The Electroretinogram of a Diurnal Gecko

Abstract

A B $ T R A C T Using the electroretinogram as the criterion of retinal activity the flicker fusion frequency, course of dark adaptation, and spectral sensitivity of the pure cone retina of the diurnal gecko, Phelsuma inunguis, were investigated.Both the curve relating flicker fusion frequency to stimulus intensity and that relating the amplitude of the flicker response to stimulus intensity showed a break as the intensity was increased. The dark adaptation curve was that typical of cone retinae; there was no break, adaptation was relatively rapid, and there was a total increase of sensitivity of only about 3 log units. The spectral sensitivity curve showed two maxima, a major one at about 560 m/z and another at about 460 m/~. Chromatic adaptation with red and blue lights demonstrated the presence of two independent mechanisms. Although red adaptation could not have had a direct effect on the pigment responsible for the "blue" mechanism the sensitivity of this mechanism was depressed by red adaptation. The possible relationships of the two mechanisms are discussed. I N T R O D U C T I O NThe geckos are a group of considerable interest to the visual physiologist. Most geckos are nocturnal in habit and are usually considered to possess a pure rod retina, but there are some diurnal species. Walls (1934) believes that the rods of nocturnal geckos have been transmuted from the cones of diurnal lizard ancestors. Confirmatory evidence for this theory has since been provided by Underwood (1951Underwood ( , 1954 who has made a detailed study of many gecko species. Walls has further suggested that some of those gecko species which are now diurnal in habit and which possess cone-like visual cells, have reverted from nocturnal gecko ancestors and that their visual cells have undergone a second t r a n s m u t a t i o n from rods back into cones. H e m e n t i o n e d the diurnal Phelsuma species as having u n d e r g o n e this second t r a n s m u t a t i o n and m a d e certain predictions a b o u t their ocular m o r p h o l o g y which have since been shown to be correct .In view of their evolutionary history the visual pigments of the gecko retina are of especial interest. Crescitelli (1958) extracted the retinae of eleven species of n o c t u r n a l geckos and found a set of unusual visual pigments which, al-II45

Evolution of visual pigments in geckos

“…This residue corresponds to A284 in G. gekko P521 and to T285 in dg1. An electroretinogram recording shows a sensitivity peak at 560 nm in the retina of a closely related diurnal gecko, Phelsuma inunguis [31], so it is speculated that the dg1 pigment has a λ max longer than P521 of the nocturnal gecko.…”

Convergent evolution of a blood-red nectar pigment in vertebrate-pollinated flowers

“…2) How conspicuous do these colors appear to pollinator given their specific visual capacities ( 34 )? To construct visual models, we used published cone sensitivities of two day gecko species, P. inunguis and Phelsuma madagascariensis longinsulae (ultraviolet sensitive [UVS] λ max = 380, short wave sensitive [SWS] λ max = 437, medium wave sensitive [MWS] λ max = 470, longwave sensitive [LWS] λ max = 560; ratio of UVS:SWS:MWS:LWS = 1:1:3.5:6) and the green-backed fire crown hummingbird Sephanoides sephaniodes (UVS λ max = 371, SWS λ max = 444, MWS λ max = 504, LWS λ max = 560; ratio of UVS:SWS:MWS:LWS = 1:2:2:4) ( 22 , 23 , 47 ). For both Phelsuma species and S. sephaniodes, we were unable to find data on cone abundance.…”

“…5 B and C ). Phelsuma geckos have four types of photoreceptors (UV, blue, green, and red), and hence, their visual space can be represented in the form of a tetrahedron, with each vertex corresponding to a specific photoreceptor ( 22 , 23 ). The reflectance spectrum of Nesocodon nectar ( Fig.…”

Convergent evolution of a novel blood-red nectar pigment in vertebrate-pollinated flowers

“…5B & 5C). Phelsuma have four types of photoreceptors (ultraviolet, blue, green, red) and hence their visual space can be represented in the form of a tetrahedron with each vertex corresponding to a specific photoreceptor (18,19). The reflectance spectra of Nesocodon nectar (Fig.…”