Insect Wings: Patterns upon patterns

GEOFFREY

INSECT WINGS INSECT WINGS

NORTH AND VERNON FRENCH

GEOFFREY NORTH AND VERNON FRENCH

Patterns upon patterns Butterfly wing patterns may be generated by an extra coordinate system superimposed on a basic wing-patterning system similar to that of the fruitfly Drosophila melanogaster. Given the morphological detail and precision exhibited in the body pattern of a multicellular organism such as an insect, it is not surprising that the natural generation of that pattern is achieved by a hierarchically arranged sequence of processes. Thus, in the case of the fruitfly Drosophila melanogaster, development begins with the initial specification of the primary body axes, anterior-posterior and dorsal-ventral, followed by division of the former into a sequence of embryonic segments. Small parts of some segments go on to form adult appendages such as legs and wings. The spatial organization of these appendage primordia, the imaginal discs, derives from that of the early segment and is elaborated by pattern-generating processes operating during larval development. Many of the molecular players in these processes are now known, and our understanding of their function is increasing rapidly [1]. While we are still far from completely understanding Drosophila development, biologists are now looking to other insects for clues to the ways in which developmental processes are modified during evolution to give novel morphologies. Such studies can use genes identified and cloned in Drosophila as probes to isolate homologues from other species. In this way, a molecular study is beginning of how the colour patterns of butterfly wings are generated. As reported at the spring meeting of the British Society for Developmental Biology (Edinburgh, 5-8 April), and described in a paper in Science [2], the initial results are striking and informative. They suggest that the mechanism of wing development in the butterfly is similar to that in the fruitfly, but that there is another, superimposed system that specifies the colour pattern of the butterfly wing. To begin with, we shall outline relevant aspects of Drosophilawing development [1]. The adult wings differentiate from imaginal discs, which are formed within the embryonic mesothoracic segment from some of the cells around the intersection of stripes of expression of two genes, wingless (wg) and decapentaplegic (dpp). Both these genes encode secreted signalling molecules (members of the WNT and TGF-3 families, respectively) with important roles in patterning the two axes of the embryonic segment. The wing imaginal discs grow throughout larval stages and evert into wings at metamorphosis. In the 1970s, regeneration experiments led to the formulation of the 'polar coordinate' model [3], in which positional information is organized circumferentially and radially on the larval imaginal disc, and is generated by

short-range cell-cell interactions during development and regeneration. The developing disc is divided by lineage restrictions into 'compartments', and Meinhardt [4] proposed that interactions at compartment borders have a primary role in establishing the framework of positional information. More recently, genetic and molecular studies have identified at least some of the components of the pattern-forming system (Fig. 1). Thus, the posterior compartment is specified by expression of the engrailed (en) gene and, as shown recently [5], the dorsal compartment is specified by the apterous (ap) gene; en and ap both encode homeodomain proteins. This broad subdivision of the disc seems to provide a framework for cell-cell interactions that further elaborate the pattern. In an elegant recent study [6], Basler and Struhl have obtained evidence that en-expressing cells in the posterior compartment of the Drosophila wing disc signal to neighbouring anterior cells by a secreted protein encoded by the hedgehog (hh) gene [7]. This induces expression of another signalling molecule, the product of the dpp gene. Basler and Struhl used a specific recombination system that could be activated by heat shock to express hh ectopically in anterior cells. They found that this led to the induction of ectopic dpp expression and to a striking reorganization of the anterior wing pattern. The wing pattern can be described in terms of specific veins: the normal pattern of veins is 1-2-3 in the anterior compartment, then 4-5 in the posterior compartment (Fig. 1). Ectopic hh expression could induce a secondary, duplicated anterior compartment with vein pattern 1-2-3-3-2-1, centred on the hh-expressing clone. Together with genetic data, the results suggest that the hh signal induces a stripe of dpp expression that provides a signal transmitted both to more anterior cells and back to posterior cells. Boundaries between dorsal and ventral compartments seem also to be important in pattern elaboration. Thus, Williams et al. [8] have recently provided evidence that expression of the vestigial (vg) gene is induced in a stripe centred on the boundary between the dorsal cells that express ap and the ventral cells that do not. Two signals may be involved, from ap+ to ap- cells and vice versa; Williams et al. suggest further that vg, which is required for wing development and encodes a nuclear protein, activates expression of other secreted signalling molecules that play a part in dorsal-ventral patterning. One candidate here is

