Specification of primary pigment cell and outer photoreceptor fates by BarH1 homeobox gene in the developing Drosophila eye.

In the developing Drosophila eye, BarH1 and BarH2, paired homeobox genes expressed in R1/R6 outer photoreceptors and primary pigment cells, are essential for normal eye morphogenesis. Here, we show evidence that BarH1 ectopically expressed under the control of the sevenless enhancer (sev-BarH1) causes two types of cone cell transformation: transformation of anterior/posterior cone cells into outer photoreceptors and transformation of equatorial/polar cone cells into primary pigment cells. sev-BarH1repressed the endogenous expression of the rough homeobox gene in R3/R4 photoreceptors, while the BarH2 homeobox gene was activated by sev-BarH1 in an appreciable fraction of extra outer photoreceptors. In primary pigment cells generated by cone cell transformation, the expression of cut, a homeobox gene specific to cone cells, was completely replaced with that of Bar homeobox genes. Extra outer photoreceptor formation was suppressed and enhanced, respectively, by reducing the activity of Ras/MAPK signaling and by dosage reduction of yan, a negative regulator of the pathway, suggesting interactions between Bar homeobox genes (cell fate determinants) and Ras/MAPK signaling in eye development.