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1.  Regional modulation of a stochastically expressed factor determines photoreceptor subtypes in the Drosophila retina 
Developmental cell  2013;25(1):93-105.
Stochastic mechanisms are sometimes utilized to diversify cell fates, especially in nervous systems. In the Drosophila retina, stochastic expression of the PAS-bHLH transcription factor Spineless (Ss) controls photoreceptor subtype choice. In one randomly distributed subset of R7 photoreceptors, Ss activates Rhodopsin4 (Rh4) and represses Rhodopsin3 (Rh3); counterparts lacking Ss express Rh3 and repress Rh4. In the dorsal third region of the retina, the Iroquois Complex transcription factors induce Rh3 in Rh4-expressing R7s. Here, we show that Ss levels are controlled in a binary On/Off manner throughout the retina, yet are attenuated in the dorsal third region to allow Rh3 co-expression with Rh4. Whereas the sensitivity of rh3 repression to differences in Ss levels generates stochastic and regionalized patterns, the robustness of rh4 activation ensures its stochastic expression throughout the retina. Our findings show how stochastic and regional inputs are integrated to control photoreceptor subtype specification in the Drosophila retina.
doi:10.1016/j.devcel.2013.02.016
PMCID: PMC3660048  PMID: 23597484
2.  Genetic Dissection of Photoreceptor Subtype Specification by the Drosophila melanogaster Zinc Finger Proteins Elbow and No ocelli 
PLoS Genetics  2014;10(3):e1004210.
The elbow/no ocelli (elb/noc) complex of Drosophila melanogaster encodes two paralogs of the evolutionarily conserved NET family of zinc finger proteins. These transcriptional repressors share a conserved domain structure, including a single atypical C2H2 zinc finger. In flies, Elb and Noc are important for the development of legs, eyes and tracheae. Vertebrate NET proteins play an important role in the developing nervous system, and mutations in the homolog ZNF703 human promote luminal breast cancer. However, their interaction with transcriptional regulators is incompletely understood. Here we show that loss of both Elb and Noc causes mis-specification of polarization-sensitive photoreceptors in the ‘dorsal rim area’ (DRA) of the fly retina. This phenotype is identical to the loss of the homeodomain transcription factor Homothorax (Hth)/dMeis. Development of DRA ommatidia and expression of Hth are induced by the Wingless/Wnt pathway. Our data suggest that Elb/Noc genetically interact with Hth, and we identify two conserved domains crucial for this function. Furthermore, we show that Elb/Noc specifically interact with the transcription factor Orthodenticle (Otd)/Otx, a crucial regulator of rhodopsin gene transcription. Interestingly, different Elb/Noc domains are required to antagonize Otd functions in transcriptional activation, versus transcriptional repression. We propose that similar interactions between vertebrate NET proteins and Meis and Otx factors might play a role in development and disease.
Author Summary
The eyes of many animals contain groups of photoreceptor cells with different chromatic sensitivities that can be arranged in complex patterns. It is of great interest to identify the genes and pathways shaping these ‘retinal mosaics’ which include stochastically distributed groups of cells, versus highly localized ones. In many insect eyes, which are composed of large numbers of unit eyes, or ommatidia, specialized photoreceptors are found only in the dorsal periphery, where they face the sky. These ommatidia are responsible for detecting linearly polarized skylight, which serves as an important navigational cue for these animals. Here we describe how two closely related proteins called Elbow and No ocelli interact with the transcription factors Homothorax and Orthodenticle to correctly specify the polarization detectors at the dorsal rim of the retina of Drosophila melanogaster. Interestingly, all four proteins are evolutionarily conserved from worms to humans, and they appear to be involved in similar developmental processes across species. Furthermore, human homologs of Elbow and No ocelli have been identified as promoters of luminal breast cancer. The newly identified role of these two proteins within a regulatory network might therefore enable new approaches in a number of important processes.
doi:10.1371/journal.pgen.1004210
PMCID: PMC3953069  PMID: 24625735
3.  Dual mode of embryonic development is highlighted by expression and function of Nasonia pair-rule genes 
eLife  2014;3:e01440.
