This Week in Science

Science  30 Jun 2000:
Vol. 288, Issue 5475, pp. 2281
  1. Martian Groundwater

    It has been thought that Mars had abundant surface water at times early in its history. Now an analysis by Malin and Edgett (p. 2330; see the cover and the Perspective by Tanaka) of images from the Mars Global Surveyor Mars Orbiter Camera at 2 to 12 meters per pixel resolution implies that groundwater has even recently seeped and run off onto the surface in a few regions, mostly in the southern hemisphere. These sites are concentrated along the walls of impact craters, in the south polar region, and in several small valleys. The fresh morphology of the gullies and their common occurrence on the poleward-facing walls of craters and valleys suggest that they were formed by transport of water from small aquifers or reservoirs toward vertical cliffs where the water could have reached the surface. Some of the escaping water may have initially frozen, but eventually it seeped out in a slurry of sediment, ice, and liquid that traveled only a short distance to form the observed gullies and other structures.

  2. Magma Supply Rates at Kilauea

    Kilauea volcano, Hawaii, is in the midst of its longest continuous eruption cycle, which started in 1983 at Pu'u O'o along the east rift zone. Cayol et al. (p. 2343) modeled the deformation and seismicity associated with the 1975 (magnitude 7.2) Kalapana earthquake to estimate the rate of magma supply between this large event and the longest eruption cycle. They found that rift zone spreading was greater, which led to higher magma supply rates than were previously estimated after the large earthquake. Thus, the current eruption is probably a direct consequence of increased magma storage after the large earthquake.

  3. Underwater Microrobots

    Microrobots are millimeter- to submillimeter-sized devices with individual parts of micrometer size. Silicon has been used in micromachining technology for making such devices, but silicon parts are not water-tolerant, which precludes their use in many applications, particularly in biology. Jager et al. (p. 2335) have built microrobotic arms that can pick up, move, and place micrometer-sized objects. The microrobots have individually addressable microactuators consisting of polypyrrole-gold bilayers and can be operated in water. The robots may be used, for example, in single-cell manipulation and characterization.

  4. Organic Members of the Quantum Hall Family

    The quantum Hall and fractional quantum Hall effects result from the correlated interaction of charge carriers whose motion is confined to a two-dimensional plane in the presence of a magnetic field. The effects are usually seen only in high-quality inorganic semiconductors and at temperatures in the millikelvin range. Schön et al. (p. 2338) now extend these observations to organic semiconductors—pentacene and tetracene. Moreover, the effects can be observed at the relatively high temperature of 2 kelvin.

  5. Quasar Jets and Gamma Rays

    Recently, several stellar x-ray binaries (also called microquasars) have been discovered near the sun. Paredes et al. (p. 2340; see the Perspective by Fender) combined radio, x-ray, and optical observations of the newly recognized microquasar LS 5039 in order to determine its structure and the mechanism for energetic ejections. They found two strong radio jets that may be associated with the gamma ray burst found by the Energetic Gamma Ray Experiment Telescope (EGRET). The jet structure and the high-energy releases from this system indicate that the companion of the normal star LS 5039 is probably a black hole or a neutron star. These results and the continued discovery of nearby microquasars suggest that compact sources such as black holes and neutron stars may be more common in the local neighborhood than previously thought.

  6. Up a Wrong Tree-Or Not?

    The advent of vast quantities of DNA sequence information has brought analysis of evolutionary trees to bear on a great variety of organisms for which fossils are non-existent or not useful. However, the frequent disagreement between trees inferred from fossils and those inferred from sequence information indicates that both systems are susceptible to certain errors. Using the evolution of the horned soldier caste in aphids as a test case, Huelsenbeck et al. (p. 2349) now demonstrate the value of estimating the uncertainty of a phylogenetic tree and mechanisms for adjusting the evolutionary conclusions appropriately.

  7. A Diatom's Life

    Diatoms, which form a large proportion of the unicellular marine phytoplankton, are a crucial element in marine food webs and the global carbon cycle, but little has been known about how they perceive and respond to changes in their local environment. Using technologies to measure single-cell signaling, Falciatore et al. (p. 2363) reveal exquisite sensing mechanisms for responding to a range of environmental signals, including iron, which has been proposed as the key nutrient for growth control in phytoplankton.

  8. When Receptors Lie in Wait for Ligands

    A fundamental biological question that remains largely unresolved concerns the mechanism by which binding of ligands to receptors on the cell surface causes transmission of a signal through the plasma membrane. One appealing explanation has been that ligand binding brings receptors together into multimeric complexes. Three reports describe cases in which the opposite approach is taken, and the receptors are bound and lie in wait for the ligand (see the Perspective by Golstein). Siegel et al. (p. 2354) and Chan et al. (p. 2351) have examined how the Fas and tumor necrosis factor receptors signal. They define a protein interaction domain in these receptors that mediates assembly of the receptors into complexes in the absence of ligand. Such association is shown to be necessary for ligand binding and subsequent signaling. The results also explain how abnormal forms of Fas can dominantly prevent Fas-induced signaling in the human disease known as autoimmune lymphoproliferative syndrome. When bound to their cognate receptors, interferons (IFNs) induce cellular resistance to viral infection. Takaoka et al. (p. 2357) show that certain cells, if they are to maximize this response when exposed to the cytokine IFN-γ, must express not only a functional IFN-γ receptor but a functional IFN-α/β receptor as well. Before these cells are even stimulated with IFN-γ, the IFN-γ receptor is preassociated with the IFN-α/βreceptor. The IFN-α/β receptor brings a transcription activator to the complex that is poised to become activated upon stimulation with IFN-γ. Crosstalk between two IFN receptor subtypes appears to increase the efficiency and strength of a cell's antiviral response to IFN-γ.

