What are ommatidia and how do they contribute to how well an insect can see

Ommatidia

The layout of the ommatidia is such that when a bee is facing directly abroad from the dominicus, each ommatidium is looking at the region of the heaven that contains the angle of polarized calorie-free to which it is virtually sensitive.

From: Encyclopedia of Insects (2d Edition) , 2009

Gene Regulatory Networks

Yen-Chung Chen , Claude Desplan , in Current Topics in Developmental Biological science, 2020

iii.4 The GRN that determines photoreceptor terminal features: Ommatidial subtypes

Ommatidia contain two types of photoreceptors. The outer photoreceptors R1–R6 are involved in motility detection and dim lite vision. They limited the broad-spectrum Rhodopsin Rh1. The inner photoreceptors R7 and R8 are involved in colour discrimination ( Yamaguchi, Desplan, & Heisenberg, 2010; Yamaguchi, Wolf, Desplan, & Heisenberg, 2008). Ommatidia can exist separated into subtypes based on the Rhodopsins expressed in their inner photoreceptors: pale and yellow ommatidia are distributed stochastically in the retina. In xanthous ommatidia (65%), R7 expresses UV-sensitive Rh4, and R8 expresses green-sensitive Rh6; in stake ommatidia, R7 expresses UV-sensitive Rh3, and R8 expresses blue-sensitive Rh5. In the dorsal third of the retina, R7s in yellow ommatidia co-express the UV Rhodopsins Rh3 and Rh4, perhaps for improved UV detection for solar orientation, while R7s in pale ommatidia express normally Rh3 lonely (Hardie, 1985; Mazzoni et al., 2008). In a unmarried row of ommatidia at the dorsal rim surface area, both R7 and R8 limited Rh3 and are specialized in detecting the vector of polarized UV light for navigation (Weir & Dickinson, 2012; Wernet et al., 2012, 2003). In outer photoreceptors, Sine oculis activates the expression of the transcription factor Glass (Gl) that links retina patterning to the morphogenesis of the rhabdomere and the expression of Rhodopsins (Bernardo-Garcia, Fritsch, & Sprecher, 2016; Moses, Ellis, & Rubin, 1989). Gl directly activates the expression of Rh1, forth with Orthodenticle (Otd) and Pph13 (Ellis, O'Neill, & Rubin, 1993; Mishra et al., 2010). Otd activates Defective proventriculus (Dve) at a high level that suppresses inner photoreceptor Rhodopsins. In contrast, Spalt (Sal) is expressed in inner photoreceptors and suppresses the default expression of Rh1 and Dve, thus allowing different combinations of Rhodopsins to be expressed in yellow and stake ommatidia (Cook, Pichaud, Sonneville, Papatsenko, & Desplan, 2003; Mollereau et al., 2001) (Fig. 1C). The ratio of xanthous and pale ommatidia is controlled past the stochastic activation of the transcription cistron Spineless (Ss) that specifies xanthous R7s (Johnston et al., 2011; Thanawala et al., 2013; Yan et al., 2017). In yellow R7s, Ss forms a heterodimer with Tango (Tgo) to directly actuate Rh4 and to activate Dve at a low level that suppresses Rh3 (Johnston et al., 2011; Thanawala et al., 2013; Yan et al., 2017). In pale R7s lacking Ss, Otd and Sal activate the expression of Rh3. In the yellow R7s of the dorsal third, the expression of Iro-C genes overcomes the suppression of Rh3 by Dve while nonetheless maintaining expression of Ss at a lower level (Mazzoni et al., 2008; Thanawala et al., 2013), and as a outcome, dorsal third yellowish R7s express both Rh3 and Rh4. Finally, in the dorsal rim expanse (DRA), Wingless signaling and Iro-C activate Homothorax (Hth) to suppress Ss, and consequently Rh4. As a upshot, DRA R7s only express Rh3 (Wernet et al., 2006, 2003). Interestingly, R8s in the DRA also express Hth and Rh3 (meet below and Fig. oneD).

