The Drosophila Broad-Complex plays a key role in controlling ecdysone-regulated gene expression at the onset of metamorphosis

Study of Dosage Compensation in Drosophila

Using a sensitive RT-QPCR assay, we analyzed the regulatory effects of sex and different dosage compensation mutations in Drosophila. To validate the assay, we showed that regulation for several genes indeed varied with the number of functional copies of that gene. We then confirmed that dosage compensation occurred for most genes we examined in male and female flies. Finally, we examined the effects on regulation of several genes in the MSL pathway, presumed to be involved in sex-dependent determination of regulation. Rather than seeing global alterations of either X chromosomal or autosomal genes, regulation of genes on either the X chromosome or the autosomes could be elevated, depressed, or unaltered between sexes in unpredictable ways for the various MSL mutations. Relative dosage for a given gene between the sexes could vary at different developmental times. Autosomal genes often showed deranged regulatory levels, indicating they were in pathways perturbed by X chromosomal changes. As exemplified by the BR-C locus and its dependent Sgs genes, multiple genes in a given pathway could exhibit coordinate regulatory modulation. The variegated pattern shown for expression of both X chromosomal and autosomal loci underscores the complexity of gene expression so that the phenotype of MSL mutations does not reflect only simple perturbations of genes on the X chromosome.

Temporal regulation of microRNA expression in Drosophila melanogaster mediated by hormonal signals and broad-Complex gene activity

lin-4 and let-7 are founding members of an extensive family of genes that produce small transcripts, termed microRNAs (miRNAs). In Caenorhabditis elegans, lin-4 and let-7 control the timing of postembryonic events by translational repression of target genes, permitting progression from early to late developmental programs. To identify Drosophila melanogaster miRNAs that could play similar roles in the control of developmental timing, we characterized the developmental expression profile of 24 miRNAs in Drosophila, and found 7 miRNAs that are either upregulated or downregulated in conjunction with metamorphosis. The upregulation of three of these miRNAs (mir-100, mir-125, and let-7), and the downregulation of a fourth (mir-34) requires the hormone ecdysone (Ecd) and the activity of the Ecd-inducible gene Broad-Complex. Interestingly, mir-125 is a putative homologue of lin-4. mir-100, -125, and let-7 are clustered within an 800-bp region on chromosome 2L, suggesting that these three miRNAs may be coordinately regulated via common cis-acting elements during metamorphosis. In S2 cells, Ecd and the juvenile hormone analog methoprene exert opposite effects on the expression of these four miRNAs, indicating the participation of both these hormones in the temporal regulation of mir-34, -100, -125, and let-7 expression in vivo.

E74 exhibits stage-specific hormonal regulation in the epidermis of the tobacco hornworm, manduca sexta

The transcription factor E74 is one of the early genes induced by ecdysteroids during metamorphosis of Drosophila melanogaster. Here, we report the cloning and hormonal regulation of E74 from the tobacco hornworm, Manduca sexta (MsE74). MsE74 is 98% identical to that of D. melanogaster within the DNA-binding ETS domain of the protein. The 5'-isoform-specific regions of MsE74A and MsE74B share significantly lower sequence similarity (30-40%). Developmental expression by Northern blot analysis reveals that, during the 5th larval instar, MsE74B expression correlates with pupal commitment on day 3 and is induced to maximal levels within 12h by low levels of 20-hydroxyecdysone (20E) and repressed by physiologically relevant levels of juvenile hormone I (JH I). Immunocytochemical analysis shows that MsE74B appears in the epidermis before the 20E-induced Broad transcription factor that is correlated with pupal commitment (Zhou and Riddiford, 2001). In contrast, MsE74A is expressed late in the larval and the pupal molts when the ecdysteroid titer has declined to low levels and in the adult molt just as the ecdysteroid titer begins to decline. This change in timing during the adult molt appears not to be due to the absence of JH as there was no change during the pupal molt of allatectomized animals. When either 4th or 5th instar larval epidermis was explanted and subjected to hormonal manipulations, MsE74A induction occurred only after exposure to 20E followed by its removal. Thus, MsE74B appears to have a similar role at the onset of metamorphosis in Manduca as it does in Drosophila, whereas MsE74A is regulated differently at pupation in Manduca than at pupariation in Drosophila.

GFP in living animals reveals dynamic developmental responses to ecdysone during Drosophila metamorphosis

Studies of Drosophila metamorphosis have been hampered by our inability to visualize many of the remarkable changes that occur within the puparium. To circumvent this problem, we have expressed GFP in specific tissues of living prepupae and pupae and compiled images of these animals into time-lapse movies. These studies reveal, for the first time, the dynamics and coordination of morphogenetic movements that could only be inferred from earlier studies of dissected staged animals. We also identify responses that have not been described previously. These include an unexpected variation in some wild-type animals, where one of the first pairs of legs elongates in the wrong position relative to the second pair of legs and then relocates to its appropriate location. At later stages, the antennal imaginal discs migrate from a lateral position in the head to their final location at the anterior end, as leg and mouth structures are refined and the wings begin to fold. The larval salivary glands translocate toward the dorsal aspect of the animal and undergo massive cell death following head eversion, in synchrony with death of the abdominal muscles. These death responses fail to occur in rbp(5) mutants of the Broad-Complex (BR-C), and imaginal disc elongation and eversion is abolished in br(5) mutants of the BR-C. Leg malformations associated with the crol(3) mutation can be seen to arise from defects in imaginal disc morphogenesis during prepupal stages. This approach provides a new tool for characterizing the dynamic morphological changes that occur during metamorphosis in both wild-type and mutant animals.

Control of Dopa decarboxylase gene expression by the Broad-Complex during metamorphosis in Drosophila

The induction of the Dopa decarboxylase gene (Ddc) in the epidermis of Drosophila at pupariation is a receptor-mediated response to the steroid molting hormone, ecdysone. Activity is also dependent on the Broad-Complex (BR-C), an early ecdysone response gene that functions during metamorphosis. BR-C encodes a family of zinc-finger protein isoforms, BR-C(Z1-Z4). Genetic experiments have shown that the Z2 isoform is required for epidermal Ddc to reach maximum expression at pupariation. In this paper, we report that BR-C regulates Ddc expression at two different developmental stages through two different cis-acting regions. At pupariation, BR-C acts synergistically with the ecdysone receptor to up-regulate Ddc. DNase I foot printing has identified four binding sites of the predominant Z2 isoform within a distal regulatory element that is required for maximal Ddc activity. The sites share a conserved core sequence with a set of BR-C sites that had been mapped previously to within the first Ddc intron. Using variously deleted Ddc genomic regions to drive reporter gene expression in transgenic organisms, we show that the intronic binding sites are required for Ddc expression at eclosion. At both pupariation and eclosion, BR-C releases Ddc from an active silencing mechanism, operating through two distinct cis-acting regions of the Ddc genomic domain at these stages. Transgenes, bearing a Ddc fragment from which one of the cis-acting silencers has been deleted, exhibit beta-galactosidase reporter activity in the epidermal cells prior to the appearance of endogenous DDC. Our finding that BR-C is required for Ddc activation at eclosion is the first evidence to suggest that this important regulator of the early metamorphic events, also regulates target gene expression at the end of metamorphosis.

