Many of the functions required for formation of the imaginal discs of Drosophila melanogaster larvae, from which adult structures are derived, are disc-specific and not required for formation of other larval tissues. Mutants blocked in disc-specific functions can produce mature viable larvae, indicating that larval development is not dependent on concomitant disc development. Some of the mutant larvae have no detectable discs (discless mutants), and other have small discs that cannot differentiate into adult structures (small disc mutants). From the results of genetic complementation and mapping experiments with such mutants, it was estimated that the genome of D. melanogaster contains around a thousand complementation groups, or about a fifth of all complementation groups in the genome, that are essential specifically for the development of all the discs. The developmental defects in some of the disc mutants appear to involve autonomous functions of the imaginal cells, and in other mutants functions provided by the larval environment to support disc development. This distinction was made on the basis of two functional tests. The purpose of one of the tests was to detect the presence in young embryos of determined imaginal cells capable of differentiating after transplantation to a wild-type host; the other test measured the capacity of early third-instar larvae to act as hosts for the further development of immature discs transplanted from a wild-type donor. Three discless mutants that were tested in this way appeared to be defective in autonomous functions of the imaginal cells, since the embryos contained no imaginal cells that could be detected in the first test, whereas the larvae, although devoid of discs, showed a normal capacity to support disc development in the second test. One of the small disc mutants tested was defective in a larval rather than an imaginal cell function, since the embryos had a normal complement of functional imaginal cells, whereas the larvae were incapable of supporting disc development. Thus, both types of disc-specific functions are essential for normal disc development.
ABSTRACT After fertilization, the development of a zygote depends upon both gene products synthesized by its maternal parent and gene products synthesized by the zygote itself. To analyze genetically the relative contributions of these two sources of gene products, several laboratories have been isolating two classes of mutants of Drosophila melanogaster: maternal-effect lethals and zygotic lethals. This report concerns the analysis of two temperature-sensitive mutants, OX736hs and PC025hs, which were isolated as alleles of a small-disc mutant, l(3)1902. These alleles are not only zygotic lethals, but also maternal-effect lethals. They have temperature-sensitive periods during larval life and during oogenesis. Mutant larvae exposed continuously to restrictive temperature have small discs. One- or two-day exposures to the restrictive temperature administered during the third larval instar lead to a homeotic transformation of the midlegs and hindlegs to the pattern characteristic of the forelegs. Mutant females exposed to the restrictive temperature during oogenesis produce eggs that can develop until gastrulation, but do not hatch. —The existence of these mutants, and one that was recently described by another group, implies that there may be a class of genes, heretofore unrecognized, whose products are synthesized during oogenesis, are essential for embryogenesis and are also synthesized during larval stages within imaginal disc cells.
Abstract The products of trithorax group (trxG) genes maintain active transcription of many important developmental regulatory genes, including homeotic genes. Several trxG proteins have been shown to act in multimeric protein complexes that modify chromatin structure. ASH2, the product of the Drosophila trxG gene absent, small, or homeotic discs 2 (ash2) is a component of a 500-kD complex. In this article, we provide biochemical evidence that ASH2 binds directly to Skittles (SKTL), a predicted phosphatidylinositol 4-phosphate 5-kinase, and genetic evidence that the association of these proteins is functionally significant. We also show that histone H1 hyperphosphorylation is dramatically increased in both ash2 and sktl mutant polytene chromosomes. These results suggest that ASH2 maintains active transcription by binding a producer of nuclear phosphoinositides and downregulating histone H1 hyperphosphorylation.
vein1(vn1) mutants lack portions of longitudinal wing vein 4 and the anterior crossvein. Stronger alleles, originally calleddefective dorsal discs,showvnis also required for the growth of the wing and haltere discs, as mutants for these alleles have tiny dorsal discs.vnfunctions nonautonomously and encodes a secreted EGF-like protein. Here we characterize the role ofvnin the imaginal wing disc by describing the expression pattern and correlating this pattern withvnmutant phenotypes and the requirement forvn. vnis expressed in wing discs in a complex and dynamic pattern. In larval wing discsvnis first expressed in the presumptive notum and later in the wing-pouch and hinge regions. There is a striking localization ofvntranscripts to intervein regions which begins with a stripe of expression straddling the AP boundary in late larval discs and develops in all intervein regions after puparium formation. We isolated newvnmutations including nulls and hypomorphs. Hypomorphicvnalleles revealed region-specific requirements forvnwithin the wing disc. We mapped lesions caused by 10vnmutations and defined a minimum size of 48 kb for the gene. The phenotype and expression analyses showvnhas an early role in global proliferation of the wing disc and specific roles in the development of the notum, hinge, longitudinal vein 4, and all intervein regions. The role ofvnin the EGF receptor signaling pathway is discussed.
