Mathias F. Wernet

Stochastic spineless expression creates the retinal mosaic for colour vision

Drosophila colour vision is achieved by R7 and R8 photoreceptor cells present in every ommatidium. The fly retina contains two types of ommatidia, called ‘pale’ and ‘yellow’, defined by different rhodopsin pairs expressed in R7 and R8 cells. Similar to the human cone photoreceptors, these ommatidial subtypes are distributed stochastically in the retina. The choice between pale versus yellow ommatidia is made in R7 cells, which then impose their fate onto R8. Here we report that the Drosophila dioxin receptor Spineless is both necessary and sufficient for the formation of the ommatidial mosaic. A short burst of spineless expression at mid-pupation in a large subset of R7 cells precedes rhodopsin expression. In spineless mutants, all R7 and most R8 cells adopt the pale fate, whereas overexpression of spineless is sufficient to induce the yellow R7 fate. Therefore, this study suggests that the entire retinal mosaic required for colour vision is defined by the stochastic expression of a single transcription factor, Spineless.

A class B scavenger receptor mediates the cellular uptake of carotenoids in Drosophila

Carotenoids are currently being intensely investigated regarding their potential to lower the risk of chronic disease and vitamin A deficiency. Invertebrate models in which vitamin A deficiency is not lethal allow the isolation of blind but viable mutants affected in the pathway leading from dietary carotenoids to vitamin A. Using a mutant in one of these model systems, Drosophila, the vitamin A-forming enzyme has recently been molecularly identified. We now show that the molecular basis for the blindness of a different Drosophila mutant, ninaD, is a defect in the cellular uptake of carotenoids. The ninaD gene encodes a class B scavenger receptor essential for the formation of the visual chromophore. A loss of this function results in a carotenoid-free and thus vitamin A-deficient phenotype. Our investigations provide molecular insight into how carotenoids may be distributed into cells of target tissues in animals and indicate a crucial role of class B scavenger receptors rendering dietary carotenoids available for subsequent cell metabolism, as needed for their various physiological functions.

The Growth Regulators warts/lats and melted Interact in a Bistable Loop to Specify Opposite Fates in Drosophila R8 Photoreceptors

Color vision in Drosophila relies on the comparison between two color-sensitive photoreceptors, R7 and R8. Two types of ommatidia in which R7 and R8 contain different rhodopsins are distributed stochastically in the retina and appear to discriminate short (p-subset) or long wavelengths (y-subset). The choice between p and y fates is made in R7, which then instructs R8 to follow the corresponding fate, thus leading to a tight coupling between rhodopsins expressed in R7 and R8. Here, we show that warts, encoding large tumor suppressor (Lats) and melted encoding a PH-domain protein, play opposite roles in defining the yR 8 or pR8 fates. By interacting antagonistically at the transcriptional level, they form a bistable loop that insures a robust commitment of R8 to a single fate, without allowing ambiguity. This represents an unexpected postmitotic role for genes controlling cell proliferation (warts and its partner hippo and salvador) and cell growth (melted).

Otd/Crx, a Dual Regulator for the Specification of Ommatidia Subtypes in the Drosophila Retina

Comparison between the inputs of photoreceptors with different spectral sensitivities is required for color vision. In Drosophila, this is achieved in each ommatidium by the inner photoreceptors R7 and R8. Two classes of ommatidia are distributed stochastically in the retina: 30% contain UV-Rh3 in R7 and blue-Rh5 in R8, while the remaining 70% contain UV-Rh4 in R7 and green-Rh6 in R8. We show here that the distinction between the rhodopsins expressed in the two classes of ommatidia depends on a series of highly conserved homeodomain binding sites present in the rhodopsin promoters. The homeoprotein Orthodenticle acts through these sites to activate rh3 and rh5 in their specific ommatidial subclass and through the same sites to prevent rh6 expression in outer photoreceptors. Therefore, Otd is a key player in the terminal differentiation of subtypes of photoreceptors by regulating rhodopsin expression, a function reminiscent of the role of one of its mammalian homologs, Crx, in eye development.

Iroquois complex genes induce co-expression of rhodopsins in Drosophila.

The Drosophila eye is a mosaic that results from the stochastic distribution of two ommatidial subtypes. Pale and yellow ommatidia can be distinguished by the expression of distinct rhodopsins and other pigments in their inner photoreceptors (R7 and R8), which are implicated in color vision. The pale subtype contains ultraviolet (UV)-absorbing Rh3 in R7 and blue-absorbing Rh5 in R8. The yellow subtype contains UV-absorbing Rh4 in R7 and green-absorbing Rh6 in R8. The exclusive expression of one rhodopsin per photoreceptor is a widespread phenomenon, although exceptions exist. The mechanisms leading to the exclusive expression or to co-expression of sensory receptors are currently not known. We describe a new class of ommatidia that co-express rh3 and rh4 in R7, but maintain normal exclusion between rh5 and rh6 in R8. These ommatidia, which are localized in the dorsal eye, result from the expansion of rh3 into the yellow-R7 subtype. Genes from the Iroquois Complex (Iro-C) are necessary and sufficient to induce co-expression in yR7. Iro-C genes allow photoreceptors to break the “one receptor–one neuron” rule, leading to a novel subtype of broad-spectrum UV- and green-sensitive ommatidia.

