The development of the imaginal abdomen of Drosophila melanogaster

Juvenile hormone suppresses sensory organ precursor determination to block Drosophila adult abdomen morphogenesis

Juvenile hormone (JH) has a classic "status quo" action at both the pupal and adult molts when administrated exogenously. In Drosophila, treatment with JH at pupariation inhibits the formation of abdominal bristles, which are derived from the histoblasts. However, the mechanism via which JH exerts this effect remains poorly understood. In this study, we analyzed the effect of JH on histoblast proliferation, migration, and differentiation. Our results indicated that whereas the proliferation and migration of histoblasts remained unaffected following treatment with a JH mimic (JHM), their differentiation, particularly the specification of sensor organ precursor (SOP) cells, was inhibited. This effect was attributable to downregulated proneural genes achaete (ac) and Scute (sc) expression levels, which prevented the specification of SOP cells in proneural clusters. Moreover, Kr-h1 was found to mediate this effect of JHM. Histoblast-specific overexpression or knockdown of Kr-h1, respectively mimicked or attenuated the effects exerted by JHM on abdominal bristle formation, SOP determination, and transcriptional regulation of ac and sc. These results indicated that the defective SOP determination was responsible for the inhibition of abdominal bristle formation by JHM, which, in turn, was mainly mediated via the transducing action of Kr-h1.

Tissue growth: Basement membrane degradation triggers cell proliferation

Building of the Drosophila abdomen relies on the removal of larval cells and expansion, through proliferation, of a population of progenitor epithelial cells. A new study shows that matrix metalloproteinases produced by larval cells drive basement membrane degradation and proliferative growth of the progenitor epithelial population.

ECM degradation in the Drosophila abdominal epidermis initiates tissue growth that ceases with rapid cell-cycle exit

During development, multicellular organisms undergo stereotypical patterns of tissue growth in space and time. How developmental growth is orchestrated remains unclear, largely due to the difficulty of observing and quantitating this process in a living organism. Drosophila histoblast nests are small clusters of progenitor epithelial cells that undergo extensive growth to give rise to the adult abdominal epidermis and are amenable to live imaging. Our quantitative analysis of histoblast proliferation and tissue mechanics reveals that tissue growth is driven by cell divisions initiated through basal extracellular matrix degradation by matrix metalloproteases secreted by the neighboring larval epidermal cells. Laser ablations and computational simulations show that tissue mechanical tension does not decrease as the histoblasts fill the abdominal epidermal surface. During tissue growth, the histoblasts display oscillatory cell division rates until growth termination occurs through the rapid emergence of G0/G1 arrested cells, rather than a gradual increase in cell-cycle time as observed in other systems such as the Drosophila wing and mouse postnatal epidermis. Different developing tissues can therefore achieve their final size using distinct growth termination strategies. Thus, adult abdominal epidermal development is characterized by changes in the tissue microenvironment and a rapid exit from the cell cycle.

The Microtubule Minus-End Binding Protein Patronin Is Required for the Epithelial Remodeling in the Drosophila Abdomen

In the developing Drosophila abdomen, the epithelial tissue displays extensive cytoskeletal remodeling. In stark contrast to the spatio-temporal control of the actin cytoskeleton, the regulation of microtubule architecture during epithelial morphogenesis has remained opaque. In particular, its role in cell motility remains unclear. Here, we show that minus-end binding protein Patronin is required for organizing microtubule arrays in histoblast cells that form the Drosophila abdomen. Loss of Patronin results in a dorsal cleft, indicating the compromised function of histoblasts. We further show that Patronin is polarized in these cells and is required for the formation of highly dynamic non-centrosomal microtubules in the migrating histoblasts. Thus, our study demonstrates that regulation of microtubule cytoskeleton through Patronin mediates epithelium remodeling.

miR-965 controls cell proliferation and migration during tissue morphogenesis in the Drosophila abdomen

Formation of the Drosophila adult abdomen involves a process of tissue replacement in which larval epidermal cells are replaced by adult cells. The progenitors of the adult epidermis are specified during embryogenesis and, unlike the imaginal discs that make up the thoracic and head segments, they remain quiescent during larval development. During pupal development, the abdominal histoblast cells proliferate and migrate to replace the larval epidermis. Here, we provide evidence that the microRNA, miR-965, acts via string and wingless to control histoblast proliferation and migration. Ecdysone signaling downregulates miR-965 at the onset of pupariation, linking activation of the histoblast nests to the hormonal control of metamorphosis. Replacement of the larval epidermis by adult epidermal progenitors involves regulation of both cell-intrinsic events and cell communication. By regulating both cell proliferation and cell migration, miR-965 contributes to the robustness of this morphogenetic system.

