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Fig. 1. R8 and non-R8 photoreceptors differentiate according to distinct developmental paradigms. (A) Non-R8 photoreceptors are recruited by Egfr signaling. Spitz (Spi, purple circles) is initially secreted by R8 and binds to the Egf receptor to induce photoreceptor differentiation (yellow). As the field of Spi expands outward from R8, photoreceptors differentiate in a stepwise fashion (R2/R5, R3/R4, R1/R6, R7). R8 differentiation does not require Egfr pathway activation (blue). (B) In wild-type ommatidia (top), Senseless (Sens) is expressed in the presumptive R8 (pre-R8) cell and Rough (Ro) expression is repressed. The pre-R8 cell then differentiates as an R8 photoreceptor. In sens mutant ommatidia (bottom), the pre-R8 cell expresses Ro and differentiates as a cell of the R2/R5 subtype. (C) R8 differentiation requires Ato and Sens function, while R2/R5 differentiation requires Spi/Egfr activation, which in turn induces Ro expression. Since Sens is a repressor of Ro in R8, it is possible that this prevention of Ro expression occurs via Sens-mediated repression of the Spi/Egfr pathway.





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