Temporal regulation of Drosophila salivary gland degeneration by the Broad-Complex transcription factors

The destruction of obsolete larval tissues at the onset of insect metamorphosis is a complex process triggered by the steroid hormone ecdysone. Among the genes required for the implementation of salivary gland (SG) degeneration the reduced bristles on palpus (rbp) gene of the Broad-Complex (BR-C) locus plays a critical role. This gene encodes the BR-C Z1 transcription factor and its expression is directly regulated by ecdysone through the ecdysone receptor (EcR/Usp). The BR-C locus encodes four major protein isoforms, including BR-C Z1, Z2, Z3, and Z4. With the exceptions of mutations in BR-C Z1 all mutations affecting the other BR-C isoforms produce pupal lethality. To gain insight into the function of the different BR-C isoforms on the process of SG degeneration, we used transgenes expressing each of the four major BR-C isoform proteins. This study revealed that, depending upon the period of expression relative to the major peak of ecdysone production, BR-C Z1, Z2, and Z4 first inhibited and then stimulated the process of SG degeneration. In contrast, BR-C Z3 exerted all time points an inhibition on SG degeneration.

AP-1, but not NF-kappa B, is required for efficient steroid-triggered cell death in Drosophila

Extensive studies in vertebrate cells have assigned a central role to Rel/NF-kappa B and AP-1 family members in the control of apoptosis. We ask here whether parallel pathways might function in Drosophila by determining if Rel/NF-kappa B or AP-1 family members contribute to the steroid-triggered death of larval salivary glands during Drosophila metamorphosis. We show that two of the three Drosophila Rel/NF-kappa B genes are expressed in doomed salivary glands and that one family member, Dif, is induced in a stage-specific manner immediately before the onset of programmed cell death. Similarly, Djun is expressed for many hours before salivary gland cell death while Dfos is induced in a stage-specific manner, immediately before this tissue is destroyed. We show that null mutations in the three Drosophila Rel/NF-kappa B family members, either alone or in combination, have no apparent effect on this death response. In contrast, Dfos is required for the proper timing of larval salivary gland cell death as well as the proper induction of key death genes. This study demonstrates a role for AP-1 in the stage-specific steroid-triggered programmed cell death of larval tissues during Drosophila metamorphosis.

Ecdysone triggers the expression of Golgi genes in Drosophila imaginal discs via broad-complex

One of the most significant morphogenic events in the development of Drosophila melanogaster is the elongation of imaginal discs during puparium formation. We have shown that this macroscopic event is accompanied by the formation of Golgi stacks from small Golgi larval clusters of vesicles and tubules that are present prior to the onset of disc elongation. We have shown that the fly steroid hormone 20-hydroxyecdysone triggers both the elongation itself and the formation of Golgi stacks (V. Kondylis, S. E. Goulding, J. C. Dunne, and C. Rabouille, 2001, Mol. Biol. Cell, 12, 2308). Using mRNA in situ hybridisation, we show here that ecdysone triggers the upregulation of a subset of genes encoding Golgi-related proteins (such as dnsf1, dsec23, dsed5, and drab1) and downregulates the expression of others (such as dergic53, dbeta'COP, and drab6). We show that the transcription factor Broad-complex, itself an "early" ecdysone target, mediates this regulation. And we show that the ecdysone-independent upregulation of dnsf1 and dsnap prior to the ecdysone peak leads to a precocious formation of large Golgi stacks. The ecdysone-triggered biogenesis of Golgi stacks at the onset of imaginal disc elongation offers the exciting possibility of advancing our understanding of the relationship between gene expression and organelle biogenesis.

The expression of the let-7 small regulatory RNA is controlled by ecdysone during metamorphosis in Drosophila melanogaster

In Caenorhabditis elegans, the heterochronic pathway controls the timing of developmental events during the larval stages. A component of this pathway, the let-7 small regulatory RNA, is expressed at the late stages of development and promotes the transition from larval to adult (L/A) stages. The stage-specificity of let-7 expression, which is crucial for the proper timing of the worm L/A transition, is conserved in Drosophila melanogaster and other invertebrates. In Drosophila, pulses of the steroid hormone 20-hydroxyecdysone (ecdysone) control the timing of the transition from larval to pupal to adult stages. To test whether let-7 expression is regulated by ecdysone in Drosophila, we used Northern blot analysis to examine the effect of altered ecdysone levels on let-7 expression in mutant animals, organ cultures, and S2 cultured cells. Experiments were conducted to test the role of Broad-Complex (BR-C), an essential component in the ecdysone pathway, in let-7 expression. We show that ecdysone and BR-C are required for let-7 expression, indicating that the ecdysone pathway regulates the temporal expression of let-7 in Drosophila. These results demonstrate an interaction between steroid hormone signaling and the heterochronic pathway in insects.

Selective binding of Drosophila BR-C isoforms to a distal regulatory element in the hsp23 promoter

The Broad-Complex (BR-C) gene plays a key role in the ecdysone regulatory hierarchy. Together with other early ecdysone-inducible genes BR-C transmits the hormonal signal to a set of secondary response genes in a tissue-specific manner. Among its targets is the hsp23 gene. Previously we showed that expression of the hsp23 gene in late third instar is BR-C-dependent, and accompanied by the appearance of a BR-C-dependent DNase I hypersensitive site at position -1400 (DHS-1400). BR-C encodes a family of transcription factors, and we show here that at least three BR-C protein isoforms--Z1, Z2, and Z3--bind to the sequences around DHS-1400 in vitro. A DNase I footprinting assay reveals five protected regions, designated site 1 to site 5, each of which specifically associates with one or several BR-C protein isoforms. We also show that a 100 bp region overlapping site 5, which binds all three isoforms in vitro, is required for hsp23 activity in vivo. The deletion of binding site 5 in a reporter gene construct reproduced the effect of the npr class mutations, that is, hsp23 is no longer expressed in any tissue tested except brain. Thus, BR-C regulates hsp23 expression via direct interaction of the predominant isoform with the distal regulatory element.

Glue secretion in the Drosophila salivary gland: a model for steroid-regulated exocytosis

Small hydrophobic hormones like steroids control many tissue-specific physiological responses in higher organisms. Hormone response is characterized by changes in gene expression, but the molecular details connecting target-gene transcription to the physiology of responding cells remain elusive. The salivary glands of Drosophila provide an ideal model system to investigate gaps in our knowledge, because exposure to the steroid 20-hydroxyecdysone (20E) leads to a robust regulated secretion of glue granules after a stereotypical pattern of puffs (activated 20E-regulated genes) forms on the polytene chromosomes. Here, we describe a convenient bioassay for glue secretion and use it to analyze mutants in components of the puffing hierarchy. We show that 20E mediates secretion through the EcR/USP receptor, and two early-gene products, the rbp(+) function of BR-C and the Ca2+ binding protein E63-1, are involved. Furthermore, we demonstrate that 20E treatment of salivary glands leads to Ca2+ elevations by a genomic mechanism and that elevated Ca2+ levels are required for ectopically produced E63-1 to drive secretion. The results presented establish a connection between 20E exposure and changes in Ca2+ levels that are mediated by Ca2+ effector proteins, and thus establish a mechanistic framework for future studies.

