One rhodopsin per photoreceptor: Iro-C genes break the rule

Abstract

While photoreceptors usually contain a single type of rhodopsin, two rhodopsins are sometimes expressed. This bi-allelic expression appears to be under genetic control, an example of which is discussed in this Primer.

Figure 1. Diagrams of the Eye and Retinal Organization of the Fruit Fly Drosophila

(A) The two compound eyes are composed of anatomically identical building blocks, the ommatidia, arranged in a hemispherical shell. The wild-type fruit fly eye is red due to pigment in pigment cells that optically isolate the ommatidia from each other. The angle between the visual axes of adjacent ommatidia (dashed blue lines), about 5°, determines the spatial acuity of the eye. (B) Each fly ommatidium contains eight photoreceptor cells, R1–R8. The six large, outer, or peripheral photoreceptors, R1–R6, have long and fat rhabdomeres, and the two slender, inner, or central photoreceptors, R7 and R8, have thinner rhabdomeres, arranged in tandem, with R7 distal and R8 proximal. (C) Light from a distant point source ((A) red lines with arrows) projected into the R7 of a certain ommatidium is received also by six photoreceptors of the R1–R6 class ((C) red rhabdomeres), each in a different, neighboring ommatidium. The signals of these R1–R6 photoreceptors are summed in the lamina, the optical ganglion adjacent to the retina, thus providing a high-sensitivity motion vision system. The R7s and R8s feed into a color vision system.

Figure 2. Absorption Spectra of the Six Rhodopsins Expressed by the Fruit Fly Drosophila (after [8])

Rh1 is the rhodopsin of the R1–R6 photoreceptors, Rh2 is expressed in the ocelli, Rh3 and Rh4 exist in two main classes of R7 photoreceptors, and Rh5 and Rh6 are the rhodopsins of the two corresponding classes of R8 photoreceptors. A specific, small set of R7 photoreceptors expresses both Rh3 and Rh4.