Green-sensitive Photoreceptors Research Articles

Polarisation vision in the dark: green-sensitive photoreceptors in the nocturnal ball-rolling dung beetle Escarabaeus satyrus.

Many insects utilise the polarisation pattern of the sky to adjust their travelling directions. The extraction of directional information from this sky-wide cue is mediated by specialised photoreceptors located in the dorsal rim area (DRA). While this part of the eye is known to be sensitive to the ultraviolet, blue or green component of skylight, the latter has only been observed in insects active in dim light. To address the functional significance of green polarisation sensitivity, we define the spectral and morphological adaptations of the DRA in a nocturnal ball-rolling dung beetle-the only family of insects demonstrated to orient to the dim polarisation pattern in the night sky. Intracellular recordings revealed polarisation-sensitive green photoreceptors in the DRA of Escarabaeus satyrus. Behavioural experiments verified the navigational relevance of this finding. To quantify the adaptive value of green sensitivity for celestial orientation at night, we also obtained the polarisation properties of the night sky in the natural habitat of the beetle. Calculations of relative photon catch revealed that under a moonlit sky the green-sensitive DRA photoreceptors can be expected to catch an order of magnitude more photons compared with the UV-sensitive photoreceptors in the main retina. The green-sensitive photoreceptors - which also show a range of morphological adaptations for enhanced sensitivity - provide E. satyrus with a highly sensitive system for the extraction of directional information from the night sky.

Spectral sensitivity of retinal photoreceptors of tortricid moths is not tuned to diel activity period.

Leafrollers (Lepidoptera: Tortricidae) are a large family of small moths containing over 10,000 species, many of which are crop pests. Grapholita molesta, Lobesia botrana and Cydia pomonella adults are sexually active before, during and after sunset, respectively. We wanted to determine whether being active at different times of the day and night is associated with differences in their visual system. Spectral sensitivity (SS) was measured with electroretinograms and selective adaptation with green, blue and ultraviolet light. SS curves could be fitted with a triple nomogram template which indicated the existence of three photoreceptor classes peaking at 355, 440 and 525 nm. The retinae showed clear regionalization, with fewer blue receptors dorsally. No differences among species or between sexes were found. Intracellular recordings in C. pomonella also revealed three photoreceptor classes with sensitivities peaking at 355, 440 and 525 nm. The blue photoreceptors showed inhibitory responses in the green part of the spectrum, indicating the presence of a colour-opponent system. Flicker fusion frequency experiments showed that the response speed was similar between sexes and species and fused at around 100 Hz. Our results indicate that the three species have the ancestral insect retinal substrate for a trichromatic colour vision, based upon the UV, blue and green-sensitive photoreceptors, and lack any prominent adaptations related to being active under different light conditions.

Blimp-1/PRDM1 and Hr3/RORβ specify the blue-sensitive photoreceptor subtype in Drosophila by repressing the hippo pathway.

During terminal differentiation of the mammalian retina, transcription factors control binary cell fate decisions that generate functionally distinct subtypes of photoreceptor neurons. For instance, Otx2 and RORβ activate the expression of the transcriptional repressor Blimp-1/PRDM1 that represses bipolar interneuron fate and promotes rod photoreceptor fate. Moreover, Otx2 and Crx promote expression of the nuclear receptor Nrl that promotes rod photoreceptor fate and represses cone photoreceptor fate. Mutations in these four transcription factors cause severe eye diseases such as retinitis pigmentosa. Here, we show that a post-mitotic binary fate decision in Drosophila color photoreceptor subtype specification requires ecdysone signaling and involves orthologs of these transcription factors: Drosophila Blimp-1/PRDM1 and Hr3/RORβ promote blue-sensitive (Rh5) photoreceptor fate and repress green-sensitive (Rh6) photoreceptor fate through the transcriptional repression of warts/LATS, the nexus of the phylogenetically conserved Hippo tumor suppressor pathway. Moreover, we identify a novel interaction between Blimp-1 and warts, whereby Blimp-1 represses a warts intronic enhancer in blue-sensitive photoreceptors and thereby gives rise to specific expression of warts in green-sensitive photoreceptors. Together, these results reveal that conserved transcriptional regulators play key roles in terminal cell fate decisions in both the Drosophila and the mammalian retina, and the mechanistic insights further deepen our understanding of how Hippo pathway signaling is repurposed to control photoreceptor fates for Drosophila color vision.

