also disrupted in stage 12 embryos that ectopically expressed Twist (K) and Twi-Twi (Q). At stage 10, Bap is expressed in a subset of mesodermal cells that are progenitors for visceral mesoderm and fat body. Bap is highly reduced in embryos that overexpress Twist (L) or Twi-Twi (R). Despite greatly reduced Bap, a complete loss of Fas III under these conditions was not found. Azpiazu and Frasch reported similar effects for bap hypomorphs (Azpiazu and Frasch, 1998).