Figure 13, 42 The Drosophila compound eye (A) Scanning electron micrograph showing the compound eye, com posed of about 800 individual units. Eye Development in drosophila Each unit contains eight photore- tor neurons(designated RI ough R8), which develop in a fixed The development of the compound eye of Drosophila provides a good exam- er and pattern (A Courtesy of T. ple of the role of direct cell-cell signaling in differentiation, which has been ikatesh, University of Michigan. elucidated largely by genetic analysis. The Drosophila compound eye consists of about 800 individual units each of which contains eight photoreceptor neurons (rI through R8)and 12 lens cells(Figure 13.42). The photoreceptor neurons develop in a fixed order, beginning with the differentiation of R8. R8 then induces two neighboring cells to become the R2 and R5 photoreceptors Next, R2 induces neighboring cells to become RI and R3, and R5 induces neighboring cells to become r4 and R6. The final step is differentiation of r7 which is induced by interaction with R8. Lens cells then develop from those cells that do not differentiate into photoreceptors The signaling pathway leading to development of the R7 cell has been characterized in detail, based on the isolation of mutant flies in which r7 fails to develop (Figure 13. 43). One of these mutants(sevenless)results from defects in a gene encoding a receptor protein-tyrosine kinase that is expressed by pre- cursors of R7 cells. Another mutant(called boss, which is short for bride-of-seur enless)results from defects in a gene encoding a cell surface protein expressed by R8 cells. Boss is the ligand for Sevenless, so direct cell-cell interaction 100gm between R8 and a precursor cell activates the sevenless protein-tyrosine kinase. Further studies have shown that cell differentiation induced by sig. RI Rat