Characteristics of pectens in diurnal and nocturnal birds and a new functional proposal relating to non-visual opsins.

The pecten is a fold-structured projection at the ocular fundus in bird eyes, showing morphological diversity between the diurnal and nocturnal species. However, its biological functions remain unclear. This study investigated the morphological and histological characteristics of pectens in wild birds. Additionally, the expression of non-visual opsin genes was studied in chicken pectens. These genes, identified in the chicken retina and brain, perceive light periodicity regardless of visual communication. Similar pleat numbers have been detected among bird taxa; however, pecten size ratios in the ocular fundus showed noticeable differences between diurnal and nocturnal birds. The pectens in nocturnal brown hawk owl show extremely poor vessel distribution and diameters compared with that of diurnal species. RT-PCR analysis confirmed the expression of Opn5L3, Opn4x, Rrh and Rgr genes. In situ hybridization analysis revealed the distribution of Rgr-positive reactions in non-melanotic cells around the pecten vessels. This study suggests a novel hypothesis that pectens develop dominantly in diurnal birds as light acceptors and contribute to continuous visual function or the onset of periodic behaviour.