Opsins and cell fate in the Drosophila Bolwig organ: tricky lessons in homology inference

Newly discovered harvestmen relict eyes eyeing for their functions

Most chelicerates operate the world with two kinds of visual organs, the median and lateral eyes of the arthropod ground plan. In harvestmen (Opiliones), however, members of the small and withdrawn suborder Cyphophthalmi lack eyes except for two genera with lateral eyes. In the other suborders (Eupnoi, Dyspnoi, and Laniatores), lateral eyes are absent but median eyes pronounced. To resolve the phylogenetic history of these contrasting trait states and the taxonomic position of a four-eyed harvestmen fossil, visual system development was recently studied in the daddy longleg Phalangium opilio (Eupnoi). This effort uncovered not only a highly regressed and internalized pair of lateral eyes but also a similarly cryptic pair of additional median eyes. After recounting the evo-devo discovery journey of uncompromising harvestmen taxonomists, this review explores comparative evidence that the enigmatic P. opilio relict eyes might serve the multichannel zeitgeber system of the biological clock.

Insect opsins and evo-devo: what have we learned in 25 years?

The visual pigments known as opsins are the primary molecular basis for colour vision in animals. Insects are among the most diverse of animal groups and their visual systems reflect a variety of life histories. The study of insect opsins in the fruit fly Drosophila melanogaster has led to major advances in the fields of neuroscience, development and evolution. In the last 25 years, research in D. melanogaster has improved our understanding of opsin genotype-phenotype relationships while comparative work in other insects has expanded our understanding of the evolution of insect eyes via gene duplication, coexpression and homologue switching. Even so, until recently, technology and sampling have limited our understanding of the fundamental mechanisms that evolution uses to shape the diversity of insect eyes. With the advent of genome editing and in vitro expression assays, the study of insect opsins is poised to reveal new frontiers in evolutionary biology, visual neuroscience, and animal behaviour. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

EyeVolve, a modular PYTHON based model for simulating developmental eye type diversification

Vision is among the oldest and arguably most important sensory modalities for animals to interact with their external environment. Although many different eye types exist within the animal kingdom, mounting evidence indicates that the genetic networks required for visual system formation and function are relatively well conserved between species. This raises the question as to how common developmental programs are modified in functionally different eye types. Here, we approached this issue through EyeVolve, an open-source PYTHON-based model that recapitulates eye development based on developmental principles originally identified in Drosophila melanogaster. Proof-of-principle experiments showed that this program’s animated timeline successfully simulates early eye tissue expansion, neurogenesis, and pigment cell formation, sequentially transitioning from a disorganized pool of progenitor cells to a highly organized lattice of photoreceptor clusters wrapped with support cells. Further, tweaking just five parameters (precursor pool size, founder cell distance and placement from edge, photoreceptor subtype number, and cell death decisions) predicted a multitude of visual system layouts, reminiscent of the varied eye types found in larval and adult arthropods. This suggests that there are universal underlying mechanisms that can explain much of the existing arthropod eye diversity. Thus, EyeVolve sheds light on common principles of eye development and provides a new computational system for generating specific testable predictions about how development gives rise to diverse visual systems from a commonly specified neuroepithelial ground plan.

Ultrastructure of the larval eyes of the hangingfly Terrobittacus implicatus (Mecoptera: Bittacidae)

The fine structure of the larval eyes of the hangingfly Terrobittacus implicatus (Huang & Hua) was investigated using scanning and transmission electron microscopy. The results show that the larval eyes of T. implicatus each consist of seven spaced ommatidia. Each ommatidium is composed of a corneal lens with about 45 lamellae, a tetrapartite eucone type of crystalline cone, eight retinula cells, two primary pigment cells, and an undetermined number of secondary pigment cells. The rhabdomeres of eight retinula cells effectively fuse into a centrally-fused, tiered funnel-shaped rhabdom extending from the base of the crystalline cone deeply into the ommatidium. In light of different positions in the ommatidium, the retinula cells can be divided into four distal and four proximal retinula cells. Pigment cells envelop the entire ommatidium. Electron-lucent vesicles are abundant throughout the cytoplasm of the eight retinula cells. The larval ommatidia of T. implicatus are similar to those of the Panorpidae, except for the distal retinula cells that also participate in the formation of the proximal rhabdom. In this case, the larval eyes of T. implicatus may lie in the transitional stage during the larval eye evolution of insects from ommatidia to stemmata.