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Current Biology 1994, Vol 4 No 7 Fig. 1. (a) Drosophila wings develop from imaginal discs, which are initially specified during embryogenesis, grow during larval stages and differentiate into the adult structures during metamorphosis. Anterior-posterior and dorsal-ventral compartment borders are indicated on the larval imaginal disc and the adult structure. During metamorphosis the disc evaginates: the central wing region bulges out and flatterns, apposing its dorsal and ventral surfaces and bringing together the notum and pleura. (b) A number of the genes that control Drosophila wing development are known; the expression patterns of these genes provide positional information in the discs that guides subsequent wing differentiation. For some genes (for example wingless and vestigial only one component of the expression pattern is shown. Cell-cell interactions across boundaries between 'compartments' of the disc appear to be particularly important in pattern elaboration.

tlg, which is expressed along the border in late wing discs

and is required for formation of the wing margin [9]. Although there are many similarities between wing and leg discs [1], proximal-distal patterning may occur rather differently in the sheet-like wing and tube-like leg. The gene Distalless (DII) is expressed in the central (presumptive distal) region of the leg disc [10] and the homeodomain protein that it encodes is thought to play an important role in pattern formation along the proximal-distal axis [1,10]. The Dll gene is also expressed along the dorsal-ventral margin of the late wing disc, although its role is not clear. There is now an intriguing indication, reported by Sean Carroll (University of Wisconsin, Madison) in Edinburgh, that the Dll gene has a special role in the generation of the colour patterns of butterfly wings. These wings are very much larger than Drosophila wings, and have the extra feature of lavish colour patterns. The patterns seem bewildering in their complexity and variety but, as clearly explained by Fred Nijhout [11], they can be understood as variations on a basic 'groundplan' of

pattern elements, mostly bands and concentric eyespots. Analysis of the patterns suggests that the wing veins, the distal wing margin and specific patterning 'foci' may be important in their generation. Butterfly wings develop from imaginal discs, and Nijhout has shown, by simple and elegant transplantation and cautery experiments on Precis coenia (Fig. 2), that the eyespot pattern is indeed specified at a late stage (on the pupal wing) by signals from a central focus [12]. In order to understand in molecular detail how butterfly wing patterns are generated, Carroll and his colleagues cloned a number of P coenia homologues of Drosophila genes important in appendage development [2]. These included Dll, ug, dpp, ap and invested (inv, an en-related gene similarly expressed in the posterior disc compartment). In situ hybridization was used to compare the expression patterns of the genes in P. coenia wing discs with the known patterns of their homologues in Drosophila wing discs. The results strongly suggest that the dorsal-ventral, anterior-posterior and proximal-distal axes of the wing

DISPATCH Fig. 2. The dorsal surfaces of the wings of the butterfly Precis coenia display a

colour pattern of eyespots and bands. On the larval wing disc (which is already evaginated and shows the vein pattern) expression patterns of apterous (dorsal), invested (posterior) and wingless (distal) resemble those in Drosophila. The early

expression pattern of Distalless resolves into a spot between veins Cul and Cu2, corresponding in position to the 'focus' that later signals to specify the posterior eyespot (arrows); the nature of this signalling process is, as yet, unknown. (Photographs courtesy of Stephen Paddock and Sean Carroll.)

are specified in similar ways in butterflies and fruitflies. For example, in the P? coenia forewing and hindwing discs, inv is expressed in posterior cells, ap is expressed only by dorsal cells and ug is expressed in presumptive wing-margin cells. These patterns are all strikingly similar to those in which their homologues are expressed in Drosophila wing discs, suggesting that their products may provide a similar framework of primary positional information in the 1? coenia wing discs. Obtaining direct evidence for this will require the development of techniques for the genetic manipulation of P coenia. It is unlikely that the mechanisms will be completely conserved and, indeed, there is no indication in the P? coenia discs of a longitudinal stripe of dpp expression along the anterior-posterior border. The expression pattern of vg in the P? coenia discs is not yet known. As in Drosophila, Dll is initially expressed in a distal zone in the wing discs of P? coenia, but then its expression resolves into a simple pattern that corresponds strikingly to the future wing colour pattern. Thus, in the mid-fifth instar discs, Dll is expressed in spots that correspond to the foci of the posterior (but, curiously, not the anterior) eyespots of the forewing and hindwing. There is, as yet, no direct evidence that Dll plays any role in the later

