Colour in the eyes of insects

Many insect species have darkly coloured eyes, but distinct colours or patterns are frequently featured. A number of exemplary cases of flies and butterflies are discussed to illustrate our present knowledge of the physical basis of eye colours, their functional background, and the implications for insect colour vision. The screening pigments in the pigment cells commonly determine the eye colour. The red screening pigments of fly eyes and the dorsal eye regions of dragonflies allow stray light to photochemically restore photoconverted visual pigments. A similar role is played by yellow pigment granules inside the photoreceptor cells which function as a light-controlling pupil. Most insect eyes contain black screening pigments which prevent stray light to produce background noise in the photoreceptors. The eyes of tabanid flies are marked by strong metallic colours, due to multilayers in the corneal facet lenses. The corneal multilayers in the gold-green eyes of the deer fly Chrysops relictus reduce the lens transmission in the orange-green, thus narrowing the sensitivity spectrum of photoreceptors having a green absorbing rhodopsin. The tapetum in the eyes of butterflies probably enhances the spectral sensitivity of proximal long-wavelength photoreceptors. Pigment granules lining the rhabdom fine-tune the sensitivity spectra.     

Regulation of eye and jaw colouration in three‐spined stickleback Gasterosteus aculeatus

Fish can change their skin and eye colour for background matching and signalling. Males of Gasterosteus aculeatus develop ornamental blue eyes and a red jaw during the reproductive season, colours that are further enhanced during courtship. Here, the effects of different hormones on physiological colour changes in the eyes and jaws of male and female G. aculeatus were investigated in vitro. In an in vivo experiment, G. aculeatus were injected with a receptor blocker of a pivotal hormone (noradrenaline) that controls colour change. In males, noradrenaline had aggregating effects on melanophore and erythrophore pigments resulting in blue eyes and a pale jaw, whereas melanocyte‐concentrating hormone (MCH) and melatonin resulted in a pale jaw only. When noradrenalin was combined with melanocyte stimulating hormone (MSH) or prolactin, the jaw became red, while the eyes remained blue. In vivo injection of yohimbine, an alpha‐2 adrenoreceptor blocker, resulted in dispersion of melanophore pigment in the eyes and inhibited the blue colouration. Altogether, the data suggest that noradrenalin has a pivotal role in the short‐term enhancement of the ornamental colouration of male G. aculeatus, potentially together with MSH or prolactin. This study also found a sex difference in the response to MCH, prolactin and melatonin, which may result from different appearance strategies in males, versus the more cryptic females.     

Coloring cells of the cornea of trout]

Cells from the eye cornea of Hexagrammos octagrammus which are responsible for changes of the cornea colour from bright orange to colourless, depending on the light conditions, are described. It was shown that the change in cornea colour was due to a shift of red pigment from the cell body into its processes (in the light) and in the opposite direction at the dark adaptation of animals. The ultrastructural constitution of these cells has a number of characteristics. The whole cell cytoplasm is filled up with fine lipid droplets wherein carotenoid pigments giving red colour to these cells are presumably dissolved; the cytoplasmic membrane forms numerous deep and branched folds into the cell and has a lot of pinocytose visicles; the cell body and especially the process display many microtubes arranged regularly. The described cells differ greatly in their form, size and ultrastructural constitution from the known types of pigment cells (melanophores, xanthophores and erythrophores). This makes it possible to consider them as chromatophores of an independent type.     

"Basic brown", a too little considered iris color]

The eye colour is mainly effected by the two components melanin and structural blue of the iris stroma which generally are present in various mixtures respectively combinations. Are these components lacking the dark pigment epithelium can produce a so-called "basic brown". The exact distinction between "brown" and "basic brown" is without doubt of considerable value for anthropological as well as for genetical examinations of eye colours.     

