The polar coordinate model goes molecular

Compensatory proliferation in Drosophila imaginal discs requires Dronc-dependent p53 activity

BACKGROUND

The p53 transcription factor directs a transcriptional program that determines whether a cell lives or dies after DNA damage. Animal survival after extensive cellular damage often requires that lost tissue be replaced through compensatory growth or regeneration. In Drosophila, damaged imaginal disc cells can induce the proliferation of neighboring viable cells, but how this is controlled is not clear. Here we provide evidence that Drosophila p53 (dp53) has a previously unidentified role in coordinating the compensatory growth response to tissue damage.

RESULTS

We find that dp53, the sole p53 ortholog in Drosophila, is required for each component of the response to cellular damage, including two separate cell-cycle arrests, changes in patterning gene expression, cell proliferation, and growth. We demonstrate that these processes are regulated by dp53 in a manner that is independent of DNA-damage sensing but that requires the initiator caspase Dronc. Our results indicate that once induced, dp53 amplifies and sustains the response through a positive feedback loop with Dronc and the apoptosis-inducing factors Hid and Reaper.

CONCLUSIONS

How cell death and cell proliferation are coordinated during development and after stress is a fundamental question that is critical for an understanding of growth regulation. Our data suggest that dp53 may carry out an ancestral function that promotes animal survival through the coordination of responses leading to compensatory growth after tissue damage.

Homology, limbs, and genitalia

Similarities in genetic control between the main body axis and its appendages have been generally explained in terms of genetic co-option. In particular, arthropod and vertebrate appendages have been explained to invoke a common ancestor already provided with patterned body outgrowths or independent recruitment in limb patterning of genes or genetic cassettes originally used for purposes other than axis patterning. An alternative explanation is that body appendages, including genitalia, are evolutionarily divergent duplicates (paramorphs) of the main body axis. However, are all metazoan limbs and genitalia homologous? The concept of body appendages as paramorphs of the main body axis eliminates the requirement for the last common ancestor of limb-bearing animals to have been provided with limbs. Moreover, the possibility for an animal to express complex organs ectopically demonstrates that positional and special homology may be ontogenetically and evolutionarily uncoupled. To assess the homology of animal genitalia, we need to take into account three different sets of mechanisms, all contributing to their positional and/or special homology and respectively involved (1) in the patterning of themain body axis, (2) in axis duplication, followed by limb patterning mechanisms diverging away from those still patterning the main body axis (axis paramorphism), and (3) in controlling the specification of sexual/genital features, which often, but not necessarily, come into play by modifying already developed and patterned body appendages. This analysis demonstrates that a combinatorial approach to homology helps disentangling phylogenetic and ontogenetic layers of homology.

Cell biology of limb patterning

Of vertebrate organ systems, the developing limb has been especially well characterized. Morphological studies have yielded a wealth of information describing limb outgrowth and have allowed for the identification of a multitude of important factors. In terms of the latter, key signaling pathways are known to control numerous aspects of limb development, including establishment of the early limb field, determination of limb identity, elongation of the limb bud, specification of digit pattern, and sculpting of the digits. Modification of underlying signaling pathways can thus result in dramatic alterations of the limb phenotype, accounting for many of the diverse limb patterns observed in nature. Given this, it is clear that signaling pathways regulate the highly orchestrated and tightly controlled sequence of cellular events necessary for limb outgrowth; however, exactly how molecular signals interface with the cell biology of limb development remains largely a mystery. In this review we first provide an overview of a number of the morphogenetic signaling pathways that have been identified in the developing limb and then review how a subset of these signals are known to modify cell behaviors important for limb outgrowth.

Regeneration in insects

@9cIntroduction@21T issues exhibit an impressive ability to respond to a myriad of insults by repairing and regenerating complex structures. The elegant and orderly process of regeneration provides clues to the mechanisms of pattern formation but also offers the hope that the process might one day be manipulated to replace damaged body parts. To manipulate the process, it will be necessary to understand the genetic basis of the process. In the case of the insect leg, we are coming close to such a level of understanding and many of the lessons learned are relevant to vertebrate systems. A dynamic web of gene regulatory networks appears to create a robust self-organizing system that is at once extremely intricate but also perhaps simple in its reliance on a few key signaling pathways and a few simple processes, e.g. autoactivation and lateral inhibition. Here we will summarize what has been learned about the networks of gene regulation present in the Drosophila leg discs and then we will explore how the regenerative responses to different insults can be understood as predictable responses to these networks. Each of the regulatory networks could themselves serve as the subject of a detailed review and that is beyond the scope of this discussion. Here we will focus on the interplay between the regulatory networks in patterning the tissue.

