Dominant Ellipse mutant alleles of the Drosophila EGF receptor homologue (DER) dramatically suppress ommatidium development in the eye and induce ectopic vein development in the wing. Their phenotype suggests a possible role for DER in specifying the founder R8 photoreceptor cells for each ommatidium. Here we analyze the basis of Ellipse mutations and use them to probe the role of DER in eye development. We show that Elp mutations result from a single amino acid substitution in the kinase domain which activates tyrosine kinase activity and MAP kinase activation in tissue culture cells. Transformant studies confirmed that the mutation is hypermorphic in vivo, but the DER function was elevated less than by ectopic expression of the ligand spitz. Ectopic spi promoted photoreceptor differentiation, even in the absence of R8 cells. Pathways downstream of DER activation were assessed to explore the basis of these distinct outcomes. Elp mutations caused overexpression of the Notch target gene E(spl) mdelta and required function of Notch to suppress ommatidium formation. The Elp phenotype also depended on the secreted protein argos and was reverted in Elp aos double mutants. Complete loss of DER function in clones of null mutant cells led to delay in R8 specification and subsequently to loss of mutant cells. The DER null phenotype was distinct from that of either spitz or vein mutants, suggesting that a combination of these or other ligands was required for aspects of DER function. In normal development DER protein was expressed in most retinal cells, but at distinct levels. We used an antibody specific for diphospho-ERK as well as expression of the DER target gene argos to assess the pattern of DER activity, finding highest activity in the intermediate groups of cells in the morphogenetic furrow. However, studies of mutant genotypes suggested that this activity may not be required for normal ommatidium development. Since we saw distinct phenotypic effects of four different levels of DER activity associated with wild-type, null mutant, Elp mutant, or fully activated DER function, we propose that multiple thresholds separate several aspects of DER function. These include activation of N signaling to repress R8 specification, turning on argos expression, and recruiting photoreceptors R1-R7. It is possible that during normal eye development these thresholds are attained by different cells, contributing to the pattern of retinal differentiation.
The naturally occurring radioactive isotope 40K is the single largest contributor to the internal background radiation dose in living organisms. We examined cell growth and mutation rate or frequency in several strains ofEscherichia coli in (i) media containing the natural content of40K, (it) media containing potassium from which essentially all of the 40K had been removed by isotope separation, and (ii) media highly enriched in 40K. Growth rates (doubling times) were identical in the presence or absence of 40K. In more than 40 chemostat experiments, we were unable to detect any significant differences in mutation rate to bacteriophage T5 resistance or in mutation frequency to valine resistance or tryptophan prototrophy attributable to 40K. We conclude that, in the bacterial systems we have studied, 40K does not make a significant contribution to spontaneous mutation.The biological effects of low levels of ionizing radiation have received much attention in recent years (1, 2). One of the major uncertainties in such studies results from the almost total lack of direct experimental information on the genetic effects of natural background radiation. In the absence of such information, the shape of the dose-response curve at radiation levels below natural background becomes a matter ofopinion and this has generated much controversy. We have therefore undertaken an experimental approach to this important problem by a measurement of the contribution made by 4K to the spontaneous mutation rate in various strains of Escherichia coli. Potassium, the principal intracellular cation, is essential to life. Natural potassium is a mixture of isotopes, and one of these, 40K, present at an isotopic abundance of 0.012%, is radioactive with a half-life of 1.28 x 109 years. 40K decays principally by emission of (3-rays (3), but about 10% of the radioactive decay is by orbital electron capture, which produces very-low-energy Auger electrons. As NaH2PO4, 20 mM; MgSO4'7H20, 0.4 mM; CaCl2-2H2O, 6.8 jiM; FeSO4-7H20, 0.9 jiM. The pH of the medium was adjusted to 7.3-7.5 with concentrated hydrochloric acid. This medium was supplemented with potassium of the desired isotopic composition and concentration. Vogel-Bonner medium (9) and M9 medium (10) were also used as standard defined media. On occasion, we used a medium similar to M9, denoted M8, in which equimolar concentrations of sodium salts replaced the potassium salts in M9 medium. 8602The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
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