Reference no: EM132706554

The compound eyes of arthropods contain photoreceptor cells surrounded by other cells such as cone cells and pigment cells. Scientists studied how interactions between neighboring cells in the developing eyes of larval fruit flies affect the differentiation of R7 precursor cells that normally become the photoreceptors of adult wild-type flies. One of their experiments is illustrated in Figure 1. R7 and R8 cells are both present in a cluster of cells in the developing eye. R8 cells have cell-surface proteins called Boss. R7 precursor cells have cell-surface receptor tyrosine kinases (RTKs) called Sevand contain the G protein Ras that is involved in intracellular signaling. RTKs are proteins with an extracellular portion that binds to a ligand and an intracellular portion that transfers phosphate groups from ATP to other proteins. Scientists examined the fate of R7 precursor cells in fly larvae with wild-type (normal) forms of the Boss, Sev, and Ras proteins or in larvae with single mutations in the protein Sev (Sev-) or double mutations in both Sev (Sev-) and Ras RasD). RasD is a form of Ras that is always active. (a) Describe the first interaction that must occur between the R7 precursor cell and the R8 cell for the R7 precursor to differentiate to an R7 photoreceptor cell (b) Describe how the defect in the Single-Mutant (Sev-) R7 precursor cell causes the cell to become a cone cell instead of a photoreceptor cell. (c) The scientists predict that the presence of activated Ras in the Double-Mutant (Sev-RasD) R7 precursor cell will enable the cell to differentiate into an R7 photoreceptor cell. Based on Figure 1, evaluate the likely accuracy of their prediction. (d) The Sev RTK is expressed in more cell types than just R7 precursor cells, and it appears to be important in determining the phenotypes of these other cells. Explain how one receptor can induce different phenotypes in different cell types.