The expression of Rhodopsins in R8 is coordinated with that of its partner R7. In a sevenless mutant that lacks R7s, about R8s limited Rh6 and resemble yellow R8s, suggesting that pale R7 provides a indicate that allows for the expression of Rh5 in R8s (Chou et al., 1999; Papatsenko, Sheng, & Desplan, 1997). Pale R7s activate a bistable loop in R8s consisting of two genes cross-regulating each other, Warts (Wts), a tumor suppressor kinase, and Melted (Melt), a growth regulator. Sens, a critical transcription cistron for specifying R8, promotes past default the expression of Wts, which represses Melted and activates Rh6. In pR8, Activin and BMP signals from pale R7 actuate Melt, which suppresses Wts and allows Otd and Yki to actuate Rh5 (Jukam et al., 2013; Jukam & Desplan, 2011; Wells, Pistillo, Barnhart, & Desplan, 2017). In the dorsal third yellow ommatidia that express both Rh3 and Rh4 in R7, R8s still limited Rh6, suggesting that the Activin betoken from R7 is suppressed by Ss, independently from the presence of Rh3. In the dorsal rim surface area, Wingless signaling and the dorsal Iro-C genes cooperate to induce the expression of Hth non simply in R7 just also in R8. In these R8s, Hth suppresses Sens and consequently Rh6 while Hth, Extradenticle, Otd, and Sal human action cooperatively to activate Rh3 that is therefore expressed in both R7 and R8. By expressing Rh3 in both R7 and R8, DRA ommatidia are non involved in colour vision only instead compare the angle of light polarization (Wernet et al., 2003; Wernet & Desplan, 2014) (Fig. iD).

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The Colorful Visual World of Butterflies

F.D. Frentiu , in Encyclopedia of the Center, 2010

Spectral Heterogeneity of Butterfly Eyes

Although ommatidia are anatomically identical in structure, they are spectrally very heterogeneous: that is, they concord dissimilar complements of photoreceptors that limited different visual pigments; dissimilar photoreceptors inside an ommatidium are designated as R1-R9 and may limited different pigments. Using molecular genetic techniques such as in situ hybridization to visualize patterns of opsin mRNA has immune us to map the types of ommatidia present in the butterfly eye. Three types of ommatidia be in the principal retinas of butterflies, with all types of ommatidia expressing LW visual pigments but differing in the expression of the UV and B visual pigments.

The extent of spectral heterogeneity of butterfly eyes differs among the major butterfly families. For example, the eyes of Nymphalidae species are quite simple in terms of ommatidial heterogeneity. Studies of   Nymphalidae species to date evidence that they take simply three types of ommatidia in the principal retina and one type in the dorsal rim area (DRA) of the center, which is an eye region specialized for the detection of polarized skylight. All ommatidial types in the main retina express long wavelength visual pigments in their R3-R8 photoreceptor cells, just the expression of the UV and B opsins in the R1 and R2 cells is variable (the R9 prison cell expresses the LW opsin in the main retina and may limited the UV opsin in the DRA). One ommatidial type contains one UV and one blue receptor, the second has 2 blue receptors, and the 3rd has ii UV receptors. In the DRA ommatidia, the R1-R8 cells limited the UV opsin. This blazon of heart employs a straightforward, one-to-one relationship between the type of visual pigment expressed and spectral phenotype of the photoreceptor jail cell. It may also best represent the ancestral butterfly eye and information technology resembles the eyes of bees and moths.

By dissimilarity, the eyes of other families of butterflies are much more diverse in their spectral complements. For example, the butterfly Pieris rapae (family Pieridae) expresses four opsins only photoreceptors with seven dissimilar meridian sensitivities have been identified, due to the presence of filtering pigments. Peradventure the most spectrally diverse butterfly eye studied so far belongs to the papilionid butterfly, the Japanese swallowtail, Papilio xuthus. Papilio xuthus expresses five different opsins in the eye, has eight different types of photoreceptors, and employs tetrachromatic color vision. In these animals, both opsin gene duplications and pigments acting every bit spectral filters have led to spectral diversification of visual systems. Below, I consider the evolutionary mechanisms that have led to butterfly visual system diverseness.

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Essays on Developmental Biology, Part A

Nathalie Nériec , Claude Desplan , in Current Topics in Developmental Biology, 2016

2.2.one Lamina

Photoreceptors from each ommatidium involved in motion vision (outer photoreceptors) first innervate the lamina neuropil, which manifests a columnar organization in which each pixel of the visual field corresponds to one cartridge (Meinertzhagen & Sorra, 2001). The lamina is more often than not composed of interneurons, whose projections do not leave the optic lobe with their cell bodies located in the lamina cortex region. Lamina neurons can be divided in 2 populations: Five types of monopolar neurons that contact a unmarried cartridge and project retinotopically into the medulla, and amacrine cells that contact several cartridges within the lamina (Fischbach & Dittrich, 1989; Hofbauer & Campos-Ortega, 1990; Tuthill, Nern, Holtz, Rubin, & Reiser, 2013).

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Evolution

J.West. Truman , in Comprehensive Molecular Insect Scientific discipline, 2005

2.4.five.2 Growth of the Eyes

In many nymphs new ommatidia are added to the eye at each molt. In S. gregaria, for example, the number of ommatidia increases from about 2500 in the newly hatched nymph to over 9000 in the adult. The new ommatidia arise from a region along the inductive margin of the eye, which is divided into proliferation and differentiation zones (Anderson, 1978). The proliferation zone shows a basal level of cell division that probably produces photoreceptor neurons at a constant rate, since outgrowing axons from new photoreceptors are evident at all times during the intermolt and molting periods. At the starting time of the molt there is an increase in mitoses in the proliferation zone and besides a spike of proliferation in the differentiation zone, where the ommatidia are beingness assembled. The divisions in the differentiation zone, likely produce support cells, such as those that make the crystalline cone and the screening pigments. Except for responses to circulating factors, the growth of the eye is nether democratic control, every bit illustrated by normal patterns of proliferation being observed in eyes transplanted to the surface of the prothorax (Anderson, 1978).