Homothorax controls a binary Rhodopsin switch in Drosophila ocelli

Visual perception of the environment is mediated by specialized photoreceptor (PR) neurons of the eye. Each PR expresses photosensitive opsins, which are activated by a particular wavelength of light. In most insects, the visual system comprises a pair of compound eyes that are mainly associated with motion, color or polarized light detection, and a triplet of ocelli that are thought to be critical during flight to detect horizon and movements. It is widely believed that the evolutionary diversification of compound eye and ocelli in insects occurred from an ancestral visual organ around 500 million years ago. Concurrently, opsin genes were also duplicated to provide distinct spectral sensitivities to different PRs of compound eye and ocelli. In the fruit fly Drosophila melanogaster, Rhodopsin1 (Rh1) and Rh2 are closely related opsins that originated from the duplication of a single ancestral gene. However, in the visual organs, Rh2 is uniquely expressed in ocelli whereas Rh1 is uniquely expressed in outer PRs of the compound eye. It is currently unknown how this differential expression of Rh1 and Rh2 in the two visual organs is controlled to provide unique spectral sensitivities to ocelli and compound eyes. Here, we show that Homothorax (Hth) is expressed in ocelli and confers proper rhodopsin expression. We find that Hth controls a binary Rhodopsin switch in ocelli to promote Rh2 expression and repress Rh1 expression. Genetic and molecular analysis of rh1 and rh2 supports that Hth acts through their promoters to regulate Rhodopsin expression in the ocelli. Finally, we also show that when ectopically expressed in the retina, hth is sufficient to induce Rh2 expression only at the outer PRs in a cell autonomous manner. We therefore propose that the diversification of rhodpsins in the ocelli and retinal outer PRs occurred by duplication of an ancestral gene, which is under the control of Homothorax.

Sensory perception of light is mediated by specialized photoreceptor neurons of the eye. Each photoreceptor expresses unique photopigments called opsins and they are sensitive to particular wavelengths of light. In insects, ocelli and compound eyes are the main photosensory organs and they express different opsins. It is believed that opsins were duplicated during evolution to provide specificity to ocelli and the compound eye and this is corelated with their distinct functions. We show that Homothorax acts to control a binary Rhodopsin switch in the fruit fly Drosophila melanogaster to promote Rhodopsin 2 expression and represses Rhodopsin 1 expression in the ocelli. Genetic and molecular analysis showed that Homothorax acts through the promoters of rhosopsin 1 and rhosopsin 2 and controls their expression in the ocelli. We also show that Hth binding sites in the promoter region of rhodopsin 1 and rhodopsin 2 are conserved between different Drosophila species. We therefore proposed that Hth may have acted as a critical determinant during evolution which was required to provide specificity to the ocelli and compound eye by regulating a binary Rhodopsin switch in the ocelli.

A combinatorial cis-regulatory logic restricts color-sensing Rhodopsins to specific photoreceptor subsets in Drosophila

Color vision in Drosophila melanogaster is based on the expression of five different color-sensing Rhodopsin proteins in distinct subtypes of photoreceptor neurons. Promoter regions of less than 300 base pairs are sufficient to reproduce the unique, photoreceptor subtype-specific rhodopsin expression patterns. The underlying cis-regulatory logic remains poorly understood, but it has been proposed that the rhodopsin promoters have a bipartite structure: the distal promoter region directs the highly restricted expression in a specific photoreceptor subtype, while the proximal core promoter region provides general activation in all photoreceptors. Here, we investigate whether the rhodopsin promoters exhibit a strict specialization of their distal (subtype specificity) and proximal (general activation) promoter regions, or if both promoter regions contribute to generating the photoreceptor subtype-specific expression pattern. To distinguish between these two models, we analyze the expression patterns of a set of hybrid promoters that combine the distal promoter region of one rhodopsin with the proximal core promoter region of another rhodopsin. We find that the function of the proximal core promoter regions extends beyond providing general activation: these regions play a previously underappreciated role in generating the non-overlapping expression patterns of the different rhodopsins. Therefore, cis-regulatory motifs in both the distal and the proximal core promoter regions recruit transcription factors that generate the unique rhodopsin patterns in a combinatorial manner. We compare this combinatorial regulatory logic to the regulatory logic of olfactory receptor genes and discuss potential implications for the evolution of rhodopsins.

