Identification of genes required for Drosophila eye development using a phenotypic enhancer-trap.

A novel method of P-element mutagenesis is described for the isolation of mutants affecting the development of the Drosophila compound eye. It exploits the interaction between the Bride of Sevenless (Boss) ligand and the Sevenless (Sev) receptor tyrosine kinase that triggers the formation of the UV-sensitive photoreceptor neuron, R7. Transposition of a boss cDNA transgene, in an otherwise boss mutant background, was used as a "phenotypic trap" in live flies to identify enhancers expressed during a narrow time window in eye development. Using a rapid behavioral screen, more than 400,000 flies were tested for restoration of R7. Some 1,800 R7-containing flies were identified. Among these, 21 independent insertions with expression of the boss reporter gene in the R8 cell were identified by a external eye morphology and staining with an antibody against Boss. Among 900 lines with expression of the boss reporter gene in multiple cells assessed for homozygous mutant phenotypes, insertions in the marbles, glass, gap1, and fasciclin II genes were isolated. This phenotypic enhancer-trap facilitates (i) the isolation of enhancer-traps with a specific expression pattern, and (ii) the recovery of mutants disrupting development of specific tissues. Because the temporal and tissue specificity of the phenotypic trap is dependent on the choice of the marker used, this approach can be extended to other tissues and developmental stages.

Induction of Drosophila eye development by decapentaplegic.

The Drosophila decapentaplegic (dpp) gene, encoding a secreted protein of the transforming growth factor-beta (TGF-beta) superfamily, controls proliferation and patterning in diverse tissues, including the eye imaginal disc. Pattern formation in this tissue is initiated at the posterior edge and moves anteriorly as a wave; the front of this wave is called the morphogenetic furrow (MF). Dpp is required for proliferation and initiation of pattern formation at the posterior edge of the eye disc. It has also been suggested that Dpp is the principal mediator of Hedgehog function in driving progression of the MF across the disc. In this paper, ectopic Dpp expression is shown to be sufficient to induce a duplicated eye disc with normal shape, MF progression, neuronal cluster formation and direction of axon outgrowth. Induction of ectopic eye development occurs preferentially along the anterior margin of the eye disc. Ectopic Dpp clones situated away from the margins induce neither proliferation nor patterning. The Dpp signalling pathway is shown to be under tight transcriptional and post-transcriptional control within different spatial domains in the developing eye disc. In addition, Dpp positively controls its own expression and suppresses wingless transcription. In contrast to the wing disc, Dpp does not appear to be the principal mediator of Hedgehog function in the eye.

The eye-specification proteins So and Eya form a complex and regulate multiple steps in Drosophila eye development.

Sine oculis (so) and eyes absent (eya) are required for Drosophila eye development and are founding members of the mammalian Six and Eya gene families. These genes have been proposed to act with eyeless (Pax6) to regulate eye development in vertebrates and invertebrates. so encodes a highly diverged homeobox transcription factor and eya encodes a novel nuclear protein. We demonstrate that So and Eya (1) regulate common steps in eye development including cell proliferation, patterning, and neuronal development; (2) synergize in inducing ectopic eyes; and (3) interact in yeast and in vitro through evolutionarily conserved domains. We propose that an So/Eya complex regulates multiple steps in eye development and functions within the context of a network of genes to specify eye tissue identity.

Graded effect of tailless on posterior gut development: molecular basis of an allelic series of a nuclear receptor gene.

By marking cells of early gastrula stage embryos, we showed that in embryos mutant for a strong tll allele the fate map is shifted posteriorly and the hindgut anlage is deleted. We therefore used aspects of hindgut development to characterize the phenotype of new and previously described tll alleles. In embryos mutant for the various alleles, relative levels of blastoderm expression of Trg (T-related gene, required to establish the hindgut) and of mature hindgut size were determined; the results of these assays correlated with each other. Of the alleles that map to the sequence encoding the Tailless nuclear receptor protein, all (four) affect the zinc fingers of the DNA binding domain; surprisingly, substitutions of highly conserved residues allow a range of activities as detected by our bioassays.

