Distribution of photoreceptor types in the retina of a marsupial, the tammar wallaby (Macropus eugenii)

Mammalian retinae generally contain low numbers of short-wavelength-sensiti ve cones (S-cones) and higher numbers of middle- to long-wavelength-sensiti ve cones (M-cones). Some recent studies found topographic differences betwe en the different photoreceptor types and in some instances between photorec eptors and ganglion cells. To investigate this question further, we constru cted topographical maps of the different photoreceptors found in an Austral ian marsupial, the tammar wallaby. We used two polyclonal antibodies that h ave been shown to label S-cones (JH455) or M-cones (JH492) in a range of ma mmals. In the tammar wallaby, the antisera clearly distinguish two cone typ es. JH455 recognizes a small subset of cones (S-cones) with a density of le ss than 500 cells/mm(2) in the ventral retina. Their density increases towa rds the dorsal retina to about 1600-2000 cells/mm(2). JH492 recognizes all remaining cones (M-cones), but also faintly labels most cone cells recogniz ed by JH455. The distribution of M-cones, unlike that of the S-cones, shows a clear horizontal streak of high cell density through the central retina, just like the ganglion cells. Unlike the ganglion cells, however, the M-co nes do not peak in the temporal retina but show a very broad peak (12,000-1 8,000 cells/mm2) in the central or even slightly nasal retina. Based on our findings, the retina of the tammar can be divided into three distinct regi ons: firstly, the dorsal retina, which has a low ganglion and low cone cell density but a high percentage of S-cones (30%), is thought to provide good spectral sensitivity; secondly, the central horizontal band of retina, whi ch has a high ganglion and high cone cell density and therefore provides go od spatial resolution; and thirdly, the ventral retina, which has a low gan glion cell but high cone cell density with few S-cones (5%) and is therefor e thought to have a high contrast sensitivity but low acuity.