Genetic Dissection Reveals Two Separate Retinal Substrates for Polarization Vision in Drosophila

BACKGROUND Linearly polarized light originates from atmospheric scattering or surface reflections and is perceived by insects, spiders, cephalopods, crustaceans, and some vertebrates. Thus, the neural basis underlying how this fundamental quality of light is detected is of broad interest. Morphologically unique, polarization-sensitive ommatidia exist in the dorsal periphery of many insect retinas, forming the dorsal rim area (DRA). However, much less is known about the retinal substrates of behavioral responses to polarized reflections. SUMMARY Drosophila exhibits polarotactic behavior, spontaneously aligning with the e-vector of linearly polarized light, when stimuli are presented either dorsally or ventrally. By combining behavioral experiments with genetic dissection and ultrastructural analyses, we show that distinct photoreceptors mediate the two behaviors: inner photoreceptors R7+R8 of DRA ommatidia are necessary and sufficient for dorsal polarotaxis, whereas ventral responses are mediated by combinations of outer and inner photoreceptors, both of which manifest previously unknown features that render them polarization sensitive. CONCLUSIONS Drosophila uses separate retinal pathways for the detection of linearly polarized light emanating from the sky or from shiny surfaces. This work establishes a behavioral paradigm that will enable genetic dissection of the circuits underlying polarization vision.

A green fluorescent protein enhancer trap screen in Drosophila photoreceptor cells

The Drosophila ommatidia contain two classes of photoreceptor cells (PRs), the outer and the inner PRs. We performed an enhancer trap screen in order to target genes specifically expressed in PRs. Using the UAS/GAL4 method with enhanced green fluorescent protein (eGFP) as a vital marker, we screened 180000 flies. Out of 2730 lines exhibiting new eGFP patterns, we focused on 16 lines expressing eGFP in particular subsets of PRs. In particular, we describe three lines inserted near the spalt major, m-spondin and furrowed genes, whose respective expression patterns resemble those genes. These genes had not been reported to be expressed in the adult eye. These examples clearly show the ability of our screen to target genes expressed in the adult Drosophila eye.

Proximodistal subdivision of Drosophila legs and wings: the elbow-no ocelli gene complex

Appendages are thought to have arisen during evolution as outgrowths from the body wall of primitive bilateria. In Drosophila, subsets of body wall cells are set aside as appendage precursors through the action of secreted signaling proteins that direct localized expression of transcription factors. The Drosophila homeodomain protein Distal-less is expressed in the leg primordia and required for formation of legs, but not wings. The homeodomain protein Nubbin is expressed in the wing primordia and required for formation of wings, but not legs. Given that insect legs and wings have a common developmental and evolutionary origin, we sought to identify genes that underlie the specification of all appendage primordia. We present evidence that the zinc-finger proteins encoded by the elbow and no ocelli genes act in leg and wing primordia to repress body wall-specifying genes and thereby direct appendage formation.

The evolutionary diversity of insect retinal mosaics: common design principles and emerging molecular logic.

Independent evolution has resulted in a vast diversity of eyes. Despite the lack of a common Bauplan or ancestral structure, similar developmental strategies are used. For instance, different classes of photoreceptor cells (PRs) are distributed stochastically and/or localized in different regions of the retina. Here, we focus on recent progress made towards understanding the molecular principles behind patterning retinal mosaics of insects, one of the most diverse groups of animals adapted to life on land, in the air, under water, or on the water surface. Morphological, physiological, and behavioral studies from many species provide detailed descriptions of the vast variation in retinal design and function. By integrating this knowledge with recent progress in the characterization of insect Rhodopsins as well as insight from the model organism Drosophila melanogaster, we seek to identify the molecular logic behind the adaptation of retinal mosaics to the habitat and way of life of an animal.

Homothorax and Extradenticle alter the transcription factor network in Drosophila ommatidia at the dorsal rim of the retina

A narrow band of ommatidia in the dorsal periphery of the Drosophila retina called the dorsal rim area (DRA) act as detectors for polarized light. The transcription factor Homothorax (Hth) is expressed in DRA inner photoreceptors R7 and R8 and is both necessary and sufficient to induce the DRA fate, including specialized morphology and unique Rhodopsin expression. Hth expression is the result of Wingless (Wg) pathway activity at the eye margins and restriction to the dorsal eye by the selector genes of the Iroquois complex (Iro-C). However, how the DRA is limited to exactly one or two ommatidial rows is not known. Although several factors regulating the Drosophila retinal mosaic are expressed in DRA ommatidia, the role of Hth in this transcriptional network is uncharacterized. Here we show that Hth functions together with its co-factor Extradenticle (Exd) to repress the R8-specific factor Senseless (Sens) in DRA R8 cells, allowing expression of an ultraviolet-sensitive R7 Rhodopsin (Rh3). Furthermore, Hth/Exd act in concert with the transcriptional activators Orthodenticle (Otd) and Spalt (Sal), to activate expression of Rh3 in the DRA. The resulting monochromatic coupling of Rh3 between R7 and R8 in DRA ommatidia is important for comparing celestial e-vector orientation rather than wavelengths. Finally, we show that Hth expression expands to many ommatidial rows in regulatory mutants of optomotorblind (omb), a transcription factor transducing Wg signaling at the dorsal and ventral eye poles. Therefore, locally restricted recruitment of the DRA-specific factor Hth alters the transcriptional network that regulates Rhodopsin expression across ommatidia.