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In Drosophila, the eye and antenna originate from a single epithelium termed the eye-antennal imaginal disc. Illumination of the mechanisms that subdivide this epithelium into eye and antenna would enhance our understanding of the mechanisms that restrict stem cell fate. We show here that Dip3, a transcription factor required for eye development, alters fate determination when misexpressed in the early eye-antennal disc, and have taken advantage of this observation to gain new insight into the mechanisms controlling the eye-antennal switch. Dip3 misexpression yields extra antennae by two distinct mechanisms: the splitting of the antennal field into multiple antennal domains (antennal duplication), and the transformation of the eye disc to an antennal fate. Antennal duplication requires Dip3-induced under proliferation of the eye disc and concurrent over proliferation of the antennal disc. While previous studies have shown that overgrowth of the antennal disc can lead to antennal duplication, our results show that overgrowth is not sufficient for antennal duplication, which may require additional signals perhaps from the eye disc. Eye-to-antennal transformation appears to result from the combination of antennal selector gene activation, eye determination gene repression, and cell cycle perturbation in the eye disc. Both antennal duplication and eye-to-antennal transformation are suppressed by the expression of genes that drive the cell cycle providing support for tight coupling of cell fate determination and cell cycle control. The finding that this transformation occurs only in the eye disc, and not in other imaginal discs, suggests a close developmental and therefore evolutionary relationship between eyes and antennae.

The conjugation of the ubiquitin-like protein SUMO to lysine side chains plays widespread roles in the regulation of nuclear protein function. Since little information is available about the roles of SUMO in development, we have screened a collection of chromosomal deficiencies to identify developmental processes regulated by SUMO. We found that flies heterozygous for a deficiency uncovering vestigial (vg) and mutations in any of several genes encoding components of the SUMO conjugation machinery exhibit severe wing notching. This phenotype is due to an interaction between sumo and vg since it is suppressed by expression of Vg from a transgene, and is also observed in flies doubly heterozygous for vg hypomorphic alleles and sumo. In addition, the ability of Vg to direct the formation of ectopic wings when misexpressed in the eye field is enhanced by simultaneous misexpression of SUMO. In S2 cell transient transfection assays, overexpression of SUMO and the SUMO conjugating enzyme Ubc9, but not a catalytically inactive form of Ubc9, results in sumoylation of Vg and augments the activation of a Vg-responsive reporter. These findings are consistent with the idea that sumoylation stimulates Vg function during wing morphogenesis.

The pivotal role of twist in mesoderm determination in the Drosophila embryo depends upon two processes--the transcriptional activation of twist in the ventrally located mesodermal anlage and the regulation of downstream gene expression by the twist transcription factor. To elucidate the molecular mechanisms involved in these processes, we have compared both the coding and regulatory regions of the twist genes from Drosophila melanogaster and Drosophila virilis. Within the coding region, the basic-helix-loop-helix DNA binding and dimerization motif is highly conserved, consistent with the functional importance of this domain. A comparison of the transcriptional regulatory regions reveals a high degree of conservation in the more distal of the two ventral activator regions that have been mapped in the twist 5' flanking region. On the other hand, the more proximal ventral activator region is absent at the corresponding position in the D. virilis twist gene. Instead, there is a region in the second intron of the D. virilis gene that resembles the proximal element of the D. melanogaster gene, in that it consists of little more than a series of whole and half binding sites for the dorsal morphogen. In transformation experiments, the intronic D. virilis element directs an expression pattern that is indistinguishable from that directed by the D. melanogaster proximal VAR. Thus, the twi genes from these two species appear to have evolved enhancer elements with very similar structural and functional properties. These findings suggest that apparently redundant spatially regulated enhancer elements may each play essential roles in fine tuning the level and/or pattern of gene expression.