Embryonic anterior–posterior patterning is well understood in Drosophila, which uses ‘long germ’ embryogenesis, in which all segments are patterned before cellularization. In contrast, most insects use ‘short germ’ embryogenesis, wherein only head and thorax are patterned in a syncytial environment while the remainder of the embryo is generated after cellularization. We use the wasp Nasonia (Nv) to address how the transition from short to long germ embryogenesis occurred. Maternal and gap gene expression in Nasonia suggest long germ embryogenesis. However, the Nasonia pair-rule genes even-skipped, odd-skipped, runt and hairy are all expressed as early blastoderm pair-rule stripes and late-forming posterior stripes. Knockdown of Nv eve, odd or h causes loss of alternate segments at the anterior and complete loss of abdominal segments. We propose that Nasonia uses a mixed mode of segmentation wherein pair-rule genes pattern the embryo in a manner resembling Drosophila at the anterior and ancestral Tribolium at the posterior.
DOI: http://dx.doi.org/10.7554/eLife.01440.001
eLife digest
Networks of genes that work together are widespread in nature. The conservation of individual genes across species and the tendency of their networks to stick together is a sign that they are working efficiently. Furthermore, it is common for existing gene networks to be adapted to perform new tasks, instead of new networks being invented every time a similar but distinct demand arises. One important question is: how can evolution use the same building blocks—such as the genes in a functioning network—in different ways to achieve new outcomes?
The gene network that sets up the ‘body plan’ of insects during development has been well studied, most deeply in the fruit fly, Drosophila. Like all insects, the body of a fruit fly is divided into three main parts—the head, the thorax and the abdomen—and each of these parts is made up of several smaller segments. There is a remarkable diversity of insect body plans in nature, and yet, these seem to arise from the same gene networks in the embryo.
When a Drosophila embryo is growing into a larva, all the different body segments develop at the same time. In most other insects, however, segments of the abdomen emerge later and sequentially during the development process. The ancestors of most insects are also thought to have developed in this way, which is known as ‘short germ embryogenesis’. So how did the so-called ‘long germ embryogenesis’, as observed in Drosophila, evolve from the short germ embryogenesis that is observed in most other insects?
The gene network that controls development includes the ‘pair-rule genes’ that are expressed in a pattern of alternating stripes that wrap around, top to bottom, along most of the length of the embryo. These stripes mark where the edges of each body segment will eventually develop. In fruit flies, this pattern extends along the entire length of the embryo and the stripes all appear at one time. However, in the abdominal region of short germ insects, the pair-rule genes are expressed in waves that pass through the posterior region as it grows, with new segments being added one behind the other.
Now, Rosenberg et al. have attempted to explain how the same genes can be used to direct the segmentation process in such different ways by studying another long germ insect species, the jewel wasp. Analysis of the expression of pair-rule genes in the jewel wasp shows that it uses a mixed strategy to control segmentation. The development of segments at the front of its body is directed in the same way as the fruit fly, with all these segments laid down together. However, the segments at the rear of the body are only patterned later, one after the other, like most other insects.
The work of Rosenberg et al. suggests that the jewel wasp represents an intermediate step between ancestral insects and Drosophila in the evolution of the gene network that patterns the ‘body plan’. Identifying and studying these intermediate forms allows us to understand the ways in which evolution can innovate by building upon what has come before.
DOI: http://dx.doi.org/10.7554/eLife.01440.002
doi:10.7554/eLife.01440
PMCID: PMC3941026  PMID: 24599282
Nasonia vitripennis; Tribolium; embryonic patterning; evolution; segmentation; pair-rule genes; D. melanogaster; other
4.  Dying to Entrain: Regulating ipRGC Spacing 
Developmental cell  2013;24(4):338-340.
In a recent issue of Neuron,Chen et al. (2013) show that apoptosis is required to ensure the even distribution of a class of retinal ganglion cells (ipRGCs), which sense luminance both intrinsically and through input from rods and cones. Disrupting apoptosis impairs photoentrainment mediated by rods/cones, but not that mediated by ipRGC-expressed melanopsin.
doi:10.1016/j.devcel.2013.02.001
PMCID: PMC3744582  PMID: 23449468
5.  Temporal Patterning of Neural Progenitors in Drosophila 
Drosophila has recently become a powerful model system to understand the mechanisms of temporal patterning of neural progenitors called neuroblasts (NBs). Two different temporal sequences of transcription factors (TFs) have been found to be sequentially expressed in NBs of two different systems: the Hunchback, Krüppel, Pdm1/Pdm2, Castor, and Grainyhead sequence in the Drosophila ventral nerve cord; and the Homothorax, Klumpfuss, Eyeless, Sloppy-paired, Dichaete, and Tailless sequence that patterns medulla NBs. In addition, the intermediate neural progenitors of type II NB lineages are patterned by a different sequence: Dichaete, Grainyhead, and Eyeless. These three examples suggest that temporal patterning of neural precursors by sequences of TFs is a common theme to generate neural diversity. Cross-regulations, including negative feedback regulation and positive feedforward regulation among the temporal factors, can facilitate the progression of the sequence. However, there are many remaining questions to understand the mechanism of temporal transitions. The temporal sequence progression is intimately linked to the progressive restriction of NB competence, and eventually determines the end of neurogenesis. Temporal identity has to be integrated with spatial identity information, as well as with the Notch-dependent binary fate choices, in order to generate specific neuron fates.