signalling activities of the eyespot focus, but the similarity between its central expression patterns here and in the Drosophila leg disc invites speculation that the eyespot and distal leg patterning involve similar mechanisms [2] - the 'flat leg' view of eyespots! Much more work and different techniques will be required to reveal the molecular basis of focal signalling and also of the processes that occur earlier, in the growing imaginal disc, to establish the eyespot foci. It seems probable, therefore, that the basic morphological pattern of Pl?coenia wings is probably specified in a similar way to that of Drosophila wings, with a set of genes specifying compartments, the borders of which may signal to generate further positional information. But superimposed on this mechanism are additional systems for generating the colour patterns. The whole insect early embryo forms a primary 'field', within which positional information is specified along two axes anterior-posterior and dorsal-ventral - and the appendage discs can be considered to be secondary fields with their own coordinate system [3]. The new work begins to show how, within secondary fields, further subfields can be established, within which even finer details of the pattern can be specified.

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Current Biology 1994, Vol 4 No 7 Acknowledgements: The authors would like to thank Stephen Paddock and Sean Carroll for their help in preparing this article.

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References

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1. 2. 3. 4. 5. 6. 7.

French V, Daniels G: The beginning and end of insect limbs. Curr Biol 1994, 3:34-37. Carroll SB, Gates J, Keys D, Paddock SW, Panganiban GEF, Selegue I, Williams JA: Pattern formation and eyespot determination in butterfly wings. Science, in press. French V, Bryant PJ,Bryant SV: Pattern regulation in epimorphic fields. Science 1976, 193:969-981. Meinhardt H: Models of Biological Pattern Formation. London: Academic Press; 1982. Diaz-Benjumea F, Coehn SM: Interaction between dorsal and ventral cells in the imaginal disc directs wing development in Drosophila. Cell 1993, 75:741-752. Basler K, Struhl G: Compartment boundaries and the control of Drosophila limb pattern by hedgehog protein. Nature 1994, 368:208-214. Ingham PW: Hedgehog points the way. Curr Biol 1994, 4:347-350.

10. 11. 12.

Williams JA, Paddock SW, Vorwerk K, Carroll SB: Organization of wing formation and induction of a wing-patterning gene at the dorsal/ventral compartment boundary. Nature 1994, 368:299-305. Couso JP, Bishop SA, Martinez-Arias A: The wingless signalling pathway and the patterning of the wing margin in Drosophila. Development 1994, 120:621-636. Cohen SM: Imaginal disc development. In Development of Drosophila. Edited by Martinez-Arias A, Bate M. Cold Spring Harbor: Cold Spring Harbor Press; 1993. Nijhout HF: The Development and Evolution of Butterfly Wing Patterns. Washington: Smithsonian Institution Press; 1991. Nijhout HF: Pattern formation on lepidopteran wings: determination of an eyespot. Dev Biol 1980, 80:267-274.

Geoffrey North is Deputy Editor of Current Biology. Vernon French, Institute of Cell, Animal and Population Biology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK.

THE AUGUST 1994 ISSUE (VOL. 4, NO. 4) OF CURRENT OPINION IN GENETICS AND DEVELOPMENT will include the following reviews, edited by Barbara Meyer and Janet Rossant, on Pattern Formation and Developmental Mechanisms: Dorsal-ventral patterning in Drosophila by T. Schiipbach Neural induction in Xenopus by R. Harland Initiating asymmetries in C. elegans development by J. Priess Cell migration in C. elegant by M. Stern and G. Garriga C. elegans cell fate determination by C. Kenyon Recent advances in vertebrate limb patterning by C. Tabin Axon guidance by chemoattractants in the vertebrate central nervous system by M. Tessier-Lavigne Axon guidance mechanisms in C. elegans by J. Culotti Vulval development in C. elegans by S. Kim Pattern formation in plants by E. Meyerowitz TGFJ3-like genes in development by B. Hogan Cell death in C. elegans by R. Horvitz Notch-related genes and cell interactions by I. Greenwald Genetic approaches of cell-matrix interactions and cell adhesion in development by R. Hynes Wnt gene function, frogs/mice by A. McMahon Wingless pathways in Drosophila by S. DiNardo Retinoic acid and Hox gene regulation by L. Gudas