The extramacrochaetae gene provides information for sensory organ patterning.

The Drosophila adult epidermis displays a stereotyped pattern of bristles and other types of sensory organs (SOs). Its generation requires the proneural achaete (ac) and scute (sc) genes. In the imaginal wing disc, the anlage for most of the thoracic and wing epidermis, their products accumulate in groups of cells, the proneural clusters, whose distribution prefigures the adult pattern of SOs. These proteins then induce the emergence of SO mother cells (SMCs). Here, we show that the extramacrochaetae (emc) gene, an antagonist of the proneural function, is another agent that contributes to SO positioning. In the wing disc, emc is expressed in a complex and evolving pattern. SMCs appear not only within proneural clusters but also within minima of emc expression. When one of these spatial restrictions is eliminated, by ubiquitously expressing ac-sc, SMCs still emerge within minima of emc. When in addition, the other spatial restriction is reduced by decreasing emc expression, many ectopic SMCs emerge in a relatively even spaced and less constant pattern. Thus, the heterogeneous distribution of the emc product is one of the elements that define the positions where SMCs arise. emc probably refines SMC (and SO) positioning by reducing both the size of proneural clusters and the number of cells within clusters that can become SMCs.     

Larval legs of mulberry silkworm Bombyx mori are prototypes for the adult legs

Morphological diversity of leg appendages is one of the hallmarks of developmental evolution. Limbs in insects may develop either from their embryonic prototypes or from imaginal discs harbored inside the larva. Bombyx mori (B. mori), a Lepidopteran insect, develops adult wings from larval wing imaginal discs. However, it has been debated whether the adult legs of B. mori arise from imaginal discs or from the larval legs. Here we addressed how the larval legs relate to their adult counterparts. We present the morphological landmarks during early leg development. We used expression of developmental genes like Distalless and extradenticle to mark leg primordia. Finally, we employed classical excision approach to develop a fate map of the adult leg. Excision and ablation of thoracic legs along proximo-distal axis at various times during larval development resulted in the loss of corresponding adult leg segments. Our data suggest that B. mori legs develop from larval appendages rather than leg imaginal discs.     

Distribution of cuticular protein mRNAs in silk moth integument and imaginal discs

The distributions of mRNAs for two cuticular proteins of Hyalophora cecropia were examined with RT-PCR and in situ hybridization. For major regions of larval and pupal cuticle, there was a strong correspondence between the type of cuticle and the predominant cuticular protein message found. Epidermal cells underlying soft cuticle had mRNA for HCCP12, with a RR-1 consensus attributed to soft cuticle, while the epidermal cells associated with hard cuticle had predominantly mRNA for HCCP66, a protein with the RR-2 consensus attributed to hard cuticle. Both messages were found in all areas of the pupal fore- and hind-wings, with modest area-specific difference in concentration being much less than differences in the relative abundance of these cuticular proteins.

mRNA for HCCP12 was present in imaginal discs of feeding larvae of H cecropia. Data from Bombyx mori available at SilkBase (http://www.ab.a.u-tokyo.ac.jp/silkbase/) revealed that imaginal discs from feeding larvae had abundant mRNA for RR-1 cuticular proteins, representing six distinct gene products. Only discs from spinning larvae had mRNAs that coded for RR-2 proteins arising from 10 distinct genes. Thus, lepidopteran wing imaginal discs can no longer be regarded as inactive in larval cuticle production.     

The role of nutrition in creation of the eye imaginal disc and initiation of metamorphosis in Manduca sexta

With the exception of the wing imaginal discs, the imaginal discs of Manduca sexta are not formed until early in the final larval instar. An early step in the development of these late-forming imaginal discs from the imaginal primordia appears to be an irreversible commitment to form pupal cuticle at the next molt. Similar to pupal commitment in other tissues at later stages, activation of broad expression is correlated with pupal commitment in the adult eye primordia. Feeding is required during the final larval instar for activation of broad expression in the eye primordia, and dietary sugar is the specific nutritional cue required. Dietary protein is also necessary during this time to initiate the proliferative program and growth of the eye imaginal disc. Although the hemolymph titer of juvenile hormone normally decreases to low levels early in the final larval instar, eye disc development begins even if the juvenile hormone titer is artificially maintained at high levels. Instead, creation of the late-forming imaginal discs in Manduca appears to be controlled by unidentified endocrine factors whose activation is regulated by the nutritional state of the animal.     