The developmental basis for allometry in insects

Within all species of animals, the size of each organ bears a specific relationship to overall body size. These patterns of organ size relative to total body size are called static allometry and have enchanted biologists for centuries, yet the mechanisms generating these patterns have attracted little experimental study. We review recent and older work on holometabolous insect development that sheds light on these mechanisms. In insects, static allometry can be divided into at least two processes: (1) the autonomous specification of organ identity, perhaps including the approximate size of the organ, and (2) the determination of the final size of organs based on total body size. We present three models to explain the second process: (1) all organs autonomously absorb nutrients and grow at organ-specific rates, (2) a centralized system measures a close correlate of total body size and distributes this information to all organs, and (3) autonomous organ growth is combined with feedback between growing organs to modulate final sizes. We provide evidence supporting models 2 and 3 and also suggest that hormones are the messengers of size information. Advances in our understanding of the mechanisms of allometry will come through the integrated study of whole tissues using techniques from development, genetics, endocrinology and population biology.

A polarity field is established early in the development of the Drosophila compound eye

The photoreceptors within the ommatidia of the Drosophila compound eye form a trapezoid. This occurs in two chiral forms in the dorsal and ventral half of the eye. We have used two manipulations to induce ectopic ommatidia, in combination with molecular markers for specific positions in the retinal field. We find that ectopic morphogenetic furrows induced on the eye field margin (or midline) and those induced in the body of the field have different consequences for the establishment of retinal polarity. Furthermore, the dorsal/ventral vector field is established early in development, prior to and independent of the initiation of the morphogenetic furrow. An 'early equator' model is presented to account for these and previously published data.

Expression patterns of developmental genes reveal segment and parasegment organization of D. melanogaster genital discs

We have used the expression patterns of genes known to be important during early Drosophila development to determine the segment-parasegment organization of the genital discs and to localize the three primordia in the male and female genital discs, engrailed (en) and hedgehog (hh) were used to locate posterior compartments in A8-A10, while cubitus interrupts (ci) localized the anterior compartments for each segment, decapentaplegic (dpp) identified the anterior cells that abut en and hh at the anterior-posterior border. abdominal-A (abd-A) identified the anterior compartment for abdominal segment 8 (aA8) in females but was not detected in the repressed female primordium in male discs. Abdominal-B (Abd-B) was expressed throughout the discs except for a small area along the edge of the posterior lobes, leaving open the possibility that A11 may contribute to the genital discs, caudal (cad) was expressed segmentally in the anal primordium of A10, extending through the Abd-B unstained region, wingless (wg) and gooseberry (gsb) may have assumed an added role in the discs perhaps providing proximal-distal cues. Models are presented to show how the segments and parasegments may fuse together during embryogenesis to form the mature male and female genital discs.

Control of the gene optomotor-blind in Drosophila wing development by decapentaplegic and wingless

Diffusible factors of several protein families control appendage outgrowth and patterning in both insects and vertebrates. In Drosophila wing development, the gene decapentaplegic (dpp) is expressed along the anteroposterior compartment boundary. Early wingless (wg) expression is involved in setting up the dorsoventral boundary. Interaction between dpp- and wg-expressing cells promotes appendage outgrowth. Here, it is shown that optomotor-blind (omb) expression is required for distal wing development and is controlled by both dpp and wg. Ectopic omb expression can lead to the growth of additional wings. Thus, omb is essential for wing development and is controlled by two signaling pathways.

Expression of engrailed in an array of identified sensory neurons: comparison with position, axonal arborization, and synaptic connectivity

engrailed (en) is expressed in the posterior region of embryonic segments and appendages of the cockroach, Periplaneta americana. By 23% of embryogenesis En immunoreactivity is apparent in the dorsal half of the cercus, appendages of segment A11. By 40% of development, En staining is present in the dorsomedial half of the cercus. The nucleus of the medial filiform hair sensory neuron (M), born in this region, expresses en strongly. Staining is never seen in the lateral neuron (L). En is expressed in M as the sensory axons enter the terminal ganglion and begin to form their different arborizations and synaptic connections. This pattern of expression persists through development to the second instar. In mutant animals with supernumerary filiform hair sensilla, En immunoreactivity is only seen in the medial neurons. In second-instar and adult cerci en expression is also seen in medially located neurons. We compared the levels of En staining in the array of 25 second instar neurons with their position, axonal arbor type, and synaptic connections. Staining intensity correlates with distance from the cercal midline, suggesting that en is regulated by other circumferential positional determinants. The expression of en does not correlate with the formation of an M-type arbor. Although 10 to 12 sensory neurons that express en form synapses with giant interneuron 5, the correlation is not precise. These results suggest that, if En does form part of a combinatorial system of positional information in the cercus, its actions are modulated by other gene products.

