Positional information in imaginal discs transformed by homoeotic mutations

Summary Mutations of the bithorax complex result in segmental transformations in the thorax and abdomen ofDrosophila. The haltere discs from larvae homozygous forbx ³ orpbx are transformed so that the discs contain cells that will produce wing cuticle as well as cells that produce haltere cuticle. The pattern regulation behavior of these discs has been examined. The fate maps of the two discs were established, and then the regulative behavior of a number of fragments from both types of mutant discs was established by culturing the fragments in vivo prior to metamorphosis. The most important conclusion from this work is that the cells producing, haltere cuticle and wing cuticle within the same disc share the same positional information and that they communicate during pattern regulation.     

A DETAILED FATE MAP OF THE WING DISC OF SARCOPHAGA PEREGRINA

In order to construct an anlageplan of the imaginal disc, various fragments of the wing disc from mature larvae of Sarcophaga peregrina were implanted into host larvae of the same age. The implants were metamorphosed with the host and the differentiated adult structures were analyzed.
On the basis of the results obtained from the transplanted disc fragments, a fate map of the wing disc was satisfactorily constructed.
The fate map of S. peregrina is similar to an earlier study of Drosophila , but the PAA, PWP, and AP are located in different positions from those of Drosophila .     

Early development of the thoracic discs ofDrosophila

Summary We estimate the number of blastoderm cells which generate the thoracic imaginal discs ofDrosophila. At hatching the wing disc is twice the size of the haltere disc, but the results suggest that both discs develop from a similar number of blastoderm cells. Two homeotic mutations, which transform the haltere into wing, affect embryonic growth but not the primordial number. All the segmental primordia may be of similar size and each may be similarly subdivided into a larger anterior, and a smaller posterior polyclone.     

Gynandromorph analysis of the thoracic disc primordia inDrosophila melanogaster

Summary Cell lineage relationships observable inDrosophila gynandromorphs have been used to locate the primordia of the individual thoracic disc relative to each other in the embryo. Three observations indicate that the borders of the individual disc primordia lie very close to each other, separated by few if any non-disc cells.First, the frequency of mosaicism within each disc indicates large primordia, of similar magnitude to the distances between the centres of adjacent primordia.Second, very few XX/XO-borders cut between adjacent discs without also cutting into one or the other disc.Third, sturt distances between points on adjacent discs are often much smaller than distances measured within individual discs. The proximity of disc primordia suggests that the individual discs might share common precursor cells in the early embryo.     

Analysis of morphogenetic movements in the development of the notum anlage of Drosophila melanogaster

Summary A comparison of the morphogenetic maps of the notum anlage of Drosophila melanogaster derived from the gynandromorph data and mosaics induced by somatic crossing-over during the first instar larval stage revealed that practically no major morphogenetic movements occur in the development of the anlage between the blastoderm and first instar larval stages and the adult stage. By comparing the morphogenetic map derived from gynandromorphs and the fate map derived from data on the transplantation of fragments of the mature wing imaginal disc, it was observed that no major morphogenetic movements occur in the notum anlage between the stages of the allocation of the disc and the mature disc. The results are consistent with the observations of other authors concerning the larval development of eye-antenna, wing and leg discs.     

Early development of leg and wing primordia in the Drosophila embryo

The development of the leg and wing primordia in the Drosophila embryo has been traced using molecular markers. Distal-less and disconnected gene expression provide molecular labels for the leg primordia throughout embryonic development, disconnected expression in the developing leg primordia depends on Distal-less activity. The leg primordia arise as discrete clusters of cells that occupy well defined positions in the embryonic ectoderm. At later stages of embryogenesis the primordia become morphologically recognizable and are intimately associated with the development of the Keilin's organs. The presumptive leg disc and the Keilin's organ appear to derive from a common primordium. Similarly the Abnormal leg pattern gene provides a molecular label for the wing and haltere primordia. The dorsal thoracic primordia appear to be of independent origin from the legs.     

