Fig. 2. Toll^10b mutant embryos specify the FC and FCM fate, and respond to alterations in the Ras and Notch signaling pathways. (A) Representation of the steps involved in FC specification (1, 2, 3, 4) and some aspects of terminal muscle differentiation (5). Dark blue indicates clusters of equipotent myoblasts in which the Ras signaling pathway is active, whereas an increase in the thickness of the outline of the cell represents activation of the Notch signaling pathway. Cells (in 1) are depicted in a transitional state when the coordinated activities of the Notch and Ras pathways (and Argos activity, not represented) single out the muscle progenitor cell (green in 2). In these clusters (1), all cells show some Notch activity, but a particular cell achieves a stronger Delta signaling ability, which leads to a strong activation of the Notch pathway in surrounding cells (dark blue, 2). Surrounding cells are therefore prevented from becoming muscle progenitors. Concomitantly with this process, a burst of Ras signaling activity in the progenitor cell (green in 2) leads to activation of progenitor cell markers, such as Eve, and feedback loops, such as Argos. Muscle progenitor cells (P1 and P2 in 3) undergo asymmetric division giving rise to two FCs represented as green and orange cells in 4. Finally, FCs fuse to surrounding FCMs (represented as blue cells with processes in 5) to form syncytial muscles, growth cones extend to innervate the muscles, and muscle precursors extend towards their normal epidermal attachment sites. (B-J) All panels show 5- to 9-hour AEL embryos with anterior to the left. Anti-Slouch (B-D) and anti-Vg (E-G) antibody staining of Toll^10b mutant embryos (B,E), and similar embryos expressing an activated form of Notch (UAS-N^intra; C,F) or Ras (UAS-ras^V12; D,G) under the control of the twist-Gal4 driver at 29°C. Activation of the Notch pathway throughout the embryo led to a complete inhibition of the FC fate (note lack of staining in C,F), whereas activation of the Ras pathway enhanced FC fate as shown by an increased staining (D,G; arrowheads). Conversely, the FCM marker Sns was readily expressed in Toll^10b embryos (arrowheads, H), and showed more (bent arrow, I), or less (J, bent arrow indicates residual staining), expression upon Notch or Ras activation, respectively. Cartoons beneath the panels represent schematically the results of experimental manipulation. Color scheme is the same as in A.