Organising activities of engrailed, hedgehog, wingless and decapentaplegic in the genital discs of Drosophila melanogaster

 The genes engrailed (en), hedgehog (hh), wingless (wg) and decapentaplegic (dpp) have been shown to play vital organising roles in the development and differentiation of thoracic imaginal discs. We have analysed the roles of these genes in organising the development and differentiation of the genital discs, which are bilaterally symmetrical and possess different primordia, namely, the male and female genital primordia and an anal primordium. Our results suggest that the organising activity of en in genital discs programs the normal development and differentiation of the genital disc by regulating the expression of hh. Hh in turn induces wg and dpp, the genes whose products act as secondary signalling molecules. Moreover, the complementary patterns of wg and dpp expression are essential for the bilateral symmetry and are maintained by mutual repression. Received: 20 April 1998 / Accepted 24 June 1998     

Patterning of the Drosophila nervous system: the achaete-scute gene complex.

The genes of the achaete-scute complex (AS-C) confer on cells the ability to become neural precursors. Their expression is restricted to groups of cells, the proneural clusters, which occupy specific positions within the embryo neural anlagen and the larva imaginal discs. Neuroblasts or sensory organ mother cells are born within these clusters. Thus, the patterns of expression of the AS-C genes help to define the topology of the nervous system.     

Intrinsic growth control in the imaginal primordia of Drosophila, and the autonomous action of a lethal mutation causing overgrowth

Cell proliferation in Drosophila imaginal discs appears to be regulated by a disc-intrinsic mechanism involving local cell interactions that also control the formation of patterns of differentiation. This growth-control mechanism breaks down in animals homozygous for the mutation lethal (2) giant discs (l(2)gd) which remain as larvae for up to 9 days longer than normal. During this time cell proliferation continues in the imaginal discs as well as in the imaginal rings for the salivary glands, foregut, and hindgut, so that these tissues become greatly overgrown. When wild-type wing discs from mid-third instar larvae were removed and cultured for up to 28 days in wild-type female adult hosts, they grew and terminated growth at a cell number close to that which would be attained in situ by the time of pupariation. On the other hand, wing discs from l(2)gd homozygotes grew rapidly and continuously when cultivated in wild-type hosts, reached an enormous size, and acquired abnormal folding patterns. Overgrowth of mutant imaginal rings also continued during culture of these tissues in wild-type hosts. We conclude that overgrowth in this mutant is due to an autonomous defect in the imaginal primordia, which requires an extended larval period for its expression in situ.     

Establishment of imaginal discs and histoblast nests in Drosophila

In Drosophila the homeotic genes of the bithorax-complex (BX-C) and Antennapedia-complex (ANT-C) specify the identity of segments. Adult segment primordia are established in the embryo as the histoblast nests of the abdomen and the imaginal discs of the head, thorax and terminalia. We have used a molecular probe for the limb primordia and in vivo culture to describe the nature of the adult primordia in mutants in which the pattern of homeotic gene expression was altered. The results suggest that the histoblast or disc 'mode' of development is initiated by the extended germ band stage through activity of the BX-C and ANT-C and is relatively inflexible thereafter [corrected].     