Studies on the Relation between the Pigment Migration and the Sensitivity Changes during Dark Adaptation in Diurnal and Nocturnal Lepidoptera

The functional significance of the pigment migration in the compound insect eye during dark adaptation has been studied in diurnal and nocturnal Lepidoptera. Measurements of the photomechanical changes were made on sections of eyes which had been dark-adapted for varying periods of time. In some experiments the sensitivity changes during dark adaptation were first determined before the eye was placed in the fixation solution. No change in the position of the retinal pigment occurred in Cerapteryx graminis until the eye had been dark-adapted for about 5 minutes. The start of the migration was accompanied by the appearance of a break in the dark adaptation curve. During longer periods of dark adaptation the outward movement of the pigment proceeded in parallel with the change in sensitivity, the migration as well as the adaptive process being completed within about 30 minutes. In the diurnal insects chosen for the present study (Erebia, Argynnis) the positional changes of the retinal pigment were insignificant in comparison with the movement of the distal pigment in Cerapteryx graminis. On the basis of these observations the tentative hypothesis is put forward that the second phase of adaptive change in nocturnal Lepidoptera is mediated by the migration of the retinal pigment while the first phase is assumed to be produced by the resynthesis of some photochemical substance. In diurnal insects which have no appreciable pigment migration the biochemical events alone appear to be responsible for the increase in sensitivity during dark adaptation.     

Short-term cycling of skin colouration in the blackspotted rockskipper Entomacrodus striatus

Physiological colour change was investigated in the blackspotted rockskipper Entomacrodus striatus in Moorea, French Polynesia. Fish colour cycled with significant autocorrelation over the 30 min observation period and was not affected by observation temperature (27 and 31° C). Cycling depended most on dark and yellow pigments (as assayed by separation of colours via software), and therefore, it was hypothesized that short-term cycling was driven by melanophores and xanthophores.     

Tyndall blue and surface white of tent caterpillars, Malacosoma SPP.

Blues and surface whites of tent caterpillars are structural colours resulting from the scattering of light by small, transparent, cuticular filaments. The filaments are small enough to be Tyndall-active, that is they scatter the short wavelengths of incident light more effectively than the long wavelengths. Immediately beneath the surface filaments of blue-coloured cuticle there is a layer of dark pigment which absorbs transmitted light. Therefore, only Tyndall-scattered light is reflected from the cuticle and since this light is diminished in long wavelengths it appears blue. Cuticle that is surface white lacks the layer of dark pigment. Light passing through the surface layer is therefore subject to scattering within the cuticle and by underlying tissues. Sufficient complementary long wavelengths are backscattered from below the surface mat of filaments that the reflected light appears white. Tyndall blue systems found in other insects are briefly discussed.     

Preliminary study on eye colour in Japanese macaques <Emphasis Type="Italic">(Macaca fuscata)</Emphasis> in their natural habitat

Eye colour in Japanese macaques shows apparent differences between individuals, continuously ranging from orange (bright), through shades of yellow and hazel-blue to dark blue (dark). We arbitrarily classified them into either 'yellow' eyes or 'blue' eyes based on the yellow area occupying in the iris' peripupillary ring. Most Japanese macaques have yellow eyes after infant phase, whilst 19, 17, 12, and 15% of monkeys (>6 months, sexes combined) have blue-eye in studied two groups of Shodoshima and two groups of Takasakiyama, respectively. Frequency of eye colour did not differ between males and females, but significantly differed in each age class. Blue eyes significantly more frequently occurred in newborns, infants and aged monkeys than in juveniles and prime adults. Data from mother-infant pairs indicated eye colour could be inherited from their parents. A case of asymmetric eye colour in Japanese macaques was found from a sample of 1962 individuals. Eye colour variation of Japanese macaques was discussed in relation to those of humans and rhesus macaques. A possible evolutionary model of eye colour in Japanese macaques was discussed.     