four-jointed is required for intermediate growth in the proximal-distal axis in Drosophila

Genes capable of translating positional information into regulated growth lie at the heart of morphogenesis, yet few genes with this function have been identified. Mutants in the Drosophila four-jointed (fj) gene show reduced growth and altered differentiation only within restricted sectors of the proximal-distal (PD) axis in the leg and wing, thus fj is a candidate for a gene with this coordination function. Consistent with a position-sensitive role, we show that fj is expressed in a regional pattern in the developing leg, wing, eye and optic lobe. The fj gene encodes a novel type II membrane glycoprotein. When the cDNA is translated in an in vitro translation system in the presence of exogenous microsomal membranes, the intralumenal portion of some of the molecules is cleaved, yielding a secreted C-terminal fragment. We propose that fj encodes a secreted signal that functions as a positive regulator of regional growth and differentiation along the PD axis of the imaginal discs.

Wingless and patched are negative regulators of the morphogenetic furrow and can affect tissue polarity in the developing Drosophila compound eye

In the developing Drosophila compound eye, a wave of pattern formation and cell-type determination sweeps across the presumptive eye epithelium. This ‘morphogenetic furrow’ coordinates the epithelial cells' division cycle, shape and gene expression to produce evenly spaced neural cell clusters that will eventually form the adult ommatidia. As these clusters develop, they rotate inwards to face the eye's equator and establish tissue polarity. We have found that wingless is strongly expressed in the dorsal margin of the presumptive eye field, ahead of the morphogenetic furrow. We have shown that inactivation of Wingless results in the induction of an ectopic furrow that proceeds ventrally from the dorsal margin. This ectopic furrow is normal in most respects, however the clusters formed by it fail to rotate, and we propose a two-vector model to account for normal rotation and tissue polarity in the retina. A second consequence of this inactivation of Wingless is that the dorsal head is largely deleted. We have also found that patched loss-of-function mosaic clones induce circular ectopic morphogenetic furrows (consistent with the observations of other workers with the hedgehog, and PKA genes). We use such patched induced furrows to test the two-vector model for cluster rotation and tissue polarity.

Axes, boundaries and coordinates: the ABCs of fly leg development

Recent studies of gene expression in the developing fruitfly leg support a model--Meinhardt's Boundary Model--which seems to contradict the prevailing paradigm for pattern formation in the imaginal discs of Drosophila--the Polar Coordinate Model. Reasoning from geometric first principles, this article examines the strengths and weaknesses of these hypotheses, plus some baffling phenomena that neither model can comfortably explain. The deeper question at issue is: how does the fly's genome encode the three-dimensional anatomy of the adult? Does it demarcate territories and boundaries (as in a geopolitical map) and then use those boundaries and their points of intersection as a scaffolding on which to erect the anatomy (the Boundary Model)? Or does it assign cellular fates within a relatively seamless coordinate system (the Polar Coordinate Model)? The existence of hybrid Cartesian-polar models shows that the alternatives may not be so clear-cut: a single organ might utilize different systems that are spatially superimposed or temporally sequential.

Dual functions of wingless in the Drosophila leg imaginal disc

The Drosophila gene wingless is a member of the Wnt gene family, a group of genes that are involved in embryonic development and the regulation of cell proliferation. wingless encodes a secreted glycoprotein that plays a role in embryogenesis as well as in the development of adult structures. In the primordia of the adult limbs, the imaginal discs, wingless is expressed in an anterior ventral sector and is required for specification of ventral fate. Ectopic expression of low levels of Wingless in the leg discs leads to partial ventralization and outgrowths of the proximodistal axis. Wingless has thus been proposed to specify ventral fate in a concentration dependent manner (i.e., as a morphogen) and to organize the proximodistal axis. We have extended the analysis of Wingless function in the leg primordium through targeted ectopic expression. We find that Wingless has two functions in the leg disc. In the specification of ventral fate, our data indicate that Wingless does not function as a morphogen but instead appears to collaborate with other factors. In addition to its role in ventral fate specification, Wingless inhibits the commitment of dorsal cells toward a determined state and influences the regulation of proliferation. We propose a model in which Wingless achieves separate functions via spatially regulated mechanisms and discuss the significance of these functions during axial patterning and organization.

dachshund encodes a nuclear protein required for normal eye and leg development in Drosophila

