Retinal ganglion cell topography and spatial resolving power in the oriental fire-bellied toad Bombina orientalis.

Abstract

The vertebrate visual system is determined by two main factors, a species' lifestyle and phylogenetic legacy. Studying the visual system in outgroup lineages may shed some light on the balance of these factors within a certain radiation. We studied the topography of retinal ganglion cells (RGCs) in the retina of the oriental fire-bellied toad Bombina orientalis. These toads belong to the ancient superfamily Discoglossoidea, a sister group to all extant Anura except for two small families. RGCs were retrogradely labeled with tetramethylrhodamine- dextran amine (TMR-DA) and examined in retinal wholemounts. RGCs occurred all over the retina except for the far periphery. Their total number was [Formula: see text] ([Formula: see text], [Formula: see text]). They comprised 73-77% of all cells in the ganglion cell layer. The spatial density of GCs increased gradually from the dorsal and ventral retinal periphery toward the equator to form a weak visual streak and a moderately pronounced area centralis. The minimum density was [Formula: see text], and the maximum, [Formula: see text]. The maximum density gradient was [Formula: see text]. The spatial resolution was minimum in the dorsal and ventral periphery ([Formula: see text] and [Formula: see text] cycles per degree in water and air, respectively). Intermediate values of spatial resolving power were found within the visual streak ([Formula: see text] and [Formula: see text] cycles per degree) and reached a peak in area centralis ([Formula: see text] and [Formula: see text] cycles per degree). This is sufficient for efficient prey location and capture. The relatively high RGC density and the presence of specialized retinal regions in oriental fire-bellied toads are consistent with their highly visual behavior. A brief review comparing the phylogeny and ecology of this with other anuran species suggests that the main factor shaping the RGC distribution in Anura is phylogenetic legacy; the environmental pressure results mainly in adjusting the maximum spatial density of RGCs (and hence the visual acuity) to meet the species' needs.