Ras controls growth, survival and differentiation in the Drosophila eye bydifferent thresholds of MAP kinase activity

Ras mediates a plethora of cellular functions during development. In the de veloping eye of Drosophila, Ras performs three temporally separate function s. In dividing cells, it is required for growth but is not essential for ce ll cycle progression, In postmitotic cells, it promotes survival and subseq uent differentiation of ommatidial cells. In the present paper, we have ana lyzed the different roles of Ras during eye development by using molecularl y defined complete and partial loss-of-function mutations of Ras, We show t hat the three different functions of Ras are mediated by distinct threshold s of MAPK activity. Low MAPK activity prolongs cell survival and permits di fferentiation of R8 photoreceptor cells while high or persistent MAPK activ ity is sufficient to precociously induce R1-R7 photoreceptor differentiatio n in dividing cells.