Thus I was able to ask how the expression of this single gene limits the size
of the haltere.
My first insight came from removing Ubx function from random clusters
of haltere cells (2).
Ubx reduces the size
of the haltere imaginal disc relative to that of the wing by decreasing the production and mobility of growth - promoting morphogens (for example, Dpp).
As a consequence, morphogen mobility is severely impaired in the posterior side of the tissue, causing the posterior
of the haltere to be smaller than the anterior (5)(see the figure).
The alterations in transcription of dpp and tkv account for much of the reduced size
of the haltere relative to the wing (2).
Not exact matches
To determine how
halteres work, Leif Ristroph
of Cornell University in Ithaca, New York, placed tiny magnetic rods on the backs
of fruit flies and released them.
But while Greek Olympians had the help
of stone or metal hand weights known as
halteres, gibbons leap up to 10 meters without assistance.
We know that all differences between these appendages are specified by the expression
of the Hox gene Ultrabithoax (Ubx) in the
haltere and its absence from the wing (1).
In the
haltere, the same is true
of the regulator
of the size regulators: Heterozygous Ubx mutant flies have enlarged
halteres (1), and flies with extra copies
of the Ubx locus have shrunken
halteres (7).
Despite (or perhaps because
of) the silencing
of subsets
of Ubx enhancers, Ubx protein levels remain normal in the outcrossed progeny, and their
halteres develop perfectly.
Ubx converts one tkv repressor into a repressor
of a second tkv repressor, thereby upregulating tkv levels in all
haltere cells.
Because receptor binding impedes morphogen mobility, I examined the expression pattern
of the Dpp receptor thickveins (tkv) and found it to be strongly up - regulated in the
haltere compared with the wing.
The mechanism by which Ubx orchestrates these changes in the
haltere is a telling example
of selector gene function.
Furthermore, the pattern
of Dpp pathway activation is altered between the
haltere and the wing, not just quantitatively but also qualitatively (see the figure).
Drosophila has two true wings that develop from imaginal discs
of 50,000 cells and two smaller balancing organs called
halteres that form from imaginal discs
of 10,000 cells.
I was able to show that the Dpp mobility restriction in the
haltere is due in large part to transcriptional up - regulation
of tkv.
Figure 5: The regulation
of some genes differs between the wing (left) and
haltere (right) imaginal discs
of Drosophila and is under the control
of the Ultrabithorax Hox gene.