Opponent processing in the retinal mosaic of nymphalid butterflies.

The eyes of nymphalid butterflies, investigated with incident illumination, show colourful facet reflection patterns—the eye shine—which is uniform or heterogeneous, dependent on the species. Facet colours suggest that the ommatidia contain different sets of photoreceptors and screening pigments, but how the colours and the cell characteristics are associated has not been clearly established. Here, we analyse the retinae of two nymphalids, Apatura ilia, which has a uniform eyeshine, and Charaxes jasius, a species with a heterogeneous eye shine, using single-cell recordings, spectroscopy and optical pupillometry. Apatura has UV-, blue- and green-sensitive photoreceptors, allocated into three ommatidial types. The UV- and blue-sensitive cells are long visual fibres (LVFs), receiving opponent input from the green-sensitive short visual fibres (SVFs). Charaxes has an expanded set of photoreceptors, allocated into three additional, red-reflecting ommatidial types. All red ommatidia contain green-sensitive LVFs, receiving opponent input from red receptors. In both species, the SVFs do not receive any opponent input. The simple retina of Apatura with three ommatidial types and two colour-opponent channels can support trichromatic vision. Charaxes has six ommatidial types and three colour-opponent channels. Its expanded receptor set can support tetrachromatic vision.This article is part of the theme issue ‘Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods’.

Red-green opponency in the long visual fibre photoreceptors of brushfoot butterflies (Nymphalidae).

In many butterflies, the ancestral trichromatic insect colour vision, based on UV-, blue- and green-sensitive photoreceptors, is extended with red-sensitive cells. Physiological evidence for red receptors has been missing in nymphalid butterflies, although some species can discriminate red hues well. In eight species from genera Archaeoprepona, Argynnis, Charaxes, Danaus, Melitaea, Morpho, Heliconius and Speyeria, we found a novel class of green-sensitive photoreceptors that have hyperpolarizing responses to stimulation with red light. These green-positive, red-negative (G+R–) cells are allocated to positions R1/2, normally occupied by UV and blue-sensitive cells. Spectral sensitivity, polarization sensitivity and temporal dynamics suggest that the red opponent units (R–) are the basal photoreceptors R9, interacting with R1/2 in the same ommatidia via direct inhibitory synapses. We found the G+R– cells exclusively in butterflies with red-shining ommatidia, which contain longitudinal screening pigments. The implementation of the red colour channel with R9 is different from pierid and papilionid butterflies, where cells R5–8 are the red receptors. The nymphalid red-green opponent channel and the potential for tetrachromacy seem to have been switched on several times during evolution, balancing between the cost of neural processing and the value of extended colour information.

Fragmentary Blue: Resolving the Rarity Paradox in Flower Colors.

Blue is a favored color of many humans. While blue skies and oceans are a common visual experience, this color is less frequently observed in flowers. We first review how blue has been important in human culture, and thus how our perception of blue has likely influenced the way of scientifically evaluating signals produced in nature, including approaches as disparate as Goethe’s Farbenlehre, Linneaus’ plant taxonomy, and current studies of plant-pollinator networks. We discuss the fact that most animals, however, have different vision to humans; for example, bee pollinators have trichromatic vision based on UV-, Blue-, and Green-sensitive photoreceptors with innate preferences for predominantly short-wavelength reflecting colors, including what we perceive as blue. The subsequent evolution of blue flowers may be driven by increased competition for pollinators, both because of a harsher environment (as at high altitude) or from high diversity and density of flowering plants (as in nutrient-rich meadows). The adaptive value of blue flowers should also be reinforced by nutrient richness or other factors, abiotic and biotic, that may reduce extra costs of blue-pigments synthesis. We thus provide new perspectives emphasizing that, while humans view blue as a less frequently evolved color in nature, to understand signaling, it is essential to employ models of biologically relevant observers. By doing so, we conclude that short wavelength reflecting blue flowers are indeed frequent in nature when considering the color vision and preferences of bees.