Elucidation of the regulation of an adult cuticle gene Acp65A by the transcription factor Broad

Broad (BR), an ecdysone-inducible transcription factor, is a major determinant of the pupal stage. The misexpression of BR-Z1 isoform (BR-Z1) during adult development of Drosophila melanogaster prevents the expression of the adult cuticle protein 65A gene (Acp65A). We found that the proximal 237 bp of the 5' flanking region of Acp65A were sufficient to mediate this suppression. A targeted point mutation of a putative BR-Z1 response element (BRE) within this region showed that it was not involved. Drosophila hormone receptor-like 38 (DHR38) is required for Acp65A expression. We found that BR-Z1 repressed DHR38 expression and that BR's inhibition of Acp65A expression was rescued by exogenous expression of DHR38. Thus, BR-Z1 suppresses Acp65A expression by preventing the normal up-regulation of DHR38 at the time of adult cuticle formation.

Extrinsic and intrinsic mechanisms directing epithelial cell sheet replacement during Drosophila metamorphosis

The fusion of epithelial sheets is an essential morphogenetic event. Here, we study the development of the abdomen of Drosophila as a model of bounded epithelia expansion and uncover a complex multistep process for the generation of the adult epidermis from histoblasts, founder cells that replace the larval cells during metamorphosis. We find that histoblasts experience a biphasic cell cycle and emit apical projections that direct their invasive planar intercalation in between larval cells. Coordinately, the larval cells extrude from the epithelia by apical constriction of an actomyosin ring and as a consequence die by apoptosis and are removed by circulating haemocytes. We demonstrate that the proliferation of histoblasts and the death of larval cells are triggered by two independent extrinsic Ecdysone hormonal pulses. Finally, we show that histoblast spreading and the death of larval cells depend on a mutual exchange of signals and are non-autonomous processes.

Arrowhead encodes a LIM homeodomain protein that distinguishes subsets of Drosophila imaginal cells

The Arrowhead gene encodes a LIM-homeodomain transcription factor required for establishment of a subset of imaginal tissues: the abdominal histoblasts and the salivary gland imaginal rings. Consistent with its role in development, during embryogenesis Arrowhead is expressed in each abdominal segment and in the labial segment. Late in embryonic development, expression is refined to the abdominal histoblasts and salivary gland imaginal ring cells themselves. When ectopically expressed in imaginal disc cells, Arrowhead causes programmed cell death and loss of corresponding adult structures. Therefore, Arrowhead expression is required for development of one set of imaginal cells and is incompatible with development of another, emphasizing the specificity of Arrowhead and the sensitivity of different target cells to its expression. Loss-of-function mutations in Arrowhead affect conserved or invariant amino acids in the LIM- and homeo-domains demonstrating the importance of these residues in LIM homeodomain protein activity.

Establishment of Drosophila imaginal precursor cells is controlled by the Arrowhead gene

Metamorphosis in Drosophila melanogaster requires synchronization of numerous developmental events that occur in isolated imaginal precursor tissues. The imaginal primordia are established during embryonic stages and are quiescent for much of larval life. The Arrowhead gene is necessary for establishment of proper numbers of cells within a subset of imaginal precursor tissues. Loss-of-function mutations in Arrowhead reduce the number of abdominal histoblasts and salivary gland imaginal ring cells before the proliferative stages of their development. The number of abdominal histoblasts in mutant animals is approximately half that of wild-type, as might result from failure of a single early division of these cells. A neomorphic Arrowhead allele results in the specific loss of the retinal precursors by the early third instar, before they have begun to differentiate. Since Arrowhead mutations affect only subsets of imaginal tissue, there must be distinctions in the developmental regulation of different imaginal precursors. Arrowhead may be part of a regulatory pathway responsible for establishing the proper number of abdominal histoblasts and salivary gland imaginal ring cells. The neomorphic Arrowhead allele, which may cause misexpression of the Arrowhead gene in the eye-antenna imaginal disc, interferes with the establishment or proliferation of retinal precursor cells.

