This Week in Science

Science  27 Feb 1998:
Vol. 279, Issue 5355, pp. 1277
  1. Night light

    Circadian rhythms maintain vertebrate day-night cycles and are reflected by a large nocturnal increase in circulating levels of melatonin. Gastel et al. (p. 1358) now show that brief exposure to light in the middle of the night causes a dramatic reduction in the activity of one of the enzymes needed to produce melatonin, pineal serotonin N-acetyltransferase. This regulation occurs through proteasomal degradation, which destroy the enzyme very rapidly.

  2. Recovering from disaster

    After the Cretaceous-Tertiary extinction of the dinosaurs and many other species, marine and terrestrial communities recovered rapidly on a global scale. Jablonski (p. 1327; see the commentary by Erwin, p. 1324) examined the evolutionary diversity of the marine molluscan faunas just before and after the extinction event in four provinces, the Gulf Coast of North America, northern Europe, northern Africa, and Pakistan-northern India. He found that the mollusks rapidly recovered in North America compared to the other three provinces. He then hypothesizes that this extraordinary recovery is directly related to the Gulf Coast region being the closest to the purported asteroid impact into the Yucatan Peninsula at Chicxulub that caused the mass extinction. Not only do these results support the Chicxulub impact event, but they also suggest that the relation of biodiversity to biogeography can be complicated by random events.

  3. Abrupt shifts whatever the climate

    Recent detailed studies of ice cores and deep sea cores has shown that abrupt changes in temperature and climate occurred every few thousand years or so during the last 50,000 to 100,000 years. Because the patterns persist through the most recent deglaciation and into the Holocene, the climatic patterns seem to be independent of the presence of large ice sheets. Whether these climatic patterns characterized earlier parts of the Pleistocene or developed just during the last glaciation has not been clear. Oppo et al. (p. 1335; see the news story by Kerr, p. 1304) show that such millennial-scale climate cycles are recorded in a core from the North Atlantic dating from 300,000 to 540,000 years ago and that the cycles affected sea surface temperature and deep ocean circulation. This record spans an extreme glaciation and a warm interglacial. Thus, millennial-scale climate variability persisted during the past 500,000 years at least, regardless of the global climate, and seems to have involved ocean circulation.

  4. A perturbed budget

    Nitrous oxide (N2O) is one of the most important greenhouse gases. Natural and anthropogenic sources contribute to its atmospheric budget, and identifying their contributions to the global N2O concentrations is necessary if we are to understand this century's increase in N2O concentrations and attempt to control the emissions. Chemical models are used to infer anthropogenic and natural sources, because N2O emissions are difficult to quantify in observations. Prather (p. 1339) shows how the photochemical coupling between N2O and stratospheric ozone can be used to identify the atmospheric perturbation due to anthropogenic N2O sources in a one-dimensional (vertical diffusion) model that couples the stratosphere, troposphere, and the ocean boundary layer. This perturbation decays 10 to 15 percent more rapidly than the overall N2O atmospheric lifetime. Such a faster decay may affect the inference of anthropogenic sources from chemical models.

  5. Ice core isotopes and geomagnetism

    The geomagnetic field shields the Earth from cosmic rays. One detailed record of the past variation of Earth's geomagnetic field may thus be provided by the abundance of cosmogenic isotopes in ice cores. Baumgartner et al. (p. 1330) show that the chlorine-36 record from the GRIP ice core in Greenland, which extends back to about 100,000 years ago, fits well with production rates inferred from a reconstruction of Earth magnetic field based on paleomagnetic data from marine sediments. The data support the inference that the magnetic field was particularly weak about 38,000 years ago.

  6. Cool tropics

    During the last glacial maximum, about 21,000 years before present, temperatures across the globe were lower than they are now, but the exact magnitude of the cooling in the tropics is still subject to debate, especially over the tropical oceans. Climate models have in the past generally only shown a moderate cooling of the tropical sea surface temperatures. Bush and Philander (p. 1341) now show that with a more realistic coupled atmosphere-ocean model that includes important feedbacks between the oceans and the atmosphere, a significant cooling is obtained for the last glacial maximum. This analysis lends support to the interpretation of some paleoclimate data that have indicated sea surface temperature coolings of similar magnitude.

