Somatic and dendritic mosaics formed by large ganglion cells in the retina of the common house gecko (Hemidactylus frenatus)

Recent studies of large ganglion cells in fishes and frogs have identified a shared inventory of three basic types, with characteristic forms and spatially independent mosaic distributions. These anamniote types and mosaics are hard to match to the large ganglion cell types and mosaics of mammals, implying that the underlying developmental programmes have diverged during evolution. Reptiles and mammals both belong to the amniote lineage, so the point of divergence can be investigated by comparing the large ganglion cells of reptiles with those of mammals, taking fishes and frogs as outgroups. With this aim, ganglion cells of the common house gecko, Hemidactylus frenatus, were labelled with horse-radish peroxidase by an in vitro method and studied in retinal flatmounts. Two prominent, regular, spatially independent mosaics were consistently present. One (alpha a) was characterized by somata displaced into the inner nuclear layer and dendrites forming planar trees in sublamina a; the other (alpha ab) comprised large orthotopic somata and distinctive, bistratified dendrites that formed discrete planar subtrees in sublaminae a and b. These subtrees were joined by up to 40 vertical link segments, whose distribution was found to correlate with the underlying photoreceptor mosaic. Some specimens also contained patches of a third mosaic (alpha c), characterized by large orthotopic somata and very large flat trees in sublamina c, but the labelling of this type was inconsistent. These reptilian mosaics share several distinctive characters with anamniote alpha-cell mosaics but differ markedly from the ganglion cell mosaics of any known mammal. The most parsimonious conclusion is that those mosaic features that are shared by the ganglion cells of all nonmammals are homologous and primitive (symplesiomorphic), while those that are shared by all therian mammals are homologous and derived (synapomorphic). This is consistent with other differences between mammalian and nonmammalian eyes. Mosaic formation itself, however, seems to be a universal characteristic of large ganglion cells.