The wings ofBombyx mori develop from larval discs exhibiting an early differentiated state: a preliminary report

Lepidopteran insects present a complex organization of appendages which develop by various mechanisms. In the mulberry silkworm,Bombyx mori a pair of meso- and meta-thoracic discs located on either side in the larvae gives rise to the corresponding fore- and hind-wings of the adult. These discs do not experience massive cell rearrangements during metamorphosis and display the adult wing vein pattern. We have analysed wing development inB. mori by two approaches, viz., expression of patterning genes in larval wing discs, and regulatory capacities of larval discs following explantation or perturbation. Expression of Nubbin is seen all over the presumptive wing blade domains unlike inDrosophila, where it is confined to the hinge and the wing pouch. Excision of meso- and meta-thoracic discs during the larval stages resulted in emergence of adult moths lacking the corresponding wings without any loss of thoracic tissues suggesting independent origin of wing and thoracic primordia. The expression of wingless and distal-less along the dorsal/ventral margin in wing discs correlated well with their expression profile in adultDrosophila wings. Partially excised wing discs did not showin situ regeneration or duplication suggesting their early differentiation. The presence of adult wing vein patterns discernible in larval wing discs and the patterns of marker gene expression as well as the inability of these discs to regulate growth suggested that wing differentiation is achieved early inB. mori. The timings of morphogenetic events are different and the wing discs behave like presumptive wing buds opening out as wing blades inB. mori unlike evagination of only the pouch region as wing blades seen inDrosophila.     

Testing homology with morphology, development and gene expression: sex-specific thoracic appendages of the ant Diacamma

Females of the ants belonging to the queenless genus Diacamma have a pair of unique tiny thoracic appendages, called "gemmae," located on the mesothoracic segment. They are covered with sensory hairs, filled with exocrine glands and are involved in the behavioral regulation of reproduction. We report here a morphological, developmental, and genetic study of the development of the gemmae. Both male and female larvae have dorsal mesothoracic discs, although differing in shape and fate. In Diacamma ceylonense, we show that, contrary to butterflies, these discs specify parts of the adult thorax in addition to wing tissues, as in Drosophila. We have cloned and studied the expression of wingless (wg) and scalloped (sd), two genes known to play a critical role in wing morphogenesis in Drosophila. In the fly's mesothoracic dorsal disc, sd is specifically expressed in the wing pouch. In Diacamma, we show that sd is also expressed in male dorsal thoracic discs, whereas its expression was undetectable in females. From this result and observations of shape and growth of cultured isolated discs, we suggest that gemmae originate from a more ventral part of the dorsal disc than the wing pouch and discuss the pro and cons of gemma/wing homology.     

The role of Wg signaling in the patterning of embryonic leg primordium in Drosophila

Cellular interaction between the proximal and distal domains of the limb plays key roles in proximal-distal patterning. In Drosophila, these domains are established in the embryonic leg imaginal disc as a proximal domain expressing escargot, surrounding the Distal-less expressing distal domain in a circular pattern. The leg imaginal disc is derived from the limb primordium that also gives rise to the wing imaginal disc. We describe here essential roles of Wingless in patterning the leg imaginal disc. Firstly, Wingless signaling is essential for the recruitment of dorsal-proximal, distal, and ventral-proximal leg cells. Wingless requirement in the proximal leg domain appears to be unique to the embryo, since it was previously shown that Wingless signal transduction is not active in the proximal leg domain in larvae. Secondly, downregulation of Wingless signaling in wing disc is essential for its development, suggesting that Wg activity must be downregulated to separate wing and leg discs. In addition, we provide evidence that Dll restricts expression of a proximal leg-specific gene expression. We propose that those embryo-specific functions of Wingless signaling reflect its multiple roles in restricting competence of ectodermal cells to adopt the fate of thoracic appendages.     

