A strong correlation exists between the distribution of retinal ganglion cells and nose length in the dog

The domestic dog, CANIS LUPUS FAMILIARIS, is a subspecies of the gray wolf, CANIS LUPUS, with almost identical mitochondrial DNA. The dog is the most diverse species on earth, with skull length varying between 7 and 28 cm whereas the wolf skull is around 30 cm long. However, eye size in dogs does not appear to vary as much. For example, small dogs such as the chihuahua appear to have very large eyes in proportion to the skull. Our aim was to examine eye size and retinal cell numbers and distribution to determine whether the dog eye exhibits as much variation as the skull. We found a correlation between eye radius and skull dimensions. However, the most surprising finding was that the distribution of ganglion cells in the eye varied tremendously from a horizontally aligned visual streak of fairly even density across the retina (as seen in the wolf) to a strong area centralis with virtually no streak (for example, as observed in a pug from the current series). This variation in ganglion cell density within a single species is quite unique. Intriguingly, the ratio of peak ganglion cell density in the area centralis to visual streak was highly negatively correlated with skull length (r = -0.795, n = 22) and positively correlated with cephalic index (r = 0.687, n = 22). The orientation of eyelid aperture was also correlated with cephalic index (r = 0.648, n = 22). Therefore, the genetic manipulation of selective breeding, which has produced an abnormal shortening of the skull and eyelids with less lateral apertures, has also produced a considerably more pronounced area centralis in the dog.