Unlike nymphs, the eyes of larvae consist of only a pocket-size prepare of separated facets, the stemmata. These likely arose from the most posterior ready of ommatidia in the eyes of the bequeathed larva. The stemmata, though, add neither facets nor more photoreceptors equally the larva progresses through its larval instars. The compound eye of the adult appears but at metamorphosis and forms immediately anterior to the stemmata.

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Invertebrate and Vertebrate Eye Development

Marker Charlton-Perkins , Tiffany A. Cook , in Current Topics in Developmental Biology, 2010

ii.2.4.4 Ommatidial subtype specification: Hth, IroC, Otd, Ss, Melt, and Wts

4 singled-out subtypes of ommatidia, called pale (p), yellow (y), dorsal yellow (dy), and dorsal rim area (DRA) ommatidia, are present in the developed eye and are defined based on which rhodopsins are expressed the R7 and R8 cells ( Fig. five.viB ). DRA ommatidia are the to the lowest degree arable of the subtypes, and these are restricted to one to two rows of ommatidia along the dorsal one-half of the eye. Unlike the other 3 subtypes, DRA ommatidia are not involved in color discrimination, but instead are involved in discerning the vector of polarized lite to aid during navigation (Hardie, 1985; Labhart and Meyer, 1999; Wernet et al., 2003; Wunderer and Smola, 1982). This is facilitated past the fact that both IPRs express the same UV Rhodopsin, Rhodopsin three, and because the membranes of the 2 IPR rhabdomeres form two crossed-over polarizing filters (Labhart and Meyer, 1999; Wernet et al., 2003; Wunderer and Smola, 1982).The TALE homeoprotein, Homothorax (Hth), is necessary and sufficient to induce all known DRA characteristics (Wernet et al., 2003); however, the responsible mechanisms and the target genes utilized past Hth to accomplish this function remains unexplored.

Figure 5.6. Ommatidial subtypes express unlike inner photoreceptor Rhodopsins. (A) A whole-mount staining of an adult retina stained with phalloidin (grayness) shows the trapezoidal organisation of the actin-rich rhabdomeres of the half-dozen outer photoreceptors and the random distribution of the pale and yellow ommatidia are revealed by immunostaining for Rh5 (blue) and Rh6 (greenish) that are expressed in the central R8 cells. (B) Diagram of the Dorsal Rim Expanse (DRA), dorsal xanthous, pale, and yellow subsets found in the Drosophila centre, defined by the Rhodopsins expressed in the R7 and R8 inner photoreceptors. All outer photoreceptors limited the same Rhodopin, Rhodopsin one. (C) Transverse sections of developed eyes, dorsal left, stained with R7 Rhodopsins (left), Rh3 (cyan) and Rh4 (cherry), or R8 Rhodopsins (right), Rh5 (blue) and Rh6 (dark-green). Annotation that two rows of ommatidia at the dorsal side of the center express Rh3 in the R7 and R8 layers, representing the DRA ommatidia. Rh3 and Rh4 expression in the dy ommatidia are weaker than in the residue of the eye. (D) Schematic representing the factors that straight inner photoreceptor identity, differentiation, and rhodopsin expression. The relative position of the nuclei that would be in the cell body for the dissimilar cell types are also indicated. Come across text for detail.