Functional analysis of regulatory elements controlling the expression of the ecdysone-regulated Drosophila ng-1 gene

The steroid hormone ecdysone controls multiple aspects of insect development, including larval moults and metamorphosis, and can induce specific genetic responses in different tissues. The definition of the molecular mechanisms able to mediate this tissue-specific responsiveness may greatly contribute to understanding how such an accurate genetic response is achieved. In this work we have identified, by transgenic analysis, the regulatory elements directing the expression of ng-1, an ecdysone-regulated Drosophila gene showing a highly specific developmental expression profile. Our results show that an ecdysone-responsive element located within the ng-1 coding region is necessary for high-level gene expression, whereas the gene's spatial and temporal expression profile is fully controlled by a distinct upstream regulatory region. This region binds a set of transcriptional factors, including the FKH regulatory protein, which can potentially modulate the ecdysone genetic regulated response.

Broad-complex, but not ecdysone receptor, is required for progression of the morphogenetic furrow in the Drosophila eye

The progression of the morphogenetic furrow in the developing Drosophila eye is an early metamorphic, ecdysteroid-dependent event. Although Ecdysone receptor-encoded nuclear receptor isoforms are the only known ecdysteroid receptors, we show that the Ecdysone receptor gene is not required for furrow function. DHR78, which encodes another candidate ecdysteroid receptor, is also not required. In contrast, zinc finger-containing isoforms encoded by the early ecdysone response gene Broad-complex regulate furrow progression and photoreceptor specification. br-encoded Broad-complex subfunctions are required for furrow progression and proper R8 specification, and are antagonized by other subfunctions of Broad-complex. There is a switch from Broad complex Z2 to Z1 zinc-finger isoform expression at the furrow which requires Z2 expression and responds to Hedgehog signals. These results suggest that a novel hormone transduction hierarchy involving an uncharacterized receptor operates in the eye disc.

Dynamic expression of broad-complex isoforms mediates temporal control of an ecdysteroid target gene at the onset of Drosophila metamorphosis

Metamorphosis in Drosophila melanogaster is orchestrated by the steroid hormone ecdysone, which triggers a cascade of primary-response transcriptional regulators and secondary effector genes during the third larval instar and prepupal periods of development. The early ecdysone-response Broad-Complex (BR-C) gene, a key regulator of this cascade, is defined by three complementing functions (rbp, br, and 2Bc) and encodes several distinct zinc-finger-containing isoforms (Z1 to Z4). Using isoform-specific polyclonal antibodies we observe in the fat body a switch in BR-C isoform expression from the Z2 to the other three isoforms during the third instar. We show that the 2Bc(+) function that corresponds presumably to the Z3 isoform is required for the larval fat body-specific expression of a transgenic construct (AE) in which the lacZ gene is under the control of the ecdysone-regulated enhancer and minimal promoter of the fat body protein 1 (Fbp1) gene. Using hs(BR-C) transgenes, we demonstrate that overexpression of Z1, Z3, or Z4, but not Z2, is able to rescue AE activity with faithful tissue specificity in a BR-C null (npr1) genetic context, demonstrating a partial functional redundancy between Z1, Z3, and Z4 isoforms. We also show that continuous overexpression of Z2 during the third instar represses AE, while conversely, expression of Z3 earlier than its normal onset induces precocious expression of the construct. This finding establishes a tight correlation between the dynamic pattern of expression of the BR-C isoforms and their individual repressive or inductive roles in AE regulation. Altogether our results demonstrate that the balance between BR-C protein isoforms in the fat body mediates, in part, the precise timing of the ecdysone activation of the AE construct but does not modulate its tissue specificity.

The mutation without children(rgl) causes ecdysteroid deficiency in third-instar larvae of Drosophila melanogaster

Larvae homozygous for the recessive lethal allele without children(rgl) (woc(rgl)) fail to pupariate. Application of exogenous 20-hydroxyecdysone elicits puparium formation and pupation. Ecdysteroid titer measurements on mutant larvae show an endocrine deficiency in the brain-ring gland complex, which normally synthesizes ecdysone, resulting in a failure of the larvae to achieve a threshold whole body hormone titer necessary for molting. Ultrastructural investigation revealed extensive degeneration of the prothoracic cells of the ring gland in older larvae. The woc gene, located in polytene chromosomal region 97F, consists of 11 exons. A 6.8-kb transcript is expressed throughout development but is absent in the mutant woc(rgl) larvae. The woc gene encodes a protein of 187 kDa. Eight zinc fingers of the C2-C2 type point to a possible function as a transcription factor. The woc protein shows considerable homology to human proteins which have been implicated in both mental retardation and a leukemia/lymphoma syndrome.

The E23 early gene of Drosophila encodes an ecdysone-inducible ATP-binding cassette transporter capable of repressing ecdysone-mediated gene activation

At the onset of Drosophila metamorphosis, the steroid hormone 20-OH ecdysone directly induces a small number of early puffs in the polytene chromosomes of the larval salivary gland. Proteins encoded by the early genes corresponding to these transcriptional puffs then regulate the activity of both the early puffs themselves and a much larger set of late puffs. Three of these early genes encode transcription factors that play critical regulatory roles during metamorphosis. Here we report the cloning, DNA sequence, genomic structure, ecdysone inducibility, and temporal expression of an early gene residing in the 23E early puff and denoted E23 (Early gene at 23). In contrast to other early genes, E23 encodes a protein with similarity to ATP-binding cassette transporters. Using heat shock-inducible transgenes, we found that E23 overexpression not only produces phenotypic abnormalities and lethality, but also interferes with ecdysone-mediated gene activation, demonstrating that E23 is capable of modulating the ecdysone response. Our results suggest the existence of a previously unrecognized regulatory mechanism for modulating steroid hormone signaling in Drosophila.

A juvenile hormone agonist reveals distinct developmental pathways mediated by ecdysone-inducible broad complex transcription factors

Juvenile hormone (JH) is an important regulator of insect development that, by unknown mechanisms, modifies molecular, cellular, and organismal responses to the molting hormone, 20-hydroxyecdysone (20E). In dipteran insects such as Drosophila, JH or JH agonists, administered at times near the onset of metamorphosis, cause lethality. We tested the hypothesis that the JH agonist methoprene acts by interfering with function of the Broad Complex (BRC), a 20E-regulated locus encoding BTB/POZ-zinc finger transcription factors essential for metamorphosis of many tissues. We found that methoprene, administered by feeding or by topical application, disrupts the metamorphic reorganization of the central nervous system, salivary glands, and musculature in a dose-dependent manner. As we predicted, methoprene phenocopies a subset of previously described BRC defects; it also phenocopies Deformed and produces abnormalities not associated with known mutations. Interestingly, methoprene specifically disrupts those metamorphic events dependent on the combined action of all BRC isoforms, while sparing those that require specific isoform subsets. Thus, our data provide independent pharmacological evidence for the model, originally based on genetic studies, that BRC proteins function in two developmental pathways. Mutations of Methoprene-tolerant (Met), a gene involved in the action of JH, protect against all features of the "methoprene syndrome." These findings have allowed us to propose novel alternative models linking BRC, juvenile hormone, and MET.

