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Fig. 2. Toll10b 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 Toll10b mutant embryos (B,E),
and similar embryos expressing an activated form of Notch
(UAS-Nintra; C,F) or Ras (UAS-rasV12;
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
Toll10b 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.
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