Somatic mutation induced by heliotrine in Drosophila

The pyrrolizidine alkaloid heliotrine has been shown to be a powerful mutagen in Drosophila. This report has evaluated the teratogenicity of heliotrine in this organism. The alkaloid was fed to larvae and its teratogenic effects measured in various developmental stages of the insect. The pupal stage is predominantly affected. The main consequences of treatment were failed eclosions at higher alkaloid concentrations (10(-4) M), while lower concentrations (10(-5) M) permitted the eclosion of adults, but these showed abdominal abnormalities ranging from severe distortions to reduced numbers of tergite bristles. mei-9 strains of Drosophila were more sensitive to the production of somatic chromosomal changes as well as the teratogenic effects of the alkaloid. These strains also showed reduced numbers of cells in histoblast nests of 6-hour-old prepupae. It is suggested that reduced numbers of histoblast cells in prepupae may be a consequence of genetic damage and this in turn leads to the abdominal distortions and reduced bristle numbers observed.

The segment polarity gene costal-2 in Drosophila. II. The origin of imaginal pattern duplications

Imaginal pattern duplications caused by hypomorphic expression of the segment polarity gene costal-2 are described. These affect the anteroposterior coordinate of the imaginal disc. A very small part of the pattern is deleted and a large number of additional pattern elements arise in a progressive order, anterior-most first followed by more and more posterior structures. Mosaic analyses show that the duplications arise nonautonomously in the larval stages but that the costal-2 gene is not required after early embryogenesis. Arguments that the duplications are the result of cell interactions and intercalary growth that themselves arise from an abnormal polarity of the embryonic segment are presented.

The Drosophila sex determination gene daughterless has different functions in the germ line versus the soma

As a regulator of the female-specific gene Sxl, da+ provides an essential maternal component in the control of sex determination and dosage compensation; nevertheless, neither the maternal nor zygotic phenotypes of the original mutant da allele is sex-specific. Here we clarify the role of da+ in Drosophila development, finding: this sex determination gene is indeed pleiotropic; zygotic functioning of da+ is essential in both sexes for somatic cell development, but not for germ cell development; da female sterility results from a somatic, rather than germ-line, defect; and expression of da+ in the maternal germ line is required only for daughters in the subsequent generation, as expected for a specific regulator of Sxl+. These conclusions follow from the characterization of new da null alleles isolated by a selection for defects in maternally acting positive regulators of Sxl.

Alternative processing and developmental control of the transcripts of the Drosophila abl oncogene homologue

Drosophila sequences homologous to the abl oncogene are located near the 5' end of a gene (Dash). The Dash gene is transcribed to give long RNAs (5-6 kb) and short RNAs (3.0 kb) that lack some of the internal exons of the gene including some of the sequences coding for the protein kinase domain. The gene is composed of at least five short exons and a long 3' exon. The 3' exon is processed in several alternative ways. It contains an intronic sequence which is spliced out in approximately 50% of the transcripts. S1 mapping shows the existence of five different 3' ends, presumed polyadenylation sites, differing by up to 1 kb. Three of these are maternal-specific while the other two are utilised during development. Dash RNA is most abundant in eggs and early embryos, becomes very rare during larval development and returns in a burst of activity in early pupae.

