Analysis of the Expression of the Rhodopsin Gene Rh1 in Retinas of Nocturnal and Diurnal Dipsadidae Snakes and Comparative Study of the Density of Photoreceptors and GCL Cells

Abstract

 

Purpose

 

Diurnal Dipsadidae snakes have pure cone retinas while nocturnal have duplex retinas with rods and cones. The aim of this study was to verify the expression of the Rh1 gene in diurnal and nocturnal snakes and compare the density of photoreceptors and cells in the ganglion cell layer (GCL) in retinas of different species of Dipsadidae snakes.

 

Methods

 

To determine the expression of the Rh1 gene, the RNA of 8 species was extracted and converted to cDNA, which was used in PCR. The gene was sequenced in the nocturnal species: Atractus pantostictus, Dipsas petersi, Sibynomorphus mikanii, S. neuwiedii and Oxyrhopus guibei, and in the diurnal: Erythrolamprus poecilogyrus, Helicops modestus and Tomodon dorsatus. Photoreceptors were labeled by immunohistochemistry and cells of the GCL were labeled with Nissl stain in wholemount retinas. Thirty nine retinas of 12 species were evaluated, 26 of which to count photoreceptors and 19 to quantify GCL cells (Table 1). The visual acuity was estimated based on the GCL cells peak density.

 

Results

 

The Rh1 gene is expressed in the retinas of all species, both nocturnal and diurnal. The spectral sensitivity, estimated based on the amino acid sequences, presented a λmax of 500 nm in the nocturnal species and 484 nm in the diurnal species. In the nocturnal species the total photoreceptor population presented higher cell density compared to the diurnal snakes (Table 1). The population of cells in the GCL presented small variation among species, but the visual acuity varied among nocturnal and diurnal species (Table 1).

 

Conclusions

 

Compared to the diurnal species, nocturnal snakes presented a higher density of photoreceptors and higher convergence between photoreceptors and GCL cells, since density of cells in the GCL did not differ between nocturnal and diurnal species. The architecture of the retinae of nocturnal snakes is therefore compatible with great sensitivity to light and adaptation to scotopic vision. The absence of rods in the retinas of diurnal snakes indicates that the Rh1 gene is expressed in cones and might contribute to the color vision in these species.

 

 

Table 1. Photoreceptors and GCL cells mean density in retinas of nocturnal and diurnal Dipsadidae snakes, and visual acuity values estimated based on the GCL cells peak density.

 

Keywords: 470 color pigments and opsins • 648 photoreceptors • 516 evolution