In all mammalian species studied to date, marked differences have been reported in the distribution of color-specific cone types. From the early histochemical experiments of De Monasterio et al.
31 and Ahnelt
32 to the first immunocytochemical analyses,
8 33 putative, then identified, S cones were reported to make only approximately 5% to 10% of all cones. Later, the total absence of short-wave cones was reported in a number of species,
12 13 14 15 16 17 and marked topographical asymmetries of cone densities were found in the mouse, guinea pig and the rabbit.
19 21 22 23 34 In these latter reports, a common feature of the investigated species was the occurrence of smaller or larger retinal areas with the underexpression or absence of M-cone visual pigment.
More recently, Calderone and Jacobs
27 have contrasted the retina of the Siberian hamster with that of the Syrian hamster—the latter species being free of any S cones. Especially interesting is the retina of the Siberian hamster in which M- and S-cone densities were found to be practically identical. This phenomenon can be explained by assuming either a similar distribution of distinct M and S cone populations or the simultaneous presence of S and M cone visual pigments in a single cone population.
The present study provides evidence for the second possibility. We showed the coexpression of two cone opsins in all cones, by using different approaches, such as double staining with either PNA and one-cone-type–specific antibody or with two cone-type–specific antibodies produced in different species. The colocalization of both markers in these labeling studies as well as in those in which the two markers were applied on consecutive semithin sections provides irrefutable evidence for the existence of dual cones all over the retina of the Siberian hamster. When screening other rodent species for a similar coexpression pattern, we found the pouched mouse (
S. campestris) to possess cones expressing both M- and S-cone opsins, very similar to those in the Siberian hamster. We must emphasize that the present findings were obtained with the same antibodies and dilutions as those used in our previous studies.
15 19 23 34 35 In these two species other antibodies
9 10 have produced equivalent results. Therefore, the cone pattern presently described in the Siberian hamster and in the pouched mouse is not an artifact due to a possible cross reaction or poorly characterized antibodies.
When comparing this cone opsin expression pattern with that of other rodents, it becomes clear that the uniform expression of both M and S opsins in all cones of the Siberian hamster and the pouched mouse is novel and unique. The first report describing the coexistence of two cone opsins in one cone cell (dual cone) was published by Röhlich et al.
23 who identified these cones in the transition zone between the dorsal and ventral part of the retina in mouse, rabbit, and guinea pig. In the latter species, the presence of dual cones was not confined only to the transition zone, but these cones were present also in the lower half of the retina. Later, others reported a similar pattern in the mouse as well.
24 The elegant physiological experiments of Lyubarsky et al.
25 were also more compatible with a higher occurrence of dual cones in the lower half of the retina in this species. Applebury et al.
11 have presented yet another model for the expression of mouse cone visual pigments. In their study a dorsoventral M-pigment gradient was proposed with all cones expressing this pigment, albeit at various levels. The S pigment, in contrast, was reported to be expressed in a constant manner with the exception of a few dorsal cones that apparently failed to synthesize this pigment. These studies show that in species in which dual cones have been described, these cones are as a rule confined to certain retinal areas
23 or restricted to a narrow developmental time window,
26 and there is usually a marked dorsoventral gradient in the expression of type-specific cone opsins.
11 23 In contrast, dual cones in the Siberian hamster and pouched mouse have a single cone population with each cone expressing both M and S opsins. A dorsoventral gradient in the expression of color-specific opsins is missing.
When the Syrian hamster is compared with the Siberian hamster,
16 27 it becomes clear that in both species the pure S cone population is missing. Both species have a single cone population that expresses only the M opsin in the Syrian hamster and both M and S opsins in the Siberian hamster. Because there is only one cone population, real color vision cannot be present. Although in the Syrian hamster the photopic sensitivity is confined to the green range of the spectrum, in the Siberian hamster, this range is extended toward the blue and UV part.
27 Whether the broadening of the spectral sensitivity of this single cone population offers any advantage for the Siberian hamster or not, remains to be elucidated. A further question is whether the hamster can use this photopic sensitivity in addition to the dominating scotopic sensitivity to collect additional visual information.
During ontogenesis, the determination of the cone cell line from progenitor cells, the separation of the M- and S-cone population and subsequently the expression of the cone opsins are probably controlled by sequential, complex gene regulations.
36 37 38 In the M-cone type, one of the last steps seems to be the turning on of the M opsin gene with the turning off of the S opsin gene. If this control mechanism is not working, both M and S opsins are expressed, or the expression of the opsins is shifted toward the blue (or UV) range. From an evolutionary point of view, it would be interesting to reveal the control factors that govern the opsin switch in cones. In this respect the Siberian hamster (together with the pouched mouse) deserves special attention as a suitable model, because it possesses a single cone population which coexpresses both M and S opsins. The molecular genetic comparison with the Syrian hamster, in which this single cone population expresses the M opsin only, offers a favorable opportunity to study molecular control mechanisms in a pure system and to understand developmental, genetic and evolutionary aspects of opsin coexpression.
The authors thank Jeremy Nathans and Willem de Grip for the kind donation of antibodies JH455 and CERN956, and Margit Kutasi, Katalin Löcsey, and Naura Chounlamountri for valuable assistance.