Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 152

development of identical symmetrical snappers ( Mellon, 1999 ). This result, as well
as experiments of the removal of one claw and/or denervation ( Read and Govind,
1997 ), have shown the following:
[T]he normal bilateral differentiation in these animals is maintained as a result of
the imbalance in the nature and amount of the sensory traffic returning to the cen-
tral ganglia from the snapper and pincer claws.
Young et al. (1994)
Myogenesis in Invertebrates
Myogenesis in insects is epigenetically determined. In Drosophila , local innervation
is required for the proliferation and distribution of myoblasts; hence, it is essential
for muscle development ( Currie and Bate, 1991 ). The indirect flight muscles (IFMs)
in this insect consist of the dorsal longitudinal muscles (DLMs) and dorsoventral
muscles (DVMs). Their development occurs in two stages. In the first stage, they
form the myoblast pool in a nerve-dependent mode, and in the second stage, which
is also nerve-dependent, motoneurons establish the critical threshold of the pool and
regulate myoblast patterning and muscle formation ( Figure 3.26 ). According to Roy
and VijayRaghavan:
[I]nductive instructions from the metamorphosing motor nerves mediate splitting
(of larval muscles—N.C.) and are essential for the process to proceed. Roy and
VijayRaghavan (1998) .
Denervation of DVMs causes a decline in the proliferation rate of myoblasts
and prevents myoblast patterning and formation of the muscle Anlagen. In contrast,
DLMs develop even after denervation. The reason for the difference is that pupal
DLMs develop from persisting larval muscles, which serve as template for myoblast
and muscle patterning, whereas DVMs are eliminated during metamorphosis and
have to develop de novo ( Fernandes and Keshishian, 2005 ). In the tobacco hawk-
moth ( Manduca sexta ), neurectomy of the larval leg nerve prevents proliferation and
accumulation of myoblasts in the DLM Anlage and the development of muscle fiber
bundles for muscle development ( Bayline et al., 2001 ; Consoulas and Levine, 1997 ).
Another paradigmatic example of the crucial role of the innervation in mus-
cle development is the development of the dorsal oblique 1 (DEO1) in Manduca .
Larval DEO1 consists of five muscle fibers; all of them but one are lost during meta-
morphosis, with the leftover fiber serving as an Anlage for the development of the
insect's adult muscle. Biologists now know why this fiber alone is privileged to sur-
vive and lead to the adult DEO1: it is the only one in which a particular ecdysone
receptor, known as EcR-B1, is expressed. Binding of ecdysone to the receptor
induces local myoblast proliferation. The fiber is a beneficiary of an extrinsic savvy,
rather than its own. The credit goes to the motoneuron innervating larval muscle;
in the process of metamorphosis, its axonal arbor recedes from the four other larval