Visual system evolution and the nature of the ancestral snake.

The dominant hypothesis for the evolutionary origin of snakes from 'lizards' (non-snake squamates) is that stem snakes acquired many snake features while passing through a profound burrowing (fossorial) phase. To investigate this, we examined the visual pigments and their encoding opsin genes in a range of squamate reptiles, focusing on fossorial lizards and snakes. We sequenced opsin transcripts isolated from retinal cDNA and used microspectrophotometry to measure directly the spectral absorbance of the photoreceptor visual pigments in a subset of samples. In snakes, but not lizards, dedicated fossoriality (as in Scolecophidia and the alethinophidian Anilius scytale) corresponds with loss of all visual opsins other than RH1 (λmax 490-497 nm); all other snakes (including less dedicated burrowers) also have functional sws1 and lws opsin genes. In contrast, the retinas of all lizards sampled, even highly fossorial amphisbaenians with reduced eyes, express functional lws, sws1, sws2 and rh1 genes, and most also express rh2 (i.e. they express all five of the visual opsin genes present in the ancestral vertebrate). Our evidence of visual pigment complements suggests that the visual system of stem snakes was partly reduced, with two (RH2 and SWS2) of the ancestral vertebrate visual pigments being eliminated, but that this did not extend to the extreme additional loss of SWS1 and LWS that subsequently occurred (probably independently) in highly fossorial extant scolecophidians and A. scytale. We therefore consider it unlikely that the ancestral snake was as fossorial as extant scolecophidians, whether or not the latter are para- or monophyletic.

A de novo mutation in KIT causes white spotting in a subpopulation of German Shepherd dogs.

Although variation in the KIT gene is a common cause of white spotting among domesticated animals, KIT has not been implicated in the diverse white spotting observed in the dog. Here, we show that a loss-of-function mutation in KIT recapitulates the coat color phenotypes observed in other species. A spontaneous white spotting observed in a pedigree of German Shepherd dogs was mapped by linkage analysis to a single locus on CFA13 containing KIT (pairwise LOD = 15). DNA sequence analysis identified a novel 1-bp insertion in the second exon that co-segregated with the phenotype. The expected frameshift and resulting premature stop codons predicted a severely truncated c-Kit receptor with presumably abolished activity. No dogs homozygous for the mutation were recovered from multiple intercrosses (P = 0.01), suggesting the mutation is recessively embryonic lethal. These observations are consistent with the effects of null alleles of KIT in other species.

Characterization of photoreceptor cell types in the little brown bat Myotis lucifugus (Vespertilionidae).

We report the expression of three visual opsins in the retina of the little brown bat (Myotis lucifugus, Vespertilionidae). Gene sequences for a rod-specific opsin and two cone-specific opsins were cloned from cDNA derived from bat eyes. Comparative sequence analyses indicate that the two cone opsins correspond to an ultraviolet short-wavelength opsin (SWS1) and a long-wavelength opsin (LWS). Immunocytochemistry using antisera to visual opsins revealed that the little brown bat retina contains two types of cone photoreceptors within a rod-dominated background. However, unlike other mammalian photoreceptors, M. lucifugus cones and rods are morphologically indistinguishable by light microscopy. Both photoreceptor types have a thin, elongated outer segment. Using microspectrophotometry we classified the absorption spectrum for the ubiquitous rods. Similar to other mammals, bat rhodopsin has an absorption peak near 500 nm. Although we were unable to confirm a spectral range, cellular and molecular analyses indicate that M. lucifugus expresses two types of cone visual pigments located within the photoreceptor layer. This study provides important insights into the visual capacity of a nocturnal microchiropteran species.

Optimal mechanisms for finding and selecting mates: how threespine stickleback ( Gasterosteus aculeatus) should encode male throat colors.

Male threespine stickleback ( Gasterosteus aculeatus) use nuptial colors to attract mates and intimidate rivals. We quantified stickleback color and environmental lighting using methods independent of human perception to evaluate the information transmitted by male signals in a habitat where these signals are displayed. We also developed models of chromatic processing based on four cone photopigments (peak absorptions at 360, 445, 530, and 605 nm) characterized microspectrophotometrically in G. aculeatus and three other stickleback species. We show that a simple opponent mechanism receiving equally weighted inputs from cones with peak absorptions at 445 nm and 605 nm efficiently encodes variation in male throat colors. An orthogonal opponent mechanism-the difference between outputs of 530-nm cones and mean of outputs of 445- and 605-nm cones-produces a neural signal that could be used for species recognition and would be largely insensitive to variation in male throat color. We also show that threespine stickleback throats/photopigments are optimized for this coding scheme. These and other findings lead to testable hypotheses about the spectral processing mechanisms present in the threespine stickleback visual systems and the evolutionary interactions that have shaped this signal/receiver system.

