The nature of the gecko visual pigment

Retinal extracts of the Australian gecko, Phyllurus milii (White), have revealed the presence of a photosensitive pigment, unusual for terrestrial animals, because of its absorption maximum at 524 mmicro. This pigment has an absorption spectrum which is identical in form with that of other visual chromoproteins. It is not a porphyropsin, for bleaching revealed the presence, not of retinene(2), but of retinene(1) as a chromophore. Photolabile pigments with characteristics similar to those of the Phyllurus visual pigment were also detected in retinal extracts of six other species of nocturnal geckos. The presence of this retinal chromoprotein adequately accounts for the unusual visual sensitivity curve described by Denton for the nocturnal gecko. This pigment may have special biological significance in terms of the unique phylogenetic position of geckos as living representatives of nocturnal animals which retain some of the characteristics of their diurnal ancestors. The occurrence of this retinene(1) pigment, intermediate in spectral position between rhodopsin and iodopsin, is interpreted in support of the transmutation theory of Walls. The results and interpretation of this investigation point up the fact that, from a phylogenetic point of view, too great an emphasis on the duplicity theory may serve to detract attention from the evolutionary history of the retina and the essential unitarianism of the visual cells.

A new photosensitive pigment of the euryhaline teleost, Gillichthys mirabilis

Retinal extracts have been prepared from dark-adapted mudsuckers by treatment of retinal tissue or of isolated outer segments of the visual cells with digitonin solution. The extracts were examined spectrophotometrically and found to absorb light maximally between the wave lengths of 488 and 510 mmicro, depending on the proportion of yellow impurities and light-sensitive pigment present. This photosensitive pigment was shown to be homogeneous by partial bleaching of the extracts with monochromatic light of various wave lengths from 390 to 660 mmicro. The mudsucker pigment was specifically demonstrated not to be a mixture of rhodopsin and porphyropsin; the adequacy of the method used to analyze such mixtures was shown by performing a control experiment with an artificial mixture of bullfrog rhodopsin and carp porphyropsin. Comparison of the hydroxylamine difference spectrum and of the absorption maximum of the purest retinal extract located the mudsucker photosensitive pigment maximum at 512 +/- 1 mmicro. Extraction of retinal tissue with a fat solvent after exposure to white light gave a preparation which after the addition of antimony chloride reagent developed the absorption band maximal near 664 mmicro, which is characteristic of retinene(1). If an hour intervened between exposure of the retinal tissue to light and extraction of the carotenoid, the antimony trichloride test gave a color band maximal at 620 mmicro, characteristic of vitamin A(1). No evidence of retinene(2) or vitamin A(2) was obtained. The euryhaline mudsucker has, therefore, a photosensitive retinal pigment with an absorption maximum halfway between the peaks of rhodopsins and of porphyropsins and belonging to the retinene(1) system characteristic of rhodopsins. The pigment is therefore named a retinene(1) pigment 512 of the mudsucker, Gillichthys mirabilis. It is uncertain whether this type of photosensitive pigment will be found in other euryhaline fishes.

The nature of the lamprey visual pigment

From the retina of the land-locked population of the sea lamprey, Petromyzon marinus, a photolabile pigment was extracted which was identified spectrophotometrically as a member of the rhodopsin group of pigments. Using the absorption spectrum of a relatively pure solution and analysis by means of difference spectra, the peak of this pigment was placed at about 497 mmicro. The method of selective bleaching by light of different wave lengths revealed no significant amounts of any other pigment in the extracts. A similar pigment was also detected in retinal extracts of the Pacific Coast lamprey, Entospenus tridentatus. These results are significant for two reasons: (a) the lamprey is shown to be an example of an animal which spawns in fresh water but which is characterized by the presence of rhodopsin, rather than porphyropsin, in the retina; (b) the primitive phylogenetic position of the lamprey suggests that rhodopsin was the visual pigment of the original vertebrates.