The development and evolution of exaggerated morphologies in insects

We discuss a framework for studying the evolution of morphology in insects, based on the concepts of "phenotypic plasticity" and "reaction norms." We illustrate this approach with the evolution of some of the most extreme morphologies in insects: exaggerated, sexually selected male ornaments and weapons, and elaborate social insect soldier castes. Most of these traits scale with body size, and these scaling relationships are often nonlinear. We argue that scaling relationships are best viewed as reaction norms, and that the evolution of exaggerated morphological traits results from genetic changes in the slope and/or shape of these scaling relationships. After reviewing literature on sexually selected and caste-specific structures, we suggest two possible routes to the evolution of exaggerated trait dimensions: (a) the evolution of steeper scaling relationship slopes and (b) the evolution of sigmoid or discontinuous scaling relationship shapes. We discuss evolutionary implications of these two routes to exaggeration and suggest why so many of the most exaggerated insect structures scale nonlinearly with body size. Finally, we review literature on insect development to provide a comprehensive picture of how scaling relationships arise and to suggest how they may be modified through evolution.

Epidermal tendon cells require Broad Complex function for correct attachment of the indirect flight muscles in Drosophila melanogaster

Drosophila Broad Complex, a primary response gene in the ecdysone cascade, encodes a family of zinc-finger transcription factors essential for metamorphosis. Broad Complex mutations of the rbp complementation group disrupt attachment of the dorsoventral indirect flight muscles during pupal development. We previously demonstrated that isoform BRC-Z1 mediates the muscle attachment function of rbp(+) and is expressed in both developing muscle fibers and their epidermal attachment sites. We now report two complementary studies to determine the cellular site and mode of action of rbp(+) during maturation of the myotendinous junctions of dorsoventral indirect flight muscles. First, genetic mosaics, produced using the paternal loss method, revealed that the muscle attachment phenotype is determined primarily by the genotype of the dorsal epidermis, with the muscle fiber and the ventral epidermis exerting little or no influence. When the dorsal epidermis was mutant, the vast majority of muscles detached or chose ectopic attachment sites, regardless of the muscle genotype. Conversely, wild-type dorsal epidermis could support attachment of mutant muscles. Second, ultrastructural analysis corroborated and extended these results, revealing defective and delayed differentiation of rbp mutant epidermal tendon cells in the dorsal attachment sites. Tendon cell processes, the stress-bearing links between the epidermis and muscle, were reduced in number and showed delayed appearance of microtubule bundles. In contrast, mutant muscle and ventral epidermis resembled the wild type. In conclusion, BRC-Z1 acts in the dorsal epidermis to ensure differentiation of the myotendinous junction. By analogy with the cell-cell interaction essential for embryonic muscle attachment, we propose that BRC-Z1 regulates one or more components of the epidermal response to a signal from the developing muscle.

The function of the broad-complex during Drosophila melanogaster oogenesis

The Broad-Complex (BR-C) is an early ecdysone response gene that functions during metamorphosis and encodes a family of zinc-finger transcription factors. It is expressed in a dynamic pattern during oogenesis. Its late expression in the lateral-dorsal-anterior follicle cells is related to the morphogenesis of the chorionic appendages. All four zinc-finger isoforms are expressed in oogenesis, which is consistent with the abnormal appendage phenotypes resulting from their ectopic expression. We investigated the mechanism by which the BR-C affects chorion deposition by using BrdU to follow the effects of BR-C misexpression on DNA replication and in situ hybridization to ovarian mRNA to evaluate chorion gene expression. Ectopic BR-C expression leads to prolonged endoreplication and to additional amplification of genes, besides the chorion genes, at other sites in the genome. The pattern of chorion gene expression is not affected along the anterior-posterior axis, but the follicle cells at the anterior of the oocyte fail to migrate correctly in an anterior direction when BR-C is misexpressed. We conclude that the target genes of the BR-C in oogenesis include a protein essential for endoreplication and chorion gene amplification. This may provide a link between steroid hormones and the control of DNA replication during oogenesis.

The acquisition of competence to respond to ecdysone in Drosophila is transcript specific

The steroid hormone ecdysone induces a precise sequence of gene activity in Drosophila melanogaster salivary glands in late third larval instar larvae. The acquisition of competence for this response does not result from a single event or pathway but requires factors that accumulate throughout the instar. Individual transcripts become competent to respond at different times and their expression is differentially affected in ecd1, dor22 and BR-C mutants. The induction of early-late transcripts, originally assumed to necessarily follow early transcripts, is partially independent of early transcript activation. Attempts to inhibit the synthesis of regulatory proteins reveal transcript-specific superinduction effects. Furthermore these inhibitors lead to the induction of betaFTZ-F1 and E93 transcripts at levels normally found in prepupal glands. These studies reveal the complexity of the processes underlying the establishment of a hormonal response.

The RXR homolog ultraspiracle is an essential component of the Drosophila ecdysone receptor

Pulses of the steroid hormone ecdysone function as key temporal signals during insect development, coordinating the major postembryonic developmental transitions, including molting and metamorphosis. In vitro studies have demonstrated that the EcR ecdysone receptor requires an RXR heterodimer partner for its activity, encoded by the ultraspiracle (usp) locus. We show here that usp exerts no apparent function in mid-third instar larvae, when a regulatory hierarchy prepares the animal for the onset of metamorphosis. Rather, usp is required in late third instar larvae for appropriate developmental and transcriptional responses to the ecdysone pulse that triggers puparium formation. The imaginal discs in usp mutants begin to evert but do not elongate or differentiate, the larval midgut and salivary glands fail to undergo programmed cell death and the adult midgut fails to form. Consistent with these developmental phenotypes, usp mutants show pleiotropic defects in ecdysone-regulated gene expression at the larval-prepupal transition. usp mutants also recapitulate aspects of a larval molt at puparium formation, forming a supernumerary cuticle. These observations indicate that usp is required for ecdysone receptor activity in vivo, demonstrate that the EcR/USP heterodimer functions in a stage-specific manner during the onset of metamorphosis and implicate a role for usp in the decision to molt or pupariate in response to ecdysone pulses during larval development.

Juvenile hormone prevents ecdysteroid-induced expression of broad complex RNAs in the epidermis of the tobacco hornworm, Manduca sexta

A cDNA homolog of the Drosophila melanogaster Broad Complex (BRC) gene was isolated from the tobacco hornworm, Manduca sexta, which shows a predicted 88% amino acid identity with Drosophila BRC in the N-terminal BTB domain. Three zinc finger domains encoding homologs of the Drosophila Z2, Z3, and Z4 domains (93, 100, and 85% identity, respectively) were obtained by RT-PCR. In Manduca dorsal abdominal epidermis, BRC RNAs were not observed during the larval molt. Three BRC transcripts-6.0, 7.0, and 9.0 kb-first appeared at the end of the feeding stage of the fifth (final) instar when the epidermis is exposed to ecdysteroids in the absence of juvenile hormone (JH) and becomes committed to pupal differentiation. These RNAs were induced in day 2 fifth larval epidermis in vitro by 20-hydroxyecdysone (20E) in the absence of JH with dose-response and time courses similar to the induction of pupal commitment. This induction by 20E in vitro was prevented by the presence of JH I at levels seen in vivo during the larval molt. In the wing discs, the BRC RNAs appeared shortly after ecdysis to the fifth instar and coincided with the onset of metamorphic competence of these discs. Application of a JH analogue pyriproxifen during the fourth instar molt delayed and reduced the levels of BRC mRNAs seen in the wing discs in the early fifth instar, but did not completely prevent their appearance in this tissue that first differentiates at metamorphosis. The expression of the BRC transcription factors thus appears to be one of the first molecular indications of the genetic reprogramming of the epidermis necessary for insect metamorphosis. How JH prevents BRC expression in this epidermis may provide the key to understanding how this hormone controls metamorphosis.

