Michael A. Russell

Pattern formation in the imaginal discs of a temperature-sensitive cell-lethal mutant of Drosophila melanogaster☆

Abstract To explore the effects of cell death on pattern formation in the developing imaginal discs of Drosophila melanogaster , I have isolated a number of cell-autonomous temperature-sensitive lethal mutants. Sex-linked temperature-sensitive lethals were screened for cell-autonomy by scoring the survival of lethal-bearing clones in genetic mosaics. The mutant with the strongest effect on clone viability gave rise to a high frequency of structural deficiencies and duplications in the derivatives of the eye-antennal discs, when subjected to pulse-treatments at the nonpermissive temperature during the late second and third instars. The patterns produced were nonrandom, with some structures showing a tendency to become deficient, and others a tendency to duplicate. Duplicated structures were only found in heads in which other structures were missing. Genetic tests identified the lethal as a point mutation at the suppressor-of-forked locus. Recombination, and complementation tests with a small duplication of this region showed that a second mutational lesion is in all probability not involved in the generation of abnormal patterns in the imaginal discs. It is therefore proposed that the cell-lethal action of the mutant is sufficient to account for phenotypic effects described. According to this hypothesis, cell death primarily causes deficiencies, and duplications occur as a response of the discs to injury. In agreement with this, it was found that in gynandromorphs, pattern duplications can be found in wild-type tissue in the presence of lethal tissue in the same disc. Thus, a cell-autonomous lethal may affect the process of pattern formation in a nonautonomous way.

The correlation of lysosomal activity and adult phenotype in a cell-lethal mutant of Drosophila.

Abstract The mutant l(1)ts726 is a temperature-sensitive cell-lethal that causes pattern abnormalities in the cuticular derivatives of the leg and eye-antennal discs when larvae are subjected to sublethal heat treatment during the temperature-sensitive period. The derivatives of other discs are unaffected. Cytoplasmic bodies staining strongly with toluidine blue were found in the epithelial cells of all discs examined from heat-treated larvae. They were identified as late secondary lysosomes and residual bodies on the basis of ultrastructural and histochemical tests. Their spatial distribution was quantified in the eye-antennal, leg, and wing discs by serial sectioning, and the results were compared with the morphological phenotypes induced. Within each kind of disc, lysosomal activity is clustered rather than uniform. The position of maximal lysosomal activity varies from disc to disc in an apparently random way, but underlying patterns can be discerned when the average distributions are calculated. In the eye-antennal disc reconstructions, the region most often affected by cell death corresponds to the part of the head most often deficient in adults and also to the high point of the hypothetical gradient of developmental capacity.

A clonal analysis of pattern duplication in a temperature-sensitive cell-lethal mutant ofDrosophila melanogaster

Abstract Clonal analysis was used to determine the numbers of cells involved in the initiation and growth of pattern duplications in mesothoracic legs that result from a 29°C treatment of a cell-lethal mutant. The frequency, size, and location of clones induced before, during, and after the 29°C treatment were analyzed. The results indicate that duplications arise from a small number of cells. These cells become committed to form the duplicate within 48 hr after the end of the 29°C treatment and begin rapid growth at about the same time. The cell number and growth rate of newly initiated duplicate legs are remarkably similar to those of normal leg discs at early stages in development, supporting the hypothesis that pattern duplicates develop by a reiteration of normal growth processes.

Hephaestus encodes a polypyrimidine tract binding protein that regulates Notch signalling during wing development in Drosophila melanogaster.

We describe the role of the Drosophila melanogaster hephaestus gene in wing development. We have identified several hephaestus mutations that map to a gene encoding a predicted RNA-binding protein highly related to human polypyrimidine tract binding protein and Xenopus laevis 60 kDa Vg1 mRNA-binding protein. Polypyrimidine tract binding proteins play diverse roles in RNA processing including the subcellular localization of mRNAs, translational control, internal ribosome entry site use, and the regulation of alternate exon selection. The analysis of gene expression in imaginal discs and adult cuticle of genetic mosaic animals supports a role for hephaestus in Notch signalling. Somatic clones lacking hephaestus express the Notch target genes wingless and cut, induce ectopic wing margin in adjacent wild-type tissue, inhibit wing-vein formation and have increased levels of Notch intracellular domain immunoreactivity. Clones mutant for both Delta and hephaestus have the characteristic loss-of-function thick vein phenotype of Delta. These results lead to the hypothesis that hephaestus is required to attenuate Notch activity following its activation by Delta. This is the first genetic analysis of polypyrimidine tract binding protein function in any organism and the first evidence that such proteins may be involved in the Notch signalling pathway.

