Fig. 6. Model of the roles of sal and svp in the specification of R3/R4 versus R7 [based on our present findings and on Domingos et al. (Domingos et al., 2004)]. sal is expressed in R3/R4 from row three to row seven, after which it is progressively repressed. sal expression in R7 starts from row seven to nine. Expression of svp in R3/R4 starts in row four. From row three to row seven, sal is required for svp expression in R3/R4, for R3/R4 specification and for PCP establishment. After rows seven to nine, sal is necessary and sufficient for R7 differentiation. Repression of sal by svp in R3/R4 is necessary for the maintenance of R3/R4 identity and the inhibition of R7 fate.