Chromatic and achromatic stimulus discrimination of long wavelength (red) visual stimuli by the honeybee Apis mellifera

It has long been assumed that bees cannot see red. However, bees visit red flowers, and the visual spectral sensitivity of bees extends into wavelengths to provide sensitivity to such flowers. We thus investigated whether bees can discriminate stimuli reflecting wavelengths above 560 nm, i.e., which appear orange and red to a human observer. Flowers do not reflect monochromatic (single wavelength) light; specifically orange and red flowers have reflectance patterns which are step functions, we thus used colored stimuli with such reflectance patterns. We first conditioned honey bees Apis mellifera to detect six stimuli reflecting light mostly above 560 nm and found that bees learned to detect only stimuli which were perceptually very different from a bee achromatic background. In a second experiment we conditioned bees to discriminate stimuli from a salient, negative (un-rewarded) yellow stimulus. In subsequent unrewarded tests we presented the bees with the trained situation and with five other tests in which the trained stimulus was presented against a novel one. We found that bees learned to discriminate the positive from the negative stimulus, and could unambiguously discriminate eight out of fifteen stimulus pairs. The performance of bees was positively correlated with differences between the trained and the novel stimulus in the receptor contrast for the long-wavelength bee photoreceptor and in the color distance (calculated using two models of the honeybee colors space). We found that the differential conditioning resulted in a concurrent inhibitory conditioning of the negative stimulus, which might have improved discrimination of stimuli which are perceptually similar. These results show that bees can detect long wavelength stimuli which appear reddish to a human observer. The mechanisms underlying discrimination of these stimuli are discussed. Handling Editor: Lars Chittka.     

Neural Photoreception in a Lamellibranch Mollusc

The pallial nerves of Spisula solidissima each contain a single afferent nerve fiber which responds directly to illumination of the nerve, and apparently mediates the "shadow" response of siphon retraction. These units show constant-frequency spontaneous activity in the dark; illumination abruptly inhibits this discharge, and cessation of the light stimulus then evokes a prolonged burst of impulses at high frequency (the off-response). Impulses are initiated at a point near the visceral ganglion, and propagated unidirectionally toward it. Stimulation with monochromatic light has revealed that more than one photoreceptor pigment is involved, since the discharge patterns evoked are wavelength-specific. Inhibition is relatively prominent at short wavelengths, excitation at long wavelengths. Following selective adaptation with blue light, "on" responses can be produced with red stimuli, demonstrating the unmasking of an excitatory event which takes place during illumination. The two photoreceptor pigments may be segregated in two or more cells presynaptic to the recorded unit, or,—more likely—may both be contained in the same cell. The spectral sensitivity function for inhibition shows a single maximum at 540 mµ, and is probably dependent upon a carotenoid pigment. No photoreceptor function has been demonstrated for a hemoprotein, apparently identical with cytochrome h, which occurs in high concentration in Spisula nerve.     

Evidence that the long-lifetime photointermediate of s-rhodopsin is a receptor for negative phototaxis in Halobacterium halobium

The effect of blue background light on behavioral response of Halobacterium halobium to step-like stimulation with green-orange attractant light was examined. The results strongly support the previously proposed hypothesis that a long-lifetime photointermediate of s-rhodopsin is the photoreceptor for repellent light: the step-like increase in green-orange light was convertible from attractant stimulus to repellent one, when the cells were constantly illuminated with blue light. No difference of the threshold intensity of the blue background light was observed between the mutant strain that lacks both bacteriorhodopsin and halorhodopsin and the wild type strain, suggesting that the two light-driven ion pumps are not participant in sensing attractant light.     

Phycomyces: An Increase in Mechanical Extensibility during the Avoidance Growth Response

The sporangiophore of Phycomyces shows a transient response to a double barrier, the avoidance growth response. Tensile tests conducted on the stage IV sporangiophore demonstrate that an increase in mechanical extensibility occurs about a minute after a double barrier stimulus. This change in mechanical extensibility is similar to the one that occurs after a light stimulus. We have concluded that the avoidance stimulus occurs somewhere on the same pathway between the photoreceptor mechanism and the final growth response.     

