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E Hafen

Publications and source records attributed to E Hafen.

69 records · Page 4Linked to original sources
PubMed1990

Mechanisms of positional signalling in the developing eye of Drosophila studied by ectopic expression of sevenless and rough.

In the developing eye of Drosophila cell fate is controlled by a cascade of inductive interactions. Little is known about how the specificity of positional signalling is achieved such that directly adjacent progenitor cells reproducibly choose distinct developmental pathways. The determination of the R7 photoreceptor in each ommatidium depends on the presence of the sevenless protein which acts as a receptor for positional information on the R7 precursor. The rough gene encodes a homeodomain protein that plays an instructive role in the determination of the R3 and R4 photoreceptor cells. The use of ectopic expression of sevenless and rough has provided insight into the mechanisms of positional signalling and the normal function of rough. Ubiquitous expression of sevenless does not alter cell fate suggesting that the inducing signal is both spatially and temporally controlled. Conversely, ectopic expression of rough in the R7 precursor causes a transformation of R7 cells into R1-6 type cells. This indicates that rough acts, similar to other homeobox genes, as a selector gene that determines the fate of single cells.

Animals↗
PubMed1989

Ubiquitous expression of sevenless: position-dependent specification of cell fate.

Specification of cell fate in the compound eye of Drosophila appears to be controlled entirely by cell interactions. The sevenless gene is required for the correct determination of one of the eight photoreceptor cells (R7) in each ommatidium. It encodes a transmembrane protein with a tyrosine kinase domain and is expressed transiently on a subpopulation of ommatidial precursor cells including the R7 precursors. It is shown here that heat shock-induced indiscriminate expression of a sevenless complementary DNA throughout development can correctly specify R7 cell identity without affecting the development of other cells. Furthermore, discontinuous supply of sevenless protein during eye development leads to the formation of mosaic eyes containing stripes of sevenless+ and sevenless- ommatidia, suggesting that R7 cell fate can be specified only within a relatively short period during ommatidial assembly. These results support the hypothesis that the specification of cell fate by position depends on the interaction of a localized signal with a receptor present on many undifferentiated cells, and that the mere presence of the receptor alone is not sufficient to specify cell fate.

Animals↗
PubMed1989

The spatial and temporal expression pattern of sevenless is exclusively controlled by gene-internal elements.

The sevenless gene controls the fate of a single photoreceptor cell type in the developing eye of Drosophila. Its RNA and protein accumulate transiently in a subpopulation of cells in the developing eye imaginal disc. We have used P-element transformation with different minigenes and marker gene constructs to identify cis-acting regulatory regions required for the complex sevenless expression pattern. Our results indicate that the upstream region of the sevenless gene is devoid of any detectable regulatory elements and that sequences located 3' to the transcription start site are sufficient to promote the sevenless expression pattern. These gene-internal sequences function in both orientations on heterologous promoters, also when placed at the 3' end of a lacZ reporter gene.

Animals↗
PubMed1989

Dynamics of Drosophila eye development and temporal requirements of sevenless expression.

The development of the compound eye of Drosophila consists of a linear, stereotyped program starting at the posterior end of the eye imaginal disc and progressing towards the anterior border. The determination of the R7 photoreceptor cells is part of this process and is dependent on the sevenless gene. In this study, we used a heat-shock-inducible sevenless gene as a conditional allele to determine the exact temporal requirements of sevenless gene expression and to reveal the stages of ommatidial development during which the presumptive R7 cell can respond to the presence of sevenless protein. Our results indicate that sevenless gene function is only required during a brief, defined period for the initiation of R7 development; subsequently sevenless is dispensable for both differentiation and function of the R7 photoreceptors. Furthermore, using rescue of R7 cells as an internal marker to monitor the progression of eye development we could examine when and at what rate ommatidial columns form.

Alleles↗
PubMed1988

Control of photoreceptor cell fate by the sevenless protein requires a functional tyrosine kinase domain.

The sevenless (sev) gene determines the fate of a single photoreceptor cell type in the eye of Drosophila. It encodes a putative cell-surface protein with homology to tyrosine kinases. Here we have determined the complete structure of the sev gene and have demonstrated that the role of the sev protein in this developmental decision is critically dependent on the tyrosine kinase function. In comparison with other known tyrosine kinases, the sev gene product is unique in size and structure. It is a polypeptide of 2554 amino acids with two putative transmembrane segments. A single amino acid substitution in the ATP-binding site of the putative kinase domain results in the synthesis of an inactive sev protein unable to determine cell fate.