Distinction of p and y ommatidia was originally observed past the presence of the random distribution of a screening paint in ~   70% of ommatidia that appeared yellow under white light illumination versus the pale appearance in the remaining xxx% of ommatidia (Kirschfeld et al., 1978). Later molecular analysis of Rh gene expression in Drosophila noted that the 30:70 ratio corresponded to the ratio of R7 cells expressing Rh3 and Rh4, (Fortini and Rubin, 1990) and R8 cells expressing Rh5 and Rh6, respectively (Chou et al., 1996; Papatsenko et al., 1997). Indeed, ~   30% of ommatidia ("pale" ommatidia) express coupled Rh3:Rh5 expression in the R7 and R8, respectively, while the remaining ~   70% of ommatidia ("yellow" ommatidia) express coupled Rh4:Rh6 in the R7 and R8 together with an additional screening pigment that gives the yellowish color under white illumination (Chou et al., 1996, 1999; Mazzoni et al., 2008; Papatsenko et al., 1997; Stark and Thomas, 2004; Fig. 5.6A–C). Interestingly, Mazzoni et al. (2008) recently noted that a subset of "yellow" ommatidia that are restricted to the dorsal third of the eye coexpress Rh3 and Rh4 in the R7, merely notwithstanding express Rh6 in the underlying R8. Thus, these dorsal-restricted ommatidia are referred to equally dorsal yellowish (dy) ommatidia. These are a especially curious subset of ommatidia, as they do not attach to the normal "one sensory receptor per sensory cell" paradigm commonly adopted in sensory systems to avoid overlapping signals (Mazzoni et al., 2004), and are non distributed throughout the eye, but instead are regionally localized. Molecularly, the Iroquois complex of transcription factors (Iro-C) specify the dy ommatidia, consistent with the fact that Iro-C factors are repeatedly used during other dorsal–ventral patterning events in the fly heart (Cavodeassi et al., 2000; Mazzoni et al., 2008; Singh and Choi, 2003). Functionally, these ommatidia are probable to recognize a broader spectrum of wavelengths in the UV (Feiler et al., 1992), and are positioned to a region of the eye that is well-nigh usually found facing the sky. Behaviorally, how the fly takes advantage of this subtype, still awaits exploration although it has been proposed that it serves to detect the solar orientation. Yamaguchi et al. (2010) recently established a useful method for testing the contribution of different IPRs to wavelength discrimination in Drosophila, which could be practical to address this exciting question in the near future.

Over the past few years, several factors have been identified that are necessary for creating the Drosophila retinal mosaic (summarized in Fig. 5.half-dozenD). These studies indicate that the p versus y decision is first fabricated in R7 cells, and requires the stochastic activation of the transcription factor Spineless in yR7s (Ss; Wernet et al., 2006). Spineless is necessary and sufficient to activate Rh4 if expressed in IPRs or OPRs, and Ss-negative R7s (pR7s) express Rh3 past default (Wernet et al., 2006). However, mutation of a potential binding site for Ss in the Rh4 promoter does non affect reporter expression in vivo, and Ss is not able to regulate Rh4 promoter activity in vitro (T. Melt, unpublished results), indicating that Ss is likely to activate some other factor to directly command Rh4 expression. Once the p versus y conclusion in R7s is fabricated, pR7s sends an inductive point to the underlying R8 (pR8s) to activate Rh5. In the absenteeism of this signal, such equally in eyes defective all R7 cells, R8 cells limited Rh6 past default (Chou et al., 1999). Therefore, although the pale fate in R7s is the default conclusion, the default determination in R8 cells is the yellow fate. Currently, the "pale" signaling molecule in pR7s remains unknown, but what is articulate is that the activation of both stake opsins, Rh3 and Rh5, is directly controlled through K50 homeodomain binding sites by the transcription cistron Otd (Fig. 5.five; Tahayato et al., 2003). Since Otd is expressed in all PRs, this suggests that a pale-specific coactivator is disquisitional for this function.

Although the R7-dependent stake signaling pathway is not known, some of the signaling molecules that are required for mediating Rh5 versus Rh6 expression in the receiving R8 cell accept been identified. These include the membrane-associated pleckstrin homology-containing poly peptide Melted and the serine/threonine cytoplasmic kinase Warts (Wts, a.k.a. Lats; Mikeladze-Dvali et al., 2005). Melt expression is necessary and sufficient to induce Rh5 expression and to repress Wts expression in pR8s, whereas Wts is necessary and sufficient to induce Rh6 expression and repress Melt expression in yR8s. The bistable repression loop between Melt and Wts thereby ensures the mutually exclusive expression of Rh5 and Rh6 in different R8 subtypes. Consequent with Rh6 being the default R8 opsin, however, Wts appears to mediate the last output of the loop, while Melt is primarily involved in preventing Wts expression in pR8s. Since neither Melt nor Wts are Dna-binding factors, electric current work is focused on identifying the transcriptional mediators of the Melt/Wts pathway. This is a particularly interesting question, because Melt and Wts are nearly recognized for their roles in two contained growth regulatory pathways—the TOR and Hippo pathways, respectively (Harvey and Tapon, 2007; Hergovich and Hemmings, 2009; Reis and Hariharan, 2005; Teleman et al., 2005; Yin and Pan, 2007). Thus, further clarification of the part of these proteins in fly PR specification may take far-reaching implications in other fields of biology. Other questions that remain unanswered relate to how the initial stochastic conclusion for p versus y prison cell fate is fabricated in the R7 layer, and what signaling pathway transmits this conclusion to the underlying R8 cell.

