This Week in Science

Science  27 Sep 2002:
Vol. 297, Issue 5590, pp. 2165
  1. In Brevia

    Xenografting of mouse ovarian tissue to a different species by Snow et al. (p. 2227) produced mature oocytes that are fertilizable and yield embryos that can develop into live young.

  2. Drier Hydrogen Clathrates

    Hydrogen has generally been thought not to form conventional water clathrates in which the hydrogen is contained in a water cage. Also, typically, clathrates at ambient pressure are thought to be limited to a ratio of caged to water molecules of no more than 1:6. Mao et al. (p. 2247) now report a hydrogen clathrate that can be synthesized at high pressures yet is stable at ambient pressures and has a ratio of 1:2. Such high concentrations of hydrogen in a stable clathrate have implications for the composition of the interiors of icy planets, and potentially for hydrogen storage.

  3. Past the Brink of Collapse

    Bose-Einstein condensates (BECs) can be generated through sympathetic cooling, in which escaping atoms carry away heat, in part because the bosons, which occupy the same quantum state, interact with one another. For fermions, additional particles occupy higher energy states, and thus collapsed states are more difficult to form. One approach is to mix fermions with bosons and borrow the latter's energy dissipation. Modugno et al. (p. 2240) now present experimental evidence that as a mixed rubidium-87 boson and potassium-40 fermion gas undergoes evaporative cooling, a point is reached where the number of potassium atoms lost abruptly changes. This collapse of the Fermi gas is attributed to attractive three-body interactions between two Rb atoms and one K atom that turns on when a critical number of particles is reached.

  4. The Cooler Side of Thermoelectrics

    Semiconductor-based devices that cool down when current is passed through them have long been promised as efficient, clean, and quiet refrigeration units. However, the realization of such devices has been limited somewhat by the rather low efficiencies achieved to date. Optimizing the figure of merit that characterizes thermoelectrics, the ZT factor (that takes into account the electronic and thermal properties of the materials), often involves gains in one property at the expense of the other. To date, ZT values around 1 are typical, but higher values are required if thermoelectrics are to live up to their expectation. Harman et al. (p. 2229) show that quantum dot superlattice structures, based on ternary and quaternary compounds of lead, tin, selenium, and tellurium, have ZT values of 1.3 to 1.6 at 300 kelvin. An initial demonstration of a simple device structure reveals a significant improvement of the cooling ability over existing bulk thermoelectric materials.

  5. Visibly Improved

    The efficient dissociation of water into hydrogen and oxygen by sunlight over a suitable catalyst is an important goal in renewable energy production. Most materials to date, such n-doped TiO2, are inefficient because they only generate the charged carriers needed to complete the reaction from photons in the ultraviolet, thus wasting the visible portion of the spectrum. Khan et al. (p. 2243; see the news story by Service) produced an n-type TiO2 that absorbs visible light by the controlled burning of titanium metal in a natural gas flame. This carbon-doped TiO2, biased at 0.3 volt, was about eight times more efficient than n-type TiO2 biased at 0.6 volt.

  6. Asian Soot

    During the past few decades, while most of the world has been getting warmer, there has been a trend toward increased summer drought in North China and increased flooding in South China, as well as moderate cooling in China and India. Many of these changes have been attributed to overgrazing, overfarming, and destruction of forests, but now it seems that another cause may be more important. Menon et al. (p. 2250; see the Perspective by Chameides and Bergin) report results from climate model simulations of the direct radiative effect of aerosols in the region of China and India. The observed changes in temperature and precipitation in China and India can be attributed to the absorption of sunlight by black carbon (BC) particles, or “soot,” produced by low-temperature household burning of biofuels and coal.

  7. Global Insecticide Resistance

    The selection pressure exerted by human use of insecticides is ubiquitous, and there has been a global spread of resistance among insects. So far, resistance has been identified as originating in the selection of increasingly insensitive targets for the pesticides in the nervous system or in the accelerated metabolism of the chemicals before they reach their targets. Daborn et al. (p. 2253; see the Perspective by Denholm et al.) have identified another route. A form of this resistance, as seen in the fruit fly Drosophila melanogaster, is mediated by the overexpression of a single allele of cytochrome P450 that has now spread worldwide.

