The mammalian photoreceptor mosaic-adaptive design

Unlike in birds and cold-blooded vertebrates' retinas. the photoreceptors o f mammalian retinas were long supposed to be morphologically uniform and di fficult to distinguish into subtypes. A number of new techniques have now b egun to overcome the previous limitations. A hitherto unexpected variabilit y of spectral and morphological subtypes and topographic patterns of distri bution in the various retinas are being revealed. We begin to understand th e design of the photoreceptor mosaics, the constraints of evolutionary hist ory and the ecological specialization of these mosaics in all the mammalian subgroups. The review discusses current cytological identification of mamm alian photoreceptor types and speculates on the likely "bottleneck-scenario " for the origin of the basic design of the mammalian retina. It then provi des a brief synopsis of current data on the photoreceptors in the various m ammalian orders and derives some trends for phenomena such as rod/cone dual ism, spectral range, preservation or loss of double cones and oil droplets, photopigment co-expression and mono- and tri-chromacy. Finally, we attempt to demonstrate that, building on the limits of an ancient rod dominant (pr obably dichromatic) model, mammalian retinas have developed considerable ra diation. Comparing the nonprimate models with the intensively studied prima te model should provide us with a deeper understanding of the basic design of the mammalian retina. (C) 2000 Elsevier Science Ltd. Ail rights reserved .