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THE Compound Middle

Adrian Horridge , David Blest , in Insect Biology in the Future, 1980

I. HOW RHABDOM Cross-Section AFFECTS Centre Functioning

The anatomy of the ommatidium of a representative insect, the locust, is shown in Fig. 1. Effectively, the tip of the rhabdom catches calorie-free that is focussed upon it by the lens. Therefore information technology acts as does a silvery halide grain in a photographic film (Fig. 2b). The athwart field size and sensitivity of the grain (and also of the tip of the rhabdom) can be calculated from the dimensions of the optical system. In item, the field is the angle subtended by the rhabdom tip in the outside globe. This is the bending projected through the posterior nodal indicate of the lens (Δσ, in Fig. 2a), increased by a small angle because the lens contributes some diffraction (Fig. 2b).

FIGURE 1. Summary of the critical region of the locust ommatidium. ax axons; bm basement membrane, c convex corneal surface, cc crystalline cone; exc extension of the cone, ppc master paint cell, ppcp processes of ditto: ppcn nuclei of ditto: rcn retinula cell nuclei: rh rhabdom. The cells are numbered according to Wilson & al. 1978.

FIGURE ii. Summary of the interaction between the size of the rhabdom tip and the aperture of the lens in determining the field size. a. The rhabdom subtends an angle d/f at the nodal point. b. A distant signal source is focused to an Blusterous disc of width Δα = λ/D subtended at the nodal point. c. Equally the point source moves through an angle ɛ the Blusterous disc moves across the rhabdom tip. d. When the rhabdom is wider, the field is correspondingly wider than in (b).

The field of the rhabdom tip, measured in the outside earth, is divers as the relation (in angular co-ordinates) of the sensitivity of the rhabdom to a indicate source of constant intensity plotted every bit a part of the direction of the point source with respect to the optical axis. Sensitivity, in this context, is divers as the reciprocal of the intensity computed to exist required to give a chosen, constant amplitude of response. Thus, sensitivity to a betoken source at each angle, ϕ, is the reciprocal of the adulterate intensity caused by moving the indicate source off-axis. Therefore, the field tin be depicted graphically in terms of the attenuation of light equally a function of angle off-axis, and described every bit a surface with a height which resembles a Gaussian function.

The cross section of the rhabdom which captures light is also taken as a Gaussian function, of total angular width Δσ = d/f radians at the fifty% level of constructive absorption subtended at the posterior nodal bespeak. A point source of calorie-free of the kind used for measuring the fields produces on the tip of the rhabdom a patch of low-cal which is non of uniform intensity (the Airy disc). The distribution of intensity across the Airy disc closely resembles a Gaussian distribution with an angular width of Δσ radians at the 50% level of intensity subtended at the nodal point (Fig. 2b). The field of width Δρ radians is the convolution of these two Gaussian functions and is given past the approximation

(ane) $\Delta {\rho }^2=\Delta {\alpha }^2+\Delta {\sigma }^{\mathrm{ii}}\mathrm{.....................................}$

where the angle Δρ can be measured outside the eye considering the other angles are subtended at the nodal point.

The width of the field of the silvery halide grain or rhabdom is therefore e'er a little larger than its geometrical angle subtended through the posterior nodal point. If diffraction is discounted the most important parameter of the centre, the width of the field, is proportional to the rhabdom diameter.

The sensitivity (S) of the grain or rhabdom to extended sources which fill the field is proportional to the integral of all the low-cal in the field multiplied by the surface area of the lens which gathers the low-cal. As the integral inside a Gaussian field is proportional to the square of the field width Δρ, we take:-

(ii) $\text{S}=\text{K}+\Delta {\rho }^2{\text{D}}^{\mathrm{two}}\mathrm{.....................................}$

where D is the aperture of the lens. When the diffraction component is negligible compared to the size of the rhabdom or grain, this reduces to the well-known formula for a photographic camera,

(3) $\text{South}=\text{K}{\text{d}}^2{\text{D}}^{\mathrm{two}}/{\text{f}}^2=\text{K}{\text{d}}^2/{\text{F}}^{\mathrm{two}}\mathrm{.....................................}$

where d is the diameter of the halide grain or rhabdom, f is the focal length (distance from posterior nodal point to focal plane), and F is the focal ratio (F/D). Thus, the capture cross-sectional area of the rhabdom d^two exerts its full effect on the sensitivity to extended sources.

Field width and sensitivity to lengthened sources are two of the bones parameters of the eye. Considering both depend on rhabdom diameter, its cyclical changes necessarily affect vision, as is axiomatic if Figs. 2c and 2d are compared. Night vision is accompanied by a new set of functional relationships, generated in part by changes in rhabdom diameter. The field width volition increase roughly in proportion to the increase in diameter and sensitivity volition increase in proportion to the surface area of the rhabdom tip. The eye becomes better at seeing in dim low-cal, but loses resolution because the large rhabdom tip smooths out the highest spatial frequencies that are passed during the day.

