Pupal Wing Research Articles

Genes expressed during the differentiation of the pupal wings are also transiently expressed during the larval moult in Bombyx mori.

By a differential screening of cDNA libraries made with RNAs present in differentiating imaginal wing discs of Bombyx mori, we have isolated clones whose expression is dramatically modified during the formation of the pupal wings. The RNAs corresponding to all but one of these clones are also transiently accumulated in the wing primordia during the last larvo-larval moult and in response to 20-hydroxyecdysone injections. If the injections are made to juvenilized larvae the expression of the same genes is stimulated but the range of the stimulation is reduced. It is suggested that in Bombyx, cells of the wings reach the pupal stage of differentiation by steps corresponding to the phases during which high levels of ecdysteroids are present.

Frizzled Gene expression and development of tissue polarity in the Drosophila wing

Almost every cell in the Drosophila pupal wing forms a single, distally pointing cuticular hair. The function of the frizzled (fz) gene is essential for the elaboration of the normal wing hair pattern. In the absence of fz function hairs develop, but they display an abnormal polarity. We have examined the developmental expression of the fz gene at the RNA level via in situ hybridization and at the protein level via Western blotting. We have found that fz is expressed in all regions of the epidermis before, during, and after the fz cold sensitive period. We have also found that fz function is not required for normal fz expression. We have further found that mutations in several other tissue polarity genes do not noticeably alter the expression or the modification state of the Fz protein.

Tissue polarity genes of Drosophila regulate the subcellular location for prehair initiation in pupal wing cells.

The Drosophila wing is decorated with a regular array of distally pointing hairs. In the pupal wing, the hairs are formed from micro-villus like prehairs that contain large bundles of actin filaments. The distal orientation of the actin bundles reveals the proximal-distal polarity within the pupal wing epithelium. We have used F-actin staining to examine early stages of prehair development in both wild-type and mutant pupal wings. We have found a striking correlation between hair polarity and the subcellular location for assembly of the prehair. In a wild-type wing, all of the distally pointing hairs are derived from prehairs that are formed at the distal vertex of the hexagonally shaped pupal wing cells. Mutations in six tissue polarity genes result in abnormal hair polarity on the adult wing, and all also alter the subcellular location for prehair initiation. Based on their cellular phenotypes, we can place these six genes into three phenotypic groups. Double mutant analysis indicates that these phenotypic groups correspond to epistasis groups. This suggests that the tissue polarity genes function in or on a pathway that controls hair polarity by regulating the subcellular location for prehair formation.

Analyses of PS integrin functions during Drosophila development.

The Drosophila position-specific (PS) antigens are homologues of the vertebrate integrins, a family of transmembrane proteins that function in cell-matrix and cell-cell adhesion. The common beta subunit of PS integrins (PS beta) is encoded by the lethal(l)myospheroid gene (mys) and is required during wing, eye and muscle development. By expressing PS beta protein at defined developmental periods, we have shown that PS integrins are required throughout pupation, but not earlier, for normal development of wings. In contrast, the key requirement for PS integrins in eye development occurs only in the late pupa. Furthermore, PS integrins are apparently not required for the differentiation of the ommatidial cells; only for their organization. These results are consistent with roles for PS integrins in the interactions between the wing epithelia during the two phases of pupal wing expansion and in maintaining the attachment of a fully formed fenestrated membrane to the basement membrane of the retina. We have also examined the functional significance of alternative splicing of the transcript of the mys gene using P element-mediated transformation to introduce transgenes producing only one of the two spliced forms of PS beta. We find that either form is sufficient to rescue postembryonic mys phenotypes in the wing, eye and muscle but that both of the two splice forms are necessary to rescue the mys embryonic defects. This result indicates a requirement for the alternative splicing of mys during embryogenesis. The location of the alternative exons suggests that the two forms of the PS beta integrin subunit may interact with alternative alpha subunits and/or ligands.