Retinal photoreceptors and visual pigments in Boa constrictor imperator.

The photoreceptors of Boa constrictor, a boid snake of the subfamily Boinae, were examined with scanning electron microscopy and microspectrophotometry. The retina of B. constrictor is duplex but highly dominated by rods, cones comprising 11% of the photoreceptor population. The rather tightly packed rods have relatively long outer segments with proximal ends that are somewhat tapered. There are two morphologically distinct, single cones. The most common cone by far has a large inner segment and a relatively stout outer segment. The second cone, seen only infrequently, has a substantially smaller inner segment and a finer outer segment. The visual pigments of B. constrictor are virtually identical to those of the pythonine boid, Python regius. Three different visual pigments are present, all based on vitamin A(1.) The visual pigment of the rods has a wavelength of peak absorbance (lambda(max)) at 495 +/- 2 nm. The visual pigment of the more common, large cone has a lambda(max) at 549 +/- 1 nm. The small, rare cone contains a visual pigment with lambda(max) at 357 +/- 2 nm, providing the snake with sensitivity in the ultraviolet. We suggest that B. constrictor might employ UV sensitivity to locate conspecifics and/or to improve hunting efficiency. The data indicate that wavelength discrimination above 430 nm would not be possible without some input from the rods.

Visual pigments in the early life stages of Pacific northwest marine fishes.

Microspectrophotometry was used to measure the visual pigments in the rods and cones of 22 species of marine fish larvae netted from the surface waters off Friday Harbor Laboratories, Washington, USA. 13 species had rods, 12 of which contained visual pigments with a wavelength of maximum absorbance near 500 nm, while one, the sand lance (Ammodytes hexapterus), had its absorbance maximum at 482 nm. The 22 species of fish larvae possessed varied combinations of single, double and twin cones, ranging in peak absorbance from 353 nm to 584 nm. Of these, green-sensitive single cones were present in 20 of the 22 species, and were the dominant cone type. Double and twin cones were present in 13 of the species. Most common were identical green-sensitive (twin) cones (in 11 species). Green/yellow-sensitive double cones occurred in four species. In a single instance (Hemilepidotus hemilepidotus) twin blue-sensitive, twin green-sensitive and double blue/yellow-sensitive cones were recorded. Of particular interest was the finding that 18 of the species had ultraviolet- and/or violet-absorbing single cones. It has been suggested that short-wavelength photosensitivity may be beneficial for planktivory by extending the spectral range over which vision can occur. The high percentage (82%) of ultraviolet and violet visual pigments in Pacific northwest fish larvae supports the prediction that short-wavelength sensitivity may be common in marine fish larvae.

Photoreceptors and visual pigments in the red-eared turtle, Trachemys scripta elegans.

Absorbance spectra of cone outer segments and oil droplets were recorded microspectrophotometrically in the retina of the red-eared turtle, Trachemys scripta elegans. There are four cone visual pigments, with lambda(max) = 617 nm (red sensitive), 515 nm (green sensitive), 458 nm (blue sensitive), and 372 nm (UV-sensitive). The red-sensitive pigment resides in single cones with red or orange oil droplets, and in both members of double cones. The principal member of the double cone contains an orange oil droplet, and the accessory member is droplet free. The green-sensitive pigment is situated in single cones with orange/dark yellow droplets. The blue-sensitive pigment is combined with the UV-absorbing oil droplet in single cones. The UV-sensitive pigment resides in single cones with clear oil droplets that exhibited virtually no absorbance down to 325 nm. Thus, seven types of cones can be identified based on their morphology, oil droplet color, and the visual pigment absorbance. At the moment, this is the most complex cone system described for vertebrates.

An ethogram of body patterning behavior in the biomedically and commercially valuable squid Loligo pealei off Cape Cod, Massachusetts.