These conclusions are implicit in the more detailed handling given by Horridge (1978), where information technology is stressed that in existent compound eyes there must always be a compromise betwixt sensitivity and resolution, the signal of residuum being determined by natural selection as a lifestyle evolves. In fact, other mechanisms are involved in the modulation of sensitivity of which movement of screening paint is an instance. Compounded with rhabdom growth and diminition, they hateful that the analysis of the eye mechanisms of even a single species is both difficult and enervating; we are a long fashion from achieving a satisfactory natural history of the compound eye. Let us consider, briefly, some of the problems which accept barely been touched upon, and to which nosotros do not have satisfactory answers.

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Invertebrate and Vertebrate Eye Development

Andreas Jenny , in Current Topics in Developmental Biology, 2010

3.2.3 Ommatidial rotation

As described to a higher place, dorsal and ventral ommatidia rotate 90° clockwise and counterclockwise, respectively ( Figs. 7.one and vii.2). The initial rotation of 45° is relatively fast and occurs over ommatidial rows four–seven/ix, while a 2d, slower phase is completed around rows fifteen–18 (Gaengel and Mlodzik, 2003; Wolff and Ready, 1991). During rotation, groups of adhering photoreceptors move betwixt stationary, interommatidial cells (IOCs; Fiehler and Wolff, 2007). To date, it is unknown whether this specialized type of prison cell migration is dependent on protrusive activities of cells such as lamellipodia and/or filopodia (equally during convergent extension; Wallingford et al., 2002; Yin et al., 2009), or whether rotation is mainly achieved via restructuring of apical junctions (as in germband extension during Drosophila embryogenesis; Bertet et al., 2004; Blankenship et al., 2006). Unfortunately, no in vivo imaging of the rotation process has been reported to date, mainly considering of the difficulties in culturing eye disks. It is, withal, expected that rotation is controlled by cytoskeletal proteins, cell adhesion, and the extracellular matrix. In addition at that place are mechanisms regulating the start and stop of the procedure. Indeed, genes affecting rotation have been described for each of these classes and include nemo, argos, drok, cadherins, and laminin A, but their interplay with the core genes and each other is non well understood (Choi and Benzer, 1994; Henchcliffe et al., 1993; Mirkovic and Mlodzik, 2006; Winter et al., 2001).

Mutations in nemo (nmo), a distant MAPK relative and Wnt antagonist, abort rotation at about 45° and nmo regulates the speed of rotation (note it is unclear whether available alleles are null mutations; Choi and Benzer, 1994; Fiehler and Wolff, 2008). Non much is known about the mechanism of action of Nmo during PCP signaling, simply it could potentially phosphorylate members of the core PCP proteins or components of the cytoskeletal or adhesion machineries to regulate their activities. Mutations in argos (aos), a diffusible inhibitor of EGFR signaling initially identified as relevant for PCP signaling due to the phenotype of the aos ^roulette (aos ^rlt ) allele, pb to strong over- and under-rotation of ommatidia (Dark-brown and Freeman, 2003; Choi and Benzer, 1994; Gaengel and Mlodzik, 2003; Strutt and Strutt, 2003). Initially, since nmo/aos double mutants abort rotation at 45°, information technology was causeless that Aos controlled the 2nd phase of rotation. More recently, information technology appears that precise control of EGFR signaling is crucial for the correct extent of rotation. Not only the inhibitor Aos but also weak alleles of egfr (a.grand.a. torpedo, top) show rotation defects. Multiple explanations, which are non necessarily mutually exclusive, were proposed to explain the observed phenotypes. First, a higher frequency of mystery cells, an R-like cell usually only briefly associated with ommatidial preclusters, was observed. These mystery cells besides remain function of the photoreceptor precluster for longer than normal and affect Fz and Fmi subcellular localization and thus might perturb Fz–PCP activity (Strutt and Strutt, 2003). In improver, a genetic interaction between EGFR signaling components and E-cadherin was observed, suggesting a regulation of cell adhesion by EGF signaling (Brown and Freeman, 2003).

The EGF signal is ordinarily mediated via the modest GTPase Ras that activates unlike downstream branches such as Raf/Rolled MAPK, PI3 Kinase, Rgl, or Canoe (Cno)/AF-half-dozen (Prober and Edgar, 2002). Using Ras effector loop mutants that are able to activate just subsets of the different Ras branches, it was shown that EGFR can touch rotation non simply via the Raf/Rolled MAPK cascade but also via Canoe/AF-half-dozen and potentially Rgl/Ral or PI3 Kinase (Gaengel and Mlodzik, 2003). Indeed, mutations in the adherens- and tight-junction associated protein Canoe show over- and underrotation defects similar to aos ^rlt , fifty-fifty early during the larval third instar stage. Although non assessed in PCP signaling, cno genetically interacts with scabrous (sca), an endosome-associated protein involved in N signaling (Li et al., 2003; Miyamoto et al., 1995). Ommatidia posterior to sca mutant clones overrotate and sca thus appears to be required non-cell autonomously for ommatidia to stop at the correct position (Chou and Chien, 2002). It remains puzzling to explain the genetic interaction of cno and sca mechanistically. However, consequent with an involvement of EGFR signaling in rotation, mutations in the phospholipase Cγ, small-wing, which is involved in the ER retention of the processed EGFR ligand Spitz, testify rotation defects (Schlesinger et al., 2004).