Amino Acid Sequence↗
PubMed1987

Localization of the sevenless protein, a putative receptor for positional information, in the eye imaginal disc of Drosophila.

The Drosophila gene sevenless encodes a putative trans-membrane receptor required for the formation of one particular cell, the R7 photoreceptor, in each ommatidium of the compound eye. Mutations in this gene result in the cell normally destined to form the R7 cell forming a non-neuronal cell type instead. These observations have led to the proposal that the sevenless protein receives at least part of the positional information required for the R7 developmental pathway. We have generated antibodies specific for sevenless and have examined expression of the protein by light and electron microscopy. sevenless protein is present transiently at high levels in at least 9 cells in each developing ommatidium and is detectable several hours before any overt differentiation of R7. The protein is mostly localized at the apices of the cells, in microvilli, but is also found deeper in the tissue where certain cells contact the R8 cell. This finding suggests that R8 expresses a ligand for the sevenless protein.

Animals↗
PubMed1987

Sevenless, a cell-specific homeotic gene of Drosophila, encodes a putative transmembrane receptor with a tyrosine kinase domain.

The determination of cell fates during the assembly of the ommatidia in the compound eye of Drosophila appears to be controlled by cell-cell interactions. In this process, the sevenless gene is essential for the development of a single type of photoreceptor cell. In the absence of proper sevenless function the cells that would normally become the R7 photoreceptors instead become nonneuronal cells. Previous morphological and genetic analysis has indicated that the product of the sevenless gene is involved in reading or interpreting the positional information that specifies this particular developmental pathway. The sevenless gene has now been isolated and characterized. The data indicate that sevenless encodes a transmembrane protein with a tyrosine kinase domain. This structural similarity between sevenless and certain hormone receptors suggests that similar mechanisms are involved in developmental decisions based on cell-cell interaction and physiological or developmental changes induced by diffusible factors.

Amino Acid Sequence↗
PubMed1985

Rescue of the Drosophila phototransduction mutation trp by germline transformation.

Phototransduction is the process by which light-stimulated photoreceptor cells of the visual system send electrical signals to the nervous system. Many of the steps that follow the initial event in phototransduction, absorption of light by rhodopsin, are ill-defined. The fruitfly, Drosophila melanogaster, provides a means to dissect phototransduction genetically. Mutations such as transient receptor potential (trp) affect intermediate steps in phototransduction. In order to facilitate molecular studies of phototransduction, the trp gene was isolated and its identity was confirmed by complementing the mutant trpCM allele of the trp gene by P-element mediated germline transformation of a 7.1-kilobase DNA fragment. Expression of the trp gene begins late in pupal development and appears to be limited to the eyes and ocelli.

Animals↗
PubMed1984

Cloning and transcriptional analysis of the segmentation gene fushi tarazu of Drosophila.

In the course of studying the Antennapedia (Antp) locus, we found that one of the 3' Antp exons has weak cross-homology to another gene affecting segmentation, fushi tarazu (ftz; meaning "not enough segments"), which is 30 kb to the left of Antp. Homozygous ftz- embryos die before hatching and lack alternate body segments. The reduced number of segments results from the fusion of the anterior portion of one segment with the posterior portion of the next segment. The ftz gene encodes a single 1.9 kb poly(A)+ RNA expressed exclusively from the early blastoderm to gastrula stages of embryonic development. The structure of the ftz gene has been analyzed by S1 nuclease mapping and by restriction mapping of a cDNA clone. The ftz gene consists of two exons, and it is the 3' exon that cross-hybridizes with the 3' exon of Antp. The role of ftz in cell determination is discussed.

Animals↗
PubMed1984

Spatial distribution of transcripts from the segmentation gene fushi tarazu during Drosophila embryonic development.

The locus fushi tarazu appears to be involved in the establishment of the segmentation pattern of the Drosophila embryo. The cuticle of ftz mutant embryos is missing structures in alternating segments such that only half the normal number of segments are present. We have localized ftz+ transcripts in tissue sections of wild-type Drosophila embryos by in situ hybridization. Transcripts from the ftz+ gene were first detected during nuclear cleavage prior to cell formation. During the last two nuclear divisions ftz+ transcription becomes gradually restricted such that at the cellular blastoderm stage the ftz+ transcripts are localized in seven evenly spaced bands of cells. The size of each band is similar to the size of the segment primordia at the blastoderm. By the time segmentation becomes morphologically distinct ftz+ transcripts are no longer detected. These results suggest that the ftz+ gene plays a key role in the determination of the segmentation pattern in the embryo.

