Pattern specification in imaginal discs ofDrosophila melanogaster: Developmental analysis of a temperature-sensitive mutant producing duplicated legs

The suppressor of forked gene of Drosophila, which encodes a homologue of human CstF-77K involved in mRNA 3'-end processing, is required for progression through mitosis

The Suppressor of forked (Su(f)) protein of Drosophila melanogaster is a homologue of the 77K subunit of human cleavage stimulation factor required for cleavage of pre-mRNAs before addition of poly(A). We have previously shown that the Su(f) protein is not ubiquitously distributed: it accumulates in dividing cells at various stages of Drosophila development. In this paper, we show that phenotypes of su(f) temperature-sensitive mutants result from a defect in cell proliferation. Analysis of the mitotic phenotype of su(f) temperature-sensitive alleles in larval brain and in imaginal discs reveals an increase in the number of metaphases with overcondensed chromosomes and asymmetric or reduced mitotic spindles. In contrast, neural differentiation in eye imaginal discs of the same mutant flies does not appear to be affected. These results indicate that su(f) is required during cell division for progression through metaphase. Taken together, these data suggest that a decrease in su(f) activity preferentially affects 3'-end formation of particular mRNAs, some of which are involved in mitosis, and are in agreement with a role of su(f) in the regulation of poly(A) site utilization.

The suppressor of forked protein of Drosophila, a homologue of the human 77K protein required for mRNA 3'-end formation, accumulates in mitotically-active cells

The suppressor of forked (Su(f)) protein of Drosophila melanogaster is highly homologous to two proteins involved in mRNA 3'-end formation, the yeast RNA14 protein and the 77K subunit of human cleavage stimulation factor (CstF). This suggests a role for su(f) in mRNA 3'-end-processing, probably as part of Drosophila CstF. We have investigated the expression pattern of su(f) during Drosophila development and found that the su(f) gene product is not detected ubiquitously. The Su(f) protein accumulates in mitotically-active cells, but does not in non-dividing cells. This expression pattern corroborates earlier data suggesting that the phenotypes of su(f) mutants could result from a defect in cell proliferation. Our results suggest that, in Drosophila, Su(f) is involved in the regulatory function of CstF.

Axes, boundaries and coordinates: the ABCs of fly leg development

Recent studies of gene expression in the developing fruitfly leg support a model--Meinhardt's Boundary Model--which seems to contradict the prevailing paradigm for pattern formation in the imaginal discs of Drosophila--the Polar Coordinate Model. Reasoning from geometric first principles, this article examines the strengths and weaknesses of these hypotheses, plus some baffling phenomena that neither model can comfortably explain. The deeper question at issue is: how does the fly's genome encode the three-dimensional anatomy of the adult? Does it demarcate territories and boundaries (as in a geopolitical map) and then use those boundaries and their points of intersection as a scaffolding on which to erect the anatomy (the Boundary Model)? Or does it assign cellular fates within a relatively seamless coordinate system (the Polar Coordinate Model)? The existence of hybrid Cartesian-polar models shows that the alternatives may not be so clear-cut: a single organ might utilize different systems that are spatially superimposed or temporally sequential.

Homology with Saccharomyces cerevisiae RNA14 suggests that phenotypic suppression in Drosophila melanogaster by suppressor of forked occurs at the level of RNA stability

The suppressor of forked [su(f)] locus of Drosophila melanogaster encodes at least one cell-autonomous vital function. Mutations at su(f) can affect the expression of unlinked genes where retroviral-like transposable elements are inserted. Changes in phenotype are correlated with changes in mRNA profiles, indicating that su(f) affects the production and/or stability of mRNAs. We have cloned the su(f) gene by P-element transposon tagging. Alterations in the DNA map of eight lethal alleles were detected in a 4.3-kb region. P-element-mediated transformation using a fragment including this interval rescued all aspects of the su(f) mutant phenotype. The gene is transcribed to produce a major 2.6-kb RNA and minor RNAs of 1.3 and 2.9 kb, which are present throughout development, being most abundant in embryos, pupae, and adult females. The major predicted gene product is an 84- kD protein that is homologous to RNA14 of Saccharomyces cerevisiae, a vital gene where mutation affects mRNA stability. This suggests that phenotypic modification by su(f) occurs at the level of RNA stability.