Determination and Regulation Capacities of the Wing Disc of the Fleshfly, Sarcophaga peregrina

Unilateral extirpation of the wing discs was performed on mid- and late-third instar larvae of Sarcophaga peregrina to investigate the regulation capacities of the thoracic discs. Out of 636 operated larvae, 175 became adult and all of them exhibited hemithoracic structures, lacking any adult structures in the operated side, which were normally produced from a wing disc. These findings suggest that the wing disc does not possess the capacity of bicentric regulation to produce the thoracic structures of the opposite side.     

Chironomus tentans epithelial cell lines sensitive to ecdysteroids, juvenile hormone, insulin and heat shock

A new culture medium, ZW, and the preparation of an extract of adult Drosophila, FX, are described, which for the first time allow the in vitro proliferation of normal Drosophila cells in the absence of undefined heterologous components. Cells from 6-hour-old Drosophila embryos can extensively differentiate and/or proliferate in ZW supplemented with FX and insulin. Cells isolated from wing discs of 90–120-hour-old larvae require ecdysterone for proliferation in ZW, in addition to FX and insulin. Explanted ovaries, testes, genital discs and intact or halved wing discs of 100-hour-old larvae grow in the same medium, at least in part due to cell proliferation. High concentrations of ecdysterone prevent differentiation and/or proliferation of cells from embryos and from wing discs and cause the lysis of most isolated imaginal disc cells grown in vitro, while cuticular differentiations are induced in wing discs and disc fragments grown in vitro.     

The influence of morphogene Wg on the formation of an ectopic eye in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

Due to the ectopic expression of the ey gene in the wing imaginal disc under the action of the 1096-Gal4 driver, a part of the wing disc cells change their fate and become eye cells. Ectopic eyes are induced in definite regions of the wing disc and form a stable pattern on the wing of an adult fly. Here, we have shown that the ectopic expression of Wg inhibits the formation of ectopic eyes, and conversely the expression of Wg is reduced in the sites of ectopic Ey expression. Experiments with overexpression of the vesicular traffic protein Hrs capable of inhibiting the Wg signaling agree with the notion on antagonism of Wg and Ey in ectopic eyes. Our results confirm that the processes of formation of normal and ectopic eyes are principally similar with regard to genetic control.     

A gross study of the compartmentalized stomach of two new-world camelids, the llama and guanaco

The origin of supernumerary gonapophyses in a polymorphic variant of Aedes stimulans has been determined histologically. Genotypic males (Mm) will develop gonapophyses on abdominal segment 8 when primordia on the larvae are subjected to a high-to-low thermal regimen. The gonapophyses develop from the normally evanescent podal buds of the eighth imaginal disc. Their course of development is similar to that of the normal gonapophyses that arise from the podal buds of the ninth imaginal disc.     

Transdetermination of imaginal wing disc fragments of Drosophila melanogaster

Fragments of the imaginal wing disc of Drosophila melanogaster were cultured in adult hosts before transfer to larvae for metamorphosis. Transdetermination occurred only after at least 2 weeks of culture in vivo, producing structures of the leg, antenna, head, and thoracic spiracle. Details of the transdetermined structures and their locations with respect to normal wing disc structures are reported. We present evidence suggesting that regulation can occur between the wing and the second leg imaginal discs, and we propose that many transdeterminations which involve neighboring discs may result from such interdisc regulation.     

Over-expression of Ultrabithorax alters embryonic body plan and wing patterns in the butterfly Bicyclus anynana

In insects, forewings and hindwings usually have different shapes, sizes, and color patterns. A variety of RNAi experiments across insect species have shown that the hox gene Ultrabithorax (Ubx) is necessary to promote hindwing identity. However, it remains unclear whether Ubx is sufficient to confer hindwing fate to forewings across insects. Here, we address this question by over-expressing Ubx in the butterfly Bicyclus anynana using a heat-shock promoter. Ubx whole-body over-expression during embryonic and larvae development led to body plan changes in larvae but to mere quantitative changes to adult morphology, respectively. Embryonic heat-shocks led to fused segments, loss of thoracic and abdominal limbs, and transformation of head limbs to larger appendages. Larval heat-shocks led to reduced eyespot size in the expected homeotic direction, but neither additional eyespots nor wing shape changes were observed in forewings as expected of a homeotic transformation. Interestingly, Ubx was found to be expressed in a novel, non-characteristic domain – in the hindwing eyespot centers. Furthermore, ectopic expression of Ubx on the pupal wing activated the eyespot-associated genes spalt and Distal-less, known to be directly repressed by Ubx in the fly?s haltere and leg primordia, respectively, and led to the differentiation of black wing scales. These results suggest that Ubx has been co-opted into a novel eyespot gene regulatory network, and that it is capable of activating black pigmentation in butterflies.     