  7. Carbonic acid chemistry

    Carbonic acid, H2CO3, is a key intermediate species in many reactions involving carbon dioxide, water, and bicarbonate and carbonate ions. In most astrophysical and planetary settings, it has been thought to decay too rapidly to carbon dioxide and water to be present in sufficient amounts for detection. Hage et al. (p. 1332) report results of laboratory experiments and thermodynamic calculations that imply that carbonic acid can sublimate and recondense without decomposition. Carbonic acid may thus be present in and account for some of the spectral features that have been observed from comets, Mars' ice caps, Earth's upper atmosphere, and the icy planets. In some of these settings, high-energy irradiation of water and carbon dioxide ices may lead directly to the formation of amorphous carbonic acid.

  8. Molecular mimicry and autoimmune disease

    A major cause of human blindness in the United States is herpes stromal keratitis, which is thought to have autoimmune components. Zhao et al. (p. 1344; see the news story by Dickman, p. 1305) have shown in a mouse model that the T cells that mediate destruction of the corneal tissue also recognize an epitope of the coat protein of herpes simplex virus-type 1 (HSV-1). When viruses were constructed that did not express this epitope, the virus could no longer induce herpes stromal keratitis. Thus, investigations into a mouse model of blindness led to insights into the pathogenesis of the disease and provide evidence that molecular mimicry is important in a virally induced autoimmune disease.

  9. A place for working memory

    Although there is general agreement on the existence and location of a monkey brain area that mediates working memory-the storage of information for immediate use-the situation in humans has been debated. Through the use of functional imaging, Courtney et al. (p. 1347) provide evidence for the existence of such a region and for its unexpected location. An area in the superior frontal sulcus was found to be specialized for working memory of spatial locations, as compared to working memory of faces, and to be distinct from a neighboring brain area (the frontal eye field) that subserves eye movements. Although the relation of the human working memory domain to the frontal eye field is similar to that in monkeys, this domain is located more superior and posterior with respect to the whole brain than the working memory domain in monkeys.

  10. Modeling brain waves

    The connectivity of excitatory neurons can, in response to a discrete stimulus, yield waves of depolarization that travel quickly across a network of cells. Recent results from electrophysiological studies of brain slices indicate that inhibitory neurons can trigger such synchronous activity as well. Rinzel et al. (p. 1351) construct a model of such a network and reproduce the wave-like recruitment of neurons. They go on to explore the influence of cellular properties, such as the reversal potential, on the propagation speed, frequency, and spatial patterning.

  11. A time for flowers

    Plants make the transition from vegetative growth to production of flowers in response to developmental cues and seasonal changes in the length of the day. Guo et al. (p. 1360; see the commentary by Suárez-López, p. 1323) have found that blue- and red-light photoreceptors work together to regulate the time of flowering. In Arabidopsis, the CRY2 gene, which encodes a blue-light photoreceptor, is the same gene as affected in the late-flowering mutant fha. The photoreceptors together work through regulation of the gene CONSTANS to determine the shift to the reproductive phase.

  12. Synaptic specialization

    Can the same nerve form different types of synapses with different targets? At the anatomical level, the answer is a clear yes. Maccaferri et al. (p. 1368) now show that, in the hippocampus mossy fiber, synapses with pyramidal cells have distinct biochemical and electrophysiological characteristics from the synapses formed with interneurons. Thus targets of the same neuron will respond differentially to the same stimulus, which will then influence the computational properties of the hippocampus.

  13. Inside the auxin pathways

    Auxin, a plant hormone, is vital to the regulation of plant growth and development. Several genes are important in the response to auxin, and biochemical studies have identified certain nuclear proteins that change their expression with auxin treatment. However, linkage between these two experimental approaches has been elusive. Rouse et al. (p. 1371) have now cloned one of the genes that regulates the auxin response and show that the encoded protein resembles those that are induced in response to auxin. The gene, AA17/AXR3, thus forms a link in the early parts of the auxin signaling pathway.

  14. Critical Candida gene

    Infection with the fungus Candida albicans can be fatal in premature infants, diabetics, surgical patients, and immunocompromised individuals. The ability of C. albicans to adhere to cells and its ability to change to a filamentous form are linked to its pathogenicity. Gale et al. (p. 1355) determined that one gene, INT1, is necessary for both processes and the ability of C. albicans to kill mice. Disruption of the activity of this gene may be important in the design of therapeutics.

  15. Src and long-term potentiation

    The role of tyrosine phosphorylation in the induction of the cellular correlate of learning, long-term potentiation or LTP, has been established for some time. Lu et al. (p. 1363) now clarify which kinase is likely to be responsible, and identify the nonreceptor protein tyrosine kinase, Src, as necessary and sufficient for the induction of hippocampal LTP.

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