Developmental biology. Fly fishing downstream.
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Animals↗
Biomedical subjects
G Struhl
Publications and source records attributed to G Struhl.
Are cross-regulatory interactions between homoeotic genes functionally significant?
The first instar larva of Drosophila consists of a chain of segments or parasegments in which the morphological pattern characteristic of each metamere is determined by the homoeotic genes, which are active in overlapping domains and are known to interact among themselves. The interactions occur at the level of transcription and allow some homoeotic genes to control the patterns and levels of expression of others. The best known among them are the down-regulation of Antennapedia (Antp) by Ultrabithorax (Ubx) and that of Ubx by abdominal-A (abd-A) and Abdominal-B (Abd-B). It has been proposed that these cross-regulatory interactions play a part in specifying cell pattern, and hence the identity of each metamere. Here we assess the functional significance of some of these interactions by expressing the Antp, Ubx or both homoeotic genes under the control of the heat-shock promoter. Predictably, we find that homoeotic gene products evade normal regulatory controls and can be maximally expressed in regions where they are normally down-regulated but, surprisingly, we find that interruption of the normal down-regulation of Antp and Ubx has no phenotypic consequences in the epidermis, where homoeotic phenotypes are normally manifest. Hence our results challenge the view that these, and possibly other cross-regulatory interactions have a role in determining segmental identity.
Animals↗
Decoding positional information: regulation of the pair-rule gene hairy.
In the series of local gene activations that occur during early Drosophila development, the striped expression patterns of the pair-rule genes provide the first indication of segmental periodicity. The experiments that we report here address the question of how these patterns arise, by studying the regulation of one of these genes, hairy. We show that each of the seven stripes of hairy expression is controlled by a distinct subset of cis-acting regulatory elements, some mediating transcriptional activation and others transcriptional repression. In general, elements necessary and sufficient for triggering a particular stripe response are clustered on the DNA and appear to overlap or be interspersed with elements involved in at least one other stripe response. Our results extend previous findings suggesting that periodic hairy expression arises by a decoding process in which each stripe is triggered by particular combinations or concentrations of regulatory factors. These regulatory factors are likely to include the products of the gap class of segmentation genes that are required for activating or positioning particular subsets of hairy stripes and are expressed with overlapping distributions during early embryogenesis.
Animals↗
Structure of the Drosophila BicaudalD protein and its role in localizing the posterior determinant nanos.
Mutations in the BicaudalD (BicD) gene lead to a global reorganization of the Drosophila body pattern such that the head, thoracic, and anterior abdominal segments are replaced by posterior abdominal segments and terminalia. We first provide evidence that the primary cause of this phenotype is the inhibition of two anterior factors, bicoid and hunchback, by mislocalized activity of the posterior determinant nanos. We then describe the isolation of the BicD gene and show that it encodes a coiled-coil protein similar to the carboxy-terminal portion of the myosin heavy chain. Finally, we find that BicD protein is uniformly distributed throughout wild-type oocytes but is concentrated at the anterior pole of BicD mutant oocytes together with ectopic nanos activity. Taken together, these results suggest that BicD encodes a cytoskeleton-like protein involved in transporting or anchoring the nanos morphogen within the oocyte cytoplasm.
Amino Acid Sequence↗
The gradient morphogen bicoid is a concentration-dependent transcriptional activator.
The bicoid (bcd) protein is expressed in an anteroposterior gradient in early Drosophila embryos and controls the zygotic activation of the segmentation gene hunchback (hb) in a broad but precisely bounded anterior domain. Here we show that the hb gene contains multiple regulatory elements that mediate transcriptional activation in response to bcd protein. Further, we demonstrate that the resulting patterns of expression in vivo depend critically on both the bcd gradient profile and the number and quality of these hb elements. Finally, we show that these same elements mediate bcd-dependent transcriptional activation in yeast and that this interaction requires distinct DNA binding and activating regions in the bcd protein. Our results argue that bcd protein normally binds and activates the hb gene in a concentration-dependent fashion, thereby allowing the gradient of bcd protein to dictate where the hb gene is initially turned on in early embryos. They also suggest that the bcd gradient has the instructive capacity to activate other subordinate control genes by the same mechanism, each in a distinct spatial domain according to its affinity for bcd protein.
Animals↗
Differing strategies for organizing anterior and posterior body pattern in Drosophila embryos.
Opposing anterior and posterior morphogen systems specify the segmented body pattern of Drosophila. The anterior morphogen, bicoid, exerts a direct, instructive influence on head and thoracic pattern by triggering different outcomes according to changes in its concentration along the body. In contrast, the posterior morphogen, nanos, simply defines where abdominal patterning can occur by eliminating an otherwise ubiquitous repressor, hunchback protein, from the posterior half of the embryo. Within this hunchback-free domain the pattern of abdominal segments must be specified by other morphogens, possibly by shorter range gradients of the products of zygotic gap genes Kruppel, knirps and tailless.
Animals↗
Morphogen gradients and the control of body pattern in insect embryos.
