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Fig. 3. wg signalling provides a genetic switch for the specification
of leg versus tracheal primordia. (A,B) Wild-type expression
pattern of trh in the salivary glands and tracheal placodes, and of
btd in cephalic segments and cell clusters in thoracic and abdominal
segments. (C,D) In a wg mutant, trh expression is
expanded along the anteroposterior axis, whereas btd expression is
abolished in the thoracic and abdominal segments. (E,F) Conversely,
upon ectopic expression of wg, trh expression in the tracheal
placodes is suppressed and btd expression is expanded. (G,H)
Restricted ectopic activation of the wg pathway reduces the domains
of trh expression and expands those of btd expression.
(I,J) Restricted inactivation of the wg pathway expands
trh expression and reduces btd expression. All embryos are
at stage 11. (K) Detail of wg mutant at a somewhat later
stage, showing that the ectopic trh-expressing cells begin to
invaginate. (L) Detail of an embryo at germ band extension upon ectopic
expression of wg at germ band retraction, indicating that expansion
of btd occurs only in part of the embryonic ectoderm, while the
segmental pattern persists in the central nervous system. (M) Schematic
representation of the role of wg (transcribed in the yellow domain)
in promoting appendage and repressing tracheal fates.
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