Ultrastructural changes of Drosophila larval and prepupal salivary glands cultured in vitro with ecdysone

Alterations in the ultrastructure of in vitro cultured larval salivary glands of Drosophila melanogaster in response to the steroid hormone ecdysone were studied in relation to complex changes in puffing patterns. We found that the changes in the fine structure of cultured glands reflected progression of the puffing pattern, and they paralleled those seen in vivo. We observed that glue secretion by exocytosis, the main function of salivary glands, took place between puff stage 5 (PS5) and PS7. Glue could not be expectorated under culture conditions but was slowly released from the lumen through a duct into the medium. After the cultured glands reached PS13/PS14, further progress of puffing and fine structural alterations required that the ecdysteroid titer be transiently extremely low or absent. Under in vitro conditions we did not observe the putative new secretory program(s) described for glands in vivo after PS12. However, ultrastructural changes which unambiguously indicated that an autohistolytic process had begun in vitro started to appear after PS17. Many salivary gland cells developed numerous features of progressive self-degradation between PS18 and PS21. Actual degradation of salivary glands in vivo seemed to be rapid, but in vitro degradation was never completed, probably due to a lack of exogenous factors from the hemolymph. Manipulations of ecdysone titer in vitro in the culture medium, known during the larval puffing cycle to cause premature induction of developmentally specific puffing patterns, did not affect the normal development of ultrastructural features of the cytoplasm and nucleus.

Ecdysone pathway is required for furrow progression in the developing Drosophila eye

In Drosophila, secretion of the steroid hormone ecdysone from the prothoracic ring gland coordinates and triggers events such as molting and metamorphosis. In the developing Drosophila compound eye, pattern formation and cell-type specification initiate at a moving boundary known as the morphogenetic furrow. We have investigated the role of ecdysone in eye development and report here that the ecdysone signaling pathway is required for progression of the morphogenetic furrow in the eye imaginal disc of Drosophila. Genetic disruption both of the ecdysone signal in vivo with the ecdysoneless1 (ecd1) mutant and of ecdysone response with a Broad-Complex mutant result in disruption of morphogenetic furrow progression. In addition, we show that ecdysone-dependent gene expression, both of a reporter of transcriptional activity of the Ecdysone Receptor and of the Z1 isoform of the Broad Complex, are localized in and close to the furrow. These results suggest that, in the morphogenetic furrow, temporal hormonal signals are integrated into genetic pathways specifying spatial pattern.

Parallel molecular genetic pathways operate during CNS metamorphosis in Drosophila

Insect metamorphosis provides a valuable model for studying mechanisms of steroid hormone action on the nervous system during a dynamic phase of functional remodeling. The Drosophila Broad Complex (BRC) holds a pivotal position in the gene expression cascade triggered by the molting hormone 20-hydroxyecdysone (20E) at the onset of metamorphosis. We previously demonstrated that the BRC, which encodes a family of zinc-finger transcription factors, is essential for transducing 20E signals into the morphogenetic movements and cellular assembly that alter the CNS from juvenile to adult form and function. We set out to examine the relationship of BRC to two other genes, IMP-E1 and Deformed (Dfd), involved in the metamorphic transition of the CNS. Representatives of the whole family of BRC transcript isoforms accumulate in the CNS during the larval-to-pupal transition and respond directly to 20E in vitro. IMP-E1 is also directly regulated by 20E, but its induction is independent of BRC, revealing that 20E works through at least two pathways in the CNS. DFD expression is also independent of BRC function. Surprisingly, BRC and DFD proteins are expressed in distinct, nonoverlapping subsets of neuronal nuclei of the subesophageal ganglion even though both are required for its migration into the head capsule. This suggests that the segment identity and ecdysone cascades operate in parallel to control region-specific reorganization during metamorphosis.

The DHR78 nuclear receptor is required for ecdysteroid signaling during the onset of Drosophila metamorphosis

Pulses of ecdysteroids direct Drosophila through its life cycle by activating stage- and tissue-specific genetic regulatory hierarchies. Here we show that an orphan nuclear receptor, DHR78, functions at the top of the ecdysteroid regulatory hierarchies. Null mutations in DHR78 lead to lethality during the third larval instar with defects in ecdysteroid-triggered developmental responses. Consistent with these phenotypes, DHR78 mutants fail to activate the mid-third instar regulatory hierarchy that prepares the animal for metamorphosis. DHR78 protein is bound to many ecdysteroid-regulated puff loci, suggesting that DHR78 directly regulates puff gene expression. In addition, ectopic expression of DHR78 has no effects on development, indicating that its activity is regulated post-translationally. We propose that DHR78 is a ligand-activated receptor that plays a central role in directing the onset of Drosophila metamorphosis.

crooked legs encodes a family of zinc finger proteins required for leg morphogenesis and ecdysone-regulated gene expression during Drosophila metamorphosis

Drosophila imaginal discs undergo extensive pattern formation during larval development, resulting in each cell acquiring a specific adult fate. The final manifestation of this pattern into adult structures is dependent on pulses of the steroid hormone ecdysone during metamorphosis, which trigger disc eversion, elongation and differentiation. We have defined genetic criteria that allow us to screen for ecdysone-inducible regulatory genes that are required for this transformation from patterned disc to adult structure. We describe here the first genetic locus isolated using these criteria: crooked legs (crol). crol mutants die during pupal development with defects in adult head eversion and leg morphogenesis. The crol gene is induced by ecdysone during the onset of metamorphosis and encodes at least three protein isoforms that contain 12–18 C2H2 zinc fingers. Consistent with this sequence motif, crol mutations have stage-specific effects on ecdysone-regulated gene expression. The EcR ecdysone receptor, and the BR-C, E74 and E75 early regulatory genes, are submaximally induced in crol mutants in response to the prepupal ecdysone pulse. These changes in gene activity are consistent with the crol lethal phenotypes and provide a basis for understanding the molecular mechanisms of crol action. The genetic criteria described here provide a new direction for identifying regulators of adult tissue development during insect metamorphosis.

Two signalling pathways specify localised expression of the Broad-Complex in Drosophila eggshell patterning and morphogenesis

The Drosophila eggshell, which has a pair of chorionic appendages (dorsal appendages) located asymmetrically along both the anterior/posterior and dorsal/ventral axes, provides a good model to study signal instructed morphogenesis. We show that the Broad-Complex, a gene encoding zinc-finger transcription factors, is essential for the morphogenesis of dorsal appendages and is expressed in a bilaterally symmetrical pattern in the lateral-dorsal-anterior follicle cells during late oogenesis. This is induced and specified along the dorsoventral axis by an epidermal growth factor receptor signalling pathway, which includes a localised transforming growth factor-alpha like molecule, Gurken, in the oocyte and the Drosophila EGF receptor homologue, Torpedo, in the surrounding somatic follicle cells. Furthermore, the precisely localised expression of BR-C along the AP axis requires a separate signalling pathway, initiated by a transforming growth factor-beta homologue, Decapentaplegic, in nearby follicle cells. These two signalling pathways, one from the oocyte and the other from the follicle cells, co-ordinately specify patches of follicle cells to express the Broad-Complex in a unique position in respect to both major axes, which in turn directs the differentiation of the dorsal appendages in the correct position on the eggshell.