Each type of sensory receptor neuron in our body expresses a specific sensory receptor protein, which allows us to detect and discriminate a variety of environmental stimuli. The regulatory logic that controls this spatially precise and highly restricted expression of sensory receptor proteins remains poorly understood. As a model system, we study the mechanisms that control the expression of different color-sensing Rhodopsin proteins in distinct subtypes of Drosophila photoreceptors, which is the basis for color vision. Compact promoter regions of less than 300 base pairs are sufficient to reproduce the non-overlapping rhodopsin patterns. However, the regulatory logic that underlies the combination (sometimes called ‘grammar’) of the cis-regulatory motifs (sometimes called ‘vocabulary’) within the rhodopsin promoters remains poorly understood. Here, we find that specific combinations of cis-regulatory motifs in the distal and the proximal core promoter regions of each rhodopsin direct its unique expression pattern.

Dedicated photoreceptor pathways in Drosophila larvae mediate navigation by processing either spatial or temporal cues

To integrate changing environmental cues with high spatial and temporal resolution is critical for animals to orient themselves. Drosophila larvae show an effective motor program to navigate away from light sources. How the larval visual circuit processes light stimuli to control navigational decision remains unknown. The larval visual system is composed of two sensory input channels, Rhodopsin5 (Rh5) and Rhodopsin6 (Rh6) expressing photoreceptors (PRs). We here characterize how spatial and temporal information are used to control navigation. Rh6-PRs are required to perceive temporal changes of light intensity during head casts, while Rh5-PRs are required to control behaviors that allow navigation in response to spatial cues. We characterize how distinct behaviors are modulated and identify parallel acting and converging features of the visual circuit. Functional features of the larval visual circuit highlight the principle of how early in a sensory circuit distinct behaviors may be computed by partly overlapping sensory pathways.

Organization of the Drosophila larval visual circuit

Visual systems transduce, process and transmit light-dependent environmental cues. Computation of visual features depends on photoreceptor neuron types (PR) present, organization of the eye and wiring of the underlying neural circuit. Here, we describe the circuit architecture of the visual system of Drosophila larvae by mapping the synaptic wiring diagram and neurotransmitters. By contacting different targets, the two larval PR-subtypes create two converging pathways potentially underlying the computation of ambient light intensity and temporal light changes already within this first visual processing center. Locally processed visual information then signals via dedicated projection interneurons to higher brain areas including the lateral horn and mushroom body. The stratified structure of the larval optic neuropil (LON) suggests common organizational principles with the adult fly and vertebrate visual systems. The complete synaptic wiring diagram of the LON paves the way to understanding how circuits with reduced numerical complexity control wide ranges of behaviors.

Embryonic development of the larval eyes of the Sunburst Diving Beetle, Thermonectus marmoratus (Insecta: Dytiscidae): a morphological study

Stemmata, the larval eyes of holometabolous insects are extremely diverse, ranging from full compound eyes, to a few ommatidial units as are typical in compound eyes, to sophisticated and functionally specialized image-forming camera-type eyes. Stemmata evolved from a compound eye ommatidial ancestor, an eye type that is morphologically well conserved in regards to cellular composition, and well studied in regards to development. However, despite this evolutionary origin it remains largely unknown how stemmata develop. In addition, it is completely unclear how development is altered to give rise to some of the functionally most complex stemmata, such as those of the sunburst diving beetle, Thermonectus marmoratus. In this study, we used histological methods to investigate the embryonic development of the functionally complex principal stemmata Eye 1 and Eye 2 of the larval visual system of T. marmoratus. To gain insights into how cellular components of their sophisticated camera-type eyes might have evolved from the cellular components of ommatidial ancestors, we contrast our findings against known features of ommatidia development, which are particularly well understood in Drosophila. We find many similarities, such as the early presence of a pseudostratified epithelium, and the order in which specific cell types are recruited. However, in Thermonectus each cell type is represented by a large number of cells from early on and major tissue re-orientation occurs as eye development progresses. This study provides insights into the timing of morphological features and represents the basis for future molecular studies.