Allocation and specification of the genital disc precursor cells in Drosophila

The adult structures of Drosophila melanogaster are derived from larval imaginal discs, which originate as clusters of cells within the embryonic ectoderm. The genital imaginal disc is composed of three primordia (female genital, male genital, and anal primordia) that originate from the embryonic tail segments A8, A9, and A10, respectively, and produce the sexually dimorphic genitalia and analia. We show that the genital disc precursor cells (GDPCs) are first detectable during mid-embryogenesis as a 22-cell cluster in the ventral epidermis. Analysis of mutant and double mutant phenotypes of embryonic patterning genes in the GDPCs, together with their expression patterns in these cells, revealed the following with respect to the origins and specification of the GDPCs. The allocation of the GDPCs from the ventral epidermis requires the function of ventral patterning genes, including the EGF receptor and the spitz group of genes. The ventral localization of the GDPCs is further restricted by the action of dorsal patterning genes. Along the anterior-posterior axis, several segment polarity genes (wingless, engrailed, hedgehog, and patched) are required for the proper allocation of the GDPCs. These segment polarity genes are expressed in some, but not all of the GDPCs, indicating that anterior and posterior compartments are not fully established in the GDPCs. In addition, we found that the three primordia of the larval genital disc have already been specified in the GDPCs by the coordinated actions of the homeotic (Hox) genes, abdominal-A, Abdominal-B, and caudal. By identifying how these different patterning networks regulate the allocation and primordial organization of the 22 embryonic precursors of the compound genital disc, we demonstrate that at least some of the organization of the larval disc originates as positional information in the embryo, thus providing a context for further studies on the development of the genital disc.     

Transdetermination: Drosophila imaginal disc cells exhibit stem cell-like potency

Drosophila imaginal discs, the primordia of the adult fly appendages, are an excellent system for studying developmental plasticity. Cells in the imaginal discs are determined for their disc-specific fate (wingness, legness) during embryogenesis. Disc cells maintain their determination during larval development, a time of extensive growth and proliferation. Only when prompted to regenerate do disc cells exhibit lability in their determined identity. Regeneration in the disc is mediated by a localized region of cell division, known as the regeneration blastema. Most regenerating disc cells strictly adhere to their disc-specific identity; some cells however, switch fate in a phenomenon known as transdetermination. Similar regeneration and transdetermination events can be induced in situ by misexpression of the signaling molecule wingless. Recent studies indicate that the plasticity of disc cells during regeneration is associated with high morphogen activity and the reorganization of chromatin structure. Here we provide both a historical perspective of imaginal disc transdetermination, as well as discuss recent findings on how imaginal disc cells acquire developmental plasticity and multipotency. We also highlight how an understanding of imaginal disc transdetermination can enhance an understanding of developmental potency exhibited by stem cells.     

The morphostatic actions of juvenile hormone

The maintenance of "status quo" in larvae by juvenile hormone (JH) involves both the programming of ecdysteroid-dependent synthesis during the molt and the suppression of morphogenetic growth during the intermolt. The latter morphostatic action does not require ecdysteroids, and has been studied in the formation of imaginal discs in Manduca sexta. Preultimate larval instars have both invaginated discs and imaginal primordia, both of which grow isomorphically with the larva. In the last instar, the young discs/primordia initiate the morphogenesis and patterning that results in a mature disc. JH suppresses both the initiation and progression of the signaling that transforms immature discs or primordia into a fully patterned imaginal disc. This transformation normally occurs in the context of the rapid growth of the last larval stage, and nutrient-dependent factors appear to be able to override the JH suppression. The morphostatic action of JH may have been important for the evolution of the larval stage. Studies on embryos of basal, hemimetabolous insects show that their premature exposure to JH can truncate patterning programs and cause precocious tissue maturation, factors essential for organizing a novel larval form. This suppression of embryonic patterning then results in embryonic fields that remain dormant as long as JH is present. These are the primordia that can transform into imaginal discs once JH disappears in preparation for metamorphosis.     