Relationships between protein isoforms and genetic functions demonstrate functional redundancy at the Broad-Complex during Drosophila metamorphosis

Metamorphosis in holometabolous insects is an ecdysone-dependent process by which the larval form is replaced by a reproductive, adult form. At the onset of metamorphosis ecdysone induces a set of early genes which coordinate tissue-specific responses to hormone. The Broad-Complex (BR-C) early gene, which acts as a global regulator of tissue-specific responses to ecdysone, encodes a family of zinc-finger DNA binding proteins known as Z1, Z2, Z3, and Z4. Genetically the BR-C encodes three complementing functions, br, rbp, and 2Bc, and a class of npr1 alleles that fail to complement any of the other genetic functions. The effects of BR-C mutations on metamorphic development are highly pleiotropic, yet little is known about the roles of individual BR-C proteins in directing the required responses to ecdysone. Because the BR-C is a vital regulator of metamorphosis it is essential to establish the relationships between BR-C genetic functions and protein products. We present here the first general and definitive study of these relationships. Using heat-inducible transgenes we have rescued lethality associated with each of the complementing genetic functions and have restored transcriptional activity of tissue-specific BR-C(+)-dependent target genes. Our data lead us to conclude that br+ function is only provided by the Z2 isoform. We find that Z1 transgenes provide full rbp+ function, while Z4 provides partial function. Likewise, while Z3 provides full 2Bc+ function, Z2 also provides partial function. These results indicate possible functional redundancy or regulatory dependence (via autoregulation) associated with the rbp+ and 2Bc+ functions. The establishment of these relationships between BR-C genetic functions and protein isoforms is an important step toward understanding the roles of BR-C proteins in directing metamorphic responses to ecdysone.

Coordination of larval and prepupal gene expression by the DHR3 orphan receptor during Drosophila metamorphosis

The DHR3 orphan receptor gene is induced directly by the steroid hormone ecdysone at the onset of Drosophila metamorphosis. DHR3 expression peaks in early prepupae, as the early puff genes are repressed and betaFTZ-F1 is induced. Here we provide evidence that DHR3 directly contributes to both of these regulatory responses. DHR3 protein is bound to many ecdysone-induced puffs in the polytene chromosomes, including the early puffs that encode the BR-C and E74 regulatory genes, as well as the E75, E78 and betaFTZ-F1 orphan receptor loci. Three DHR3 binding sites were identified downstream from the start site of betaFTZ-F1 transcription, further indicating that this gene is a direct target of DHR3 regulation. Ectopic expression of DHR3 revealed that the polytene chromosome binding pattern is of functional significance. DHR3 is sufficient to repress BR-C, E74A, E75A and E78B transcription as well as induce betaFTZ-F1. DHR3 thus appears to function as a switch that defines the larval-prepupal transition by arresting the early regulatory response to ecdysone at puparium formation and facilitating the induction of the betaFTZ-F1 competence factor in mid-prepupae. This study also provides evidence for direct cross-regulation among orphan members of the nuclear receptor superfamily and further implicates these genes as critical transducers of the hormonal signal during the onset of Drosophila metamorphosis.

A steroid-triggered switch in E74 transcription factor isoforms regulates the timing of secondary-response gene expression

The steroid hormone 20-hydroxyecdysone (referred to here as ecdysone) directs Drosophila metamorphosis by activating a series of genetic regulatory hierarchies. ETS domain transcription factors encoded by the ecdysone-inducible E74 early gene, E74A and E74B, act at the top of these hierarchies to coordinate the induction of target genes. We have ectopically expressed these E74 isoforms to understand their regulatory functions during the onset of metamorphosis. We show that E74 can regulate its own transcription, most likely through binding sites within its gene. Ectopic expression of E74B can partially repress the E78B and DHR3 orphan receptor genes, suggesting a role for E74 in the appropriate timing of early-late gene expression. Furthermore, E74A is both necessary and sufficient for E78B induction, implicating E74A as a key regulator of E78B expression. We also show, consistent with our studies of E74 loss-of-function mutations, that E74B is a potent repressor of late gene transcription and E74A is sufficient to prematurely induce the L71-1 late gene. However, ectopic expression of both Broad-Complex and E74A activators in an E74B mutant background is not sufficient to prematurely induce all late genes, indicating that other factors contribute to this regulatory circuit. These observations demonstrate that the steroid-triggered switch in E74 transcription factor isoforms plays a central role in the proper timing of secondary-response gene expression.

Two new regulatory elements controlling the Drosophila Sgs-3 gene are potential ecdysone receptor and fork head binding sites

We identified two regulatory elements in the upstream region of the Drosophila Sgs-3 gene which are both able to bind the ecdysone receptor (EcR/USP) and the product of the fork head gene. Interestingly, only one of the EcR/USP binding sites is able to recognize in vitro-translated EcR/USP, which provides evidence for the existence of different receptor forms having different DNA binding specificities. Deletions of the elements lead to a reduced accumulation of Sgs-3 mRNA without altering the temporal expression profile of the gene. The data are consistent with the hypothesis that the ecdysone receptor directly contributes to the transcriptional activation of Sgs-3 by binding to at least one of the two elements. Since also the Sgs-4 gene is controlled by a functional EcR/USP binding site, a direct participation of EcR/USP in the formation of regulatory complexes may be of general importance for the hormonal control of Sgs genes.

Flies on steroids--Drosophila metamorphosis and the mechanisms of steroid hormone action

Recent studies have provided new insights into the molecular mechanisms by which the steroid hormone ecdysone triggers the larval-to-adult metamorphosis of Drosophila. Ecdysone-induced transcription factors activate large sets of secondary-response genes and provide the competence for subsequent regulatory responses to the hormone. It seems likely that similar hormone-triggered regulatory hierarchies exist in other higher organisms and that Drosophila is providing our first glimpses of the complexities of these gene networks.

The Drosophila fork head factor directly controls larval salivary gland-specific expression of the glue protein gene Sgs3

The Drosophila Fork head protein participates in salivary gland formation, since salivary glands are missing in fork head embryos. Here we show that the fork head encoded protein binds to an upstream regulatory region of the larval salivary gland glue protein gene Sgs3. Mobility shift assay in the presence of an anti-Fork head antibody demonstrated that the Fork head factor interacts with the TGTTTGC box shown to be involved in tissue-specific Sgs3 expression. Experiments employing a set of oligonucleotide competitors revealed that Fork head binding was prevented by the same single base substitutions that were previously shown to interfere with the TGTTTGC element function in vivo. Furthermore, the anti-Fork head antibody bound to >60 sites of polytene chromosomes, including the puffs of all Sgs genes and Fork head protein was detected in the nuclei of salivary glands of larvae of all examined stages. These data provide experimental evidence for the hypothesis that the protein encoded by the fork head gene is required initially for salivary gland formation and is utilized subsequently in the control of larval genes specifically expressed in this organ.