doi:10.1016/B978-0-12-396968-2.00003-8
PMCID: PMC3927947  PMID: 23962839
6.  Temporal patterning of Drosophila medulla neuroblasts controls neural fates 
Nature  2013;498(7455):456-462.
In the Drosophila optic lobes, the medulla processes visual information coming from inner photoreceptors R7 and R8 and from lamina neurons. It contains ~40,000 neurons belonging to over 70 different types. We describe how precise temporal patterning of neural progenitors generates these different neural types. Five transcription factors--Homothorax, Eyeless, Sloppy-paired, Dichaete and Tailless--are sequentially expressed in a temporal cascade in each of the medulla neuroblasts as they age. Loss of either Eyeless, Sloppy-paired or Dichaete blocks further progression of the temporal sequence. We provide evidence that this temporal sequence in neuroblasts, together with Notch-dependent binary fate choice, controls the diversification of the neuronal progeny. Although a temporal sequence of transcription factors had been identified in Drosophila embryonic neuroblasts, our work illustrates the generality of this strategy, with different sequences of transcription factors being used in different contexts.
doi:10.1038/nature12319
PMCID: PMC3701960  PMID: 23783517
7.  Stochastic spineless expression creates the retinal mosaic for colour vision 
Nature  2006;440(7081):10.1038/nature04615.
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.
doi:10.1038/nature04615
PMCID: PMC3826883  PMID: 16525464
8.  Deterministic or stochastic choices in retinal neuron specification 
Neuron  2012;75(5):739-742.
There are two views on vertebrate retinogenesis: a deterministic model dependent on fixed lineages, and a stochastic model in which choices of division modes and cell fates cannot be predicted. In this issue of Neuron, He et al. (2012) address this question in zebra fish using live imaging and mathematical modeling.
doi:10.1016/j.neuron.2012.08.008
PMCID: PMC3438524  PMID: 22958814
vertebrate retinogenesis; competence; stochasticity; retinal progenitor; birth order
9.  Power tools for gene expression and clonal analysis in Drosophila 
Nature methods  2011;9(1):47-55.
The development of two-component expression systems in Drosophila melanogaster, one of the most powerful genetic models, has allowed the precise manipulation of gene function in specific cell populations. These expression systems, in combination with site-specific recombination approaches, have also led to the development of new methods for clonal lineage analysis. We present a hands-on user guide to the techniques and approaches that have greatly increased resolution of genetic analysis in the fly, with a special focus on their application for lineage analysis. Our intention is to provide guidance and suggestions regarding which genetic tools are most suitable for addressing different developmental questions.
doi:10.1038/nmeth.1800
PMCID: PMC3574576  PMID: 22205518
11.  The retinal mosaics of opsin expression in invertebrates and vertebrates 
Developmental neurobiology  2011;71(12):1212-1226.
Colour vision is found in many invertebrate and vertebrate species. It is the ability to discriminate objects based on the wavelength of emitted light independent of intensity. As it requires the comparison of at least two photoreceptor types with different spectral sensitivities, this process is often mediated by a mosaic made of several photoreceptor types. In this review, we summarize the current knowledge about the formation of retinal mosaics and the regulation of photopigment (opsin) expression in the fly, mouse and human retina. Despite distinct evolutionary origins, as well as major differences in morphology and phototransduction machineries, there are significant similarities in the stepwise cell-fate decisions that lead from progenitor cells to terminally differentiated photoreceptors that express a particular opsin. Common themes include i) the use of binary transcriptional switches that distinguish classes of photoreceptors, ii) the use of gradients of signaling molecules for regional specializations, iii) stochastic choices that pattern the retina and iv) the use of permissive factors with multiple roles in different photoreceptor types.
doi:10.1002/dneu.20905
PMCID: PMC3190030  PMID: 21557510
photoreceptor; opsin; colour vision; retinal mosaic; transcription factors
12.  Binary Regulation of Hippo Pathway by Merlin/NF2, Kibra, Lgl, and Melted Specifies and Maintains Post-mitotic Neuronal Fate 
Developmental cell  2011;21(5):874-887.