MRNAs of wing epidermis of the oak silkmoth Antheraea polyphemus and other silkmoths during pupal development

The mRNA populations of wing epidermal cells of the silkmoth Antheraea polyphemus during juvenile hormone induced second pupal development have been analyzed by two techniques. The analysis of the data derived from hybridization of cDNAs to the corresponding mRNAs demonstrate that the mRNAs are complex during the entire period of pupal development. Each mRNA population hybridizes to its cDNA over five orders of Rot and belong to three abundance classes. A comparison of the hybridization of two mRNAs derived from the wing epidermis at the beginning of development and at the end of pupal development with each other's cDNAs shows that only about 60% of the messages are common between the two stages. The 40% of the mRNAs that are new and abundant at the time of second pupal cuticle synthesis have been isolated as pupal specific (null) cDNAs and characterized by hybridization to various mRNAs. Quantitative analysis of the data shows that the mRNAs corresponding to these specific cDNAs are about 200 in number and appear at 4 days of pupal development. The mRNAs derived from the pupal wing epidermis of three other silkmoths can also hybridize to the polyphemus pupal specific cDNAs to varying extents demonstrating the presence of similar mRNAs in their wing epidermis at the appropriate time. The differences in the hybridization patterns seen with the various mRNA populations and the appearance of new and abundant mRNAs at 4 days of pupal development have been correlated with two dimensional gel electrophoretic patterns of the in vitro translation products from the same mRNAs. A comparison of these patterns demonstrate the appearance of several new mRNAs at the time of pupal cuticle synthesis. The translation products of these mRNAs appear to have isoelectric points and molecular weights similar to those of the major pupal cuticular polypeptides. The translation products of the mRNAs derived from the pupal wing epidermis of other silkmoths also yield 2-D gel patterns quite similar to that of polyphemus silkmoth. The hybridization data and the 2-D gel patterns of the translation products of mRNAs isolated from the abdominal and pupal wing epidermis derived from primary pupae are indistinguishable from those of mRNAs derived from secondary pupal wing epidermis.

The development of eyespot patterns on butterfly wings: morphogen sources or sinks?

ABSTRACT We have studied the development of the eyespot colour pattern on the adult dorsal forewing of the nymphalid butterflies, Bicyclus safitza and B. anynana, by cauterising the presumptive eyespot centres (the foci) on the pupal wing. The effects on pattern depended on age at cautery. Early focal cautery (at 1-12 hours after pupation) usually reduced or eliminated the eyespot, while cautery at a non-focal site usually had no effect. These results resemble those of a previous study on another species but, in addition, we find that a later cautery (at 12-24 hours) had the converse effect of generating pattern, so that focal cautery enlarged the anterior eyespot (but usually not the large posterior eyespot) and nonfocal cautery induced a new ectopic eyespot. The effects of cautery on patterning are more extensive by an order of magnitude than the cell death which is caused, so implicating a long-range mechanism, such as a morphogen gradient, in eyespot development. The focus clearly acts to establish the normal eyespot pattern, but a simple source/diffusion model is not supported by the response to late cautery. We suggest two alternative forms of gradient model in which late damage can mimic and augment the action of a focus. In the Source/Threshold model, the focus is a morphogen source, and cautery can remove the focus but also transiently lowers the response threshold in surrounding cells. In the Sink model, the focus generates the gradient by removing morphogen, and cautery can eliminate the focus but it also causes a transient destruction or leakage of morphogen. These models can explain most features of the results of cautery.

Programmed cell death in the wing of Orgyia leucostigma (lepidoptera: Lymantriidae)

Programmed cell death is an integral and ubiquitous phenomenon of development that is responsible for the reduction of wing size in female moths of Orgyia leucostigma (Lymantriidae). Throughout larval and pupal life, cells of the wing epithelium proliferate and interact to form normal imaginal discs and pupal wings in both sexes. But at the onset of adult development, most cells in female O. leucostigma wings degenerate over a brief, 2-day period. Lysosomes and autophagic vacuoles appear in cells of the wing epithelium shortly after it retracts from the pupal cuticle. Hemocytes actively participate in removing the resulting cellular debris. By contrast, epithelial cells in wings of developing adult males of O. leucostigma do not undergo massive cell death. Wing epithelium of female pupae transferred to male pupal hosts behaves autonomously in this foreign environment. By pupation, cells of the female wing apparently are committed to self-destruct even in a male pupal environment. Normal interactions among epithelial cells within the plane of a wing monolayer as well as between the upper and lower monolayers of the wing are disrupted in female O. leucostigma by massive cell degeneration. Despite this disruption, the remaining cells of the wing contribute to the formation of a diminutive, but reasonably proportioned, adult wing with scales and veins.

Components of positional information in the developing wing margin of the Lyra mutant of Drosophila.