Gonadotropin-releasing hormone-independent effects of recombinant vertebrate ancient long opsin in the goldfish Carassius auratus reveal alternative reproduction pathways.

Vertebrate ancient long (VAL)-opsin is a green-sensitive photoreceptor that shows high sequence similarity to vertebrate ancient opsin, which is considered to play a role in sexual maturation via gonadotropin-releasing hormone (GnRH); however, the role of VAL-opsin in vertebrate sexual maturity remains unclear. Therefore, we investigated the possible role of VAL-opsin in reproduction in the goldfish Carassius auratus under a state of GnRH inhibition. Goldfish were injected with recombinant VAL-opsin protein (0.5μg/g body mass) and/or the GnRH antagonist cetrorelix (0.5μg/fish), and changes in the mRNA expression levels of genes associated with goldfish reproduction were measured by quantitative polymerase chain reaction, including those involved in the hypothalamus-pituitary-gonad (HPG) axis, VAL-opsin, GnRH, the gonadotropins (GTHs) luteinizing hormone and follicle-stimulating hormone, and estrogen receptor (ER). Moreover, the fish were irradiated with a green light-emitting diode (520nm) to observe the synergistic effect on the HPG axis with VAL-opsin. Green LED exposure significantly and slightly increased the VAL-opsin and GnRH levels, respectively; however, these effects were blocked in groups injected with cetrorelix at all time points. Cetrorelix significantly decreased the mRNA levels of GTHs and ER, whereas these hormones recovered by co-treatment with VAL-opsin. These results indicate that green LED is an effective light source to promote the expression of sex hormones in fish. Moreover, VAL-opsin not only affects activity of the HPG axis but also appears to act on the pituitary gland directly to stimulate a new sexual maturation pathway that promotes the secretion of GTHs independent of GnRH.

Candidates for photic entrainment pathways to the circadian clock via optic lobe neuropils in the Madeira cockroach.

The compound eye of cockroaches is obligatory for entrainment of the Madeira cockroach's circadian clock, but the cellular nature of its entrainment pathways is enigmatic. Employing multiple-label immunocytochemistry, histochemistry, and backfills, we searched for photic entrainment pathways to the accessory medulla (AME), the circadian clock of the Madeira cockroach. We wanted to know whether photoreceptor terminals could directly contact pigment-dispersing factor-immunoreactive (PDF-ir) circadian pacemaker neurons with somata in the lamina (PDFLAs) or somata next to the AME (PDFMEs). Short green-sensitive photoreceptor neurons of the compound eye terminated in lamina layers LA1 and LA2, adjacent to PDFLAs and PDFMEs that branched in LA3. Long UV-sensitive compound eye photoreceptor neurons terminated in medulla layer ME2 without direct contact to ipsilateral PDFMEs that arborized in ME4. Multiple neuropeptide-ir interneurons branched in ME4, connecting the AME to ME2. Before, extraocular photoreceptors of the lamina organ were suggested to send terminals to accessory laminae. There, they overlapped with PDFLAs that mostly colocalized PDF, FMRFamide, and 5-HT immunoreactivities, and with terminals of ipsi- and contralateral PDFMEs. We hypothesize that during the day cholinergic activation of the largest PDFME via lamina organ photoreceptors maintains PDF release orchestrating phases of sleep-wake cycles. As ipsilateral PDFMEs express excitatory and contralateral PDFMEs inhibitory PDF autoreceptors, diurnal PDF release keeps both PDF-dependent clock circuits in antiphase. Future experiments will test whether ipsilateral PDFMEs are sleep-promoting morning cells, while contralateral PDFMEs are activity-promoting evening cells, maintaining stable antiphase via the largest PDFME entrained by extraocular photoreceptors of the lamina organ.

Changes in electrophysiological properties of photoreceptors in Periplaneta americana associated with the loss of screening pigment.