Maturation and aging of adult fat body and oenocytes in Drosophila as revealed by light microscopic morphometry

A morphological and cytometric analysis of the adult fat body cells and oenocytes was made on sections of abdomens from immature, mature and senescent Drosophila melanogaster of both sexes. There are about 18,000 fat body cells in abdomens of female and mature male flies. Immature and senescent males have about 12,000 and 15,000 cells, respectively. The size of the cells is almost the same for immature flies of both sexes and increases about six-fold to approximately 2600 micron2, so that mature flies of both sexes have equivalent amounts of fat body tissue. The proportions of lipid, glycogen, and background cytoplasm of fat body cells also remain relatively constant throughout adult life, but dense, proteinaceous granules are observed in cells of senescent flies. The amounts of cellular components change dramatically due to change of cell size with age; the amount of lipid shows the greatest sexual difference with about 2x more in the females at all stages studied. The oenocytes number about 6,000 in the abdomens of all but immature male flies, which have approximately 4,000. Although the cells of both sexes triple in size to about 700 micron 2, the oenocytes of males reach maximum size earlier than those of females. The major features of oenocytes appear to be dense background cytoplasm, putative lipid droplets found only in mature flies, and pigmented granules first seen in the cells of mature flies which accumulate with age to 33% of the cytoplasm. The number of cells and their anticipated capacity for protein synthesis is discussed in relation to the production of yolk protein precursors.

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.

Developmental use of gene products in Drosophila: the maternal-zygotic transition

Recent results suggest that activity of a large fraction of the Drosophila genome is needed at multiple developmental stages. The timing of the transition from dependence on maternally stored gene products to reliance on zygotically coded products has been examined for several zygotic-lethal mutations in the z-w region of the X chromosome. The mutants differ in zygotic sensitivity to reduced maternal activity, and they have a wide range of times of lethality. Nevertheless, both temperature shift experiments and clonal analysis indicate that all of the maternal-zygotic transitions occur around the time of blastoderm formation.

On the dispersion of imaginal progenitor cells in theDrosophilablastoderm

The number of blastoderm cells inDrosophilawhose descendants form adult structures has frequently been estimated from genetic mosaics. Data from somatic recombination (method I) and gynandromorph (method II) mosaics both yield very low estimates, e.g. about 10–20 progenitor cells for the eye and antenna, wing or leg.

In gynandromorphs the mosaic dividing line has a random orientation on the blastoderm. In the 6000 cell blastoderm it should be very unlikely that the mosaic dividing line passes through any small patch of only 10–20 cells. Yet it has been reported that 10–25% of eye/antenna, wing or leg disks in gynandromorphs are mosaic. Thus the frequency of mosaicism data seems to be in contradiction to the progenitor population estimates. Similar discrepancies are found in the data for other adult structures.

In this paper we derive a formula for estimating the number of cells in a blastoderm patch from the frequency with which the gynandromorph dividing line passes through it (method III). In a second method (method IV) we use the maximum distances inside the progenitor areas on a fate map to estimate the progenitor patch size. These two estimates agree closely with each other. We find, e.g. that 50–100 cells are in the patches from which the eye/antenna, wing or leg disks derive.

We examine a number of possible explanations for why the first two estimates are so much smaller than the last two. The former estimates refer to the number of progenitor cells which actually have descendants in the adult structure; the latter estimates refer to the total patch area in which the progenitor cells sit. With the present information the most reasonable conclusion is that the progenitor cells for the adult structures are dispersed among other cells which have different developmental fates. If confirmed by experiment, this result has many implications for the process of determination.

The effect of inversions on mitotic recombination in Drosophila melanogaster

The effect of inversions on mitotic recombination outside the inversion was studied in inversion-heterozygotes. Seven euchromatic inversions of the X-chromosome, with breakpoints within the interval between two cell markers, were chosen. The size of the inverted region and the distance from the proximal breakpoint to the proximal cell marker varied. Mitotic recombination was X-ray induced in larvae and clones scored in the tergites of emerged adults. The frequency of recombinants between both cell markers and the frequency of recombinants proximal to the proximal cell marker was used to estimate the effect of interference in pairing caused by the inversions. Such an effect only occurs in small chromosome intervals. This indicates that homologous sequences are tightly paired in the interphase nuclei of somatic cells. This conclusion is derived from data based on X-ray induced mitotic recombination. The possibility of extending this conclusion to non-irradiated cells is discussed.