A number of parameters characteristic of the wing margin precursor in imaginal discs of Drosophila are known: the zone of non-proliferating cells or ZNC (O'Brochta and Bryant 1985), aldehyde oxidase (AO) and other enzyme staining patterns (Sprey et al. 1982), E1C antigen localization in a narrow band along the margin (Piovant and Lena 1988). To test our hypothesis that such parameters, and others, act in concert to determine margin identity and the positional information that specifies the bristles and hairs appropriate to the anterior, posterior and distal margins, we have examined these parameters in the dominant mutant Lyra, in which much of the anterior and posterior margins is missing. After establishing that Lyra phenotype is already evident in the early pupal wing, we tested the known imaginal disc parameters and found that only Mab E1C (Piovant and Lena 1988) distribution differs from wild type, suggesting that E1C antigen may be a component of positional information. Sibatani's (1983) model for specification of positional information (PI) applied to wing discs predicts the Lyra adult wing shape as well as the reduced distribution of E1C antigen in Lyra wing discs. The model is based on the assumption that specification of positional information depends on interactions of multiple, independent factors. Clonal analysis with shaggy (Simpson et al. 1988 and Ripoll et al. 1988) indicates that factors in addition to E1C antigen contribute to margin PI in Lyra wings and should allow us to test the multi-component hypothesis further.

Characterization of cAMP-dependent protein kinase from the pupal wing epidermis of Manduca sexta

Cyclic AMP-dependent protein kinase (cAMP-PK) activity in the wing epidermis of day zero pupae of Manduca sexta was characterized. The preferred exogenous substrates were histones, subfractions H1 and H2b, casein and protamine sulfate; histone H2a was only phosphorylated moderately, while free base protamine and bovine serum albumin were poor substrates for cAMP-PK. cAMP-PK activity required Mg 2+ and was optimal in the presence of 1 mM Mg 2+. Co 2+ and Mn 2+ did not substitute for Mg 2+, and Ca 2+ inhibited cAMP-PK activity. The effective concentration of cAMP for activation of the cAMP-PK was substantially lower than that of cGMP (EC 50 1.3 × 10 −8 and 1.2 × 10 −6 M, respectively). The type II isozyme of cAMP-PK comprised approx. 75% of the total cytosolic wing cAMP-PK as determined by DEAE anion exchange chromatography. Photoaffinity labeling of the whole cell homogenate with 8-azido cAMP revealed the presence of only type II isozyme. The distribution of the cAMP-PK isozymes was also determined for whole cell homogenates of brain, prothoracic glands, hemolymph, trachea, nerve cord, fat body, muscle and midgut.

Scale Arrangement Pattern in a Lepidopteran Wing. 1. Periodic Cellular Pattern in the Pupal Wing of Pieris rapae: (scale/cellular pattern/regular spacing/wing epidermis/Pieris rapae).

In most species of lepidopteran insects, anteroposterior rows formed by scales are arranged at regular intervals in the adult wing; within each row two kinds of scales are alternately arranged. To investigate the cellular basis for the scale arrangement pattern, we examined cell arrangement in the epidermal monolayer of the pupal wing of a small white cabbage butterfly, Pieris rapae, by scanning electron microscopy and light microscopy. The arrangement of scale precursor cells, closely resembling that of scales in the adult wing, was observed in the wing epidermis of the early pupa. Scale precursor cells are proximodistally elongated and form anteroposterior rows. Within a row two kinds of scale precursor cells are nearly alternately arranged, which is not so precise as the alternation of scales in the adult wing. Individual rows of scale precursor cells are separated by rows of single or double undifferentiated general epidermal cells. Occasionally, arrangement abnormalities occur both in the adult and the pupal wing. The cellular basis for the regular spacing of scale rows is discussed.

Microtubule polarities indicate that nucleation and capture of microtubules occurs at cell surfaces in Drosophila.

Hook decoration with pig brain tubulin was used to assess the polarity of microtubules which mainly have 15 protofilaments in the transcellular bundles of late pupal Drosophila wing epidermal cells. The microtubules make end-on contact with cell surfaces. Most microtubules in each bundle exhibited a uniform polarity. They were oriented with their minus ends associated with their hemidesmosomal anchorage points at the apical cuticle-secreting surfaces of the cells. Plus ends were directed towards, and were sometimes connected to, basal attachment desmosomes at the opposite ends of the cells. The orientation of microtubules at cell apices, with minus ends directed towards the cell surface, is opposite to the polarity anticipated for microtubules which have elongated centrifugally from centrosomes. It is consistent, however, with evidence that microtubule assembly is nucleated by plasma membrane-associated sites at the apical surfaces of the cells (Mogensen, M. M., and J. B. Tucker. 1987. J. Cell Sci. 88:95-107) after these cells have lost their centriole-containing, centrosomal, microtubule-organizing centers (Tucker, J. B., M. J. Milner, D. A. Currie, J. W. Muir, D. A. Forrest, and M.-J. Spencer. 1986. Eur. J. Cell Biol. 41:279-289). Our findings indicate that the plus ends of many of these apically nucleated microtubules are captured by the basal desmosomes. Hence, the situation may be analogous to the polar-nucleation/chromosomal-capture scheme for kinetochore microtubule assembly in mitotic and meiotic spindles. The cell surface-associated nucleation-elongation-capture mechanism proposed here may also apply during assembly of transcellular microtubule arrays in certain other animal tissue cell types.