Brilliant skyblue pigment formation from gardenia fruits

It is well known that blue pigment is formed by the reaction of amino acids with genipin, the hydrolyzate of geniposide from gardenia fruits. We studied the effect of the amino acid on blue pigment formation and found a linear relationship between the molecular weight of the neutral amino acid and the λ_max of the blue pigment formed. Thin layer chromatographic analysis revealed brilliant skyblue components of the blue pigments formed from glycine, alanine, leucine, phenylalanine and tyrosine. Furthermore, a brilliant skyblue color was obtained by a reverse phase column chromatography (HP-20) of blue pigments formed from glycine, leucine and phenylalanine. The λ_max of these purified pigments lay above 600 nm, and the peaks were sharper than those of crude pigments. After standing for two weeks at 40°C in 40% ethanol solution, the brilliant skyblue pigment formed from genipin and glycine remained stable, losing none of its initial absorbance.     

Flower colours and pigments in Disa hybrid (Orchidaceae)

Flower colours and the composition of pigments in the perianths of five cultivars of Disa orchids were analyzed. Carotenoids were major pigment components in the orange-red flowers of ‘Dawn Angel’. We identified two types of pigment composition in the red flowered cultivars: ‘San Francisco’ contained more carotenoids and less anthocyanins, while ‘Marlene’ contained more anthocyanins than carotenoids. The red-purple flowered cultivars, only contained slight amounts of carotenoids, and the red-purple colour was attributed to the relatively high density of a cyanidin-based anthocyanin. The importance of the characterization of pigments in the perianths of orchid has been discussed in both breeding for flower colour improvement and chemotaxonomy.     

Quantitative variation of melanins in llama (Lama glama L.)

The amount of melanin pigments was investigated in 80 Argentinean llama, representative of seven phenotypes and four different fleece colours, by means of spectrophotometric assays: SpEM (spectrophotometric eumelanin), SpPM (spectrophotometric pheomelanin), SpASM (spectrophotometric alkali soluble melanin), and SpTM (spectrophotometric total melanin). It was found that, although to a different extent, the quantitative variation of these pigments was affected both by phenotypes and fleece colours and, hence, it was possible to identify and to distinguish homogeneous groups on the basis of these two factors. In particular, SpEM revealed the most reliable parameter for a discrimination among these groups. Low concentrations of this pigment characterize red, wild, and red black phenotypes and distinguish them from black and tan and non-agouti black ones, both showing high concentrations; low concentrations also distinguish reddish brown fleece colour from very dark red and black ones.     

The mechanistic,genetic and evolutionary causes of bird eye colour variation

Birds display a rainbow of eye colours, but this trait has been little studied compared with plumage coloration. Avian eye colour variation occurs at all phylogenetic scales: it can be conserved throughout whole families or vary within one species, yet the evolutionary importance of this eye colour variation is under-studied. Here, we summarize knowledge of the causes of eye colour variation at three primary levels: mechanistic, genetic and evolutionary. Mechanistically, we show that avian iris pigments include melanin and carotenoids, which also play major roles in plumage colour, as well as purines and pteridines, which are often found as pigments in non-avian taxa. Genetically, we survey classical breeding studies and recent genomic work on domestic birds that have identified potential ‘eye colour genes’, including one associated with pteridine pigmentation in pigeons. Finally, from an evolutionary standpoint, we present and discuss several hypotheses explaining the adaptive significance of eye colour variation. Many of these hypotheses suggest that bird eye colour plays an important role in intraspecific signalling, particularly as an indicator of age or mate quality, although the importance of eye colour may differ between species and few evolutionary hypotheses have been directly tested. We suggest that future studies of avian eye colour should consider all three levels, including broad-scale iris pigment analyses across bird species, genome sequencing studies to identify loci associated with eye colour variation, and behavioural experiments and comparative phylogenetic analyses to test adaptive hypotheses. By examining these proximate and ultimate causes of eye colour variation in birds, we hope that our review will encourage future research to understand the ecological and evolutionary significance of this striking avian trait.     

