Meeting Abstract
The eye is an ideal model for studying the mechanisms that control the evolution of complex traits in changing environments. Geckos are an exemplary vertebrate group to investigate this question, because they have undergone many evolutionary transitions between diel activity patterns, exposing them to different light levels throughout their history. One way to modify the light sensitivity of an eye is to modulate the convergence of photoreceptors onto retinal ganglion cells: the higher the convergence, the brighter the image. We therefore predicted nocturnal geckos to have high convergence, similar to other vertebrates active in dim light. We examined retinal ganglion cell and photoreceptor densities in the nocturnal leopard gecko (Eublepharis macularius). After isolating and wholemounting the retina, we stained ganglion cells with cresylviolet and mapped cell topography using R. Retinal ganglion cells were distinguished from displaced amacrine cells by anatomical criteria. Photoreceptors were visible due to pigmented epithelium outlining individual photoreceptors, a finding confirmed by H&E staining and immunohistochemistry. Our results reveal a surplus of retinal ganglion cells. The low convergence is consistent across the entire retina, as the topographic distribution of ganglion cells and photoreceptors is similar across the retina, with an area centralis nasal to the optic nerve. Hence, high retinal convergence does not contribute to light sensitivity in the leopard gecko. Instead, large receptive fields and high light sensitivity are achieved by increasing photoreceptor size. Future comparative studies are required to test whether this is a pattern found across all geckos.
