| Members
of the Six family of transcription factors are known to play
crucial roles in the development of both flies and vertebrates
including mammals. Each family member is characterized by
the presence of both the SIX protein-protein interaction domain
(SD) and the homeobox DNA-binding domain (HD). The fruit fly
Drosophila melanogaster contains three members of
this family: sine oculis, optix and DSix4
(Figure 1).
Of these three
genes only sine oculis and optix play roles
in early eye development (Figure 2). Both genes have additional
roles in the development of the embryonic head and larval
visual systems. DSix4, on the other hand, functions
during adult testis and muscular development. In mammals Six3
and Six6 (optix orthologs) also function
to specify the retina while Six1, Six2,
Six4 and Six5 (orthologs of sine oculis
and DSix4) appear to play a minor role if any in
the early retina.
The simple structure
and stereotyped development of the fly retina along with the
genetic and molecular tools that are available to Drosophila
make the compound eye an excellent experimental system within
which to identify binding partners and transcriptional targets
of the Six family members. Using the UAS/GAL4 system I have
forcibly expressed sine oculis, optix and
DSix4 individually ahead and behind the morphogenetic
furrow using the ey-GAL4 and GMR-GAL4 drivers respectively.
In all cases, the expression of a Six family member in the
retina leads to an alteration of adult eye structure (Figure
3). Underlying the rough eye phenotypes are defects in retinal
determination and cell fate specification.
I have established
stocks of each of the above-pictured combinations of GAL4
and UAS lines and am conducting a series of genetic screens
in which I am looking for mutants that modify the parental
rough eye phenotypes. I expect to find genes that interact
specifically with each Six family member as well as more promiscuous
factors that interact with all three transcription factors.
These genes would be predicted to be either binding partners
or transcriptional targets of Six family members. Other labs
have identified these types of genes by using yeast two-hybrid
assays, bioinformatics and DNA microarrays. Our approach will
complement and extend these studies by identifying genes in
an in vivo functional assay.
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