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 identi fied a shared inventory of three basic types, with characteristic form s and spatially independent mosaic distributions. These anamniote type s and mosaics are hard to match to the large ganglion cell types and m osaics of mammals, implying that the underlying developmental programm es have diverged during evolution. Reptiles and mammals both belong to the amniote lineage, so the point of divergence can be investigated b y 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 ho rseradish peroxidase by an in vitro method and studied in retinal flat mounts. Two prominent, regular, spatially independent mosaics were con sistently present. One (alpha(a)) was characterized by somata displace d into the inner nuclear layer and dendrites forming planar trees in s ublamina a; the other (alpha(ab)) comprised large orthotopic somata an d distinctive, bistratified dendrites that formed discrete planar subt rees in sublaminae a and b. These subtrees were joined by up to 40 ver tical link segments, whose distribution was found to correlate with th e underlying photoreceptor mosaic. Some specimens also contained patch es of a third mosaic (alpha(c)), characterized by large orthotopic som ata and very large flat trees in sublamina c, but the labelling of thi s type was inconsistent. These reptilian mosaics share several distinc tive characters with anamniote alpha-cell mosaics but differ markedly from the ganglion cell mosaics of any known mammal. The most parsimoni ous conclusion is that those mosaic features that are shared by the ga nglion cells of all nonmammals are homologous and primitive (symple-si omorphic), while those that are shared by all therian mammals are homo logous and derived (synapomorphic). This is consistent with other diff erences between mammalian and nonmammalian eyes. Mosaic formation itse lf, however, seems to be a universal characteristic of large ganglion cells.