Positional information in insect segments

Abstract Many kinds of multicellular animals are made of a series of reiterated developmental units called segments. Although a different pattern may differentiate in every segment, there is evidence in insects that the system of positional information underlying the pattern of differentiation is similar or homologous in all segments. The patterns regenerated after surgical experiments on epidermal tissue have been interpreted as evidence for an anteroposterior gradient of positional information repeated in each segment and controlling the type and orientation of the cuticular structures that differentiate after each molt. However, this model is unable to account for some recent results of D. A. Wright and P. A. Lawrence (1981 Dev. Biol. 85, 317–327), who argue that positional values are specified along the anteroposterior axis of a segment by a variable having the properties of an angle. Here it has been shown, that a new model for positional information in the insect segment, based on chemical wave patterns generated by reaction-diffusion, is capable of explaining the formal properties of the positional information system. Using computer simulations it has also been shown that the phenotypes of a number of embryonic lethal mutants that affect segmentation in Drosophila can be explained by various simple changes in the parameters of the model, such as wavelengths, equilibrium values, etc.

Clonal analysis of segmental and compartmental homoeotic transformations in polycomb mutants of Drosophila melanogaster.

We describe two new effects of Polycomb mutations on the determination of compartments in the wing of Drosophila. Ventral and posterior wing compartments are transformed partially to their dorsal and anterior counterparts. Although these new phenotypes are most strongly expressed in lethal pharate adults heteroallelic for Pc2 and a new allele PcT2, they are also found regularly but with low expressivity as dominant phenotypes of all the other Pc alleles we tested. Several different intersegmental homoeotic transformations caused by Polycomb have previously been described, leading to the hypothesis that Polycomb regulates the activity of certain selector genes normally active in specific segments. We now show that the degree of expression of the inter- and intrasegmental transformations are highly correlated in a range of different Pc genotypes, and that more than one determinative decision can be affected in a single compartment. This suggests that the wild-type Pc product may act as a general regulator of several different selector genes so as to influence both early embryonic and later determinative decisions in the imaginal discs. To test this idea we used clonal analysis to look at the effects of Pc on clonal restrictions at the dorsoventral and anterioposterior compartment boundaries, and its time of action with respect to each phenotype.

Positional information and the bithorax complex

Abstract Positional information considerations are applied to mutants which cause transformations involving the mesothorax, metathorax, and first and second abdominal segments of Drosophila melanogaster. The determination of these segments and their anterior and posterior compartments is the subject of a two-part model. Part I considers the developmental control relationships of the selector genes for these compartments. It is concluded that two levels of positional information gradients are involved in the determination of compartments at blastoderm, and that the interaction of these two levels is necessary for the activation of some selector genes. In Part II the genetic map of the bithorax complex is considered in light of the conclusions of Part 1. A genetic fine structure of the complex is proposed. The evolution of the bithorax complex and some of its singular properties are discussed.

Effect of the bithorax mutation on determination in duplicating Drosophila imaginal discs

Abstract Pattern duplications in the leg imaginal discs of Drosophila melanogaster can be produced by larval heat-pulse treatment of a temperature-sensitive cell lethal mutation. We have used these pattern duplications to assess the role of the bithorax gene in determination. The bithorax mutation normally transforms only the anterior compartment of the metathoracic leg to mesothorax but in duplicated legs transformation of the posterior compartment of the duplicate was often also observed. Cell lineage analysis in these duplicates demonstrated that anterior bithorax -transformed cells may participate in the formation of both mesothoracic and metathoracic posterior duplicate compartments. The results suggest that (1) bithorax + influences establishment of the posterior metathoracic state in the duplicate by activation of postbithorax + ; (2) the state of a cell (meso- or metathoracic) is not transmitted by cell heredity to its mitotic descendents when they form the duplication blastema; and (3) regardless of their compartmental origins, cells in the duplication blastema appear to make a coordinated determinative decision influenced by the bithorax locus.

THE GENETICS OF PATTERN FORMATION

Publisher Summary The development of an insect, like that of any other multicellular eukaryote, is characterized by the emergence of a spatially ordered array of differentiated cells. Because each cell carries in general the same set of genes, functional differentiation is ascribed to differential gene expression. The power of mutant analysis depends critically on the success of the first step—the isolation of a representative set of mutants blocked specifically in the process of interest. Because pattern formation involves cellular interactions that depend uponthe proper operation of a large number of elementary cellular functions, many pattern mutants are expected to define genes whose wild-type activities are necessary factors, in the pattern formation process. Such mutants have fortuitous pleiotropic effects on pattern formation. A mutant is similar to a missing or defective instruction that causes premature termination of the programs execution or an incorrect answer at the end. This chapter discusses the deduction of the informational roles of the genes involved and the logic of the program from the variety of defects pattern mutants cause.