Neural specification and differentiation in the Drosophila eye sweep across the unpatterned epithelial monolayer of the eye imaginal disc following a developmental wave termed the morphogenetic furrow. The furrow begins at the posterior margin of the eye imaginal disc and moves anteriorly as a linear front. Progression of the furrow requires the function of hedgehog, which encodes a secreted signaling protein. We characterize mutations in dachshund, a gene that encodes a novel nuclear protein required for normal cell-fate determination of imaginal disc cells. In the absence of dachshund function, cells at the posterior margin of the eye disc fail to follow a retinal differentiation pathway and appear to adopt a cuticle fate instead. These cells are therefore unable to respond to pattern propagation signals such as hedgehog and furrow initiation does not occur. In contrast, cells in more anterior portions of the eye disc are able to differentiate as retinal cells in the absence of dachshund activity and respond normally to patterning signals. These results suggest that posterior margin cells are distinct from other cells of the eye imaginal disc by early stages of development. dachshund is also necessary for proper differentiation of a subset of segments in the developing leg. Null mutations in dachshund result in flies with no eyes and shortened legs.

The role of the Distal-less gene in the development and evolution of insect limbs

BACKGROUND

Arthropod diversity is apparent in the variations in limb number, type, and position along the body axis. Among the insects, for example, butterflies and moths (Lepidoptera) develop larval abdominal and caudal appendages ('prolegs'), whereas flies (Diptera) do not. Comparative studies of the expression and regulation during development of limb-patterning genes, such as Distal-less (Dll), may provide insights into arthropod evolution.

RESULTS

We report the cloning of a Dll homolog from the butterfly Precis coenia, and present data showing that it is expressed in all developing limbs (except the mandible), including the prolegs; the relationship between Dll and wingless expression observed in Drosophila is conserved in Precis among all limbs. However, Dll is deployed in distinct spatial and temporal patterns within each limb type.

CONCLUSIONS

These data suggest that Dll function, suppressed in the abdomen early in insect evolution, has been derepressed in Lepidoptera, and also suggest that there is a common mechanism underlying the formation of all insect appendages. The limb-type-specific patterns of Dll expression (and its exclusion from the mandible) indicate that regulation of Dll expression may be critical to limb morphology, and are inconsistent with Dll functioning in a simple distal-to-proximal concentration gradient.

Drosophila wingless: a paradigm for the function and mechanism of Wnt signaling

The link between oncogenesis and normal development is well illustrated by the study of the Wnt family of proteins. The first Wnt gene (int-1) was identified over a decade ago as a proto-oncogene, activated in response to proviral insertion of a mouse mammary tumor virus. Subsequently, the discovery that Drosophila wingless, a developmentally important gene, is homologous to int-1 supported the notion that int-1 may have a role in normal development. In the last few years it has been recognized that int-1 and Wingless belong to a large family of related glyco-proteins found in vertebrates and invertebrates. In recognition of this, members of this family have been renamed Wnts, an amalgam of int and Wingless. Investigation of Wnt genes in Xenopus and mouse indicates that Wnts have a role in cell proliferation, differentiation and body axis formation. Further analysis in Drosophila has revealed that Wingless function is required in several developmental processes in the embryo and imaginal discs. In addition, a genetic approach has identified some of the molecules required for the transmission and reception of the Wingless signal. We will review recent data which have contributed to our growing understanding of the function and mechanism of Drosophila Wingless signaling in cell fate determination, growth and specification of pattern.

Interactions of decapentaplegic, wingless, and Distal-less in the Drosophila leg

The genes decapentaplegic, wingless, and Distalless appear to be instrumental in constructing the anatomy of the adult Drosophila leg. In order to investigate how these genes function and whether they act coordinately, we analyzed the leg phenotypes of the single mutants and their inter se double mutant compounds. In decapentaplegic the tarsi frequently exhibit dorsal deficiencies which suggest that the focus of gene action may reside dorsally rather than distally. In wingless the tarsal hinges are typically duplicated along with other dorsal structures, confirming that the hinges arise dorsally. The plane of symmetry in double-ventral duplications caused by decapentaplegic is virtually the same as the plane in double-dorsal duplications caused by wingless. It divides the fate map into two parts, each bisected by the dorsoventral axis. In the double mutant decapentaplegic wingless the most ventral and dorsal tarsal structures are missing, consistent with the notion that both gene products function as morphogens. In wingless Distal-less compounds the legs are severely truncated, indicating an important interaction between these genes. Distal-less and decapentaplegic manifest a relatively mild synergism when combined.

