Fig. 8

Comparison of ommatidial differentiation in adult ommatidia and larval Bolwig organ of Drosophila. a Sequential specification of cell fates in the adult ommatidium. R8 photoreceptors express either long-wavelength opsin, or a blue-sensitive opsin. The peripheral photoreceptors 1–6 expressed a long-wavelength opsin. R7 photoreceptors express a UV-sensitive opsin. b Cell fate specification in the Drosophila Bolwig organ begins with three to four photoreceptors that will express the same blue-sensitive opsin that is expressed in a fraction of the R8 cells in the adult retina. These Bolwig organ cells are essential for the specification of the next cell fates and therefore referred to as founder (F) cells similar to the adult R8 cells. The second type of Bolwig organ photoreceptors express a long-wavelength opsin and are referred to as peripheral (P) photoreceptors. The Bolwig organ P photoreceptors are homologous to photoreceptors 1, 2, 3 and 6 of the adult ommatidium. Note the lack of R7 and accessory photoreceptors in the Bolwig organ which is likely explained through precocious termination of cell fate specification compared to adult ommatidial development