Absence of screening pigment in insect compound eyes has been linked to visual dysfunction. We investigated how its loss in a white-eyed mutant (W-E) alters the photoreceptor electrophysiological properties, opsin gene expression, and the behavior of the cockroach, Periplaneta americana. Whole-cell patch-clamp recordings of green-sensitive photoreceptors in W-E cockroaches gave reduced membrane capacitance, absolute sensitivity to light, and light-induced currents. Decreased low-pass filtering increased voltage-bump amplitudes in W-E photoreceptors. Intracellular recordings showed that angular sensitivity of W-E photoreceptors had two distinct components: a large narrow component with the same acceptance angle as wild type, plus a relatively small wide component. Information processing was evaluated using Gaussian white-noise modulated light stimulation. In bright light, W-E photoreceptors demonstrated higher signal gain and signal power than wild-type photoreceptors. Expression levels of the primary UV- and green-sensitive opsins were lower and the secondary green-sensitive opsin significantly higher in W-E than in wild-type retinae. In behavioral experiments, W-E cockroaches were significantly less active in dim green light, consistent with the relatively low light sensitivity of their photoreceptors. Overall, these differences can be related to the loss of screening pigment function and to a compensatory decrease in the rhabdomere size in W-E retinae.

Opsin expression patterns coincide with photoreceptor development during pupal development in the honey bee, Apis mellifera

BackgroundThe compound eyes of insects allow them to catch photons and convert the energy into electric signals. All compound eyes consist of numerous ommatidia, each comprising a fixed number of photoreceptors. Different ommatidial types are characterized by a specific set of photoreceptors differing in spectral sensitivity. In honey bees, males and females possess different ommatidial types forming distinct retinal mosaics. However, data are lacking on retinal ontogeny and the mechanisms by which the eyes are patterned. In this study, we investigated the intrinsic temporal and circadian expression patterns of the opsins that give rise to the ultraviolet, blue and green sensitive photoreceptors, as well as the morphological maturation of the retina during pupal development of honey bees.ResultsqPCR and histological labeling revealed that temporal opsin mRNA expression differs between sexes and correlates with rhabdom elongation during photoreceptor development. In the first half of the pupal stage, when the rhabdoms of the photoreceptors are still short, worker and (dorsal) drone retinae exhibit similar expression patterns with relatively high levels of UV (UVop) and only marginal levels of blue (BLop) and green (Lop1) opsin mRNA. In the second half of pupation, when photoreceptors and rhabdoms elongate, opsin expression in workers becomes dominated by Lop1 mRNA. In contrast, the dorsal drone eye shows high expression levels of UVop and BLop mRNA, whereas Lop1 mRNA level decreases. Interestingly, opsin expression levels increase up to 22-fold during early adult life. We also found evidence that opsin expression in adult bees is under the control of the endogenous clock.ConclusionsOur data indicate that the formation of the sex-specific retinal composition of photoreceptors takes place during the second half of the pupal development, and that opsin mRNA expression levels continue to increase in young bees, which stands in contrast to Drosophila, where the highest expression levels are found during the late pupal stage and remain constant in adults. From an evolutionary perspective, we hypothesize that the delayed retinal maturation during the early adult phase is linked to the delayed transition from indoor to outdoor activities in bees, when vision becomes important.

Electrical interactions between photoreceptors in the compound eye of Periplaneta americana.

The compound eye of Periplaneta americana contains two spectral classes of photoreceptors: narrow-band UV-sensitive and broad-band green-sensitive. In intracellular recordings, stimulation of green-sensitive photoreceptors with flashes of relatively bright UV/violet light produced anomalous delayed depolarization after the end of the normal light response, whereas stimulation of UV-sensitive photoreceptors with green light elicited biphasic responses characterized by initial transient hyperpolarization followed by prolonged delayed depolarization. To explore the basis for these findings, we used RNA interference to selectively suppress expression of the genes encoding green opsin (GO1), UV opsin (UVO) or both. The hyperpolarizing component in UV-sensitive photoreceptors was eliminated and the delayed depolarization was reduced after GO1 knockdown, suggesting that the hyperpolarization represents fast inhibitory interactions between green- and UV-sensitive photoreceptors. Green-sensitive photoreceptor responses of GO1 knockdowns to flashes of UV/violet were almost exclusively biphasic, whereas residual responses to green had normal kinetics. Knockdown of UVO reduced the responses of UV-sensitive photoreceptors but had minor effects on delayed depolarization in green-sensitive photoreceptors. Angular sensitivity analysis indicated that delayed depolarization of green-sensitive photoreceptors by violet light originates from excitation of (an)other photoreceptor(s) in the same ommatidium. The angle at which the maximal delayed depolarization was observed in green-sensitive photoreceptors stimulated with violet light did not match the angle of the maximal transient depolarization. In contrast, no significant mismatch was observed for delayed depolarization elicited by green light. These results suggest that the cellular sources of the normal transient and additional delayed depolarization by violet light are separate and distinct.

