tramtrack is a transcriptional repressor required for cell fate determination in the Drosophila eye.

Abstract

Cell fate determination in the Drosophila eye is mediated by inductive events between neighboring cells in the eye imaginal disc. These inductive signals lead to differential gene expression necessary for the elaboration of different cell types in the compound eye. Several putative transcription factors have been identified previously that may be required for expression of genes that specify cell fate in the compound eye. Repression of inappropriate gene expression may be as important as transcriptional activation in the determination of cell fate. We report the identification of a mutation in the Drosophila tramtrack (ttk) locus that is required for cell fate determination in the compound eye. ttk is expressed as two proteins, p69 and p88, shown previously to bind to the regulatory regions of several segmentation genes. In ttk 1, an allele missing the mRNA encoding p88, many ommatidia contained supernumerary R7 cells and decreased numbers of RI-R6 cells, ttk len, which appears to disrupt both Ttk proteins, was characterized by early embryonic arrest as well as transformation of ommatidial cells into nonommatidial cell types in mosaic flies. Consistent with previous proposals that the Ttk proteins are transcriptional repressors of segmentation genes, we detected ectopic or increased expression of the segment polarity gene engrailed in several ttk ~ larval tissues. We propose that p69 is required to repress expression of genes that are incompatible with development of photoreceptor cell fates, whereas p88 appears to be required to repress genes that promote the R7 cell fate.[Key Words: Drosophila; development; eye; transcription; repressor; engrafted] Received December 15, 1992; revised version accepted March 23, 1993.Cell fate specification in the Drosophila compound eye appears to be mediated by signals from neighboring cells within each developing cluster of cells in the eye-imaginal disc (for review, see Greenwald and Rubin 1992). A rapidly growing number of genes required in photoreceptor cell determination, particularly that of the R7 cell, have been identified. These include sevenless (sev), which encodes a receptor tyrosine kinase required in the R7 cell, and boss, the ligand for the Sev protein that is required specifically in the R8 cell (Hafen et al. 1987;Reinke and Zipursky 1988;Simon et al. 1989;Kramer et al. 1991). The recent isolation of mutations in Rasl, and the positive and negative regulators of Rasl, Sos, and Gapl, have demonstrated that R7 cell determination in-

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“…Between four and five eyes were analyzed in semithin sections for each genotype. In ttk 1 , mRNA encoding p88 is missing (Xiong and Montell 1993). ttk 0219 is an enhancer trap allele with a P element inserted upstream of the ttk transcription unit (Lai et al 1996).…”

“…Two ETS-domain-containing transcription factors, Yan and Pointed, are direct targets of MAPK phosphorylation: Pointed is activated by phosphorylation and promotes R7 induction (O'Neil et al 1994), whereas Yan is inhibited by phosphorylation and acts as a transcriptional repressor (Rebay and Rubin 1995). Inactivation of a second repressor, Tramtrack88 (Ttk88), also is required for R7 development (Xiong and Montell 1993 …”

The BTB domain, found primarily in zinc finger proteins, defines an evolutionarily conserved family that includes several developmentally regulated genes in Drosophila.

“…It is required during embryogenesis, presumably to repress inappropriate transcription ofthese genes, and possibly other segmentation genes, including odd-skipped, hairy, and runt (4)(5)(6)(7). ttk is also required during cell fate determination in the eye (8). BR-C is an early response locus in the ecdysone-induced regulatory cascade that initiates metamorphosis, and its products are thought to activate downstream regulatory genes (3).…”

End of the line? Tramtrack and cell fate determination in Drosophila

“…Mutants in which expression of only Ttk-88 is affected, demonstrate aberrant photoreceptor development with ~5 0 % of ommatidia exhibiting extra R 7 cells and a number of ommatidia possessing either more or less than the normal six outer photoreceptors (Xiong & Montell 1993;Lai et al 1996). Clonal analysis suggests that Ttk-88 is required autonomously and suppresses inappropriate R 7 cell determination in cells that might otherwise be competent to assume this fate.…”