THE DROSOPHILA-RHOMBOID GENE MEDIATES THE LOCALIZED FORMATION OF WINGVEINS AND INTERACTS GENETICALLY WITH COMPONENTS OF THE EGF-R SIGNALING PATHWAY

The rhomboid (rho) gene, which encodes a transmembrane protein, is a m ember of a small group of genes (ventrolateral genes) required for the differentiation of ventral epidermis in the Drosophila embryo. The ve ntrolateral genes include spitz, which encodes an EGF-like ligand, and Star. The receptor for spitz may be the gene encoding the Drosophila epidermal growth factor-receptor (Egf-r) because the phenotype resulti ng from partial loss of function of Egf-r is similar to that of ventro lateral group mutants. Among ventrolateral genes encoding cell-surface or secreted proteins, rho is the only member expressed in a localized pattern corresponding to cells requiring the activity of the ventrola teral pathway. In this paper we provide evidence that spatial localiza tion of rho plays an analogous role in establishing vein pattern in th e adult wing. rho is expressed in early wing disc cells likely to be w ing vein primordia and later is sharply restricted to developing veins . Flies homozygous for the viable rho(ve) allele have missing veins an d rho fails to be expressed in rho(ve) mutant wing discs. Ectopic expr ession of rho during wing development leads to the formation of extra veins. Gene dosage studies among ventrolateral genes suggest that the rho product (Rho) may facilitate Spi-EGF-R signaling, resulting in act ivation of RAS. We discuss models for how localized expression of Rho may amplify signaling mediated by ubiquitously distributed ligand and receptor components.