Visual pigments in the sea lamprey, Petromyzon marinus

We present microspectrophotometric evidence for the existence of two distinct visual pigments residing in two different morphological types of photoreceptor of the sea lamprey. In the upstream migrant Petromyzon marinus, the pigment found in short receptors has a wavelength of peak absorbance (lambda max) of 525 nm, whereas the pigment located in long receptors has a lambda max of 600 nm. Although the former appears to be pure porphyropsin, the latter is akin to visual pigments found in the red-absorbing cones of amphibian and teleost retinae. The kinship is more than superficial pertaining to lambda max, however, because the long receptor pigment, like the others, shows the typical sensitivity to the anionic milieu. Lampreys belong to the class Cyclostomata, which now becomes the sixth phylogenetic class of vertebrates with anion-sensitive as well as anion-insensitive visual pigments. This finding strengthens the hypothesis that sensitivity to anions is an integral property of all long-wavelength-absorbing vertebrate pigments and that these pigments form a distinct group in which an external Cl- ion is utilized in tuning the lambda max of the alpha-band absorbance to its native maximum value. The presence of an anion-sensitive and an anion-insensitive pigment in a retina implies the expression of two distinct opsin genes. We infer this from several examples of correlation between anion sensitivity and opsin sequence groupings. Moreover, the presence of two distinct opsin genes expressed throughout six vertebrate classes implies their existence in a common ancestor to all.