The localized requirements for a gene affecting segmentation in Drosophila: analysis of larvae mosaic for runt

The runt gene is required in a developing Drosophila embryo for proper segmentation. Mutant embryos fail to hatch but secrete a larval cuticle in which pattern defects are apparent. In runt embryos, there are pattern deletions spaced at two segment intervals along the antero-posterior axis of the animal. The deleted regions are replaced by mirror-image duplications of the remaining regions. This paper investigates the localized requirements for runt+ activity by analyzing the segmentation patterns in larval genetic mosaics. This analysis is aided by the faintoid and shavenbaby mutations which affect larval cuticle morphology without affecting segmentation. These two mutations serve as markers of the regions of larval cuticle secreted by genetically runt cells. The analysis of the runt mosaic patterns indicates the effects of the gene on segmentation are primarily cell autonomous. This includes both the pattern deletions and the associated mirror-image duplications. This indicates the mirror-image duplications are not due to regeneration but result from a more direct effect of runt on patterning in the embryo. The mosaic patterns also reveal other aspects of the process of pattern formation in the larval epidermis. Based on these results a model is presented for the generation of the larval pattern.

Krüppel, a gene whose activity is required early in the zygotic genome for normal embryonic segmentation

Embryos homozygous for Krüppel die as late embryos with an altered segmentation pattern. In strong alleles the normal thoracic and anterior abdominal segments are replaced by a partial mirror image duplication of the posterior abdomen. Weak alleles cause smaller pattern deletions in the thorax and abdomen and are not associated with mirror image duplications. The altered segmentation pattern can be traced back to 12 min after the onset of gastrulation, when the shorter germ bands in homozygous Kr embryos provide a first indication of abnormal patterning. The mutant was mapped to position 107.6 at the tip of the right arm of the second chromosome, cytologically to bands 60F2-5. Analysis of homozygous deficiency embryos indicate that the phenotype produced by strong point mutations probably represents the amorphic condition. The requirement for Kr+ gene activity is strictly zygotic. Maternal dosage of Kr+ has no effect on the embryonic phenotype, nor does homozygosity for Kr prevent germ cells from making normal eggs capable of normal embryonic development when fertilized by wild-type sperm. The requirement for Kr+ seems restricted to embryogenesis. Homozygous clones induced in imaginal discs during larval development survive and develop normally and in vivo cultures established from homozygous embryos proliferate normally and metamorphose into adult structures of normal morphology.

Morphological and somatic clonal analyses of pattern triplications

The growth of pattern triplications induced by a 48-hr 29 degrees C treatment given to larvae homo- or hemizygous for a ts cell-lethal mutation was examined to determine which structures result from new, regulative growth and which are produced by the original imaginal disc cells. Pattern triplications contain one complete leg pattern (orthodrome) and two partial patterns (antidrome and paradrome). The results of two morphological analyses and one somatic clonal analysis suggest that in triplications in which the antidrome and paradrome become more complete distally (diverge) the paradrome is formed by a portion of the original leg pattern, and the antidrome and orthodrome are formed by extra, regulative growth. A different result is suggested for triplications in which the antidrome and paradrome become less complete distally (converge). In these, the orthodrome appears to be formed by the original leg pattern and the antidrome and paradrome by extra growth. These results agree with predictions based on the polar coordinate model of positional information.

Cell determination boundaries as organizing regions for secondary embryonic fields

A model is proposed for pattern formation in secondary embryonic fields. It is stipulated that the boundaries, resulting from the primary embryonic organization of a developing organism, act as organizing regions for secondary embryonic fields, e.g., imaginal discs in insects. This boundary mechanism would allow very reliable pattern formation in the course of development: Primary positional information leads to cells of different determination, separated by sharp borders. At these borders, in turn, positional information would be generated for the next finer subdivision, and so on. This occurs if two or more differently determined cell types (e.g., compartments) cooperate for the production of a morphogenetic substance. A high concentration of the morphogen would appear at the common boundary of the cell types involved. Many experiments reported in the literature, for instance, the formation of duplicated and triplicated insect legs and the regeneration-duplication phenomenon of imaginal disc fragments can be explained under this assumption. The proposed boundary mechanism provides a molecularly feasible basis for the polar coordinate model.

Distal regeneration and symmetry

A revision of the "polar coordinate model" shows how pattern formation in diverse regenerating systems can be understood in terms of strictly local cell interactions.

Mutations affecting segment number and polarity in Drosophila

In systematic searches for embryonic lethal mutants of Drosophila melanogaster we have identified 15 loci which when mutated alter the segmental pattern of the larva. These loci probably represent the majority of such genes in Drosophila. The phenotypes of the mutant embryos indicate that the process of segmentation involves at least three levels of spatial organization: the entire egg as developmental unit, a repeat unit with the length of two segments, and the individual segment.