Rhodopsin patterning in central photoreceptor cells of the blowfly Calliphora vicina: cloning and characterization of Calliphora rhodopsins Rh3, Rh5 and Rh6.

The ommatidia that constitute the compound eyes of flies contain eight photoreceptor cells, which are divided into two classes: the peripheral photoreceptors, R1-6, and the central photoreceptors, R7 and R8. In the fruit fly, Drosophila, R1-6 express the same rhodopsin (Rh1), whilst the R7 and R8 of a given ommatidium express either Rh3 and Rh5, or Rh4 and Rh6, respectively. We have studied whether this expression pattern of rhodopsins is conserved in the blowfly Calliphora vicina. We have cloned three novel Calliphora rhodopsins, which are homologues of Drosophila Rh3, Rh5 and Rh6, with an amino acid sequence identity of 80.7%, 60.9% and 86.1%, respectively. Immunocytochemical studies with antibodies specific for Rh3, Rh5 and Rh6 revealed that Rh3 is expressed in a subset of R7 cells, while Rh5 and Rh6 are expressed in a non-overlapping subset of R8 cells. Rh3 and Rh5 are present in most cases in the same ommatidia, which account for approximately 27% of all ommatidia, and Rh6 is found in the complementary 73%. The similarity of the rhodopsin expression pattern of Calliphora with that of Drosophila suggests that the developmental mechanism regulating the terminal differentiation of R7 and R8 cells are highly conserved between these fly species.