Colour and colour change in the grasshopper, Kosciuscola tristis

The males of the small grasshopper (Kosciuscola tristis), with a restricted range above 1830 m in the Australian Alps, exhibit a remarkable colour change. They are dark, almost black, when cold and change to a bright sky blue colour within minutes of exposure to warmth.Sections of cuticle fixed in the two conditions confirm that the cells underlying the cuticle contain two kinds of granules: large (diameter 1·0 μm) spherical, brown granules, and smaller (0·17 μm) less dense granules. In the blue (warm) condition the small granules are closely packed in the distal part of the cells, whereas the ‘black’ granules are found predominantly in the deeper proximal zones. Evidence is presented to suggest that the blue colour arises from Tyndall scattering of light by the suspension of small granules and is intensified by being seen against a dark background.In the black condition the black granules are found to have moved towards the surface, mingling with the smaller granules and ‘quenching’ the light scattering.The smaller granules are white in the isolated state. They consist of a mixture of uric acid and a pteridine, probably leucopterin.The epidermal cells contain numerous microtubules, which are directed towards the cell surface, that is, parallel to the direction of movement of the granules. It is possible that the microtubules are associated with the movement.     

Responses of Odonata chromatophores to environmental stimuli

Responses to change in temperature and light intensity were studied in three species of Australian Odonata using time-lapse photography. In each species, responses to temperature are dependent on both the instantaneous temperature and the direction of temperature change. At temperatures below those which produce unstable colour phases, the change to dark phase takes about 9 hr and is constant in rate. The reverse change is directly temperature dependent and can be much more rapid. Responses to change in light intensity are attributable to the heating effect of light rather than to true light sensitivity. All colour changes show wide individual variation in both rate and amount. They are slightly affected by temperature acclimation but are unaffected by prevailing weather, time of year, geographic location, or age.     

Site of conversion of endogenous all-trans-retinoids to 11-cis-retinoids in the bovine eye

By use of a new high-resolution high-pressure liquid chromatographic method for the separation of isomeric forms of retinol, retinal, retinyl ester and retinal oxime, various retinoids were analyzed in separated retinal pigment epithelial tissue or neural retinal tissue from fresh bleached bovine eyes after incubation in the dark at either 30 or 4°C for 90 min. 11-cis-Retinoids significantly increased during incubation at 30°C, relative to those at 4°C, in the retinal pigment epithelium, but not in the retina. The major forms of vitamin A in incubated retinal pigment epithelium and neural retina were retinyl esters (70%) and all-trans-retinol (69%), respectively. Thus, in keeping with observations on the isomerization of radioactive retinol in homogenates of eye tissues, the retinal pigment epithelium seems to be the primary site of 11-cis-retinoid formation from endogenous all-trans-retinoids in the bovine eye.     

FRACTIONATION OF THE EYE PIGMENTS OF DROSOPHILA MELANOGASTER

Eye pigments of normal and mutant types of D. melanogaster have been extracted with water and fractionated by chromatographic adsorption on powdered talc. Spectra of all the fractions obtainable in solution have been measured and the general chemical behavior of the pigments is described. Two chemically distinct groups of pigments are found, to be identified with the earlier designated red and brown components. The red component in the wild-type eye contains three well defined pigments, two of them capable of further subdivision so that the total number of fractions obtained is five. There is also present a brown component pigment which could not be treated quantitatively by the methods employed. All members of the wild-type red component are found in cinnabar eyes, unaccompanied by the brown component. Conversely, brown eyes contain a pigment indistinguishable from the wild-type brown component, virtually alone. In sepia eyes, one red component and two brown component pigments can be distinguished, all three pigments differing from those of wild-type eyes. Pigments apparently identical with those found in wild-type melanogaster eyes have also been found in D. virilis.     

Photoreceptor spectral sensitivities of the Small White butterfly <Emphasis Type="Italic">Pieris rapae crucivora</Emphasis> interpreted with optical modeling

The compound eye of the Small White butterfly, Pieris rapae crucivora, has four classes of visual pigments, with peak absorption in the ultraviolet, violet, blue and green, but electrophysiological recordings yielded eight photoreceptors classes: an ultraviolet, violet, blue, double-peaked blue, green, blue-suppressed-green, pale-red and deep-red class. These photoreceptor classes were identified in three types of ommatidia, distinguishable by the different eye shine spectra and fluorescence; the latter only being present in the eyes of males. We present here two slightly different optical models that incorporate the various visual pigments, the light-filtering actions of the fluorescent, pale-red and deep-red screening pigment, located inside or adjacent to the rhabdom, and the reflectance spectrum of the tapetum that abuts the rhabdom proximally. The models serve to explain the photoreceptor spectral sensitivities as well as the eye shine.     