Histological analysis of the dynamics of growth of imaginal discs and histoblast nests during the larval development ofDrosophila melanogaster

1. Histological analyses were made of imaginal discs and histoblasts during the larval development ofDrosophila melanogaster to determine the number of cells, the patterns of cell division and the growth dynamics in these adult primordia. Histological studies were also made of the imaginal rings which are the primordia of the adult salivary gland, fore-and hindgut, the anlage cells of the midgut and several larval and embryonic tissues. 2. In the newly-hatched larva, the immature eye-antenna, wing, haltere, leg and genital discs contain about 70, 38, 20, 36-45 and 64 cells respectively. These numbers include cells destined to form cuticular elements as well as peripodial, tracheal and nerve cells and probably the progenitors of adepithelial cells. The number of cells counted in the various imaginal disc anlagen is 1.5 to 4 times higher than the numbers deduced from genetic mosaic analyses by other investigators and reasons for these differences are given. 3. About 12 h after fertilization, mitosis ceases in all tissues of the embryo except the nervous system. After the larva hatches, mitosis resumes in most of the imaginal anlagen and in some larval tissues. The time of resumption of mitosis in the imaginal anlagen was determined after treating the larvae with colchicine for 2 h. 4. Among the imaginal discs, the eye disc is the first to begin cell division, at about 13-15 h after the hatching of the larva (first instar) followed by the wing (15-17 h), the haltere (18-20 h), the antenna, leg, and genitalia (24-26 h, early second instar), and finally the labial and dorsal prothoracic discs (52-54 h, early third instar). The cell doubling time for various discs was calculated from cell counts and the times agree closely with the doubling times deduced from clonal analyses by other workers: e.g., 7.5 h for the cells of the wing disc. 5. The imaginal ring of the hindgut first shows cell division early in the second instar. The imaginal rings of the foregut and salivary glands, the anlage cells of the midgut and the cells of the segmental lateral tracheal branches begin to divide early in the third instar. 6. The histoblasts which are the anlagen of the integument of the adult abdomen do not increase in number from the time of larval hatching until about 5 h after pupation when they begin to divide. Their behaviour contrasts with that of the histoblasts of the other dipterans such asCalliphora, Musca andDacus, which begin to divide during the second instar. 7. The histoblasts are an integral part of the larval abdominal epidermis and, unlike imaginal disc cells, secrete cuticle during larval life. Each hemisegment consists of an anterior dorsal, a posterior dorsal, and a ventral histoblast nest containing about 13, 6 and 12 cells respectively. The 62 histoblasts in each larval segment represent about 7-8% of the total number of cells that form the integument of that segment. 8. The number of cells in a particular type of histoblast nest was constant for both male and female larvae and among the different abdominal segments, except that the anterior dorsal group of the first and the seventh segments contains fewer cells than those of the other segments. Although the male and female adultDrosophila lack the first abdominal sternite and the male lacks the seventh abdominal tergite and sternite, the ventral histoblast nests of the first and the dorsal and ventral nests of the seventh abdominal segments are present in the larval stages as well as in the prepupa and have the same morphology and cell number as similar nests in the rest of the abdominal segments. 9. The cells of the imaginal discs increase in volume about six-fold and their nuclei increase in volume three-fold between the time of hatching and the initiation of mitosis. The histoblasts increase in volume about 60-fold and their nuclei increase in volume about 25-fold between larval hatching and pupariation. 10. Prior to each cell division, the nuclei of the columnar cells of the disc epithelium and of the histoblasts appear to migrate toward the apical surface of the epithelium. The cells round up and shift toward the apical region where mitosis occurs. After cytokinesis, the daughter cells move back to deeper positions in the epithelium. Because the nuclei of the non-dividing cells continue to lie deep in the epithelium, this intermitotic migration of nuclei gives these epithelia a pseudostratified appearance. 11. Analyses of the growth of larval cells and of organs confirmed the observations of earlier investigators that cell division occurs only in a few larval tissues, whereas growth in the rest of the larval tissues is by cell enlargement and polyteny. During larval life, cell division was detected only in the central nervous system, gonads, prothoracic glands, lymph glands and haemocytes. Each tissue began mitosis at a characteristic stage in larval life. The larval cells that did not divide, grew enormously, e.g., epidermal cells increased in volume 150-fold and their nuclei increased in volume 80-fold. 12. The adepithelial cells, which give rise to some of the imaginal muscles, were first identified between the thick side of the imaginal dise epithelium and the basement membrane at the beginning of the third larval instar (50-52 h). The origin of these precursors of mesodermal structures was analysed and evidence is presented that the adepithelial cells come from the disc epithelium. The question of the origin of the mesoderm of cyclorrhaphan Diptera is reviewed and it is suggested that the imaginal disc ectoderm may become segregated from the rest of the embryo before gastrulation has occurred, that is before the mesoderm has been established.