Squids have a wide repertoire of body patterns; these patterns contain visual signals assembled from a highly diverse inventory of chromatic, postural, and locomotor components. The chromatic components reflect the activity of dermal chromatophore organs that, like the postural and locomotor muscles, are controlled directly from the central nervous system. Because a thorough knowledge of body patterns is fundamental to an understanding of squid behavior, we have compiled and described an ethogram (a catalog of body patterns and associated behaviors) for Loligo pealei. Observations of this species were made over a period of three years (> or = 440 h) and under a variety of behavioral circumstances. The natural behavior of the squid was filmed on spawning grounds off Cape Cod (northwestern Atlantic), and behavioral trials in the laboratory were run in large tanks. The body pattern components--34 chromatic (including 4 polarization components), 5 postural, and 12 locomotor--are each described in detail. Eleven of the most common body patterns are also described. Four of them are chronic, or long-lasting, patterns for crypsis; an example is Banded Bottom Sitting, which produces disruptive coloration against the substrate. The remaining seven patterns are acute; they are mostly used in intraspecific communication among spawning squids. Two of these acute patterns--Lateral Display and Mate Guarding Pattern--are used during agonistic bouts and mate guarding; they are visually bright and conspicuous, which may subject the squids to predation; but we hypothesize that schooling and diurnal activity may offset the disadvantage presented by increased visibility to predators. The rapid changeability and the diversity of body patterns used for crypsis and communication are discussed in the context of the behavioral ecology of this species.

An action spectrum for the light-dependent inhibition of swimming behavior in newly hatched white sturgeon, Acipenser transmontanus.

Newly hatched white sturgeon (Acipenser transmontanus) yolk-sac larvae are negatively phototactic, photokinetic and geotactic--seeking cover in the substrate during daylight hours. If cover is unavailable and the light intensity is low, they move into the water column and alternate periods of rapid, upward swimming with periods of quiescence during which they sink. In the dark they swim almost continuously. A flash of light inhibits swimming activity totally for a period dependent on the light intensity. Using this observation, we obtained an action spectrum for swimming inhibition for 30 newly hatched larvae. This action spectrum is best fit by a single visual pigment with absorbance maximum at 537 nm. The lack of red sensitivity fits well with previous work showing that red-sensitive photoreceptor cells are absent in young sturgeon. The possible significance of red insensitivity is discussed from both ecological and practical standpoints.

The photoreceptors and visual pigments of the garter snake (Thamnophis sirtalis): a microspectrophotometric, scanning electron microscopic and immunocytochemical study.

Scanning electron microscopy, immunocytochemistry, and single cell microspectrophotometry were employed to characterize the photoreceptors and visual pigments in the retina of the garter snake, Thamnophis sirtalis. The photoreceptor population was found to be comprised entirely of cones, of which four distinct types were identified. About 45.5% of the photoreceptors are double cones consisting of a large principal member joined near the outer segment with a much smaller accessory member. About 40% of the photoreceptors are large single cones, and about 14.5% are small single cones forming two subtypes. The outer segments of the large single cones and both the principal and accessory members of the doubles contain the same visual pigment, one with peak absorbance near 554 nm. The small single cones contain either a visual pigment with peak absorbance near 482 nm or one with peak absorbance near 360 nm. Two classes of small single cones could be distinguished also by immunocytochemistry and scanning electron microscopy. The small single cones with the 360-nm pigment provide the garter snake with selective sensitivity to light in the near ultraviolet region of the spectrum. This ultraviolet sensitivity might be important in localization of pheromone trails.

Microspectrophotometric and immunocytochemical identification of ultraviolet photoreceptors in geckos.

Retinas of the nocturnal geckos, Hemidactylus turcicus, Hemidactylus garnotii, and Teratoscincus scincus, were studied with microspectrophotometry and immunocytochemistry against various visual pigment epitopes to reveal UV-sensitive photoreceptors. From 6-20% of the thinner members of type C double photoreceptors, earlier believed to be blue-sensitive, were found to contain a UV-absorbing visual pigment with lambda max at 363-366 nm. The pigment had bleaching and dichroic properties typical of other photoreceptor cell types of the retina. Presumptive UV-sensitive cells in retinal sections were "negatively" labeled as they did not react with either the cone-specific monoclonal antibody COS-1 or with the anti-rhodopsin polyclonal serum AO, which together labeled all of the remaining photoreceptor types (green-sensitive A singles, B doubles, and thicker members of C doubles, as well as the blue-sensitive majority of thinner members of C doubles). UV cells were moderately stained with the mAb K42-41 produced against the 5-6 loop of bovine rhodopsin, which also moderately labeled blue-sensitive cells. mAb OS-2 strongly stained all outer segments, including the UV-sensitive ones. Similarities between gecko UV visual pigments, and UV visual pigments of other vertebrates, as well as possible functional significance of these cells are discussed.