Characterization of downstream elements in a Raf-1 pathway.

At the poles of the Drosophila embryo, cell fate is established by a pathway that begins with the activation of a membrane-associated tyrosine kinase (the torso gene product); this then leads to activation of a serine/threonine kinase (Drosophila Raf-1). Activated Raf-1 then leads, by an undefined mechanism, to the transcriptional activation of the tailless (tll) gene; the tll gene product, itself a transcription factor, subsequently regulates the expression of an array of target genes. To further define this pathway, we have utilized sequence comparison between Drosophila melanogaster and Drosophila virilis to identify conserved elements in the tll promoter region. As assessed by DNase I footprinting and promoter dissection experiments, two of these elements are potential regulatory targets of Raf-1-activated transcription factors. Sequence comparison also reveals that the unique residues in the DNA-binding domain of the tll protein, the next component in the pathway, are conserved. One of these residues, the alanine after the last cysteine in the first zinc finger, may be responsible for part of the difference between the tll protein DNA binding site and the closely related half-site of the retinoid/estrogen receptors. Consistent with the rapid turnover of the tll protein, it contains a PEST sequence (rich in proline, glutamate and aspartate, serine, and threonine) that is also conserved.

bicoid and the terminal system activate tailless expression in the early Drosophila embryo.

In the early Drosophila embryo, the maternal terminal genes are required for formation of the acron at the anterior and the telson at the posterior. We show here that the terminal system, a signal transduction pathway active at the poles of the embryo, is required to activate transcription of the key zygotic gene tailless (tll) in two symmetrical domains. Consistent with the characterization of the tll protein as a putative transcription factor (a member of the steroid receptor superfamily) that represses segmentation genes and activates terminal-specific genes, we observe a correlation between the presence of the posterior cap of tll expression and differentiation of a telson. While the maternal patterning systems of the Drosophila embryo function for the most part independently, the maternal anterior system, in which the bicoid (bcd) protein functions as a graded morphogen, is required together with the terminal system to establish the acron. This dual requirement is evident at the molecular level in the control of tll expression. We find that bcd and the terminal system are required together to activate the anterior-dorsal stripe of tll expression that is correlated with formation of the acron. In the absence of bcd, the anterior cap of tll expression established by the terminal system persists and an ectopic telson forms at the anterior, while in the absence of terminal system activity only an abnormal anterior stripe forms. This is the first described example of how, by jointly controlling expression of the same gene, two systems of positional information function together to set unique positional values.

Dual role of the Drosophila pattern gene tailless in embryonic termini.

One of the first zygotically active genes required for formation of the terminal domains of the Drosophila embryo is tailless (tll). Expression of the tll gene is activated ectopically in gain-of-function mutants of the maternal terminal gene torso (tor); this suggests that tor normally activates the tll gene in the termini. Ectopic expression of tll under the control of an inducible promoter results in differentiation of ectopic terminal-specific structures, the Filzkörper, and leads to the activation of at least one gene, hunchback, that is required to form these structures. Ectopic expression of the tll gene also represses segmentation by repressing the gap genes Krüppel and knirps and probably also pair rule genes.

The Drosophila gene tailless is expressed at the embryonic termini and is a member of the steroid receptor superfamily.

The zygotically active tailless (tll) gene plays a key role in the establishment of nonmetameric domains at the anterior and posterior poles of the Drosophila embryo. We have cloned the tll gene and show that it encodes a protein with striking similarity to steroid hormone receptors in both the DNA binding "finger" and ligand binding domains. tll RNA is initially expressed in embryos in two mirror-image symmetrical domains; this pattern then quickly resolves into a pattern consistent with the mutant phenotype: a posterior cap and an anterior dorsal stripe. That the tll gene may also play a role in the nervous system is suggested by its strong expression in the forming brain and transient expression in the peripheral nervous system.