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Eyes and Vision

Michael F. Land , in Encyclopedia of Insects (Second Edition), 2009

Imaging Mechanisms

The structures that form the images in the ommatidia of apposition eyes are quite varied ( Fig. 6). In terrestrial insects, as in terrestrial vertebrates, the simplest mode to produce an image is to make the cornea curved (Fig. vi(A). Ordinary spherical-surface optics then employ, and an image is formed about four radii of curvature backside the front face. In aquatic insects such as the water issues Notonecta, the external surface of the cornea has picayune power because of the reduction in refractive alphabetize difference (Fig. 6B). It is augmented by two other surfaces, the rear of the lens and an unusually curved interface in the middle of the lens whose function may be to correct one of the defects of spherical surfaces—spherical aberration.

Effigy 6. Four mechanisms of prototype germination in apposition optics. (A) Corneal lens (bee, fly). (B) Multisurface lens (h2o bugs). (C) Lens/lens-cylinder afocal combination (collywobbles). Details in text.

[Reproduced with permission from State and Nilsson (2002).]

The optics of butterflies, which resemble ordinary apposition eyes in nearly all respects, have an optical organization that is subtly dissimilar from the organization in Fig. half-dozen(A.Instead of forming an image at the rhabdom tip, as in the centre of a bee or locust, the epitome lies inside the crystalline cone. The proximal part of the cone contains a very powerful lens cylinder that makes the focused light parallel over again, then that it reaches the rhabdom every bit a axle that just fits the rhabdom (Figs. 6C and 17). This system, known every bit afocal apposition because there is no external focus, has much in common with the superposition optical system of moths, to which collywobbles are closely related, and volition be considered subsequently.

Figure 17. "Afocal" apposition in butterfly eyes. (A and B) Although each ommatidium acts independently, like an apposition heart, the optical elements function every bit telescopes with an internal image, every bit in superposition eyes (Fig. 13). The broad beam of light reaching the cornea is reduced to "fit" the rhabdom (run across text). (C) A consequence of this arrangement is that the rhabdom tip is imaged onto the cornea. I, image plane; Rh, rhabdom.

[Reproduced with permission from Land and Nilsson (2002).]

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Transcriptional Switches During Development

Xiao-jiang Quan , ... Bassem A. Hassan , in Current Topics in Developmental Biological science, 2012

7 Spineless Stochasticity

During differentiation, PRs develop dissimilar morphologies, location inside the ommatidia and photopigment (Rhodopsin) expression ( Melt and Desplan, 2001; Hardie, 1985). The outer PRs, R1–R6, express Rhodopsin 1 (Rh1), which is sensitive to a broad wavelength spectrum, and are involved in move detection and vision in dim light. The inner PRs, R7 and R8, sit on top of each other at the middle of each ommatidium and differentiate into functionally distinct cells involved in color vision. R7s express 1 of the 2 UV-sensitive Rhodopsins, Rh3 and Rh4 whereas R8s express either the blue-sensitive (Rh5) or the green-sensitive (Rh6) Rhodopsins. In about ommatidia, expression of Rh4 in the R7 is primarily coupled to Rh6 in the corresponding R8 while Rh3 expressing-R7s are exclusively coupled with R8s that express Rh5. This defines two major functional subtypes of ommatidia: "pale" (Rh3/Rh5) and "xanthous" (Rh4/Rh6) that, interestingly, are stochastically located in the retina. Despite their stochastic spatial pattern, the proportion of "pale" versus "yellow" ommatidia follows is roughly thirty–70% and this ratio is conserved amid flies from Drosophila to Musca (Franceschini et al., 1981; Jukam and Desplan, 2010; Morante et al., 2007).

