| The mandarin vole, Microtus mandarinus, is one of the herbivorous animals distributed throughout Inner Mongolia, Shan, Shaanxi and Henan provinces in China. Mandarin voles usually confine their habitat to underground caves. They have small body size and short reproductive cycle, as well as intense resistance to moisture and starvation. Mandarin voles have both daytime and nocturnal activity, while they mainly exhibit the nocturnality pattern. In the absence of the behavior of hibernation, they are considered as one of the main agricultural pest rodents. In this study, we report the results of a study on the visual system of mandarin voles using histological, immune-histochemical and praxiological approaches. Specifically, our research focus on the development of the structure of visual organs, the patterns of photosensory cells and their distribution, and their capacity of color discrimination. Results of our study intend to reveal the characteristic of visual systems in mandarin voles, thereby providing novel evidences for the evolution theory regarding visual sense in rodents.Our results are as below:1. Mandarin voles follow normal development process of eyeball structure, possessing the basic structure of eyelid, cornea, iris, crystalline lens, sclera, chorioid and retina. Their retinas have clear stratification. The structure of visual organ fully develop at21days of age, which is similar to that in adult voles. Meanwhile, the structure of retinas in mandarin voles is the same as the classical structure of retinas in mammals, suggesting that mandarin voles are inferiorly degenerated in their visual organs.2. The density of outer nuclear layer cells is greater than that of ganglion cells, with a ratio of16to1. This result suggests that the activity rhythms of mandarin voles is between diurnality and nocturnality, while it mainly exhibits the nocturnality pattern. Mandarin voles have thick pigment epithelial layers in their retinas, in which pigment epithelial cells contain a large number of melanin granules. This structure gives them protection of visual cells to prevent the coup injury of hard light in the daytime.3. Immunofluorescence results indicate that photosensory cells in the retinas of mandarin voles consist of two types of cone cells, namely S-cone cells and L-cone cells. They show a classical dichromat pattern. The two types of cone cells vary greatly in the amount of cone cells, while S-cone cells are more than L-cone cells. Their distribution in the surface of retinas is inhomogeneous. From the central to the peripheral region, the density of two types of cone cells exhibit a decreasing trend. They are mostly concentrated in the central region, indicating that mandarin voles have excellent ability of receiving short wave illumination to allow their activity in dark environment. Also, they are able to receive long wave illumination to adapt to short period of daytime activity.4. Mandarin voles have more rod cells than cone cells. Given that rod cells function as receiving weak light and mediating scotopic vision, this characteristic adapts to their life habit of nocturnal activity.5. Mandarin voles’ behavior change in illuminations at different wavelengths. Under red light mandarin voles move restlessly, while under blue light they are quiet. Under blue light, mandarin voles exhibit significantly prolonged rest time and shorten moving time. On the contrary, they show considerable moving time and less rest time under red light. These results confirm the dichromat of mandarin voles, and further illustrate that they are capable of discriminating lights of different wavelengths and have specific color discrimination. Consequently, we conclude that mandarin voles follow normal development process in the structure of visual organ, which allows them be adaptive in short period of surface activity. They possess the capacity of photoreception to discriminate lights of different wavelengths, and have specific color discrimination.
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