Patterning the Drosophila retina for color vision relies on post-mitotic specification of photoreceptor subtypes. R8 photoreceptors express one of two light-sensing Rhodopsins, Rh5 or Rh6. This fate decision involves a bistable feedback loop between Melted, a PH-domain protein, and Warts, a kinase in the Hippo growth pathway. Here, we show a subset of the Hippo pathway—Merlin(Mer), Kibra(Kib), and Lethal(2)giant larvae(Lgl), but not Expanded or Fat--is required for Warts expression and activity in R8 to specify Rh6 fate. Melted represses warts transcription to disrupt Hippo pathway activity and specify Rh5 fate. R8 Hippo signaling therefore exhibits ON-or-OFF regulation, promoting mutually exclusive fates. Furthermore, Mer and Lgl are continuously required to maintain R8 neuronal subtypes. These results reveal a role for Mer, Kib, and Lgl in neuronal specification and maintenance, and show that the Hippo pathway is re-implemented for sensory neuron fate by combining canonical and non-canonical regulatory steps.
doi:10.1016/j.devcel.2011.10.004
PMCID: PMC3215849  PMID: 22055343
binary cell fate; color vision; photoreceptor; rhodopsin; Hippo pathway; neural development; neural maintenance; Merlin; Warts; Kibra; Lgl
13.  Interlocked feedforward loops control cell type-specific Rhodopsin expression in the Drosophila eye 
Cell  2011;145(6):956-968.
How complex networks of activators and repressors lead to exquisitely specific cell type determination during development is poorly understood. In the Drosophila eye, expression patterns of Rhodopsins define at least eight functionally distinct though related subtypes of photoreceptors. Here, we describe a role for the transcription factor gene defective proventriculus (dve) as a critical node in the network regulating Rhodopsin expression. dve is a shared component of two opposing, interlocked feedforward loops (FFLs). Orthodenticle and Dve interact in an incoherent FFL to repress Rhodopsin expression throughout the eye. In the R7 and R8 photoreceptors, a coherent FFL relieves repression by Dve while activating Rhodopsin expression. Therefore, this network uses repression to restrict, and combinatorial activation to induce cell type-specific expression. Further, Dve levels are finely tuned to yield cell type- and region-specific repression or activation outcomes. This interlocked FFL motif may be a general mechanism to control terminal cell fate specification.
doi:10.1016/j.cell.2011.05.003
PMCID: PMC3117217  PMID: 21663797
14.  Feedback from Rhodopsin controls rhodopsin exclusion in Drosophila photoreceptors 
Nature  2011;479(7371):108-112.
Sensory systems with high discriminatory power employ neurons that express only one of several alternative sensory receptor proteins. This exclusive receptor gene expression restricts the sensitivity spectrum of neurons and is coordinated with the choice of their synaptic targets1-3. However, little is known about how it is maintained throughout the life of a neuron. Here we show that the green-light sensing receptor Rhodopsin 6 (Rh6) acts to exclude an alternative blue-sensitive Rhodopsin 5 (Rh5) from a subset of Drosophila R8 photoreceptor neurons4. Loss of Rh6 leads to a gradual expansion of Rh5 expression into all R8 photoreceptors of the aging adult retina. The Rh6 feedback signal results in repression of the rh5 promoter and can be mimicked by other Drosophila Rhodopsins; it is partially dependent on activation of Rhodopsin by light, and relies on Gαq activity, but not on the subsequent steps of the phototransduction cascade5. Our observations reveal a thus far unappreciated spectral plasticity of R8 photoreceptors, and identify Rhodopsin feedback as an exclusion mechanism.
doi:10.1038/nature10451
PMCID: PMC3208777  PMID: 21983964
15.  Heads and Tails: Evolution of Antero-Posterior Patterning in Insects 
Biochimica et biophysica acta  2008;1789(4):333-342.
doi:10.1016/j.bbagrm.2008.09.007
PMCID: PMC2700975  PMID: 18976722
16.  The Twin Spot Generator for differential Drosophila lineage analysis 
Nature methods  2009;6(8):600-602.
