African mole-rats (Bathyergidae, Rodentia) are subterranean rodents that live in extensive dark underground tunnel systems and rarely emerge aboveground. They can discriminate between light and dark but show no overt visually driven behaviours except for light-avoidance responses. Their eyes and central visual system are strongly reduced but not degenerated. Here, we focus on retinal ganglion cells (RGCs). Sighted mammals have numerous RGC types with distinct morphological and functional properties that encode different aspects of a visual scene. We analysed the morphological diversity of 216 intracellularly dye-injected RGCs in the giant mole-rat (Fukomys mechowii) and 48 RGCs in Ansell's mole-rat (Fukomys anselli). Using a hierarchical cluster analysis on 11 morphological parameters, we show that both species possess at least five RGC types with distinct dendritic field sizes and branching patterns. These resemble some RGC types of the mouse and rat, but mole-rat RGCs feature overall sparser and more asymmetric branching patterns. The dendritic trees of most RGCs in all clusters are monostratified in the inner plexiform layer, but bistratified and multistratified/diffuse cells also exist. Thus, although RGC morphologies have become disorganized, the basic retinal organization principle of parallel information processing by distinct RGC types is retained.

Department of Game Management and Wildlife Biology Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague 165 00 Czech Republic

Department of General Zoology Faculty of Biology University of Duisburg Essen Essen 45141 Germany

Department of Pediatrics Stanford University School of Medicine Stanford CA 95305 USA

Department of Zoology Faculty of Science Charles University Prague 128 43 Czech Republic

Dr Senckenbergische Anatomie Institute for Clinical Neuroanatomy Frankfurt am Main 60590 Germany

Laboratory of Biomathematics Institute of Physiology of the Czech Academy of Sciences Prague 142 20 Czech Republic

Light Microscopy Institute of Molecular Genetics of the Czech Academy of Sciences Prague 142 20 Czech Republic

Max Planck Institute for Brain Research Frankfurt am Main 60438 Germany

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Distinct retinal ganglion cell types in strictly subterranean, naturally microphthalmic mammals