The function of the proneural genes achaete and scute in the spatio-temporal patterning of the adult labellar bristles of Drosophila melanogaster

The sensory precursors for labellar taste bristles develop from the labial disc in three distinct temporal waves occurring at 0 h, 8 h and 14 h of pupal development. In each temporal wave, transcripts for the achaete (ac) and scute (sc) genes are expressed in overlapping patterns in cells of the disc epithelium prior to the appearance of sensory mother cells (SMCs). No bristles form in mutant flies in which the ac and sc genes are absent. When the sc gene alone is deleted, a set of seven bristles fail to form. Pulses of ubiquitous sc ⁺ expression during pupal development, in a strain mutant for both ac and sc, can result in flies with all the labellar bristles at their correct positions. sc ⁺ pulses at times corresponding to the initiation of each of the waves of SMC specification in the disc was sufficient to restore bristle pattern. Bristles were not induced at ectopic positions and times as a result of the ubiquitous expression of sc ⁺. These results suggest that the proneural genes ac and sc do not themselves set the pattern of the labellar bristles. Instead, they are required for the elaboration of the pattern set by other gene products. We also show that the formation and positioning of the later waves of bristles can take place even in the absence of bristles normally specified earlier.

dishevelled is required during wingless signaling to establish both cell polarity and cell identity

The dishevelled gene of Drosophila is required to establish coherent arrays of polarized cells and is also required to establish segments in the embryo. Here, we show that loss of dishevelled function in clones, in double heterozygotes with wingless mutants and in flies bearing a weak dishevelled transgene leads to patterning defects which phenocopy defects observed in wingless mutants alone. Further, polarized cells in all body segments require dishevelled function to establish planar cell polarity, and some wingless alleles and dishevelled; wingless double heterozygotes exhibit bristle polarity defects identical to those seen in dishevelled alone. The requirement for dishevelled in establishing polarity in cell autonomous. The dishevelled gene encodes a novel intracellular protein that shares an amino acid motif with several other proteins that are found associated with cell junctions. Clonal analysis of dishevelled in leg discs provides a unique opportunity to test the hypothesis that the wingless dishevelled interaction species at least one of the circumferential positional values predicted by the polar coordinate model. We propose that dishevelled encodes an intracellular protein required to respond to a wingless signal and that this interaction is essential for establishing both cell polarity and cell identity.

Positional information and pattern formation in development

A widely used mechanism for pattern formation is based on positional information: cells acquire positional identities as in a coordinate system and then interpret this information according to their genetic constitution and developmental history. In Drosophila maternal factors establish the axes and set up a maternal system of positional information on which further patterning is built. There is a cascade of gene activity which leads both to the development of periodic structures, the segments, and to their acquiring a unique identity. This involves the binding of transcription factors to regulatory regions of genes to produce sharp thresholds. Many of the genes involved in these processes, particularly the Hox complex, are also involved in specifying the body axis and limbs of vertebrates. There are striking similarities in the mechanisms for specifying and recording positional identity in Drosophila and vertebrates.

The TGF-beta superfamily: new members, new receptors, and new genetic tests of function in different organisms

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The segment polarity gene hedgehog is required for progression of the morphogenetic furrow in the developing Drosophila eye

Cell-type specification in the Drosophila compound eye begins at the morphogenetic furrow. The furrow sweeps across the developing eye epithelium and is coincident with four classes of cellular events: coordinated changes in cell shape, changes in gene expression, synchronization of the cell cycle, and the specification of a regular array of ommatidial founder cells. The molecular mechanisms that induce these events in the developing eye have hitherto been unknown. We identify here a gene specifically required for furrow progression, hedgehog (hh). We show that hh expression posterior to the morphogenetic furrow is continuously required for its progression. We propose that forward diffusion of hh protein induces anterior cells to enter the furrow.

Pattern formation in the limbs of Drosophila: bric a brac is expressed in both a gradient and a wave-like pattern and is required for specification and proper segmentation of the tarsus

We have identified the gene bric a brac and show that it is required for pattern formation along the proximal-distal axis of the leg and antenna of Drosophila. In bric a brac mutant legs, the bristle pattern of the three central tarsal segments is transformed towards the pattern of the most proximal tarsal segment. In addition, bric a brac mutant legs and antennae have segmentation defects. bric a brac encodes a nuclear protein that shares a highly conserved domain with two transcription factors from Drosophila. bric a brac function is dosage dependent and is required in a graded manner for the specification of tarsal segments. The graded requirement for bric a brac correlates with its graded expression pattern, suggesting that the concentration of BRIC A BRAC protein specifies segment identity in the tarsus.