  9. Ready to Listen

    Even before hearing becomes active, there is a transient cholinergic innervation in the developing mammalian inner ear. The physiological significance of these synapses has never been demonstrated. Glowatzki and Fuchs (p. 2366) show that the novel neuronal nicotinic acetylcholine receptor α9 is functionally present in neonatal cochlear inner hair cells and behaves like a true acetylcholine receptor. Its stimulation results in the activation of a fast calcium-dependent potassium channel. Acetylcholine thus inhibits afferent activity and acts to impose rhythmicity onto the immature auditory pathway.

  10. Immediately Early

    Human cytomegalovirus (HCMV) is an important pathogenic herpesvirus of humans. Its genome had been thought to be composed entirely of DNA, but Bresnahan and Shenk (p. 2373; see the Perspective by Roizman) have discovered that HCMV packs RNA transcripts into its virion. When a new cell is infected, the virus can generate protein without transcription and thereby avoid inducing early adverse cellular responses. Packaging RNA, rather than mature protein, could allow retention of signal sequences on the viral protein so that proteins will be targeted to the correct cellular compartment, and thus possibly prime cells for efficient virus replication.

  11. A Perfect Host?

    The protozoan parasite that causes malaria spends a critical portion of its life cycle in the mosquito host. Schneider and Shahabuddin (p. 2376) investigated how the innate immune system in a non-host combats the parasite by identifying experimental conditions that allowed the parasite to grow in the fruit fly Drosophila. Because the fruit fly is so genetically well defined, this model should provide new insights into the pathogenesis of malaria, which may lead to new ideas for prevention and treatment.

  12. FAS(T) Route to Weight Loss

    The increasing recognition of obesity as a serious and costly public health problem has resulted in intensified efforts to understand the physiological mechanisms that regulate food intake and body weight. Loftus et al. (p. 2379; see the news story by Gura) have identified an unexpected link between appetite control and anabolic energy metabolism. Treatment of mice with a synthetic compound (C75) inhibited fatty acid synthase (FAS), an enzyme that catalyzes synthesis of long-chain fatty acids under conditions of energy surplus, and led to rapid weight loss due primarily to inhibition of feeding. C75 was well tolerated by the mice and appeared to act in a leptin-independent manner by inhibition of neuropeptide Y in the hypothalamus.

  13. Fault Structure for the Chi-Chi Earthquake

    The 20 September 1999 Chi-Chi earthquake in Taiwan produced a surface rupture of about 80 kilometers in length and a vertical displacement of about 9 meters along the northernmost transverse segment of the rupture. Kao and Chen (p. 2346) determined the source parameters of the mainshock and a sequence of aftershocks and combined their results with available seismic and borehole data, to relate the massive surface rupture with the subsurface structure of the Chelungpu fault. They suggest that the Chelungpu fault is a reactivated thrust fault dipping at 20° to 30° to the east and this fault is contributing to the deformation and mountain building in central Taiwan, perhaps even more than a shallower-dipping decollement mapped to the west of the Chelungpu fault. In addition, they suggest that a second, blind thrust fault parallels the Chelungpu fault at a depth of 20 to 40 kilometers, and this previously unrecognized structure is also involved in the deformation.

  14. Perceiving Plant Steroids

    Genome analysis of Arabidopsis and other plants has identified a family of proteins that share leucine-rich repeats and have other characteristics which suggest that they may be receptor-like kinases. He et al. (p. 2360) now present an analysis of one family member, BRI1, previously implicated in brassinosteroid signaling. They worked with a fusion protein constructed from the extracellular domain from the Arabidopsis protein and the kinase domain from a receptor from rice that modulates disease resistance. They now show that the perception of the ligand, in this case a steroid, occurs through the extracellular domain of BRI1 at the cell surface.

  15. Promoting Survival in Developing Immune Systems

    Proper development of the immune system requires complicated controls of the development of thymocytes and selection of functional T cells through positive and negative selection. Sun et al. (p. 2369) examined the role of the retinoic acid receptor-related orphan receptor (RORγ) in development of the mouse immune system. Their studies of animals that lacked RORγ indicate that RORγ promotes survival of thymocytes by promoting the expression of the anti-apoptotic protein Bcl-xL. The knockout animals lacked lymph nodes and Peyer's patches, apparently because of failed development of the CD4+ progenitor cells required for normal development of these structures. The authors propose that regulation of RORγ signals might modulate expression of Bcl-xL and thus control “death by neglect” of thymocytes that fail to interact productively with peptide-MHC complexes.

  16. Late Cretaceous True Polar Wander: Not So Fast

    Analyzing the pattern of 27 paleomagnetic poles calculated from Pacific plate seamount anomaly modeling (SAM), Sager and Koppers (Research Article, 21 January, p. 455) inferred a possible event of rapid true polar wander (TPW), on the order of 3° to 10° per million years, at approximately 84 ± 2 million years ago. Cottrell and Tarduno object that the study underestimated potential errors implicit in SAM paleopole data and “may not have adequately taken into account alternative explanations,” such as overprint magnetizations and local tectonic complexities, in explaining the observed data. They also argue that a “well-studied, highly regarded pelagic sedimentary section of the same age,” in the Umbrian Apennines of Italy, shows no evidence of the proposed shift. Sager and Koppers respond that SAM poles “are useful, if employed with proper caution,” when alternative high-quality data are scarce. They add that tectonic complexities “should cause random errors in SAM pole locations and thus do not explain the grouping of poles.” And they conclude that the test based on the Italian data is “inconclusive” in view of the data's inclination scatter and the large uncertainty in the location of the TPW rotation pole. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/288/5475/2283a

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