Single and Multiple Visual Systems in Arthropods

Extraction of two visual pigments from crayfish eyes prompted an electrophysiological examination of the role of visual pigments in the compound eyes of six arthropods. The intact animals were used; in crayfishes isolated eyestalks also. Thresholds were measured in terms of the absolute or relative numbers of photons per flash at various wavelengths needed to evoke a constant amplitude of electroretinogram, usually 50 µv. Two species of crayfish, as well as the green crab, possess blue- and red-sensitive receptors apparently arranged for color discrimination. In the northern crayfish, Orconectes virilis, the spectral sensitivity of the dark-adapted eye is maximal at about 550 mµ, and on adaptation to bright red or blue lights breaks into two functions with λ_max respectively at about 435 and 565 mµ, apparently emanating from different receptors. The swamp crayfish, Procambarus clarkii, displays a maximum sensitivity when dark-adapted at about 570 mµ, that breaks on color adaptation into blue- and red-sensitive functions with λ_max about 450 and 575 mµ, again involving different receptors. Similarly the green crab, Carcinides maenas, presents a dark-adapted sensitivity maximal at about 510 mµ that divides on color adaptation into sensitivity curves maximal near 425 and 565 mµ. Each of these organisms thus possesses an apparatus adequate for at least two-color vision, resembling that of human green-blinds (deuteranopes). The visual pigments of the red-sensitive systems have been extracted from the crayfish eyes. The horse-shoe crab, Limulus, and the lobster each possesses a single visual system, with λ_max respectively at 520 and 525 mµ. Each of these is invariant with color adaptation. In each case the visual pigment had already been identified in extracts. The spider crab, Libinia emarginata, presents another variation. It possesses two visual systems apparently differentiated, not for color discrimination but for use in dim and bright light, like vertebrate rods and cones. The spectral sensitivity of the dark-adapted eye is maximal at about 490 mµ and on light adaptation, whether to blue, red, or white light, is displaced toward shorter wavelengths in what is essentially a reverse Purkinje shift. In all these animals dark adaptation appears to involve two phases: a rapid, hyperbolic fall of log threshold associated probably with visual pigment regeneration, followed by a slow, almost linear fall of log threshold that may be associated with pigment migration.     

The fine structure of odonata chromatophores

The appearance, fine structure and pigment composition of the epidermal chromatophores of mature Austrolestes annulosus (Lestidae) are described and compared with the developing chromatophores of teneral Austrolestes and the mature chromatophores of Diphlebia lestoides (Amphipterygidae) and Ischnura heterosticta (Caenagrionidae). Mature chromatophores contain masses of near spherical light-scattering bodies and larger irregularly shaped pigment vesicles. These effect colour change by migrating in opposite directions, through a system of interconnecting granular endoplasmic reticulum tubules. The pigment, a mixture of xanthommatin and dihydroxanthommatin, has a liquid or gelatinous consistency. Developing chromatophores of teneral insects lack light-scattering bodies and well-defined migratory pigment vesicles, but contain irregular masses of pigment of similar chemical composition.     

Isolation and antisense suppression of <Emphasis Type="Italic">flavonoid 3', 5'-hydroxylase</Emphasis>modifies flower pigments and colour in cyclamen

Background  

Cyclamen is a popular and economically significant pot plant crop in several countries. Molecular breeding technologies provide opportunities to metabolically engineer the well-characterized flavonoid biosynthetic pathway for altered anthocyanin profile and hence the colour of the flower. Previously we reported on a genetic transformation system for cyclamen. Our aim in this study was to change pigment profiles and flower colours in cyclamen through the suppression of flavonoid 3', 5'-hydroxylase, an enzyme in the flavonoid pathway that plays a determining role in the colour of anthocyanin pigments.