Behavioral responses to visual overstimulation in the cockroach Periplaneta americana L.

In the visual systems of insects, different types of photoreceptors contribute to specialized visual channels that mediate distinct functions and behaviors. Large compound eyes of Periplaneta americana contain photoreceptors of two spectral classes, broadband green-sensitive photoreceptors and narrow-band UV-sensitive photoreceptors. Here, we investigated how visual stimulation by UV and green light affects locomotor, resting, and grooming behaviors in P. americana under conditions when light avoidance is not possible. We show that green but not UV light stimulates locomotor activity, inducing paradoxical positive masking. Duration of resting and grooming decreased with increasing light intensity, consistent with development of behavioral stress in response to visual overstimulation. A reaction of full immobility is described under UV light and at higher intensities of green light, with relative periods of immobility and grooming strongly negatively correlated. Low-intensity UV was more effective than low-intensity green light in suppressing grooming and inducing immobility. Our results suggest that locomotor activity in P. americana is mainly regulated by green-sensitive photoreceptors, and that dim UV light can trigger behavioral immobility, whereas both wavelengths induce stress-like reactions at high intensities. Considering the intrinsic UV sensitivity of green-sensitive photoreceptors, the contrasting behavioral responses indicate antagonistic interactions between UV and green visual channels.

Photoreceptor spectral sensitivity of the compound eyes of black soldier fly (Hermetia illucens) informing the design of LED-based illumination to enhance indoor reproduction

Mating in the black soldier fly (BSF) is a visually mediated behaviour that under natural conditions occurs in full sunlight. Artificial light conditions promoting mating by BSF were designed based on the spectral characteristics of the compound eye retina. Electrophysiological measurements revealed that BSF ommatidia contained UV-, blue- and green-sensitive photoreceptor cells, allowing trichromatic vision. An illumination system for indoor breeding based on UV, blue and green LEDs was designed and its efficiency was compared with illumination by fluorescent tubes which have been successfully used to sustain a BSF colony for five years. Illumination by LEDs and the fluorescent tubes yielded equal numbers of egg clutches, however, the LED illumination resulted in significantly more larvae. The possibilities to optimize the current LED illumination system to better approximate the skylight illuminant and potentially optimize the larval yield are discussed.

Effect of light intensity on flight control and temporal properties of photoreceptors in bumblebees.

To control flight, insects rely on the pattern of visual motion generated on the retina as they move through the environment. When light levels fall, vision becomes less reliable and flight control thus becomes more challenging. Here, we investigated the effect of light intensity on flight control by filming the trajectories of free-flying bumblebees (Bombus terrestris, Linnaeus 1758) in an experimental tunnel at different light levels. As light levels fell, flight speed decreased and the flight trajectories became more tortuous but the bees were still remarkably good at centring their flight about the tunnel's midline. To investigate whether this robust flight performance can be explained by visual adaptations in the bumblebee retina, we also examined the response speed of the green-sensitive photoreceptors at the same light intensities. We found that the response speed of the photoreceptors significantly decreased as light levels fell. This indicates that bumblebees have both behavioural (reduction in flight speed) and retinal (reduction in response speed of the photoreceptors) adaptations to allow them to fly in dim light. However, the more tortuous flight paths recorded in dim light suggest that these adaptations do not support flight with the same precision during the twilight hours of the day.