Insulin/IGF signaling regulates the change in commitment in imaginal discs and primordia by overriding the effect of juvenile hormone

At the beginning of the final larval (fifth) instar of Manduca sexta, imaginal precursors including wing discs and eye primordia initiate metamorphic changes, such as pupal commitment, patterning and cell proliferation. Juvenile hormone (JH) prevents these changes in earlier instars and in starved final instar larvae, but nutrient intake overcomes this effect of JH in the latter. In this study, we show that a molecular marker of pupal commitment, broad, is up-regulated in the wing discs by feeding on sucrose or by bovine insulin or Manduca bombyxin in starved final instar larvae. This effect of insulin could not be prevented by JH. In vitro insulin had no effect on broad expression but relieved the suppression of broad expression by JH. This effect of insulin was directly on the disc as shown by its reduction in the presence of insulin receptor dsRNA. In starved penultimate fourth instar larvae, broad expression in the wing disc was not up-regulated by insulin. The discs became responsive to this action of insulin during the molt to the fifth instar together with the ability to become pupally committed in response to 20-hydroxyecdysone. Thus, the Manduca bombyxin acts as a metamorphosis-initiating factor in the imaginal precursors.     

Developmental genetics of the Drosophila gut: specification of primordia, subdivision and overt-differentiation.

The Drosophila gut is composed of three major parts, the foregut, midgut and hindgut, which arise from anterior and posterior invaginations of the early blastoderm. We review the process of the specification of the gut primordia, subsequent subdivision and region-specific cell differentiation in terms of developmental genetics. Graded activities of maternal signals at anterior and posterior terminal domains of the blastoderm, being mediated by activities of two zygotic gap genes, tailless and huckebein, lead to the activation of key genes that determine the gut primordia: serpent (GATA factor gene) for the endodermal midgut; brachyenteron (Brachyury homolog) for the ectodermal hindgut. fork head (HNF-3 homolog) and caudal (Cdx homolog) are also essential for the development of all gut primordia or hindgut primordium, respectively. Subdivision of the midgut epithelium is regulated by inductive signals emanating from the visceral mesoderm, which is under the control of HOM-C genes. In contrast, pattern formation of the ectodermal foregut and hindgut is regulated by secreted signaling molecules, such as Wingless (Wnt homolog), Hedgehog and Decapentaplegic (Bmp-4 homolog), as in the case of segmented structures and imaginal discs. Finally, the gut is subdivided into at least 36 compartments that are recognized asminimum tissue units of regional differentiation. A few genes that are responsible for determining and maintaining the state of overt-differentiation of the compartments have also been reported. A marked feature of the genetic mechanism of the gut development is the unexpectedly wide spectrum of the similarities of relevant genes and regulatory pathways of gene expression between Drosophila and vertebrates, which may imply a prototypic style of body plan common to protostomes and deuterostomes.     

Vein-positioning in the Drosophila wing in response to Hh; new roles of Notch signaling

The Drosophila wing is a classical model for studying the generation of developmental patterns. Previous studies have suggested that vein primordia form at boundaries between discrete sectors of gene expression along the antero-posterior (A/P) axis in the larval wing imaginal disc. Observation that the vein marker rhomboid (rho) is expressed at the centre of wider vein-competent domains led to propose that narrow vein primordia form first, and produce secondary short-range signals activating provein genes in neighbouring cells (see Curr. Opin. Genet. Dev. 10 (2000) 393). Here, we examined how the central L3 and L4 veins are positioned relative to the limits of expression of Collier (Col), a dose-dependent Hedgehog (Hh) target activated in the wing A/P organiser. We found that rho expression is first activated in broad domains adjacent to Col-expressing cells and secondarily restricted to the centre of these domains. This restriction which depends upon Notch (N) signaling sets the L3 and L4 vein primordia off the boundaries of Col expression. N activity is also required to fix the anterior limit of Col expression by locally antagonising Hh activation, thus precisely positioning the L3 vein primordium relative to the A/P compartment boundary. Experiments using Nts mutants further indicated that these two activities of N could be temporally uncoupled. Together, these observations highlight new roles of N in topologically linking the position of veins to prepattern gene expression.