In Drosophila, widely-used mitotic recombination-based strategies generate mosaic flies with positive readout for only one daughter cell after division. To differentially label both daughter cells, we developed the Twin Spot Generator technique (TSG) and demonstrate that through mitotic recombination, TSG generates green and red twin spots in internal fly tissues, visible even as single cells. We discuss the wide applications of TSG to lineage and genetic mosaic studies.
doi:10.1038/nmeth.1349
PMCID: PMC2720837  PMID: 19633664
17.  The Sequence Specificity of Homeodomain-DNA Interaction 
Cell  1988;54(7):1081-1090.
Summary
The Drosophila developmental gene, engrailed, encodes a sequence-specific DNA binding activity. Using deletion constructs expressed as fusion proteins in E. coli, we localized this activity to the conserved homeodomain (HD). The binding site consensus, TCAATTAAAT, is found in clusters in the engrailed regulatory region. Weak binding of the En HD to one copy of a synthetic consensus is enhanced by adjacent copies. The distantly related HD encoded by fushi tarazu binds to the same sites as the En HD, but differs in its preference for related sites. Both HDs bind a second type of sequence, a repeat of TAA. The similarity in sequence specificity of En and Ftz HDs suggests that, within families of DNA binding proteins, close relatives will exhibit similar specificities. Competition among related regulatory proteins might govern which protein occupies a given binding site and consequently determine the ultimate effect of cis-acting regulatory sites.
PMCID: PMC2753412  PMID: 3046753
18.  Switch of rhodopsin expression in terminally differentiated Drosophila sensory neurons 
Nature  2008;454(7203):533-537.
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)1. The larval eye becomes the adult extraretinal ‘eyelet’ composed of four green-sensitive (Rh6) photoreceptors2,3. 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.
doi:10.1038/nature07062
PMCID: PMC2750042  PMID: 18594514
19.  Generating patterned arrays of photoreceptors 
One of the most fascinating topics in biology is to understand the development of highly differentiated cells such as photoreceptors (PRs). This process involves successive steps, starting with the generation of the eye primordium, recruitment and specification of PRs and finally, expression of the proper rhodopsin, the photopigment that initiates the signaling cascade underlying light input excitation. In this review, we describe the sequential steps that take place in the Drosophila eye, from the initial neuronal specification of PRs through their full maturation, focusing specifically on the transcription factors and signaling pathways involved in controlling the precise expression of different rhodopsins in specialized PRs.
doi:10.1016/j.gde.2007.05.003
PMCID: PMC2713430  PMID: 17616388
20.  The color vision circuit in the medulla of Drosophila 
Current biology : CB  2008;18(8):553-565.
Background
Color vision requires comparison between photoreceptors that are sensitive to different wavelengths of light. In Drosophila, this is achieved by the inner photoreceptors (R7 and R8) that contain different rhodopsins. Two types of comparisons can occur in fly color vision: between the R7 (UV-sensitive) and R8 (blue or green-sensitive) photoreceptor cells within one ommatidium (unit eye); or between different ommatidia that contain spectrally distinct inner photoreceptors. Photoreceptors project to the optic lobes: R1-6, which are involved in motion detection, project to the lamina, while R7 and R8 reach deeper in the medulla. This paper analyzes the neural network underlying color vision in the medulla.
Results
We reconstruct the neural network in the medulla, focusing on neurons likely to be involved in processing color vision. We identify the full complement of neurons in the medulla, including second order neurons that contact both R7 and R8 from a single ommatidium, or contact R7 and/or R8 from different ommatidia. We also examine third order neurons and local neurons that likely modulate information from second order neurons. Finally, we present highly specific tools that will allow us to functionally manipulate the network and test both activity and behavior.
Conclusions
This precise characterization of the medulla circuitry will allow us to understand how color vision is processed in the optic lobe of Drosophila, providing a paradigm for more complex systems in vertebrates.
doi:10.1016/j.cub.2008.02.075
PMCID: PMC2430089  PMID: 18403201
21.  Stochasticity and Cell Fate 
Science (New York, N.Y.)  2008;320(5872):65-68.
Summary
Fundamental to living cells is the capacity to differentiate into subtypes with specialized attributes. Understanding the way cells acquire their fates is a major challenge in developmental biology. How cells adopt a particular fate is usually thought of as being deterministic, and in the large majority of cases it is. That is, cells acquire their fate by virtue of their lineage or their proximity to an inductive signal from another cell. In some cases, however, and in organisms ranging from bacteria to humans, cells choose one or another pathway of differentiation stochastically without apparent regard to environment or history. Stochasticity has important mechanistic requirements as we discuss. We will also speculate on why stochasticity is advantageous, and even critical in some circumstances, to the individual, the colony, or the species.
doi:10.1126/science.1147888
PMCID: PMC2605794  PMID: 18388284
22.  The First Steps in Drosophila Motion Detection 
Neuron  2007;56(1):5-7.