The basic body plan of insects is set up in response to determinants initially localized at the anterior and posterior poles of the egg. Early in development, these determinants give rise to a series of morphogen gradients which in turn trigger a cascade of molecular signals determining the body pattern. This signalling process is outlined and recent experiments testing the roles of these gradient systems in determining anterior and posterior pattern are described. The results of these experiments suggest that anterior pattern is controlled by a single instructive gradient, whereas posterior pattern depends on the overlap of several gradients, each providing only one or a few distinct responses.
Animals↗
Localized surface activity of torso, a receptor tyrosine kinase, specifies terminal body pattern in Drosophila.
The subdivision of the Drosophila body into distinct terminal and central domains depends on the torso (tor) protein, a putative receptor tyrosine kinase that is active at both ends of the early embryo. We show that the tor protein is uniformly expressed along the surface membrane of early embryos despite its localized activity at both poles. Further, we present evidence that polarized activity of this protein depends on other terminal gene functions, one of which may be a localized extracellular ligand generated during oogenesis. Finally, using the temperature-sensitive gain-of-function mutation torRL3, we show that different levels of active tor protein can specify distinct portions of the terminal pattern. Thus, we argue (1) that for functions as a ubiquitous surface receptor that is activated by a spatially restricted ligand, and (2) that localized activity of the tor kinase may generate one or more gradients of intracellular signals that control body pattern.
Animals↗
cis-acting sequences responsible for anterior localization of bicoid mRNA in Drosophila embryos.
The anterior body pattern of Drosophila melanogaster is specified in large part by the protein product of the bicoid (bcd) gene which functions as a graded morphogen with its peak of expression at the anterior pole of the embryo. Formation of the gradient is dependent on prior localization of bcd messenger RNA at the anterior pole of the egg cell during oogenesis. Here we demonstrate that a discrete portion of the bcd mRNA is necessary for anterior localization of the bcd transcript and is sufficient to cause localization of heterologous transcripts. The sequences responsible for localization appear to span an interval of about 625 base pairs in the 3' untranslated portion of the bcd mRNA and to include regions capable of forming extensive secondary structure. Transcripts from bcd are synthesized predominantly, if not exclusively, in the nurse cells and then transported to the oocyte by connections at the prospective anterior pole. Our findings support the proposal that bcd transcripts are selectively recognized and trapped as they enter the anterior tip of the oocyte, and suggest that this localization process is mediated by anchored sequence-specific receptors in the oocyte cytoplasm.
Animals↗
Isolation, structure, and expression of even-skipped: a second pair-rule gene of Drosophila containing a homeo box.
We report the isolation, sequence, and transcriptional behavior of the Drosophila even-skipped (eve) gene, a member of the pair-rule class of segmentation genes. We show that the eve gene contains a homeo box and hence is related structurally to the pair-rule gene fushi tarazu and to homeotic selector genes. However, the eve homeo box differs significantly from those previously described and encodes a putative DNA recognition helix that would probably recognize different sequences. The eve gene resembles other pair-rule genes in showing a transient seven stripe zebra pattern during the blastoderm stage, but seven additional stripes arise soon thereafter. Together, these 14 stripes are required for the activation of coincident stripes of engrailed transcripts, leading to the subdivision of the embryo into compartmental and segmental units.
Animals↗
Altered distributions of Ultrabithorax transcripts in extra sex combs mutant embryos of Drosophila.
The product of the extra sex combs (esc) gene is required early in embryogenesis to ensure the correct spatial expression of the bithorax and Antennapedia gene complexes during subsequent development. Here we describe the spatial and temporal patterns of transcription of the Ultrabithorax (Ubx) gene in mutant esc embryos. In wild-type embryos, Ubx transcripts are first detected in the blastoderm primorida of particular thoracic and abdominal segments, predominately in a band of cells constituting the progenitor cells of parasegment 6: after gastrulation, high levels of transcript accumulate in a sharply restricted region of the germ band comprising parasegments 6-12. In esc- embryos, the initial pattern of Ubx transcripts appears indistinguishable from that of wild-type embryos. However, following gastrulation and germ band extension, Ubx transcripts accumulate in most of the ectodermal and mesodermal derivatives of the body, including those of all 14 parasegments. Then, the abundance of the transcripts declines so that after germ band shortening (12 h after fertilization), only low levels are detected in each parasegment. These results show that the esc gene product is not involved in choosing where the Ubx gene is to be expressed, but rather, that it acts subsequently to ensure that the gene remains off in those primordia in which it is not initially activated. In addition, they suggest the possibility that other homeotic genes may regulate Ubx expression --in this case, the accumulating products of other indiscriminately expressed homeotic genes serving to down-regulate Ubx transcription throughout the body.
Animals↗
Regulation of the Ultrabithorax gene of Drosophila by other bithorax complex genes.
