Sexual dimorphism in a photoreceptor

Abstract

The recent observation of fluorescence emission from photoreceptor cells in flies¹ offers a new opportunity to map the distribution of the various spectral types in a highly organized retinal mosaic. Previous studies had led to the conclusion that in each ommatidium the two central receptor cells are quite distinct from their six neighbours in terms of anatomical projection^2,3, visual pigment and physiology⁴. We show here that in a restricted, well defined region of the male eye only, one of these central cells resembles its six neighbours not only in having the same visual pigment and physiological properties, but also in sending its axon to the same neuropile. A clue to the function of this sexually dimorphic retinal organization is given by the fact that these male-specific cells dominate the frontal-dorsal part of the retina (fovea) which is used by the male to track females in aerobatic chases^5,6. We suggest that the male fovea, already known to drive sex-specific higher-order neurones⁷, has sacrificed colour vision for other more vital kinds of information processing such as improving quantum catch and possibly also contrast transfer or movement detection.