The visual system, with its ability to perceive motion, is crucial for most animals to walk or fly steadily. Theoretical models of motion detection exist, but the underlying cellular mechanisms are still poorly understood. In this issue of Neuron, Rister and colleagues dissect the function of neuronal subtypes in the optic lobe of Drosophila to reveal their role in motion detection.
doi:10.1016/j.neuron.2007.09.025
PMCID: PMC2633596  PMID: 17920008
23.  Stochastic neuronal cell fate choices 
Though many neuronal cell fate decisions result in reproducible outcomes, stochastic choices often lead to spatial randomization of cell subtypes. This is often the case in sensory systems where expression of a specific sensory receptor gene is selected randomly from a set of possible outcomes. Here, we describe recent findings elucidating the mechanisms controlling color photoreceptor subtypes in flies and olfactory receptor subtypes in worms and mice. Although well-known biological concepts such as lateral signaling and promoter selection play roles in these cases, fundamental questions concerning these choice mechanisms remain.
doi:10.1016/j.conb.2008.04.004
PMCID: PMC2478740  PMID: 18511260
24.  Distinct mechanisms for mRNA localization during embryonic axis specification in the wasp Nasonia 
Developmental biology  2007;306(1):134-142.
mRNA localization is a powerful mechanism for targeting factors to different regions of the cell and is used in Drosophila to pattern the early embryo. During oogenesis of the wasp Nasonia, mRNA localization is used extensively to replace the function of the Drosophila bicoid gene for the initiation of patterning along the antero-posterior axis. Nasonia localizes both caudal and nanos to the posterior pole, whereas giant mRNA is localized to the anterior pole of the oocyte; orthodenticle1 (otd1) is localized to both the anterior and posterior poles. The abundance of differentially localized mRNAs during Nasonia oogenesis provided a unique opportunity to study the different mechanisms involved in mRNA localization. Through pharmacological disruption of the microtubule network, we found that both anterior otd1 and giant, as well as posterior caudal mRNA localization was microtubule-dependent. Conversely, posterior otd1 and nanos mRNA localized correctly to the posterior upon microtubule disruption. However, actin is important in anchoring these two posteriorly localized mRNAs to the oosome, the structure containing the pole plasm. Moreover, we find that knocking down the functions of the genes tudor and Bicaudal-D mimics disruption of microtubules, suggesting that tudor's function in Nasonia is different from flies, where it is involved in formation of the pole plasm.
doi:10.1016/j.ydbio.2007.03.012
PMCID: PMC1973164  PMID: 17434472
Nasonia; mRNA localization; nanos; caudal; giant; tudor; otd1; Bicaudal; microtubules; actin
25.  Iroquois Complex Genes Induce Co-Expression of rhodopsins in Drosophila 
PLoS Biology  2008;6(4):e97.
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.
Author Summary
Most sensory systems follow the rule “one receptor molecule per receptor cell.” For example, photoreceptors in the fly eye and cones in the human eye each express only one light-sensitive rhodopsin. Rhodopsins are G-coupled protein receptors, a class of ancient signaling molecules that mediate not just vision but also the sense of smell, the inflammatory response, and other physiological processes. However, the mechanisms that regulate mutual exclusion of receptor genes in the visual and olfactory systems are poorly understood. Each ommatidium in the fly eye consists of eight photoreceptors (R1–R8); six of which mediate broad-spectrum motion vision (R1–R6) and two that mediate color vision (R7 and R8). We identified a new class of photoreceptors in the fly retina that violates the one rhodopsin–one receptor rule. This subset of ommatidia, located in the dorsal third of the eye, co-expresses two ultraviolet-sensitive rhodospins (rh3 and rh4) in R7, while maintaining discrimination between green and blue opsins in R8. We took advantage of the genetic tools offered by the fruit fly to show that this co-expression depends on the Iroquois Complex (Iro-C) genes that are both necessary and sufficient to allow the two ultraviolet-sensitive rhosopsins to be expressed in the same R7 cell. These results shed new light on the mechanisms regulating co-expression of rhodopsins in the eye, and may well have implications for regulating co-expression in olfactory receptors and other G-protein coupled systems.
Iro-C genes control the co-expression of sensory receptors.
doi:10.1371/journal.pbio.0060097
PMCID: PMC2323304  PMID: 18433293

Results 1-25 (25)