Visual opsin expression and morphological characterization of retinal photoreceptors in the pouched lamprey (Geotria australis, Gray)

Dec 18, 2020The Journal of comparative neurology

Light-sensitive protein types and cell shapes in the eye of the pouched lamprey (Geotria australis)

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Abstract

The retina of the southern hemisphere pouched lamprey, Geotria australis, possesses five distinct photoreceptor subtypes.

Five different photoreceptor subtypes were identified in the retina based on their structure and the expression of five visual opsin classes.
Each photoreceptor subtype expresses a single opsin class, indicating specialized functions.
The spatial distribution of each photoreceptor subtype suggests adaptation to varying light conditions in the environment.
These findings may indicate strong selection pressures for broad spectral sensitivity in brightly lit surface waters.

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Lampreys are extant members of the agnathan (jawless) vertebrates that diverged ~500 million years ago, during a critical stage of vertebrate evolution when image-forming eyes first emerged. Among lamprey species assessed thus far, the retina of the southern hemisphere pouched lamprey, Geotria australis, is unique, in that it possesses morphologically distinct photoreceptors and expresses five visual photopigments. This study focused on determining the number of different photoreceptors present in the retina of G. australis and whether each cell type expresses a single opsin class. Five photoreceptor subtypes were identified based on ultrastructure and differential expression of one of each of the five different visual opsin classes (lws, sws1, sws2, rh1, and rh2) known to be expressed in the retina. This suggests, therefore, that the retina of G. australis possesses five spectrally and morphologically distinct photoreceptors, with the potential for complex color vision. Each photoreceptor subtype was shown to have a specific spatial distribution in the retina, which is potentially associated with changes in spectral radiance across different lines of sight. These results suggest that there have been strong selection pressures for G. australis to maintain broad spectral sensitivity for the brightly lit surface waters that this species inhabits during its marine phase. These findings provide important insights into the functional anatomy of the early vertebrate retina and the selection pressures that may have led to the evolution of complex color vision.

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Visual opsin expression and morphological characterization of retinal photoreceptors in the pouched lamprey (Geotria australis, Gray)

Abstract

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