Performance of blue- and green-sensitive photoreceptors of the cricket Gryllus bimaculatus

The compound eye of the cricket Gryllus bimaculatus contains a specialized dorsal rim area (DRA) populated by distinct blue-sensitive photoreceptors responsible for perception of polarized light. The rest of the eye is dominated by green-sensitive photoreceptors. Using patch clamp we studied dissociated ommatidia of nocturnal adults and diurnal eight-instar nymphs with the goals (1) of characterizing the biophysical properties of cricket photoreceptors in general and (2) describing the functionally dissimilar blue- and green-sensitive photoreceptors in terms of voltage-gated channel composition and signal coding. Despite different lifestyles, adult and nymph photoreceptors were indistinguishable. No significant circadian changes were observed in K⁺ currents. In contrast, prominent differences were seen between blue- and green-sensitive photoreceptors. The former were characterized by relatively low absolute sensitivity, high input resistance, slow quantum bumps with long latencies, small light-induced and K⁺ currents and low steady-state depolarization. Information rate, a measure of photoreceptor performance calculated from voltage responses to bandwidth-limited white noise-modulated light contrast, was 87 ± 8 bits s⁻¹ in green-sensitive photoreceptors vs. 59 ± 14 bits s⁻¹ in blue-sensitive photoreceptors, implying a limited role of DRA in the perception of visual contrasts. In addition, evidence of electrical coupling between photoreceptors is presented.

Rhabdom evolution in butterflies: insights from the uniquely tiered and heterogeneous ommatidia of the Glacial Apollo butterfly,Parnassius glacialis

The eye of the Glacial Apollo butterfly, Parnassius glacialis, a 'living fossil' species of the family Papilionidae, contains three types of spectrally heterogeneous ommatidia. Electron microscopy reveals that the Apollo rhabdom is tiered. The distal tier is composed exclusively of photoreceptors expressing opsins of ultraviolet or blue-absorbing visual pigments, and the proximal tier consists of photoreceptors expressing opsins of green or red-absorbing visual pigments. This organization is unique because the distal tier of other known butterflies contains two green-sensitive photoreceptors, which probably function in improving spatial and/or motion vision. Interspecific comparison suggests that the Apollo rhabdom retains an ancestral tiered pattern with some modification to enhance its colour vision towards the long-wavelength region of the spectrum.

Differences in Photoreceptor Processing Speed for Chromatic and Achromatic Vision in the Bumblebee,Bombus terrestris

Fast detection of visual change can be mediated by visual processes that ignore chromatic aspects of the visual signal, relying on inputs from a single photoreceptor class (or pooled input from similar classes). There is an established link between photoreceptor processing speed (in achromatic vision) and visual ecology. Highly maneuverable flies, for example, have the fastest know photoreceptors, relying on metabolically expensive membrane conductances to boost performance. Less active species forgo this investment and their photoreceptors are correspondingly slower. However, within a species, additional classes of photoreceptors are required to extract chromatic information, and the question therefore arises as to whether there might be within-species differences in processing speed between photoreceptors involved in chromatic processing compared with those feeding into fast achromatic visual systems. We used intracellular recording to compare light-adapted impulse responses in three spectral classes of photoreceptor in the bumblebee. Green-sensitive photoreceptors, which are known to provide achromatic contrast for motion detection, generated the fastest impulse responses (half-width, Deltat = 7.9 +/- 1.1 ms). Blue- and UV-sensitive photoreceptors (which are involved in color vision) were significantly slower (9.8 +/- 1.2 and 12.3 +/- 1.8 ms, respectively). The faster responses of green photoreceptors are in keeping with their role in fast achromatic vision. However, blue and UV photoreceptors are still relatively fast in comparison with many other insect species, as well as vertebrate cones, suggesting a significant investment in photoreceptor processing for color vision in bees. We discuss this finding in relation to bees' requirement for accurate learning of flower color, especially in conditions of variable luminance contrast.

