This Week in Science

Science  29 Mar 1996:
Vol. 271, Issue 5257, pp. 1785
  1. Inhibited by cross talk

    Multiple signal transduction pathways are often required during development to determine cell fates. How these signaling pathways are integrated, however, is frequently unclear. Axelrod et al. (p. 1826; see the Perspective by Blair, p. 1822) examined the interaction between the Notch and Wingless signaling pathways during pattern formation in the developing Drosophila wing. During the induction of bristles on the wing margin, dishevelled, a gene known to function in the Wingless pathway, acts to inhibit signaling by Notch. Experiments in vitro suggest that this interaction is direct because Dishevelled interacts physically with the carboxyl terminus of Notch. This interaction provides a molecular mechanism for the inhibitory cross talk observed between the pathways.

  2. Out of the depths (I)

    More and more rocks exposed at the Earth's surface are now recognized as coming from deep in the mantle. These include xenoliths, which are small rocks carried upward by magmas that may come from depths of 400 kilometers or more, and larger rock packages, typically exposed by tectonic processes in continental collision zones and containing high-pressure minerals such as diamond. Dobrzhinetskaya et al. (p. 1841; see the news story by Kerr, p. 1811) show that the Alpe Arami peridotite, a large rock massif in the Alps, may have come from depths of 300 kilometers or more. Olivine in the rocks contains inclusions of a new phase of FeTiO3 (see the cover) that indicate original exsolution of the perovskite phase at high pressures.

  3. Out of the depths (II)

    The rates and methods by which silicic magmas ascend through the crust have been uncertain. Brandon et al. (p. 1845) studied the dissolution of epidote, a mineral that is stable at high pressures in magmas but dissolves at shallow depths, to evaluate magma emplacement and cooling rates. Preservation of epidote in some dikes and intrusions implies that the parent magmas were emplaced and crystallized within 1000 years or so. Such rates are too fast for ascent of these magmas in diapirs.

  4. No old greenhouse?

    The latest Cretaceous climate has often been considered to be a warm greenhouse climate. Certainly high-latitude regions were warmer than comparable regions today, but there is evidence that the tropical oceans were cooler than one would expect. D'Hondt and Arthur (p. 1838) analyzed oxygen isotopes from planktonic foraminifera in Late Cretaceous sediments from a wide range of latitudes and found that the tropical oceans were cool. There was also a small latitudinal sea surface temperature gradient, indicative of enhanced poleward heat transport but not of a greenhouse-controlled climate.

  5. What makes a queen

    The differentiation of queen and worker castes in honeybee colonies is regulated by differential pheromone production. Plettner et al. (p. 1851; see the Perspective by Robinson, p. 1824) used deuterated enzyme substrates to work out the biosynthetic pathways for these pheromones. Various isomeric compounds that perform different functions in different castes are related, and slight changes in the functioning enzymes produce the caste-specific blends.

  6. Magnetic transfer

    Higher contrast can be obtained in magnetic resonance imaging (MRI) if the nuclei being studied have a greater fraction of spins aligned “up” versus “down.” Recently, optical pumping methods have been used to align spins in the noble gases, such as 3He and 129Xe. Navon et al. (p. 1848; see the news story by Service, p. 1810) now show that when spin-polarized 129Xe is introduced into solution, it can efficiently transfer magnetization to solvent protons through the nuclear Overhauser effect. This effect may find use in conventional MRI and in protein and surface nuclear magnetic resonance.

  7. Not quite connected

    Nose whiskers are an important part of the mouse sensory system. Each whisker projects through the thalamus into barrel structures in the cortex, which respond primarily to their “center” whisker but can also integrate signals from neighboring barrels. Welker et al. (p. 1864) have identified a naturally occurring mutant in which the barreloid structures of the thalamus are altered and individual cortical barrels are absent. Although sensory processing still occurs and overall input topology is preserved, spatial and temporal discrimination between individual whisker responses is reduced.

  8. Nerve and muscle

    Formation of the neuromuscular junction occurs when the growth cone of a developing neuron contacts its muscle target and forms a synapse. Kopczynski et al. (p. 1867; see the news story by Roush, p. 1807) identified a gene (late bloomer, or lbl) in Drosophila that encodes a member of the tetraspanin family of cell surface proteins. In embryos with mutant lbl genes, synapse formation is delayed and other neurons form abnormal connections to the muscle cell targets.

  9. Linking cell growth and elongation factors

    One form of myeloid leukemia in humans is associated with a chromosomal translocation that has a breakpoint in the ELL gene, but the function of ELL has been unknown. Shilatifard et al. (p. 1873) found that ELL encodes a transcriptional elongation factor that increases the catalytic rate of RNA polymerase II transcription by suppressing the transient pausing of polymerase. Recently, the product of the von Hippel-Lindau tumor suppressor gene was also found to be an elongation factor.

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