This Week in Science

Science  11 Apr 2003:
Vol. 300, Issue 5617, pp. 209
  1. Martian Liquidity

    The Sun causes solid-body tides on Mars, and the amplitude of the tidal effects can provide information about the structure of the martian interior. Yoder et al. (p. 299; see the Perspective by Dehant) used small changes in the orbital dynamics of the Mars Global Surveyor spacecraft, attributed to these tidal effects and measured for more than 3 years, to redefine martian structure. They determined that the outermost part of the core is probably partially liquid, thus altering the current stiff, solid-core paradigm. More work will be needed to understand how this liquidity affects the evolution, size, and composition of the core as well as the mantle.

  2. Recolonizing Lake Victoria

    Lake Victoria is host to one of the largest adaptive radiations of vertebrates. This assemblage of more than 500 endemic species of haplochromine cichlid fishes has become one of the major evolutionary model systems for the study of patterns and mechanisms of diversification. However, the origins of this radiation, and the rates of speciation within it, have been much debated. Verheyen et al. (p. 325; see the cover) used a large molecular phylogenetic analysis to address these questions. They find that Lake Kivu, and its haplochromine cichlids, played a central role in the evolution and phylogeography of all of the haplochromine cichlids in East Africa. They also present a biogeographic model of how the other rift lakes were colonized from Lake Kivu.

  3. Revealing the Energy Bands of C60

    Doped fullerides display a wealth of electronic properties, including antiferromagnetism and superconductivity, and have received much attention as an ideal system to study many-body physics such as electron-electron and electron-phonon interactions. However, the determination of the details of the energy bands and their bandwidth of the material has remained elusive because of difficulties in handling the samples and in obtaining the necessary energy resolution to see these features. Using an improved angle-resolved photon emission spectroscopic technique to study specially prepared monolayers of electron-doped C60 fullerides, Yang et al. (p. 303) now reveal the details of the band dispersion. These studies pave the way for accurate comparisons between theory and experiments of these test systems.

  4. Metals That Go with the Flow

    When a bulk sample of a ductile metal or alloy is deformed, it usually undergoes work hardening due to the increase in dislocations, and surface instabilities eventually lead to necking and failure. Champion et al. (p. 310) fabricated bulk samples of pure nanocrystalline copper by generating ultrafine powders that were then cold-pressed isostatically and sintered under a hydrogen atmosphere at low temperatures. They find that when this material is plastically deformed, neither work hardening nor neck formation occurred. Instead, the samples exhibited near-perfect elastic-plastic behavior.

  5. Ironing Out Iron Age Dates

    The chronology of the Iron Age in the Near East has been controversial; it relates directly to events described in the Bible and in Egyptian texts. In particular, it has been difficult to date many artifacts and occupation layers of cities and to then relate these dates to the historical record. Bruins et al. (p. 315; see the news story by Holden) provide a series of 14-carbon dates from a city in Israel, Tel Rehov, that was occupied repeatedly during the Iron Age. The series of dates can be tied to the Egyptian record and imply that the Iron Age extended from the 10th to the 9th centuries B.C.E.

  6. Chromosomal Rearrangements and Speciation

    Although the idea that chromosomal rearrangements may help to form new species is not new, recent theories suggest that barriers to recombination induced by these rearrangements may act to reduce recombination and facilitate positive selection. To test this idea, Navarro and Barton (p. 321; see the Perspective by Rieseberg and Livingstone) studied human and chimpanzee sequence data for the coding regions of 115 genes. Protein evolution, as measured by nonsynonymous nucleotide substitution relative to neutral mutation rates, was much faster in rearranged chromosomes than in collinear chromosomes. This information should help in identifying significant early differences between humans and chimpanzees.