Ecdysone and insect immunity: the maturation of the inducibility of the diptericin gene in Drosophila larvae

The developmental analysis of the inducibility of the Drosophila diptericin gene promoter as a response to septic injury shows an important increase in the response during the third larval instar leading to a maximum in late larvae and early prepupae. This increase, or maturation, is temporally correlated with known ecdysone induced events of the salivary gland and we now present evidence, using wild type and mutant larvae, that it does indeed depend upon ecdysone. The response remains minimal in larvae carrying either the temperature sensitive ecdysone deficient late larval lethal allele ecd1, or l(1)t187, a deep orange allele known to be deficient in the ecdysone response. However, experiments with the late larval lethal Broad-Complex mutant l(1)t435 show that the regulation of this response is distinct from the developmental ecdysone regulated hierarchies.

Juvenile hormone: the status of its "status quo" action

Juvenile hormone (JH) allows larval molting in response to ecdysteroids but prevents the switching of gene expression necessary for metamorphosis. I first review our efforts to isolate the nuclear receptor for JH in the larval epidermis of Manduca sexta using photoaffinity analogs and our recent findings that the molecule isolated does not bind JH I with high affinity. The reported apparent high affinity binding of JH I by the recombinant 29 kDa protein (rJP29) was artifactual due to the presence of contaminating esterases. Purified rJP29 bound little detectable JH I, but its binding of the photoaffinity analog was prevented by JH I as well as other isoprenoids, indicating a low affinity for these compounds. Our recent studies focus on the effects of JH on the early molecular events induced by 20-hydroxyecdysone (20E). Culture of day 2 5th larval epidermis with 10(-6)M 20E for 24 h caused first pupal commitment, then the onset of the predifferentiative events necessary for pupation. Biphasic increases in the mRNAs of the two isoforms of the ecdysone receptor (EcR-A and EcR-B1) and of E75A, an ecdysteroid-induced transcription factor, coincided with these two phases. The mRNAs for Ultraspiracle (USP) and the metamorphosis-specific Broad-Complex (BR-C) increased only during the second phase. The presence of JH had no effect on the initial increases in EcR mRNAs but caused an increased accumulation of E75A mRNA. This JH also prevented the later changes in EcR, USP, and BR-C mRNAs. Thus, JH influences only certain of the early actions of 20E which then result in its preservation of the "status quo."

Ecdysone signaling cascade and regulation of Drosophila metamorphosis

Pulses of the steroid hormone 20-hydroxyecdysone (ecdysone) regulate diverse biological responses during the life history of insects. Studies of the fruit fly, Drosophila melanogaster, have provided significant insights into the mechanisms underlying ecdysone mediated regulation of development. During the dramatic metamorphosis of Drosophila, ecdysone induces the histolysis of nearly all of the larval tissues and differentiation and morphogenesis of the structures composing the adult fly. These changes are mediated by a genetic signaling cascade that was first recognized as puffs in the giant polytene chromosomes of the salivary gland. This genetic regulatory cascade is composed of early and late genes that are intricately coordinated by changes in hormone titer. Early genes encode regulatory proteins that are involved in the proper regulation of late genes, which are thought to play a more direct role in development. The regulation and function of these genes is discussed in the context of the cell- and tissue-specific changes required for the reorganization of a larva to form an adult fly.

Drosophila Sgs genes: stage and tissue specificity of hormone responsiveness

The up- and down-regulation of the salivary gland secretion protein (Sgs) genes during the third larval instar of Drosophila melanogaster are controlled by fluctuations of the titre of the steroid hormone 20-hydroxyecdysone (20E). Induction of these genes by a low hormone titre is a secondary response to 20E mediated by products of 20E-induced 'early' genes. Surprisingly, in the case of the Sgs-4 gene this response also requires a direct contribution of the 20E-receptor complex. A model is presented which proposes that the Sgs genes, and other 20E-regulated genes with similar temporal expression profiles, are regulated by complex hormone response units. The hormonal signal is effectively transmitted by these response units only after binding of additional factors, e.g. secretion enhancer binding proteins, which act together in a synergistic manner with the 20E receptor and early gene products to establish a stage- and tissue-specific expression pattern.

Hormonal induction of Dopa decarboxylase in the epidermis of Drosophila is mediated by the Broad-Complex

The 2B5 early puff locus corresponds to the Broad-Complex BR-C) and encodes a family of transcription factors whose members are induced by the molting hormone ecdysone. Mutations in the br subcomplementation group substantially reduce the levels of Dopa decarboxylase (DDC) in the epidermis of mature third instar larvae but not in mature second instar organisms. Enzyme levels are normal in the central nervous system of the two mutants examined. The specificity of these effects suggests that a product of the BR-C locus mediates the rapid appearance of DDC in mature third instar larvae experiencing an elevated titer of ecdysone. The likely identity of this protein has been confirmed by pursuing the observation that the br28 allele caused by the insertion of a Pelement into the Z2 DNA-binding domain. Both the transcript and a protein carrying this domain are present in the epidermis and a BR-C recombinant protein carrying the Z2 finger binds to the first intron of the Ddc gene. Five binding sites have been identified within the intron by DNAase I footprinting and a core consensus sequence has been derived which shares some identity with the consensus binding site of the Z2 protein to the Sgs-4 regulatory region. Our demonstration that Ddc is a target of BR-C in the epidermis is the first direct evidence of a role for this early gene in a tissue other than the salivary glands. The data reinforce the idea that BR-C, which clearly mediates a salivary gland-specific response to ecdysone, may play a widespread role in the hormone's activation of gene cascades in other target tissues.

The Drosophila 63F early puff contains E63-1, an ecdysone-inducible gene that encodes a novel Ca(2+)-binding protein

Pulses of ecdysone at the end of Drosophila larval development dramatically reprogram gene expression as they signal the onset of metamorphosis. Ecdysone directly induces several early puffs in the salivary gland polytene chromosomes that, in turn, activate many late puffs. Three early puffs, at 2B5, 74EF, and 75B, have been studied at the molecular level. Each contains a single ecdysone primary-response gene that encodes a family of widely expressed transcription factors. We report here a molecular characterization of the 63F early puff. Unexpectedly, we have found this locus to be significantly different from the previously characterized early puff loci. First, the 63F puff contains a pair of ecdysone-inducible genes that are transcribed in the larval salivary glands: E63-1 and E63-2. Second, E63-1 induction in late third instar larvae appears to be highly tissue-specific, restricted to the salivary gland. Third, E63-1 encodes a novel Ca(2+)-binding protein related to calmodulin. The discovery of an ecdysone-inducible Ca(2+)-binding protein provides a foundation for integrating steroid hormone and calcium second messenger signaling pathways and generates an additional level for potential regulation of the ecdysone response.

