PHOTORECEPTORS IN A PRIMITIVE MAMMAL, THE SOUTH-AMERICAN OPOSSUM, DIDELPHIS-MARSUPIALIS-AURITA - CHARACTERIZATION WITH ANTI-OPSIN IMMUNOLABELING

The retinas of placental mammals appear to lack the large number and m orphological diversity of cone subtypes found in diurnal reptiles. We have now studied the photoreceptor layer of a South American marsupial (Didelphis marsupialis aurita) by peanut agglutinin labeling of the c one sheath and by labeling of cone outer segments with monoclonal anti -visual pigment antibodies that have been proven to consistently label middle-to-long wavelength (COS-1) and short-wavelength (OS-2) cone su bpopulations in placental mammals. Besides a dominant rod population ( max. = 400,000/mm(2)) four subtypes of cones (max. = 3000/mm(2)) were identified. The outer segments of three cone subtypes were labeled by COS-1: a double cone with a principal cone containing a colorless oil droplet, a single cone with oil droplet, and another single cone. A se cond group of single cones lacking oil droplets was labeled by OS-2 an tibody. The topography of these cone subtypes showed striking anisotro pies. The COS-1 labeled single cones without oil droplets were found a ll over the retina and constituted the dominant population in the area centralis located in the temporal quadrant of the upper, tapetal hemi sphere. The population of OS-2 labeled cones was also ubiquitous altho ugh slightly higher in the upper hemisphere (200/mm(2)). The COS-1 lab eled cones bearing an oil droplet, including the principal member of d ouble cones, were concentrated (800/mm(2)) in the inferior, non-tapeta l half of the retina. The two spectral types of single cones resemble those of dichromatic photopic systems in most placental mammals. The a dditional set of COS-1 labeled cones is a distinct marsupial feature. The presence of oil droplets in this cone subpopulation, its absence i n the area centralis, and the correlation with the non-tapetal inferio r hemisphere suggest a functional specialization, possibly for mesopic conditions. Thus, sauropsid features have been retained but probably with a modified function.