The Transcription Factor Optomotor-Blind Antagonizes Drosophila Haltere Growth by Repressing Decapentaplegic and Hedgehog Targets

In Drosophila, decapentaplegic, which codes for a secreted signaling molecule, is activated by the Hedgehog signaling pathway at the anteroposterior compartment border of the two dorsal primordia; the wing and the haltere imaginal discs. In the wing disc, Decapentaplegic and Hedgehog signaling targets are implicated in cell proliferation and cell survival. However, most of their known targets in the wing disc are not expressed in the haltere disc due to their repression by the Hox gene Ultrabithorax. The T-box gene optomotor-blind escapes this repression in the haltere disc, and therefore is expressed in both the haltere and wing discs. Optomotor-blind is a major player during wing development and its function has been intensely investigated in this tissue, however, its role in haltere development has not been reported so far. Here we show that Optomotor-blind function in the haltere disc differs from that in the wing disc. Unlike its role in the wing, Optomotor-blind does not prevent apoptosis in the haltere but rather limits growth by repressing several Decapentaplegic and Hedgehog targets involved both in wing proliferation and in modulating the spread of morphogens similar to Ultrabithorax function but without disturbing Ultrabithorax expression.     

Wound healing in the imaginal discs of Drosophila. I. Scanning electron microscopy of normal and healing wing discs.

Scanning electron microscopy was used to investigate the morphology of intact imaginal wing discs of third-instar larvae of Drosophila melanogaster. The disc stalk, nerve and tracheal entries and the surface ultrastructure of the columnar cells, the peripodial membrane cells, and the adepithelial cells are described. The behavior of various fragments of the wing disc during culture in vivo was also studied. After injuring a wing disc by cuts with a tungsten needle, during the first day of culture the epithelium curls and the wound surface contracts. Subsequent closure of the wound in $\text{3}\text{4}$ and $\text{1}\text{4}$ sectors, in fragments generated by straight cuts and in central squares, leads to the confrontations of cells from formerly separate positions, as was proposed in connection with the polar coordinate model of French, Bryant, and Bryant [(1976). Pattern regulation in epimorphic fields. Science193, 969–981]. Wound healing comprises three steps: (1) Cell debris is removed; (2) occasional cell processes span the wound; (3) all cells at the wound edge contact cells on the opposite side. After 2–3 days, a continuous epithelium is re-established. The tissue distortion may lead to transient contacts of the columnar epithelium with the peripodial membrane and with itself. The latter can explain the occasional duplications of structures which, according to the fate map, arise from near the wound edge, and which have been previously reported from cultured imaginal disc fragments. The tissue movements appear to be due to the contractile properties of individual cells.     

Regenerative interactions between Drosophila imaginal discs of different types.

We have tested the ability of fragments of one type of imaginal disc to stimulate regeneration of another type. It has been shown by others that, when extreme proximal and distal fragments of the wing disc are combined, intercalary regeneration of the missing tissue ensues. Each fragment, if cultured alone, will merely duplicate its structures. We now find that distal fragments of other thoracic discs, haltere and leg, while retaining their autonomy for differentiation, also interact with proximal wing tissue to promote regeneration of more distal wing structures. The proximal wing tissue used in these experiments was the wingless abnormal wing disc which, in the absence of interaction, yields only proximal wing structures. These results suggest that spatial organization is controlled by similar systems in the various thoracic discs. In contrast, head and genital disc material provided no regenerative stimulus to the mutant wing disc tissue.