Biology Reference
In-Depth Information
FIGURE 18.12
Eversion of the leg disc is associated with relaxation of the previous anisotropy of cells so that
they contract in a circumferential direction and elongate in a direction corresponding to the radius of the original
disc, a direction that now corresponds to the long axis of the leg. The shapes of the disc and leg have been simplified
in this diagram in order to make the correspondence of old and new axes clear; both are in reality rather folded.
technique. The requirement for myosin in the discs cannot be tested by a simple gene
knockout because embryos carrying the spaghetti-squash (sqh) mutation, which disrupts the
gene encoding the myosin regulatory light chain, fail to develop far enough for imaginal
disc development to be studied. The mutants can be rescued, however, by a construct encod-
ing wild-type myosin regulatory light chain driven by a heat-shock promoter as long as they
are subjected to a daily heat shock. If embryos are grown with daily myosin-transcription-
inducing heat shocks up to a particular stage of development and then the heat shock is
omitted, myosin is made available for early development but can be taken away by the
time discs evaginate. When the cessation of heat shock is timed to coincide with leg disc ever-
sion, eversion fails. This suggests that myosin regulatory light chain is important to the
process.
38
Rho, which activates myosin regulatory light chain via Rho kinase in many
systems, is also required for leg eversion.
39
This suggests that the evaginating leg may
assemble its contractile machinery through the 'classical' Rho pathway used by so many
cells.
