Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 153

Figure 3.26 Diagrammatic representation of the early developmental events in the patterning
of DLMs. (A) During larval life, myoblasts (red dots) that give rise to the DLMs and other dorsal
mesothoracic muscles remain sequestered in the wing disc (asterisk) and associated with motor
nerves (green) innervating the larval muscles, dorsal oblique 1, 2, and 3. The attachment sites
for the IFMs are prefigured on the notal epithelium of the wing disc by expression of stripe in
groups of cells (short arrows). Blue , larval muscle nuclei; long arrow , a synaptic bouton on the
larval muscle. To p , anterior; vertical green arrow , dorsal midline. Orientation of the subsequent
panels is similar. (B) During early pupal development (6-10 h APF), the myoblasts migrate out
from the everting wing disk (asterisk) and swarm over the three remnant larval fibers, which
unlike other larval muscles, escape histolysis. The larval muscles appear vacuolated, and there is
a regression of the synaptic terminals of the motor neurons (long arrow). The stripe expressing
epidermal cells (purple spots) that will become the attachment sites for the DLMs position
themselves adjacent to the larval muscles (short arrows, posterior attachment sites). (C) By
16 h APF, the larval fibers have begun to split longitudinally to form the templates for DLM
development, and the motor neurons send out fresh arborizations over the splitting muscles (long
arrows). Filopodial extensions from the ends of the templates anchor them to the attachment
sites on the epidermis (short arrows).
Source : From Roy and VijayRaghavan (1998) .
muscles and remains connected to only one of them. The motoneuron releases a “dif-
fusible” agent that induces the muscle fiber to produce the specific receptor EcR-B1.
The remaining uninnervated larval fibers produce only the isoform EcRA of the
ecdysone receptor that, in a suicidal process, induces the PCD and elimination of the