  7. Porous-Metal Actuators

    The conversion of an external electrical signal into a volume change, and hence mechanical force, is known as actuation and is of considerable importance in the development of small-scale devices. This effect has been seen in ceramics such as piezoelectrics and in conducting polymers. Weissmüller et al. (p. 312; see the Perspective by Baughman) demonstrate that if platinum is generated with a continuous network of nanometer-sized pores, they can generate reversible strain amplitudes comparable to those of commercial materials. These strains are induced through surface-charging effects with potentials of about 1 volt.

  8. The Higher By-Products of Biomass Burning

    A layer of sulfate aerosol found in the stratosphere plays important roles in the dynamics and chemistry of the atmosphere, but its origin has remained obscure. Biomass burning is thought to be the source of 10 to 20% of atmospheric carbonyl sulfide (COS), which is a major source of sulfate. Notholt et al. (p. 307) present measurements of COS and other trace gases which show that concentrations of COS at the tropical tropopause are 20 to 50% higher than the values assumed in models, and that these emissions are associated with biomass burning. These results suggest that deep convection of the products of biomass burning is a more important source of the COS in the upper troposphere than previously believed.

  9. Protecting the Vascular Wall

    Atherosclerosis is characterized by aberrant growth and migration of vascular smooth muscle cells (SMCs). In a study of genetically engineered mice, Boucher et al. (p. 329) identify LRP1 (low-density lipoprotein receptor-related protein 1) as an important physiological regulator that helps to keep these cells in check. The atheroprotective effect of LRP1 was traced to its ability to control signaling events involving the platelet-derived growth factor receptor.

  10. Second Coming for Malaria

    The study of the evolution of the malaria parasite is as complex as the parasite itself. There are two contrasting estimates for the origins of Plasmodium falciparum—one of about 6000 years ago, the other to at least 100,000 years ago. Joy et al. (p. 318) reconciles these dates in an analysis of mitochondrial DNA sequences of the parasite obtained from almost 100 samples worldwide. An initial early spread of the parasite out of Africa in the Pleistocene appears to have been followed by later migration and huge expansion of the parasite population about 10,000 years ago. This expansion occurred with a wave of human migration out of Africa, the emergence and growth of agricultural societies, and the speciation of the major mosquito vector, Anopheles gambiae. A consequence of rapid population expansion is seen in the large number of recent mutations in P. falciparum, especially in Africa.

  11. Sweet Signals in Plants

    Glucose is a nutritional component for plants; its availability supports various physiological responses. Moore et al. (p. 332; see the Perspective by Frommer et al.) now show that glucose also functions as a signaling molecule to relate physiological responses such as growth, development, and senescence to photosynthetic productivity and other environmental resources. Analysis of mutant plants defective in a hexokinase enzyme shows that some of the signaling functions continue to operate even in the absence of catalytic activity for glucose processing.

  12. Early Help in Programming Immune Memories

    To generate a convincing humoral immune response to infection, most antibody-producing B cells must receive assistance from helper CD4 T cells. By contrast, the primary response of many CD8 T lymphocytes shows little dependence on T cell help, and there has been debate whether the same holds true for memory CD8 T cells. Shedlock and Shen (p. 337) and Sun and Bevan (p. 339) show that vigorous CD8 T cell memory in mice depends heavily on CD4 T cell help, although this reliance develops specifically during primary rather than secondary responses. Thus, initial priming of CD8 T cells to antigen appears to represent a crucial juncture in the production of efficient immunological memory to infection and vaccines.

  13. Polyubiquitination of p53 by p300

    The transcription factor and tumor suppressor protein p53 undergoes rapid turnover in the cell by the ubiquitin-proteasome system. Not only is it monoubiquitinated, but p53 is modified by chains of ubiquitin molecules to ensure efficient degradation. Grossman et al. (p. 342) have determined that p53 is polyubiquitnated by p300, a transcriptional regulator. This enzymatic activity settles some of the outstanding riddles of how p300 can control both p53 activation and destruction. Because p300 regulates a variety of short-lived transcription factors, the finding may have broader implications.