The bithorax complex of Drosophila is composed of three major units of gene function, Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B), each having a precise realm of action within the thorax and abdomen. Recent molecular studies have established that the transcripts and protein products of the Ubx gene are expressed principally in a restricted portion of the body, beginning in the second thoracic segment and ending in the eighth abdominal segment (i.e., in parasegments 5-13). However, within this region, both show complex, heterogeneous patterns of expression. Using mutations in the abd-A, Abd-B, and esc (extra sex combs) genes we show that these distinct patterns result in large part from regulation of Ubx gene expression by the abd-A and Abd-B gene functions.
Animals↗
Different requirements for homeotic genes in the soma and germ line of Drosophila.
Some homeotic genes of Drosophila are active only in particular developmental compartments ("selector" genes) whereas others are active in all compartments and are responsible for regulating selector gene expression. Using pole cell transplants to generate mosaic females with mutant germ lines, we examine the extent to which genes of both classes are required during oogenesis. We find that none of four selector genes (Ultrabithorax+, Antennapedia+, Sex combs reduced+, and engrailed+) is required for the development of viable eggs, and none contributes detectable product to the fertilized zygote. Similarly, neither of two regulatory genes (Polycomb+ and extra sex combs+) are required for normal oogenesis. However, both are active in the germ line and contribute functional gene product to the embryo. These results suggest that selector genes are inactive in the germ line where they may be regulated by a different mechanism than in the soma.
Animals↗
Homeotic transformations of the abdominal segments of Drosophila caused by breaking or deleting a central portion of the bithorax complex.
In Drosophila, genes in the centromere-proximal portion of the bithorax complex (BX-C) have been shown to control the development of the metathorax, and parts of the mesothorax and first abdominal segment. Here, we explore the roles of genes positioned more distally by examining the larval and adult phenotypes caused by a breakpoint and deletion in the middle of the complex. We find that both aberrations affect only abdominal segments, transforming the more anterior segments towards the first abdominal segment, and the remaining segments into a graded series of novel segment types which are partially transformed towards more anterior abdominal segments. Moreover, the adult transformations, which we have observed in somatic clones of mutant cells, are in close accord with the transformations observed in mutant first instar larvae, and appear to be expressed in a cell autonomous fashion. We discuss these results in the light of current views of the organization and function of the complex.
Abdomen↗
Role of the esc+ gene product in ensuring the selective expression of segment-specific homeotic genes in Drosophila.
The product of the extra sex combs+ (esc+) gene is required during embryogenesis for the correct determination of segments in Drosophila. If this product is absent, most segments develop like the normal eighth abdominal segment. Here, I extend previous results (Struhl, 1981a) showing that this phenotype results in large part from indiscriminate expression of the bithorax-complex genes which are normally active only in particular segments of the thorax and abdomen. In addition, I test whether the esc+ gene product is required for the correct expression of other homeotic genes. First, I have examined two genes of the Antennapedia-complex (Sex combs reduced+ and Antennapedia+): I find that both genes are normally required in only some of the body segments, but that in the absence of the esc+ gene product, both appear to function adventitiously in other segments. Second, comparing esc+ and esc- embryos lacking both these genes as well as the bithorax-complex, I find that additional homeotic genes (possibly those normally involved in specifying head segments) appear to be expressed indiscriminately when the esc+ gene product is absent. Finally, I present evidence that the products of the esc+ gene and the Polycomb+ gene (a second gene required for the correct regulation of the bithorax-complex) act independently. On the basis of these results, I propose a tentative outline of the roles and realms of action of all of these genes.
Animals↗
Decapentaplegic - hopes held out.
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Animals↗
Further studies of the engrailed phenotype in Drosophila.
Although most mutations at the engrailed locus of Drosophila cause embryonic death when homozygous, they are viable in clones of cells. We describe the phenotype of such clones in the eye-antenna, proboscis, humerus, wing, legs, and terminalia. When in anterior compartments the clones are normal, but in most posterior compartments they are abnormal and fail to respect the anteroposterior compartment boundary. We find that the yield of engrailed-lethal clones in posterior compartments is often significantly lower than expected, indicating that these clones are lost during development. Mutant clones are abnormal in the analia and rare in the humerus, suggesting that both structures are of posterior provenance. These results support the hypothesis that the engrailed+ gene is required exclusively in cells of posterior compartments to specify their characteristic cell affinities and pattern.
Animal Population Groups↗
Early role of the esc+ gene product in the determination of segments in Drosophila.
Recent studies of the esc+ gene suggest that its product acts principally in the initiation, but not the maintenance, of segmental determination. We have tested this hypothesis by examining when the esc+ gene product is required during embryogenesis, and whether the esc+ gene product is required for the stable determination of imaginal disc cells passaged in in vivo culture. We find that the esc+ gene product is required during a discrete period of embryogenesis. If the gene product is absent or inactive during this period, most segments develop like the eighth abdominal segment. In contrast, absence of active gene product before or after this period has relatively little effect on segmental development. These results support the hypothesis that the esc+ gene product has a discrete early function in the initiation of segmental determination.
Animals↗