Animals↗
PubMed1983

An improved in situ hybridization method for the detection of cellular RNAs in Drosophila tissue sections and its application for localizing transcripts of the homeotic Antennapedia gene complex.

An improved method for the detection of cellular RNAs in tissue sections has been developed. It involves in situ hybridization of tritium-labeled cloned DNA probes to tissue sections and autoradiography. The method was calibrated by using a cloned DNA probe complementary to transcripts abundant in the midgut cells of Drosophila larvae. The improved method also permitted the detection of these transcripts in sectioned embryos where they are much less abundant. The sensitivity of the method can be approximated by quantifying the signal intensities over the hybridizing embryonic midgut cells relative to the larval midgut cells for which the number of transcripts has been estimated. Based on these calculations we estimate that the method is sensitive enough to detect 100 complementary RNA molecules per cell after 3 days of autoradiographic exposure with a signal-to-noise ratio of 10. The method has been successfully applied to detect transcripts of the homeotic gene Antennapedia. Serial sections allow us to study the spatial pattern of gene expression in the course of development.

Journal Article↗
PubMed1983

Spatial distribution of Antennapedia transcripts during Drosophila development.

We have localized transcripts specified by the homeotic Antennapedia (Antp) locus within serial tissue sections of Drosophila embryos and larvae by in situ hybridization. As a hybridization probe we used a 2.2-kb cDNA sequence (903) which is complementary to at least four non-contiguous chromosomal DNA regions within a span of 100 kb derived from the Antp+ locus. The tritiated probe was directly hybridized to frozen tissue sections of wild-type embryos and larvae. Hybridization was first detected to the progenitors of the thoracic segments during the cellularization of the syncytial blastoderm, when embryonic cells first become determined to form particular adult segments. At later embryonic stages the ventral nervous system also becomes labeled in a spatially-restricted manner. Initially, accumulation of transcripts is detected in all thoracic and abdominal ganglia of the ventral cord. Subsequently, the highest concentration of transcripts is detected in the ganglion cells of the mesothorax. Proper development of this segment is known to be affected in individuals homozygous for putative Antp null alleles. In third-instar larvae, hybridization to the different imaginal disks of all three thoracic segments is observed, localized mainly to those regions that will form proximal, cuticular portions of the respective segments. These results are discussed with respect to the role of homeotic genes in the specification of particular anatomical segments.

Animals↗
PubMedDate not supplied

Regulation of Antennapedia transcript distribution by the bithorax complex in Drosophila.

The homoeotic genes of Drosophila seem to be involved in the establishment of developmental pathways and the specification of different anatomical segment identities. For example, it has been proposed that a function of the Antennapedia+ (Antp+) locus is to direct embryonic cells to follow a mesothoracic developmental pathway. Using cloned cDNA probes of the Antp+ (ref. 6) locus and an improved in situ hybridization method, we found that the neural cells of the embryonic and larval mesothorax possess higher levels of Antp+ transcripts than the neural tissue of the other segments. Here we present further evidence that Antp+ products are associated with formation of the mesothorax, obtained by analysing the distribution of Antp+ transcripts in bithorax mutants. Embryos homozygous for any one of several bithorax mutations display a transformation of the epidermis of the more posterior segments into the homologous tissue of the mesothorax. For example, the metathorax and first seven abdominal segments of embryos that completely lack the bithorax gene complex show a cuticular phenotype characteristic of the normal mesothorax. By in situ hybridization of an Antp+ cDNA probe to tissue sections of these embryos, we have found that the neural cells of the transformed segments accumulate Antp+ transcripts to a level characteristic of the normal mesothorax.

Animals↗
PubMedDate not supplied

A conserved DNA sequence in homoeotic genes of the Drosophila Antennapedia and bithorax complexes.

A repetitive DNA sequence has been identified in the Drosophila melanogaster genome that appears to be localized specifically within genes of the bithorax and Antennapedia complexes that are required for correct segmental development. Initially identified in cloned copies of the genes Antennapedia, Ultrabithorax and fushi tarazu, the sequence is also contained within two other DNA clones that have characteristics strongly suggesting that they derive from other homoeotic genes.

Animals↗