Visual pigments and spectral sensitivity of the diurnal gecko Gonatodes albogularis.

The visual pigments and oil droplets in the retina of the diurnal gecko Gonatodes albogularis were examined microspectrophotometrically, and the spectral sensitivity under various adapting conditions was recorded using electrophysiological responses. Three classes of visual pigments were identified, with lambda max at about 542, 475, and 362 nm. Spectral sensitivity functions revealed a broad range of sensitivity, with a peak at approximately 530-540 nm. The cornea and oil droplets were found to be transparent across a range from 350-700 nm, but the lens absorbed short wavelength light below 450 nm. Despite the filtering effect of the lens, a secondary peak in spectral sensitivity to ultraviolet wavelengths was found. These results suggest that G. albogularis does possess the visual mechanisms for discrimination of the color pattern of conspecifics based on either hue or brightness. These findings are discussed in terms of the variation in coloration and social behavior of Gonatodes.

Ultraviolet visual pigments in marine fishes of the family pomacentridae.

Near-UV visual pigments have been reported in single cones of several freshwater and euryhaline fishes. The presence of UV visual pigments in stenohaline marine fishes have, as yet, not been identified. In the pomacentridae near-UV visual pigments are present in single cones from the three species we examined--the tropical coral fishes Dascyllus trimaculatus and Pomacentrus coelestis, and the temperate Chromis punctipinnis. Maximum absorption of the UV pigments is centered around 360 nm. In juvenile Chromis, however, the UV visual pigment is not present. Instead there is a single cone containing a violet-sensitive pigment absorbing maximally around 420 nm. All three species are obligate diurnal planktivores. The UV sensitivity may function to enhance their ability to forage on zooplankton.

A third, ultraviolet-sensitive, visual pigment in the Tokay gecko (Gekko gekko).

Numerous extraction and microspectrophotometric studies have shown that the nocturnal Tokay gecko (Gekko gekko), has two visual pigments: a "green" with lambda max at 521 nm and a "blue" at 467 nm. In addition, similar studies on other nocturnal gecko species have found only the same two classes of visual pigment. With the finding that some diurnal species of gecko have a third visual pigment class with lambda max peaking in the UV, doubts were raised concerning the presence of only two visual pigment classes in nocturnal forms. Therefore, a microspectrophotometric re-examination of the Tokay gecko was undertaken to look specifically for a UV visual pigment. A UV-absorbing pigment (364 nm lambda max) was found in approx. 20% of the thin outer segments of type C double rods, thought previously to contain only the 467 nm pigment. That this UV-absorbing pigment was truly a visual pigment was confirmed by its dichroism, behaviour following exposure to UV radiation and "nomogram" fit. It is suggested that this visual pigment had been seen in previous microspectrophotometric studies, but its similarity to known photoproducts peaking in the same spectral region resulted in a case of mistaken identity.

Visual pigments and oil droplets in genetically manipulated and carotenoid deprived quail: a microspectrophotometric study.

The spectral absorbances of visual pigments and retinal oil droplets were studied in three groups of Japanese quail (Coturnix coturnix japonica): an unselected control population and two artificially selected strains that exhibited different early approach preferences between blue and red stimuli. The oil droplets were examined with and without prior carotenoid deprivation. Four cone pigments and five oil droplet types were identified, resembling those in other avian species. Carotenoid deprivation eliminated all pigmentation detectable in oil droplets by microspectrophotometry. Placement of chicks on normal diet gradually reintroduced normal pigmentation within the span of about a week. No statistically significant differences were found between normal and genetically selected birds in either visual pigments or oil droplet types, or in their relative proportions. It is concluded that differences in the early colour preferences of quail are unlikely to be a result of variation in the spectral properties of their photoreceptors.

Vitamin A2-based visual pigments in fully terrestrial vertebrates.