Host Plants, Temperature and Seasonal Effects on Immature Development and Adult Size of Euploea-Core-Corinna (Lepidoptera, Danainae)

The suitability of four commonly used host plants of Euploea core, i.e. Cryptostegia grandiflora, Hoya australis, Nerium oleander and Parsonsia straminea, was measured in terms of developmental rates, pupal weight and adult wing length. The experimental temperature and seasonal quality of fresh shoots of the host plant affected all these variables. The results of development rates were inconsistent across host plants at different temperatures, but pupal weight and wing length of the butterfly were consistent. Despite great differences in the weight of pupae, pupal developmental rates appeared mainly affected by temperature. Fresh foliage produced in early spring gave significantly better growth of larvae than fresh foliage produced in late spring or autumn. Larvae coped with poor food quality in two distinct ways. One group moulted at approximately the right time, but at an unusually small size, the other group delayed moulting until it reached an appropriate size for moult. Losses were heavy in both groups. Overall, host plants in the Apocynaceae were more suitable for growth than those in the Asclepiadaceae, but whether the host plants were introduced or native, or whether they contained copious quantities of latex or not, did not seem to have a significant effect. The successful use of host plants in the Asclepiadaceae, like the C. grandiflora and ornamental H. australis, is therefore probably related to other factors, such as the frequency with which they produce fresh foliage and their common presence, overriding any minor differences in suitability. Key words: Euploea, developmental rates, host plants, seasonal effects.

The formation of pattern on the wing of the moth,Ephestia kühniella

AbstractTransverse bands of coloured scales are a common feature of lepidopteran wing patterns and it has been suggested that their positions are specified by a propagated ‘Determination Wave’ (Kühn & von Englehardt), or a ‘Gradient’ of diffusible morphogen (Njjhout). We have assessed these models in an experimental study of the formation of bands in the moth, Ephestia kühniella.The banding pattern can be altered by microcautery of the pupal wing between 1 h and 48 h postpupation (at 20°C) and effects are of two types:Local modifications follow early (1–3 h postpupation) cautery located on or between the presumptive sites of the bands; operations more proximally or distally on the wing have no effect. Patterns consist of a loop deflecting the nearest band medially, around the site of cautery; or an isolated ring of band scales surrounding the operation site, with the two bands in their normal positions. The type of pattern formed depends only on the location of the cautery, with rings following medial cautery (midway between the bands).Global modifications follow a 36–48 h cautery anywhere on the wing surface and they consist of a medial displacement of both bands. The degree of displacement is very variable (with band separation reduced to 33–87 % of that on the contralateral control wing) but this does not depend on location of the cautery. Furthermore, the degree of modification does not depend on the precise age at cautery, as an equivalent range of effects is produced at 36 h, at 42 h and at 48 h.The Determination Wave model is disproved since it explicitly assumes that the different degrees of global modification form a temporal sequence (from severe early to mild late) and they do not. Furthermore, this model cannot explain adequately the formation of loop or ring local modifications.The Gradient model can account for many features of the results, but it does not readily explain the ring local modifications or the effect of (36–48 b) cautery in causing a global change in the banding pattern, independent of the site or precise time of operation.

Ecdysteroid action on moth epithelial tissues and cell lines

AbstractThe spontaneous formation of multicellular, fluid‐filled vesicles in primary cultures of pupal wings of Hyalophora cecropia is inhibited by 20‐hydroxyecdysone (20‐OH‐ecdysone). Physiological levels of ecdysteroids inhibit the ability of larval hemolymph to induce such vesicles in an established cell line (IAL‐TND1) derived from imaginal discs of Trichoplusia ni (Hübner). This cell line grew as vesicles for its first year in culture even in the absence of hemolymph, but then converted to growing as multicellular aggregates. Aggregate‐conditioned media but not vesicle‐conditioned or control media displayed ecdysteroid activity in an imaginal disc bioassay. Larval hemolymph had no influence on the imaginal discs' response to ecdysteroids. Hence, its action in promoting vesicle formation is not to prevent the action of ecdysteroids. We postulate that the change in morphology of TND1 cell cultures from vesicles to aggregates was accompanied by their attaining (or enhancing) the capacity to produce ecdysteroids.