Multiple interacting elements delineate an ecdysone-dependent regulatory region with secondary responsive character

Within the 2.2 kb region between hsp23 and gene 1 of the small heat shock gene locus 67B1 of Drosophila melanogaster, an approximately 1 kb perturbation of the chromatin architecture has previously been observed to occur in response to the steroid hormone ecdysone. Transient expression assays in hormonally-responsive Drosophila tissue culture cells utilizing hsp70-lacZ chimeric reporter constructs revealed the presence of ecdysone-dependent regulatory sequences in this hsp23-gene 1 intergenic region. The analysis delimited five functional segments: three core regions which were completely encompassed within the region of chromatin perturbation, and two gene-proximal regions which appear to be functionally equivalent under some circumstances. None of the delineated regions was capable of stimulating expression independently, while sub-maximal expression was obtained when combinations of two or three regions were monitored. This requirement for multiple DNA segments to drive maximal transcription suggested that cooperative interactions between the regions were essential for full hormonal responsiveness. Unexpectedly, no binding of the ecdysone receptor was detectable within any of the delineated regions, implying the involvement of multiple non-receptor factors in the observed hormonal responsiveness. The ecdysone-dependent activation of reporter constructs driven by these sequences showed a significant time lag and was coupled with a marked sensitivity to low concentrations of cycloheximide. The data obtained strongly suggest that the cis-acting elements delimited within the hsp23-gene 1 intergenic region respond to ecdysone in a secondary manner, presumably by requiring interaction with the product(s) of primary ecdysone-responsive genes.

The Drosophila E74 gene is required for metamorphosis and plays a role in the polytene chromosome puffing response to ecdysone

The steroid hormone ecdysone initiates Drosophila metamorphosis by reprogramming gene expression during late larval and prepupal development. The ecdysone-inducible gene E74, a member of the ets proto-oncogene family, has been proposed to play a key role in this process. E74 is encoded within the 74EF early puff and consists of two overlapping transcription units, E74A and E74B. To assess the function(s) of E74 during metamorphosis, we have isolated and characterized recessive loss-of-function mutations specific to each transcription unit. We find that mutations in E74A and E74B are predominantly lethal during prepupal and pupal development, consistent with a critical role for their gene products in metamorphosis. Phenotypic analysis reveals that E74 function is required for both pupariation and pupation, and for the metamorphosis of both larval and imaginal tissues. E74B mutants are defective in puparium formation and head eversion and die as prepupae or cryptocephalic pupae, while E74A mutants pupariate normally and die either as prepupae or pharate adults. We have also investigated the effects of the E74 mutations on gene expression by examining the puffing pattern of the salivary gland polytene chromosomes in newly formed mutant prepupae. Most puffs are only modestly affected by the E74B mutation, whereas a subset of late puffs are sub-maximally induced in E74A mutant prepupae. These observations are consistent with Ashburner's proposal that early puff proteins induce the formation of late puffs, and define E74A as a regulator of late puff activity. They also demonstrate that E74 plays a wide role in reshaping the insect during metamorphosis, affecting tissues other than the salivary gland in which it was originally identified.

The Drosophila E74 gene is required for the proper stage- and tissue-specific transcription of ecdysone-regulated genes at the onset of metamorphosis

The steroid hormone ecdysone directly induces a small set of early genes, visible as puffs in the larval salivary gland polytene chromosomes, as it signals the onset of Drosophila metamorphorsis. The products of these genes appear to function as regulators that both repress their own expression and induce a large set of secondary-response late genes. We have identified recessive loss-of-function mutations in the early gene E74, a member of the ets protooncogene family that encodes two related DNA-binding proteins, E74A and E74B. These mutations cause defects in pupariation and pupation, and result in lethality during metamorphosis. Here we extend our phenotypic characterization of the E74A and E74B mutant alleles to the molecular level by examining their effects on the transcription of over 30 ecdysone-regulated genes. We show that the transcription of most ecdysone primary-response genes during late larval and prepupal development is unaffected by the E74 mutations. Rather, we find that E74 is necessary for the appropriate regulation of many ecdysone secondary-response genes. E74B is required for the maximal induction of glue genes in mid third instar larval salivary glands, while E74A is required in early prepupae for the proper timing and maximal induction of a subset of late genes. E74 activity is also necessary for the correct regulation of genes expressed predominantly in the fat body, epidermis or imaginal discs. These observations confirm that E74 plays a critical role in regulating transcription during the early stages of Drosophila metamorphosis. In addition, the widespread effects of the E74 mutations on transcription indicate that E74 functions in regulatory hierarchies not only in the larval salivary gland, but throughout the entire organism.

Enhancer-trap targeting at the Broad-Complex locus of Drosophila melanogaster

Here, we describe the exact replacement of a defective unmarked P element by an enhancer-trap transposon marked by the miniwhite gene and carrying lacZ as a reporter gene. The original defective P element was located in an intron of the Broad-Complex (BRC), a key gene involved in metamorphosis. Replacement events resulted from conversions induced by the P-element transposase from a donor enhancer-trap element located on another chromosome. Six independent conversion events were selected. In all converted chromosomes, the enhancer-trap transposon was in the same orientation as the original P element. From the pattern of X-gal staining observed, lacZ expression likely reflects the regulatory influence of BRC enhancers on the convertant transposon. Reversion to wild type was achieved by excision of the enhancer-trap transposon. The six convertants were analyzed in detail at the nucleotide level. The occurrence of a polymorphism at position 33 of the P-element sequences led us to propose a conversion mechanism involving homologous P sequences for repair. This is in contrast to previously analyzed P-element transposase-induced conversion events and proposed models relying on sequence identity between genomic Drosophila sequences. The lack of any homology requirement other than between P element sequences means that our findings can be easily generalized. Targeting a marked P-element derivative at a precise site without loss or addition of genetic information makes it possible to exploit the hundreds of defective P elements scattered throughout the Drosophila genome by replacing them with engineered P elements, already available.

The role of the BR-C locus on the expression of genes located at the ecdysone-regulated 3C puff of Drosophila melanogaster

During the third larval instar, the steroid moulting hormone ecdysone activates three temporally distinct puff sets on the D. melanogaster salivary gland polytene chromosome: the so-called intermoult, early and late puffs. Hormonal regulation of intermoult puffs is quite complex and, so far, largely not understood. In order to further investigate this aspect, we have analysed the effects of mutations in a key regulator of the ecdysone response at the onset of metamorphosis, the Broad-Complex (BR-C) locus, on the expression of genes mapping at the 3C intermoult puff. On the basis of an accurate examination of 3C intermoult gene activity in single, carefully staged, third instar larvae of wild-type and BR-C mutant strains, we were able to subdivide these genes into two groups. Each group is characterised by a different temporal expression profile, so that at the beginning of the wandering stage the transcription of the first group declines as group II transcription is induced. Interestingly, the BR-C locus appears to play a regulatory role in establishing this transcriptional switch. By using mutants of each of the three lethal complementation groups, we precisely defined the role of BR-C functions in this developmental transition and we show that this locus also plays an essential role in the early pre-metamorphic hormonal response.

Remodeling of the insect nervous system

Our nervous systems and behavior are shaped by hormonally driven developmental changes that continue beyond the embryonic period. Key insights into this process have emerged from studies of the insect nervous system. During insect metamorphosis, the nervous system is remodeled through postembryonic neurogenesis, programmed cell death and the modification of persistent neurons. These changes are regulated to a large degree by gene cascades that are triggered by steroid hormones, the ecdysteroids. Current studies are attempting to reveal the molecular mechanisms involved in regulating these dramatic examples of developmental plasticity.