Larval and adult abdominal defects resulting from microcautery of blastoderm staged Drosophila embryos

Drosophila embryos were damaged by microcautery at the cellular blastoderm stage at the sites of presumptive histoblasts, identified from fate maps. The resulting adults were analyzed for abnormal abdominal structures in one series, and in two further series the pupal cases of the defective adults which hatched were also checked for irregularities in segmentation of the larva, both dorsally and ventrally. The relationships between the larval segmentation and adult pattern are described. A sample of pupal cases of morphologically normal flies hatching from microcautery were checked and showed that regulation only rarely occurred, i.e., abnormal larvae sometimes produced normal adults. Both tergite and sternite defects occurred, and duplications of parts of these structures were observed in both cases. In general, abnormal fusions, missing hemi-segments, and partial deletions were associated with larval defects and were therefore probably the result of damage to larval cells, or both larval cells and histoblasts. Duplications and partial segment deficiencies also resulted from apparantly normal pupal cases and were therefore probably the result of directly damaging the presumptive histoblast cells of the blastoderm. It is suggested that the various nests of histoblasts in each segment act as one morphogenetic field, with larval cells within the field.

Non-autonomy in achaete mosaics of Drosophila

Numbers of bristles are reduced in the dorsocentral regions of achaeteDrosophila melanogaster. In achaete tissue of mosaics the effect is not uniform, and near the clone boundaries bristle numbers are significantly higher than they are elsewhere in the clone. It is argued that the cause of this non-autonomy stems from ‘factors’ that spread into the achaete clone from surrounding non-achaete cells.

Isolation of temperature sensitive mutations blocking clone development inDrosophila melanogaster, and the effects of a temperature sensitive cell lethal mutation on pattern formation in imaginal discs

A method of isolating temperature-sensitive (ts) mutations blocking clone development, based on the analysis of twin spots produced by X-ray induced somatic recombination is reported. From this screen 10 ts mutations were recovered which caused an absence of the lethal-bearing clone at the restrictive temperature. Eight of these mutations were analyzed. Seven proved to be autonomous ts cell lethals and one was an autonomous ts mutation which reversibly affected cell division and growth of imaginal disc cells and growth of larval cells. The effects on development of one of the cell lethal mutations,l(1)ts-504, are described. Heat pulses (29°C) 24-72 hrs long caused a high frequency (up to 90%) of morphologically abnormal animals. The abnormalities observed were of two major kinds: deficiencies and duplications of imaginal disc derivatives. In addition, alterations of tarsal segmentations occurred. Heat pulses to larvae also delayed pupariation and eclosion by as much as four days. In general, longer pulses led to a greater delay in pupariation and eclosion and a higher frequency of deficiencies and duplications than shorter pulses. Exposure to restrictive temperature early in larval development delayed pupariation and resulted in mostly normal animals; exposure during the second and early third larval instar also delayed pupariation and led to a high frequency of duplications; exposure later in larval life, i.e. mid and late third larval instar, caused no delay in pupariation but led to a high frequency of deficiencies. These results can be explained by the occurrence of areas of cell death, which can be seen in the imaginal discs of larvae exposed to restrictive temperature by staining with trypan blue. This conclusion is further supported by the observation in gynandromorphs of duplications of female nonmutant tissue. These results are discussed in relation to current theories of pattern formation.