Restoration of wing margins in Lyra mutants of Drosophila melanogaster

Lyra is a dominant, homozygous lethal mutation of Drosophila melanogaster; in heterozygotes the wings lack portions of the anterior and posterior margins including the characteristic bristles. We have found that, in addition to the loss of bristle forming cells, there is a decrease in the number of wing surface cells that varies between 10 and 20%. However, we observed no histological evidence of excessive cell death in either the larval discs or the pupal wing precursors in Lyra flies. Restoration of all or part of the normal wing margins occurs in some, but not all, cases of morphogenetic mosaics, in which there were patches of wild-type cells in Lyra wing margins due to irradiation-induced mitotic recombination. Analysis of these restorations, using margin bristles as indicators, shows that the Lyra wild-type gene is not involved in bristle formation per se and further that its expression is not cell autonomous. Instead the effect of the Lyra mutation appears to be associated with development of a margin forming subpopulation of cells and to influence the characteristic pattern of cells and bristles in the wing margin via an inductive interaction. The dorsal-ventral boundary can be demonstrated in the de facto wing margins of Lyra mutants suggesting that its origin is independent of any function Lyra might have in normal wing margin morphogenesis. In wing margin restorations the dorsal-ventral boundary is clearly delimited by trichomes and somewhat less rigorously shown by the margin bristles. Further, in these restorations ventral clones induce dorsal bristles, as well as ventral ones, and vice versa, indicating that the influence of Lyra is not restricted by the dorsal-ventral boundary.

Ecdysone 20‐hydroxylation in imaginal wing discs of Pieris brassicae (lepidoptera): Correlations with ecdysone and 20‐hydroxyecdysone titers in pupae

AbstractEcdysone 20‐hydroxylase activity has been detected in pupal wing discs of Pieris brassicae. This activity is due to an enzyme system located in microsomal fractions. Its apparent Km is 58 nM for ecdysone. The enzyme is inhibited by the reaction product 20‐hydroxyecdysone with an apparent Ki of 2.6 μM. Its activity varied during pupal‐adult development with a maximum on day 4, when ecdysone levels are the highest in the animal. Although low, the peak activity is sufficient to assure 25% of the conversion of endogenous ecdysone into 20‐hydroxyecdysone in pupae. Ecdysone and 20‐hydroxyecdysone levels were measured in hemolymph and whole animals; ecdysone appears to be mainly located in hemolymph, whereas 20‐hydroxyecdysone seems to be equally distributed between hemolymph and tissues. All these findings are discussed in relation to the roles of ecdysone and 20‐hydroxyecdysone during pupal‐adult development.

The development of sensory nerves within the wing ofDrosophila melanogaster : A light- and electron-microscope study.

TheDrosophila wing is a simple structure bearing many sensory neurons which form an elementary pattern of nerves within the veins, and provides a simple system for studying the formation of nerve pathways. A light-and electron-microscope study of the developing pupal wing was undertaken to establish the time at which the different classes of sensillum differentiate and to determine the arrangement of tissues within the wing during sensory axon out-growth in order to assess possible candidates for directing axon outgrowth. Major findings were 1. Wing development passes through three main stages: at 6-12 h after puparium formation the wing is flantened and secreting pupal cuticle; at 15-18 h the wing is dramatically inflated but is still a simple epithelium secreting pupal cuticle; at 21-24 h the wing collapses and begins to differentiate adult structures. 2. There are no persisting larval nerves which might act as pioneers or pathfinders for the later developing adult neurons. 3. Axon bundles are first observed during the 15-18 h stage prior to the pupal moult when the wing is still secreting pupal cuticle. 4. At this stage the wing is an inflated sac, without any veins or orderly arrangements of tracheae which might act as guides for axon outgrowth. Vein formation takes placeafter formation of the axon pathways. 5. The bristle axons grow along the anterior wing margin in close contact with the basal lamina of the epithelial cells, often within a gap between the processes of the epithelial cells, which could mechanically channel their out-growth. 6. The campaniform sensillum axons appear to navigate along the inner surface of the wing epithelium rather like the pioneer axons found in embryonic appendages of other insects. 7. Differentiation of sensory neurons takes place long before differentiation of the other cells associated with sensilla; most axons are present by 18 h when the wing is still secreting pupal cuticle but morphological differentiation of the bristle shaft and socket cells is not observed until about 42 h when the wing is secreting the cuticulin layer of the adult cuticle.