Colour categorization by domestic chicks

Spectral stimuli form a physical continuum, which humans divide into discrete non-overlapping regions or categories that are designated by colour names. Little is known about whether non-verbal animals form categories on stimulus continua, but work in psychology and artificial intelligence provides models for stimulus generalization and categorization. We compare predictions of such models to the way poultry chicks (Gallus gallus) generalize to novel stimuli following appetitive training to either one or two colours. If the two training colours are (to human eyes) red and greenish-yellow or green and blue, chicks prefer intermediates, i.e. orange rather than red or yellow and turquoise rather than green or blue. The level of preference for intermediate colours implies that the chicks interpolate between the training stimuli. However, they do not extrapolate beyond the limits set by the training stimuli, at least for red and yellow training colours. Similarly, chicks trained to red and blue generalize to purple, but they do not generalize across grey after training to the complementary colours yellow and blue. These results are consistent with a modified version of a Bayesian model of generalization from multiple examples that was proposed by Shepard and show similarities to human colour categorization.     

Integration of photo- and chemosensory signaling pathways in <Emphasis Type="Italic">Chlamydomonas</Emphasis>

The behavior of Chlamydomonas reinhardtii Dangeard is regulated by both light and chemical stimuli. Generation of a transmembrane photoreceptor current is the earliest so far resolved event in phototaxis of green flagellates. Tryptone rapidly inhibits the photoreceptor current in gametes of C. reinhardtii and induces their accumulation. The time-course, concentration dependence and induction during gametogenesis of these two processes coincide. On the other hand, tryptone causes a weak stimulation of the photoreceptor current in the absence of any behavioral responses in vegetative cells. This shows that the tryptone-induced inhibition of the photoreceptor current in C. reinhardtii is due to activation of a gamete-specific chemosensory system, and that integration of the photo- and chemosensory signals already occurs at the initial steps of the signaling pathways.     

苔藓植物蓝光/紫外光-A 信号传导的研究

种子植物含有5个已分离的光受体和至少1个未鉴定的蓝光/紫外光-A受体。隐花色素(CRY1、CRY2和CRY3) 调节植物的生长发育,而向光蛋白(PHOT1和PHOT2) 调节植物对光的营养反应。黄素可以吸收蓝光和紫外光-A,是生色团。对这些光受体的结构和作用模式已了解很多。苔藓植物小立碗藓中含有2个已分离的隐花色素(CRY1a和CRY1b),负责调节侧枝形成和生长素代谢;有4个向光蛋白(PHOTA1,PHOTA2,PHOTB1,PHOTB2) 调节叶绿体的运动。苔藓细胞内蓝光/紫外光-A刺激引发的信号转导有Ca2+参与。     

Blue-Light-Regulated Expression of Genes for Two Early Steps of Chlorophyll Biosynthesis in Chlamydomonas reinhardtii

In light:dark-synchronized cultures of Chlamydomonas reinhardtii, the genes encoding the enzymes for two early steps of chlorophyll biosynthesis, glutamate-1-semialdehyde aminotransferase (gsa) and [delta]-aminolevulinic acid dehydratase (alad), are expressed at high levels early in the light phase, just prior to a rapid burst of chlorophyll synthesis. Induction of gsa mRNA in synchronized cells is totally dependent on light, whereas induction of alad mRNA occurs to approximately one-half the light-induced level even in cells kept in the dark during the light phase and appears to be dependent on the cell cycle or a circadian rhythm. gsa mRNA and alad mRNA accumulation is induced by light that was passed through blue (400-480 nm) or green (490-590 nm) filters but not by light that was passed through orange (>560 nm) or red (>610 nm) filters, indicating the participation of a blue-light photoreceptor system rather than a protochlorophyllide- or rhodopsin-based photoreceptor. Light induction of gsa mRNA accumulation is absent in a carotenoid-deficient mutant, which suggests that a carotenoid-containing blue-light photoreceptor is involved. In contrast, pretreatment of wild-type cells with either of two flavin antagonists, phenylacetic acid and KI, does not prevent the light induction. In the later part of the light phase, the gsa mRNA level decreases more rapidly than that of alad mRNA. Turnover studies indicate that the half-life of alad mRNA is twice that of gsa mRNA. This difference in mRNA stability partially accounts for the more rapid decline in gsa mRNA levels after the peak of light induction is reached. Thus, differential blue-light induction and stability of mRNAs regulates the expression of these two chlorophyll biosynthetic genes.     

