MULTIPLE RAS SIGNALS PATTERN THE DROSOPHILA OVARIAN FOLLICLE CELLS

During Drosophila oogenesis, spatially restricted activity of the TORP EDO receptor tyrosine kinase first recruits follicle cells adjacent to the oocyte to a posterior cell fate and then specifies dorsal follicl e cells. Another receptor tyrosine kinase, BREATHLESS, stimulates migr ation of the anterior follicle cells known as border cells. Since Ras is known to mediate many receptor tyrosine kinase effects, we have inv estigated the role of Ras in follicle cell fate determination, differe ntiation, and migration throughout oogenesis. Early ectopic Ras activi ty induced transient expression of posterior follicle cell markers in anterior follicle cells, but did not inhibit anterior differentiation. Later ectopic Ras activity inhibited anterior follicle cell different iation but did not induce posterior marker expression. Complete transf ormation of anterior follicle cells to posterior follicle cells requir ed early ectopic Ras activity in egg chambers where terminal different iation of anterior cells was inhibited. These results suggest that, in vivo as in vitro, Ras can have diverse effects on different cells, bu t, in addition, Ras activity can have different effects on the same ce lls at different stages in their development. (C) 1997 Academic Press.