As part of a broad study of the ocular and extraocular photoreceptors of reptiles, we have used high performance liquid chromatography (HPLC) to identify the retinoids present in whole eye extracts of the arboreal lizard Anolis carolinensis and the non-arboreal ruin lizard Podarcis sicula. Unexpectedly, only vitamin A2-derived chromophore was detected in Anolis, while a mixture of vitamin A1- and vitamin A2-derived chromophores was detected in Podarcis. These are the first examples of fully terrestrial vertebrates using vitamin A2-derived chromophore for visual pigment generation. Furthermore, microspectrophotometric (MSP) data for Anolis show a class of photoreceptor having a visual pigment with maximum absorbance at about 625 nm, some 40 nm further into the red than has been found in any terrestrial vertebrate examined to date.

The eyespot of Euglena gracilis: a microspectrophotometric study.

The eyespots in cells of streptomycin-bleached strains and of dark-grown cultures of Euglena gracilis, were examined by means of fluorescence microscopy and microspectrophotometry. When viewed with light in the region of 380-500 nm, the stigma appeared as a dark spot. Adjacent to this was a second spot, not seen with white light, but which was seen to fluoresce when excited with radiation at 370 +/- 20 nm. This fluorescence proved to be polarized in contrast to other fluorescing bodies in the cell. The absorption curves, obtained by microspectrophotometry of individual eyespots, were found to consist of two spectral maxima, an A-band in the blue and a B-band in the green. Unlike the A-band, the B-band provided evidence of originating from an anisotropic structure. Relating these data to literature findings, we conclude that the B-band is the absorbance of a pigment in the quasi-crystalline paraflagellar body and the A-band perhaps a pigment in the orange-red stigma. The spectrum of the B-band does not appear to be that of a flavoprotein or of a free carotenoid but its resemblance to the spectrum of rhodopsin is significant in relation to published data for the Chlamydomonas eyespot that suggests the presence of a rhodopsin-like pigment as the photosensitive system responsible for phototaxis in this alga.

The eyespot of Chlamydomonas reinhardtii: a comparative microspectrophotometric study.

The eyespot of Chlamydomonas reinhardtii is believed to utilize a rhodopsin-like pigment in its responses to light. This paper examines its eyespot by means of microspectrophotometry with the finding of an absorption spectrum with two bands, an A-band in the blue, and a B-band in the green. This spectrum is identical to that previously recorded from the eyespot of Euglena gracilis. As with Euglena the B-band was found to have dichroic character and its spectrum was similar to the absorption curve of rhodopsin. This A-B-spectrum was always recorded from a single granule in each cell. It is concluded that both E. gracilis and C. reinhardtii may utilize a rhodopsin-like pigment as the photopigment associated with the eyespot response to light. In both these algae a few particles in each cell were found whose spectra consisted of two other bands, C and D, blue- and red-shifted, respectively, relative to the eyespot A-B-bands. There is some reason to believe that the C-D-granules may also be involved in certain light-controlled activities of the cells.

Tibetan terrier model of canine ceroid lipofuscinosis.

Over a 10-year period, we have studied the Tibetan terrier's visual electrophysiology, light and electron microscopic (EM) retinal characteristics of a slowly evolving inherited ceroid lipofuscinosis (CL). The retinal degeneration with CL inclusions (rdi) in the inner nuclear layer (bipolar cells) and nerve fiber layer (ganglion cells) has been called "rdi" to differentiate the visual abnormality from typical early retinal degeneration (erd) reported also in the Tibetan terrier. The unique "rdi" electroretinogram (ERG) gives a predominant P III wave at age 7 weeks but becomes more characteristically depressed in all phases over several years. Nyctalopia is the only functional abnormality for the first 5 to 6 year of life. Signs are remarkably few considering the pathology. Microscopic studies of the retina show accumulations, increasing with age, of autofluorescent dense inclusion particles which stain intensely by Luxol fast blue, PAS, and acid-fast procedures. Ultrastructural studies of the retina show the dense particles to be lamellar membranes repeating every 2 to 4 nm, consistent with ceroid lipofuscin. The inner retinal layers were always the target layer to be affected first and most severely. The ganglion cells were most frequently involved. The photoreceptors eventually degenerated but relatively few particles were found in this layer. The cytosomes in the cerebral cortex and brainstem neurons resemble lipofuscin, containing granular, lamellar, and globular components. Different pigment bodies were present in the cerebellar Purkinje cells. Neuronal loss which was moderate in the cerebellum and mild in the cerebrum was accompanied by astrogliosis and a striking presence of macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)