FIGURE 7-3 The signal transduction pathway by which the development of the R7 photoreceptor cell is induced in the eye of Drosophila. This pathway is a receptor tyrosine kinase pathway. Many of the components of the pathway were first discovered in Drosophila by doing selections in mutants in which the pathway was operating close to threshold due to a component of reduced activity. These “sensitized strains” revealed secondary mutations easily, and might be useful for detecting toxicant effects on development. BOSS, the ligand presented on the surface of neighboring cells R2 and R5. SEV, the transmembrane receptor that binds the BOSS ligand protein from an adjacent cell. DRK, an adapter protein binding to the phosphorylated cytoplasmic tail of the receptor. SOS, a GTP:GDP exchange factor protein activated when binding to DRK. RAS, a small G protein active when GTP is bound. DRAF, a protein kinase that phosphorylates DSOR, which in turn phosphorylates ROLLED, which in turn phosphorylates a transcription factor, which then activates specific gene expression involved in R7 determination. MARPKKK, mitogen-activated protein kinase kinase kinase; MEK, mitogen-activated protein kinase/extracellular signal-related kinase kinase; MAPK, mitogen-activated protein kinase. See text for details. Source: Adapted from Simon et al. (1991).