One Rhodopsin per Photoreceptor: Iro-C Genes Break the Rule
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.