Switch of rhodopsin expression in terminally differentiated Drosophila sensory neurons

Specificity of sensory neurons requires restricted expression of one sensory receptor gene and the exclusion of all others within a given cell. In the Drosophila retina, functional identity of photoreceptors depends on light-sensitive Rhodopsins (Rhs). The much simpler larval eye (Bolwig organ) is composed of about 12 photoreceptors, eight of which are green-sensitive (Rh6) and four blue-sensitive (Rh5). The larval eye becomes the adult extraretinal 'eyelet' composed of four green-sensitive (Rh6) photoreceptors. Here we show that, during metamorphosis, all Rh6 photoreceptors die, whereas the Rh5 photoreceptors switch fate by turning off Rh5 and then turning on Rh6 expression. This switch occurs without apparent changes in the programme of transcription factors that specify larval photoreceptor subtypes. We also show that the transcription factor Senseless (Sens) mediates the very different cellular behaviours of Rh5 and Rh6 photoreceptors. Sens is restricted to Rh5 photoreceptors and must be excluded from Rh6 photoreceptors to allow them to die at metamorphosis. Finally, we show that Ecdysone receptor (EcR) functions autonomously both for the death of larval Rh6 photoreceptors and for the sensory switch of Rh5 photoreceptors to express Rh6. This fate switch of functioning, terminally differentiated neurons provides a novel, unexpected example of hard-wired sensory plasticity.

Senseless functions as a molecular switch for color photoreceptor differentiation inDrosophila

A major question in development is how different specialized cell types arise from a common progenitor. In the adult Drosophila compound eye, color discrimination is achieved by UV-, blue- and green-sensitive photoreceptors (PRs). These different PR subsets arise from neuronal precursors called R7 and R8 cells. Recent studies have demonstrated that R7-based UV-sensitive PRs require the repression of R8-based blue/green-sensitive PR characteristics to properly develop. This repression is mediated by the transcription factor Prospero (Pros). Here, we report that Senseless (Sens), a Drosophila ortholog of the vertebrate Gfi1 transcription factor, plays an opposing role to Pros by both negatively regulating R7-based features and positively enforcing R8-based features during terminal differentiation. In addition, we demonstrate that Pros and Sens function together with the transcription factor Orthodenticle (Otd) to oppositely regulate R7 and R8 PR Rhodopsin gene expression in vitro. These data show that sens, previously shown to be essential for neuronal specification, also controls differentiation of specific neuronal subtypes in the retina. Interestingly, Pros has recently been shown to function as a tumor suppressor, whereas Gfi1 is a well-characterized oncogene. Thus, we propose that sens/pros antagonism is important for regulating many biological processes.

Visual cycle and its metabolic support in gecko photoreceptors

Photoreceptors of nocturnal geckos are transmuted cones that acquired rod morphological and physiological properties but retained cone-type phototransduction proteins. We have used microspectrophotometry and microfluorometry of solitary isolated green-sensitive photoreceptors of Tokay gecko to study the initial stages of the visual cycle within these cells. These stages are the photolysis of the visual pigment, the reduction of all- trans retinal to all- trans retinol, and the clearance of all- trans retinol from the outer segment (OS) into the interphotoreceptor space. We show that the rates of decay of metaproducts (all- trans retinal release) and retinal-to-retinol reduction are intermediate between those of typical rods and cones. Clearance of retinol from the OS proceeds at a rate that is typical of rods and is greatly accelerated by exposure to interphotoreceptor retinoid-binding protein, IRBP. The rate of retinal release from metaproducts is independent of the position within the OS, while its conversion to retinol is strongly spatially non-uniform, being the fastest at the OS base and slowest at the tip. This spatial gradient of retinol production is abolished by dialysis of saponin-permeabilized OSs with exogenous NADPH or substrates for its production by the hexose monophosphate pathway (NADP + glucose-6-phosphate or 6-phosphogluconate, glucose-6-phosphate alone). Following dialysis by these agents, retinol production is accelerated by several-fold compared to the fastest rates observed in intact cells in standard Ringer solution. We propose that the speed of retinol production is set by the availability of NADPH which in turn depends on ATP supply within the outer segment. We also suggest that principal source of this ATP is from mitochondria located within the ellipsoid region of the inner segment.