Escaping compound eye ancestry: the evolution of single-chamber eyes in holometabolous larvae

Stemmata, the eyes of holometabolous insect larvae, have gained little attention, even though they exhibit remarkably different optical solutions, ranging from compound eyes with upright images, to sophisticated single-chamber eyes with inverted images. Such optical differences raise the question of how major transitions may have occurred. Stemmata evolved from compound eye ancestry, and optical differences are apparent even in some of the simplest systems that share strong cellular homology with adult ommatidia. The transition to sophisticated single-chamber eyes occurred many times independently, and in at least two different ways: through the fusion of many ommatidia [as in the sawfly (Hymenoptera)], and through the expansion of single ommatidia [as in tiger beetles (Coleoptera), antlions (Neuroptera) and dobsonflies (Megaloptera)]. Although ommatidia-like units frequently have multiple photoreceptor layers (tiers), sophisticated image-forming stemmata tend to only have one photoreceptor tier, presumably a consequence of the lens only being able to efficiently focus light on to one photoreceptor layer. An interesting exception is found in some diving beetles [Dytiscidae (Coleoptera)], in which two retinas receive sharp images from a bifocal lens. Taken together, stemmata represent a great model system to study an impressive set of optical solutions that evolved from a relatively simple ancestral organization.

Opsin evolution and expression in arthropod compound eyes and ocelli: insights from the cricket Gryllus bimaculatus

Background

Opsins are key proteins in animal photoreception. Together with a light-sensitive group, the chromophore, they form visual pigments which initiate the visual transduction cascade when photoactivated. The spectral absorption properties of visual pigments are mainly determined by their opsins, and thus opsins are crucial for understanding the adaptations of animal eyes. Studies on the phylogeny and expression pattern of opsins have received considerable attention, but our knowledge about insect visual opsins is still limited. Up to now, researchers have focused on holometabolous insects, while general conclusions require sampling from a broader range of taxa. We have therefore investigated visual opsins in the ocelli and compound eyes of the two-spotted cricket Gryllus bimaculatus, a hemimetabolous insect.

Results

Phylogenetic analyses place all identified cricket sequences within the three main visual opsin clades of insects. We assign three of these opsins to visual pigments found in the compound eyes with peak absorbances in the green (515 nm), blue (445 nm) and UV (332 nm) spectral range. Their expression pattern divides the retina into distinct regions: (1) the polarization-sensitive dorsal rim area with blue- and UV-opsin, (2) a newly-discovered ventral band of ommatidia with blue- and green-opsin and (3) the remainder of the compound eye with UV- and green-opsin. In addition, we provide evidence for two ocellar photopigments with peak absorbances in the green (511 nm) and UV (350 nm) spectral range, and with opsins that differ from those expressed in the compound eyes.

Conclusions

Our data show that cricket eyes are spectrally more specialized than has previously been assumed, suggesting that similar adaptations in other insect species might have been overlooked. The arrangement of spectral receptor types within some ommatidia of the cricket compound eyes differs from the generally accepted pattern found in holometabolous insect taxa and awaits a functional explanation. From the opsin phylogeny, we conclude that gene duplications, which permitted differential opsin expression in insect ocelli and compound eyes, occurred independently in several insect lineages and are recent compared to the origin of the eyes themselves.

Drosophila TRP channels and animal behavior

Multiple classes of cell surface receptors and ion channels participate in the detection of changes in environmental stimuli, and thereby influence animal behavior. Among the many classes of ion channels, Transient Receptor Potential (TRP) cation channels are notable in contributing to virtually every sensory modality, and in controlling a daunting array of behaviors. TRP channels appear to be conserved in all metazoan organisms including worms, insects and humans. Flies encode 13 TRPs, most of which are expressed and function in sensory neurons, and impact behaviors ranging from phototaxis to thermotaxis, gravitaxis, the avoidance of noxious tastants and smells and proprioception. Multiple diseases result from defects in TRPs, and flies provide an excellent animal model for dissecting the mechanisms underlying "TRPopathies." Drosophila TRPs also function in the sensation of botanically derived insect repellents, and related TRPs in insect pests are potential targets for the development of improved repellents to combat insect-borne diseases.

