The origins of colour vision in vertebrates.

The capacity for colour vision is mediated by the comparison of the signal intensities from photoreceptors of two or more types that differ in spectral sensitivity. Morphological, physiological and molecular analyses of the retina in an agnathan (jawless) fish, the lamprey Geotria australis, may hold important clues to the origins of colour vision in vertebrates. Lampreys are extant representatives of an ancient group of vertebrates, the origins of which are thought to date back to at least the early Cambrian, approximately 540 million years ago. G. australis possesses five photoreceptor types, each with cone-like ultrastructural features and different spectral sensitivities. Recent molecular genetic studies have also revealed that five visual pigment (opsin) genes are expressed in the retina, each of which is orthologous to the major classes of vertebrate opsin genes. These findings reveal that multiple opsin genes originated very early in vertebrate evolution, prior to the separation of the jawed and jawless vertebrate lineages, thereby providing the genetic basis for colour vision in all vertebrates.