Neural modification by paired sensory stimuli

With repetitive stimulation of two sensory pathways which are intact within the isolated nervous system of Hermissenda, features of a cellular conditioning paradigm were identified. Type A photoreceptors, unlike type B photoreceptors, produce fewer impulses in response to light following temporally specific pairing of light stimuli with rotation stimuli. Type A photoreceptor impulse wave-forms are also specifically changed by such stimulus regimens. These findings can be explained, at least in part, by increased inhibition of type A cells by type B cells after stimulus pairing.     

Blue flickers of hope: secondary structure, dynamics, and putative dimerization interface of the blue-light receptor YtvA from Bacillus subtilis

Bacillus subtilis is capable of responding to various kinds of extracellular, potentially harmful stimuli via a stress response pathway, which involves a signal transduction and integration hub, the stressosome, and finally leads to activation of σ(B). One of the different signals initiating the underlying phosphorylation cascade is blue light. While it is known that the bacterial photoreceptor YtvA is responsible for blue light detection, the intramolecular activation mechanism and the structure of this multidomain protein are unknown. Using solution NMR spectroscopy, we have obtained a near complete backbone assignment of the full-length protein. More importantly, we report relaxation data and data on the solvent accessibility of full-length YtvA in the dark state which are interpreted with respect to secondary structure, the mobility, and the quaternary structure of the protein. Finally, we show that YtvA adopts an elongated domain orientation with LOV-LOV and STAS-STAS interactions on either side.     

The role of terminal-link stimuli in concurrent-chain schedules: revisited using a behavioral-history procedure

A behavioral-history procedure was used to study the function of terminal-link stimuli as conditioned reinforcers in multiple concurrent-chain schedules of reinforcement. First, three pigeons were exposed to multiple concurrent-chain schedules in which the two multiple-schedule components were correlated with a blue and a white stimulus, respectively. In each component the initial links were equal independent variable-interval (VI) 15 s schedules. A fixed-interval (FI) 10 s schedule operated on the red key in one terminal link while extinction operated on the green key in the alternative terminal link. When large preferences for the red stimulus had been established, two tests were conducted. In the terminal-link test, under new initial-link stimuli--purple and brown--an FI 10 s schedule operated for both the red and green terminal-link stimuli. In the subsequent initial-link test, the blue and white initial-link stimuli were reintroduced, and, as in the terminal-link test, FI 10s operated for both the red and the green terminal-link stimuli. In the terminal-link test, the three pigeons showed no preference for the terminal links with the red stimulus, but showed clear and consistent preferences for the red stimulus when blue and white stimuli were reintroduced as initial-link stimuli in the initial-link test. This suggests that there are multiple sources of control over initial-link response allocation in concurrent-chains, including control by both terminal- and initial-link stimuli.     

Contrast gain, signal-to-noise ratio, and linearity in light-adapted blowfly photoreceptors

Response properties of short-type (R1-6) photoreceptors of the blowfly (Calliphora vicina) were investigated with intracellular recordings using repeated sequences of pseudorandomly modulated light contrast stimuli at adapting backgrounds covering 5 log intensity units. The resulting voltage responses were used to determine the effects of adaptational regulation on signal-to-noise ratios (SNR), signal induced noise, contrast gain, linearity and the dead time in phototransduction. In light adaptation the SNR of the photoreceptors improved more than 100-fold due to (a) increased photoreceptor voltage responses to a contrast stimulus and (b) reduction of voltage noise at high intensity backgrounds. In the frequency domain the SNR was attenuated in low frequencies with an increase in the middle and high frequency ranges. A pseudorandom contrast stimulus by itself did not produce any additional noise. The contrast gain of the photoreceptor frequency responses increased with mean illumination and the gain was best fitted with a model consisting of two second order and one double pole of first order. The coherence function (a normalized measure of linearity and SNR) of the frequency responses demonstrated that the photoreceptors responded linearly (from 1 to 150 Hz) to the contrast stimuli even under fairly dim conditions. The theoretically derived and the recorded phase functions were used to calculate phototransduction dead time, which decreased in light adaptation from approximately 5-2.5 ms. This analysis suggests that the ability of fly photoreceptors to maintain linear performance under dynamic stimulation conditions results from the high early gain followed by delayed compressive feed-back mechanisms.     