Drosophila as a developmental paradigm of regressive brain evolution: proof of principle in the visual system

Evolutionary developmental biology focuses heavily on the constructive evolution of body plan components, but there are many instances such as parasitism, cave adaptation, or postembryonic growth rate optimization where evolutionary regression is of adaptive value. This is particularly true in the nervous system because of its massive energy costs. However, comparatively little effort has thus far been made to understand the evolutionary developmental trajectories of adaptive nervous system reduction. This review focuses on the organization and evolution of the Drosophila larval brain, which represents an exceptional example of miniaturization, most dramatically in the visual system. It is specifically discussed how the dependency of outer optic lobe development on retinal innervation can be assumed to have facilitated a first evolutionary phase of larval visual system reduction. Afferent input-contingent development of neu- ral compartments very likely plays a widespread role in adaptive brain evolution. Understanding the complete deconstruction of the larval optic neuropiles in Drosophila awaits expanded comparative analysis but has the promise to inform about further developmental trajectories and mechanisms underlying regressive evolution of the brain.

See the light: electrophysiological characterization of the Bolwig organ's light response of Calliphora vicina 3rd instar larvae

The anatomy and development of the larval cyclorraphous Diptera visual system is well established. It consists of the internal Bolwig organ (BO), and the associated nerve connecting it to the brain. The BO contributes to various larval behaviors but was never electrophysiologically characterized. We recorded extracellulary from the Bolwig nerve of 3rd instar Calliphora vicina larvae to quantify the sensory response caused by BO stimulation with light stimuli of different wavelengths, intensities and directions. Consistent with previous behavioral experiments we found the BO most sensitive to white and green, followed by blue, yellow, violet and red light. The BO showed a phasic-tonic response curve. Increasing light intensity produced a sigmoid response curve with an approximate threshold of 0.0105 nW/cm(2) and a dynamic range from 0.105 nW/cm(2) to 52.5 nW/cm(2). No differences exist between feeding and wandering larvae which display opposed phototaxis. This excludes reduced BO sensitivity from causing the switch in behavior. Correlating to the morphology of the BO frontal light evoked the maximal reaction, while lateral light reduced the neural response asymmetrically: Light applied ipsilaterally to the recorded BO always produced a stronger response than when applied from the contralateral side. This implies that phototacic behavior is based on a tropotactic mechanism.

Spatial distribution of opsin-encoding mRNAs in the tiered larval retinas of the sunburst diving beetle Thermonectus marmoratus (Coleoptera: Dytiscidae)

Larvae of the sunburst diving beetle, Thermonectus marmoratus, have a cluster of six stemmata (E1-6) and one eye patch on each side of the head. Each eye has two retinas: a distal retina that is closer to the lens, and a proximal retina that lies directly underneath. The distal retinas of E1 and E2 are made of a dorsal and a ventral stack of at least twelve photoreceptor layers. Could this arrangement be used to compensate for lens chromatic aberration, with shorter wavelengths detected by the distal layers and longer wavelengths by the proximal layers? To answer this question we molecularly identified opsins and their expression patterns in these eyes. We found three opsin-encoding genes. The distal retinas of all six eyes express long-wavelength opsin (TmLW) mRNA, whereas the proximal retinas express ultraviolet opsin (TmUV I) mRNA. In the proximal retinas of E1 and E2, the TmUV I mRNA is expressed only in the dorsal stack. A second ultraviolet opsin mRNA (TmUV II), is expressed in the proximal retinas of E1 and E2 (both stacks). The finding that longer-wavelength opsins are expressed distally to shorter-wavelength opsins makes it unlikely that this retinal arrangement is used to compensate for lens chromatic aberration. In addition, we also described opsin expression patterns in the medial retina of E1 and in the non-tiered retina of the lensless eye patch. To our knowledge, this is also the first report of multiple UV opsins being expressed in the same stemma.