Cuticular proteins of the silkmoth Antheraea polyphemus

Proteins from untanned pupal and adult abdominal cuticles, as well as pupal wing cuticle and adult wing scales from the oak silkmoth Antheraea polyphemus have been solubilized with denaturing solvents: SDS (2%), urea (8 M) and guanidine hydrochloride (6 M). Up to 50% of the proteins can be extracted by the procedure developed and all the solvents extract an approximately similar complement of proteins from the same cuticle. Both pupal and adult cuticles contain from 30 to 40 polypeptides separable on SDS-PAGE of which about ten in each case make up the major proportion. The major proteins fall in the molecular weight range of 15,000 to 45,000. SDS-PAGE and amino acid analyses indicate that (i) the protein composition of pupal and adult cuticles differ from one another (ii) the protein composition of cuticles, from different regions of the body e.g. wing or abdomen, are similar when derived from a particular stage (iii) proteins extracted from scales of adult wings represent a true reflection of the total adult wing cuticle and (iv) only minor differences exist between the proteins of segment and intersegment cuticle. Some of the major individual proteins have been isolated and analysis of amino acids, as well as the nature of digestion products obtained by limited proteolysis, indicate these to be unique. Immunological analyses, however, indicate some structural component to be common among some of the proteins because of good antibody cross reactivity.

Protein synthesis during early adult and second pupal development in the silkmoth, Antheraea polyphemus

Experiments were performed to measure protein synthesis in wing epidermal cells during early pharate adult and second pupal development. The period for juvenile hormone (JH) determination of second pupal wings occurred between 18 and 24 hr after injection of 20-hydroxyecdysone (20-HE) plus JH. Between 12 and 24 hr, [ 35S]-methionine incorporation remained relatively constant in pupae that had received exogenous hormones (either 20-HE alone or 20-HE and JH) and there was no significant difference between these groups. For 12–48 hr after hormone injection, no variation in banding patterns specific to a particular hormone treatment were evident by one-dimensional electrophoresis, despite dramatic changes in protein content at this time. At 3–6 days, differentiation specific protein bands were detected. Two dimensional electrophoresis was then used to examine more closely specific proteins synthesized during the 18–24 period, when developmental reprogramming occurred. At 24 hr, the synthesis of two clusters of proteins and a single protein were detected in samples from 20-HE treated animals but were not detected in samples from animals injected with 20-HE plus JH. No JH-specific proteins were detected. The adult specific proteins may be the product of events concerning the mechanisms of adult commitment.

Epithelial Invagination: Adhesive Properties of Cells Can Govern Position and Directionality of Epithelial Folding

Definitive embryonic forms emerge during animal development as epithelial cell layers repeatedly fold and interact. Inductive influences of adjacent cells specify sites of folding in epithelia. The cellular properties that are altered by induction result in characteristic changes in cell form which accompany folding. The ubiquitous presence of microfilaments and microtubules in cells undergoing invagination has led to the belief that cytoskeletal elements are the causal agents for epithelial folding. Most of the research emphasizing the importance of cytoskeletal elements is based on the use of anticytoskeletal drugs and the premise that these drugs specifically disrupt the architecture of the cytoskeleton. Numerous experiments, however, have shown that these drugs do not exert their effects solely on the cytoskeleton. They also affect cell surface properties. The possibility that localized increases in cell surface adhesivity can orchestrate cellular form changes and subsequent folding movements has been repeatedly considered for theoretical reasons, but rarely tested in the laboratory. The recent finding that invagination or evagination of an epithelial monolayer can be induced following transposition of grafts between proximal and distal extremes of the moth pupal wing has provided new support for this idea. Adhesive properties of wing cells are graded along the proximo-distal axis of the wing, and the direction of induced epithelial folding is related to relative adhesiveness of graft and host cells. Graft invagination only occurs following a transfer of tissue from proximal extreme to distal extreme, whereas graft evagination only accompanies the reciprocal exchange of tissues. Emphasis on the role of cytoskeletal elements in governing the folding of epithelia should be tempered by an appreciation of the importance of cell adhesivity. The observed accumulation of cell surface molecules at sites destined to invaginate may contribute to increased adhesivity of the invaginating population of cells, and oriented microtubules and microfilaments may simply stabilize the cellular configurations assumed as cell surface properties drive changes in cellular form.