Genetic interactions with Rap1 and Ras1 reveal a second function for the fat facets deubiquitinating enzyme in Drosophila eye development

Abstract

The Drosophila fat facets gene encodes a deubiquitinating enzyme that regulates a cell communication pathway essential very early in eye development, prior to facet assembly, to limit the number of photoreceptor cells in each facet of the compound eye to eight. The Fat facets protein facilitates the production of a signal in cells outside the developing facets that inhibits neural development of particular facet precursor cells. Novel gain-of-function mutations in the Drosophila Rap1 and Ras1 genes are described herein that interact genetically with fat facets mutations. Analysis of these genetic interactions reveals that Fat facets has an additional function later in eye development involving Rap1 and Ras1 proteins. Moreover, the results suggest that undifferentiated cells outside the facet continue to influence facet assembly later in eye development.

Figure 3

Summary of mutant eye phenotypes. The numbers refer to R cells, each of which are represented by a solid circle. The faf mutant eye phenotype is qualitatively different from the Rap1 mutant phenotype and hypomorphic faf alleles (faf^hypo) enhance the mutant phenotype of Rap1.

Figure 1

Mutant eye phenotypes. Scanning electron micrographs and tangential sections through eyes of the following genotypes are shown. (A and B) Wild-type. (C and D) faf^BX3/faf^FO8. (E and F) Rap1^T58M faf^BX3/faf^FO8. (G and H) Rap1^T58M/Rap1⁻. (I and J) Rap1^R/+. (K and L) Rap1^V153M/Rap1^V153M. (M and N) Rap1⁻ faf^FO8/faf^BX3. (O and P) Ras1^E63K faf^BX3/faf^FO8. (Q and R) Ras1^E63K/Ras1⁻. (S and T) faf^BX4.

Figure 2

Amino acid alterations in Rap1 and Ras1 mutant proteins. The amino acid sequences of Drosophila Rap1 (15) and Ras1 (12) are shown. The GTP-binding regions are boxed (21) and the region thought to bind effector is underlined (22, 23). The amino acid changes in Rap1^R (15), Rap1^V153M, Rap1^T58M, and Ras1^E63K mutant proteins are indicated. The codon changes are V153M (GTG → ATG), T58M (ACG → ATG), and E63K (GAG → AAG).

Figure 4

Quantitation of mutant eye phenotypes. The average fraction of faf-type (open bars) and Rap1^R-type (shaded bars) mutant facets was calculated for each of the genotypes shown. One hundred to 500 facets in two to five eyes were counted for each genotype, except for the genotypes with no error bars where one eye was counted. The error bars represent one standard deviation above and below the mean value obtained for each eye; standard errors are large in the cases where there was significant variation in the penetrance of the phenotype. For reasons that are not understood, the mutant Rap1 alleles suppress the faf phenotype.

Figure 5

Complementation of mutant phenotypes by transgenes. The average fraction of mutant facets, indicated by the bars, was calculated for each of the genotypes shown. One hundred to 500 facets in two to five eyes were counted for each genotype, except for the genotypes with no error bars where one eye was counted. (In glrs-faf, four eyes were analyzed and the standard error was zero.) The error bars represent one standard deviation above and below the mean value obtained for each eye; standard errors are large in the cases where there was significant variation in the penetrance of the phenotype.