Eye movements in the water flea, Daphnia magna: effects of light-stimulus characteristics

ABSTRACT. Average Daphnia eye movement rates decline upon exposure to bright white light from an initial 8 Hz to 4.5 Hz after 20 min. Eye movement rates were maximal for white light of intensity 10W/m², and for blue light presented through the side of the head, though no difference could be measured between the responses to different light colours if presentation was through the top of the head.
Slow moving light stimuli rotating around the animal evoked different stereotyped eye movement patterns for blue and for orange light. Ultrastructural changes in the rectangular cross-section rhabdom were confined to the central region if the animal was exposed to strong orange light, and to the short edges if exposed to strong blue light.     

Dark regeneration of rhodopsin in crayfish photoreceptors

The eyes of crayfish were exposed to lights of known spectral composition, and the course of regeneration was followed in the dark by measuring the content of rhodopsin and metarhodopsin in single rhabdoms isolated at various times after the adaptation, using an assay that is based on the fluorescence of metarhodopsin. Complete recovery requires several days in the dark after intense adaptation to orange light, but requires less than 2 d after blue light exposure. Following an orange light exposure with blue produces recovery kinetics characteristic of the blue light exposure alone. This quickening of recovery occurs whether the receptors are exposed to blue light either immediately or many hours after the original exposure to orange. Conversely, following blue light adaptation with orange leads to slow recovery, which is characteristic of orange alone. Recovery from long-wavelength adaptation is slower principally because many rhabdoms seem to delay the onset of regeneration. We suggest that the regeneration system is itself photosensitive, and after orange light adaptation the supply of active chromophore (presumably 11-cis retinal) limits the rate of recovery. Once started, recovery proceeds slowly and continuously, and the total pigment concentration (rhodopsin plus metarhodopsin) in the rhabdomeric membrane remains approximately constant. Within hours after intense adapting exposures, the rhabdoms become altered in appearance, the surfaces become coated with accessory pigment, and the bands of microvilli are less distinct. These changes persist until recovery of rhodopsin proceeds, which suggests that visual pigment regeneration results from addition of newly synthesized rhodopsin associated with membrane turn-over.     

The signal transducing photoreceptors of plants

Light signals are amongst the most important environmental cues regulating plant development. In addition to light quantity, plants measure the quality, direction and periodicity of incident light and use the information to optimise growth and development to the prevailing environmental conditions. Red and far-red wavelengths are perceived by the photoreversible phytochrome family of photoreceptors, whilst the detection of blue and ultraviolet (UV)-A wavelengths is conferred by the cryptochromes and phototropins. Higher plants contain multiple discrete phytochromes, the apoproteins of which are encoded by a small divergent gene family. In Arabidopsis, two cryptochrome and two phototropin family members have been identified and characterized. Photoreceptor action regulates development throughout the lifecycle of plants, from seed germination through to architecture of the mature plant and the onset of reproduction. The roles of individual photoreceptors in mediating plant development have, however, often been confounded by redundant, synergistic and in some cases mutually antagonistic mechanisms of action. The isolation of mutants null for individual photoreceptors and the construction of mutants null for multiple photoreceptors have therefore been paramount in elucidating photoreceptor functions. Photoreceptor action does not, however, operate in isolation from other signalling systems. The integration of light signals with other environmental cues enables plants to adapt their physiology to changing seasonal environments. This paper summarises current understanding of photoreceptor families and their functions throughout the lifecycle of plants. The integration of light signals with other environmental stimuli is also discussed.     

Stochastic Properties of Discrete Waves of the Limulus Photoreceptor

In the dark-adapted photoreceptor of the horseshoe crab, Limulus, transient discrete depolarizations of the cell membrane, discrete waves, occur in total darkness and their rate of occurrence is increased by illumination. The individual latencies of the discrete waves evoked by a light stimulus often cannot be resolved because the discrete waves overlap in time. The latency of the first discrete wave that follows a stimulus can be determined with reasonable accuracy. We propose a model which allows us to make an estimate of the distribution of the latencies of the individual light-evoked discrete waves, and to predict the latency distribution of the first discrete wave that follows a stimulus of arbitrary intensity-time course from the latency distribution of the first discrete wave that follows a brief flash of light. For low intensity stimuli, the predictions agree well with the observations. We define a response as the occurrence of one or more discrete waves following a stimulus. The distribution of the peak amplitudes of responses suggests that the peak amplitude of individual discrete waves sometimes has a bimodal distribution. The latencies of the two types of discrete waves, however, follow similar distributions. The area under the voltage-time curve of responses that follow equal energy long (1.25 sec) and short (10 msec) light stimuli follows similar distributions, and this suggests that discrete waves summate linearly.     