The proper expression of the Rhodopsin genes, and more generally, the differentiation of PRs, involves an intricate network of transcriptional activators and repressors. The full general logic of this regulatory network is that all PRs have the competency to limited all rhodopsin genes, due to the combinatorial expression of transcriptional activators, either full general to all PRs, similar Orthodenticle (Otd), or subtype-specific, like Spalt (Sal) (inner PRs) (Mollereau et al., 2001), Prospero (R7) (Melt et al., 2003) or Senseless (R8) (Xie et al., 2007). Specific expression of Rhodopsins then results from the repression of all only i of them in specific subsets of cells. In some cases, this repression is directly achieved by transcriptional factors decision-making PR specific fates. For instance, the R8 determinant Senseless direct represses the expression of the R7-specific Rh3 and Rh4 in R8 cells (Xie et al., 2007). Similarly, the R7 determinant Prospero straight represses the expression of R8-specific rhodopsins, Rh5 and Rh6 in R7, by binding to their promoters (Cook et al., 2003). A contempo beautiful piece of piece of work from the Desplan lab shed light onto a previously unidentified key player in this process, defective proventriculus (dve), a K50 homeobox protein (Johnston et al., 2011). Johnston and colleagues found that regulation of dve expression results from the balance between two opposite signals: activation past Otd and repression by Sal. Every bit a result, Dve is strongly expressed in outer PRs where it represses the expression of inner PRs photopigments Rh3, Rh5, and Rh6. In dissimilarity, information technology is non expressed in almost inner PRs except for the yellow R7 where it represses Rh3 expression.

The expression of "pale" Rh5 versus "yellow" Rh6 in R8 depends on a signal emanating from the committed "pale" R7 (Chou et al., 1999; Papatsenko et al., 1997). The exact nature of this signal remains to be elucidated. In contrast, maintenance of the decision involves a bistable regulatory loop where the Melted pleckstrin-homology (PH) domain protein and the Ser/Thr kinase warts, respectively members of the Tor/Insulin and the Hippo/Salvador signaling pathways, play contrary roles (Mikeladze-Dvali et al., 2005). Therefore, the mosaic of stake versus yellow ommatidia derives from the presence of Dve in a subset of R7 cells (the future yellow R7s) where it escapes the repression by Spalt. Previous work from Desplan's lab had already demonstrated that the stochastic expression of the transcriptional factor Spineless represented a binary switch defining the mosaic of "yellow" versus "stake" PRs in the retina (Wernet et al., 2006). It is thus satisfying to larn from Johnston and coworkers that, Spineless activates the expression of dve in yellow R7s. In pale R7s, in contrast, dve is repressed by Sal. This, in combination with the activation of Rh3 by Otd and Sal and the repression of Rh5/6 by Pros, results in exclusive Rh3 expression in these cells. Signaling from pale R7s represses Wts activity in their neighboring R8s thus preventing Rh6 expression and leading to pale, Rh5 expressing R8. Effigy x.3 summarizes these findings as an example of the complex combination of gene regulatory networks and signaling networks resulting in terminal PR differentiation. Ultimately, the question of the stochasticity of the formation of "pale" versus "yellow" ommatidia relates to how the stochastic expression of Spineless is achieved during middle development, which remains to exist elucidated.

Figure 10.3. Schematic representation of the cistron regulatory network and signaling events thought to specify the terminal differentiation of the "pale" inner PR subtype. For a comprehensive model of how these events command the differentiation of the various PR subtypes beyond the retina the reader is referred to figure 7 in the original work by Johnston et al. (2011).

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Volume 2

Andrew W. Stoker , in Handbook of Cell Signaling (Second Edition), 2010

Drosophila

The compound eye of the fly contains near 800 light-receiving ommatidia, each with photoreceptor neurons named R1 through R8. Axons from these photoreceptors project from the eye to the optic lobe in the encephalon, where each terminates either in the lamina layer (R1 to R6) or in proximal layers (R8) or distal layers (R7) of the medulla [vi]. DPTP69D and DLAR influence these axonal termination events [2, 7, 8]. For example, if axons of R1 to R6 are fabricated DPTP69D-scarce, they will overshoot their target and terminate in the medulla. In addition, loss of DPTP69D in the R7 photoreceptor causes its axon to stop short in the R8 termination zone of the medulla. DPTP69D appears to control the ability of growth cones to de-adhere (defasciculate) from the R8 axon at correct navigational decision points (step ii, Figure 238.1). Interestingly, whereas DLAR-deficient axons from R1 to R6 terminate normally, R7 axons that lack DLAR reach, just so retract from, their medulla targets [7, 8]. This suggests a failure to establish stable adhesive or synaptic contacts with medulla target cells, indicating that DLAR is involved in this process. DLAR mutants and cadherin mutants have similar phenotypes, suggesting that they may regulate like adhesive signaling pathways [7], which is of interest given contempo evidence that a related mammalian RPTP PTPσ influences cadherin adhesion during neurite outgrowth [9]. The collective data also indicate that DPTP69D and DLAR part prison cell apart, although DLAR likewise shows evidence of non-autonomous function in R8, suggesting that it may "transport" signals through its extracellular domain. In contrast to their guidance roles, DRPTPs do non appear to be necessary for axon elongation in the visual system, unlike their vertebrate counterparts.

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