Does melanopsin bistability have physiological consequences?

Recent publications in the Journal of Biological Rhythms have focused on the hypothesis that the property of melanopsin bistability is functionally translated to in vivo mammalian physiology. Physiological consequences of photopigment bistability likely can be inferred from the more extensive invertebrate literature. In invertebrates, photopigment bistability results in (a) photoreceptor independence from specialized chromophore regenerating systems, (b) long-wavelength enhancement of a blue light effect, (c) expression of a prolonged depolarization after potential following intense blue light stimulation,and (d) photopigment endocytosis following chronic short-wavelength light exposure. If analogous physiological phenomena result from melanopsin bistability in mammals, then one can take advantage of the spectral composition of a light source to modulate its impact on photoentrainment and other light-dependent circadian phenomena. In any event,investigators studying phenomena that are affected by photic stimulation of intrinsically photosensitive retinal ganglion cells should detail the spectral composition of their light sources before, during, and after an experimental photic stimulus.     

Structure and function of the photoreceptor stentorins in Stentor coeruleus. I. Partial characterization of the photoreceptor organelle and stentorins

The unicellular ciliary protozoan, Stentor coeruleus, exhibits photophobic and phototactic responses to visible light stimuli. The pigment granule contains the photoreceptor chromoproteins (stentorins). Stentorin localized in the pigment granules of the cell serves as the primary photoreceptor for the photophobic and phototactic responses in this organism. An initial characterization of the pigment granules has been described in terms of size, absorbance spectra and ATPase activity. Two forms of the stentorin pigments have been isolated from the pigment granules. Stentorin I has an apparent molecular weight of 68,600 and 52,000 by SDS-PAGE (at 10 and 13% gel, respectively) or 102,000 by steric exclusion HPLC, whereas stentorin II is a larger molecular assembly probably composed of several proteins (mol. wt. greater than 500,000). Stentorin I is composed of at least two heterologous subunits corresponding to apparent mol. wts. of 46,000 (fluorescent, Coomassie blue negative) and 52,000 (fluorescent, Coomassie blue positive) on SDS-PAGE (13% gel). However, these values were found to be strongly dependent on the degree of crosslinking in the acrylamide gel. Stentorin II appears to be the primary photoreceptor whose absorption and fluorescence properties are consistent with the action spectra for the photoresponses of the ciliate to visible light.     

Light Induced Concentration Changes of Adenosine-Triphosphate in Phycomyces Sporangiophores

The concentrations of extractable adenosine triphosphate (ATP) following the induction of positive light-growth responses in Phycomyces sporangiophores by blue light stimuli have been measured by means of the luciferin-luciferase assay. The ATP concentration in the light-sensitive growing zone increases 30 to 50% within 30 seconds after the start of a light stimulus and returns to the normal adapted level within 1 minute after stimulation. The ATP concentration is constant for any level of light adaptation and is uniform along the length of sporangiophores even though the light sensitivity is confined to a growing zone less than 5 mm long. These results suggest that one of the initial biochemical steps after a light stimulus is the production of extractable ATP.     

CRYPTOCHROME2 in vascular bundles regulates flowering in Arabidopsis

Plants make full use of light signals to determine the timing of flowering. In Arabidopsis thaliana, a blue/UV-A photoreceptor, CRYPTOCHROME 2 (cry2), and a red/far-red photoreceptor, PHYTOCHROME B (phyB), are two major photoreceptors that control flowering. The light stimuli for the regulation of flowering are perceived by leaves. We have recently shown that phyB expression in mesophyll but not in vascular bundles suppresses the expression of a key flowering regulator, FLOWERING LOCUS T (FT), in vascular bundles. In this study, we asked where in the leaf cry2 perceives light stimuli to regulate flowering. To answer this question, we established transgenic Arabidopsis lines in which the cry2-green fluorescent protein (GFP) fusion was expressed under the control of organ/tissue-specific promoters in a cry2-deficient mutant background. Analysis of these lines revealed that expression of cry2-GFP in vascular bundles, but not in epidermis or mesophyll, rescued the late flowering phenotype. We further confirmed that cry2-GFP expressed in vascular bundles increased FT expression only in vascular bundles. Hence, in striking contrast with phyB, cry2 most likely regulates FT expression in a cell-autonomous manner.