  8. Making Heterochromatin

    The formation of higher-order chromatin domains plays a vital role in processes such as transcription, imprinting, dosage compensation, recombination, chromosome condensation, and segregation. The posttranslational modification of histone tails is central to chromatin assembly. Hall et al. (p. 2232; see the Perspective by Jenuwein) show that a small 4.3-kilobase cenH repeat sequence, which has strong homology to centromeric repeats, is both necessary and sufficient for the initiation of heterochromatin formation at the silent mating-type region of fission yeast, as well as at an ectopic location. This repeat sequence can direct posttranslational methylation of the histone H3 lysine 9 residue, a critical determinant of heterochromatin, in the absence of the heterochromatin protein Swi6. This ability is dependent on the RNA interference (RNAi) machinery. Maintenance of heterochromatin and its inheritance through mitosis and meiosis requires Swi6 but does not require the RNAi machinery.

  9. Limits to Stem Cell Plasticity

    Whether adult hematopoietic stem cells (HSCs) can alter their developmental fate and generate cells of nonhematopoietic tissues has been a point of active debate. Compared with other tissue-specific stem cells, HSCs are relatively available. Regrettably, their plasticity may not extend much beyond hematopoietic tissues in the normal animal. Wagers et al. (p. 2256) analyzed the fate of single transplanted HSCs as well as the fate of HSCs in mice with a surgically joined circulatory system. The results suggest that contributions of HSCs to nonhematopoietic tissues occur rarely, if ever.

  10. More Dangers from Designer Drugs

    In recent years, MDMA, or “ecstasy,” has become a widespread recreational drug in the Western world. Current research has suggested already that this substance is selectively damaging serotoninergic synaptic transmission. Ricaurte et al. (p. 2260; see the news story by Holden) show that ecstasy causes not only serotoninergic but also dopaminergic neurotoxicity in monkeys. MDMA might then interfere with motor function even when given in recreational doses. Because neuropsychiatric disorders are often related to dopamine dysfunction, recreational users may be putting themselves at risk for developing such diseases.

  11. Also Traveling in Pairs

    Most members of the kinesin superfamily of motor proteins move processively along microtubules as dimers. However, the Unc104/Kif1a kinesin had been observed to move as a monomer by a diffusion process. Tomishige et al. (p. 2263) show that Unc104/Kif1a develops the processive movement of conventional kinesin when dimers are generated through either fusion to a strong dimerizing coiled-coil segment or by raising their concentration. The observed velocity is comparable to that in vivo, indicating that dimers of Unc104 likely control the fast transport of synaptic vesicles.

  12. Clues to Treating Celiac Sprue

    The treatment of human autoimmune disorders is often difficult because of a lack of clear causative factors. Celiac sprue (also known as celiac disease) is a highly prevalent autoimmune disease that is triggered by gluten, such as that found in wheat, barley, and rye. A variety of in vivo and in vitro approaches were used by Shan et al. (p. 2275; see the Perspective by Schuppan and Hahn) to identify a relatively short gluten peptide that appears to be the primary initiating factor. This multivalent gluten peptide could be detoxified by exposure to a bacterial peptidase, which suggests a strategy for oral peptidase supplement therapy for this disease

  13. A Lifetime's Worth of Gene Expression

    Gene expression data provide an important resource for defining gene function and for identifying hierarchies and networks of genes that regulate specific developmental programs. Arbeitman et al. (p. 2270; see the cover) have sampled one-third of the genes in the Drosophila genome by DNA microarray analysis. This genome-wide transcriptional profile reveals patterns of gene expression in the whole organism associated with different developmental stages, from fertilized egg through metamorphosis to the aging adult.

  14. Aluminum Floc Formation

    Aluminum plays an important role in transport and deposition of heavy elements in polluted waters, yet there is much uncertainty regarding its aqueous chemistry. When acidic waters (such as those associated with mine drainage or acid rain) mix with neutral waters, aluminum-rich flocs appear. Furrer et al. (p. 2245) analyzed these flocs and simulated their formation in a series of laboratory experiments. The results imply that, in many natural waters, aluminum is complexed as a polyoxocation containing 13 aluminum ions in tetrahedral coordination.

  15. Orbital Ordering in Ruthenates

    Extensive studies of 3d transition metal oxides have revealed that strong interactions between the electron orbitals can result in various exotic electronic and magnetic phases forming as the temperature is lowered. In comparison, studies of the 4d transition metal oxides are relatively new, and the weaker crystal field effects caused by the more delocalized 4d electrons do not suggest that such orbital ordering effects are necessarily expected. However, Khalifah et al. (p. 2237) report structural and magnetic susceptibility measurements that provide strong evidence for an orbital ordering transition occurring in the newly formed layered ruthenate, La4Ru2O10. These results present a new challenge to condensed-matter theorists and experimentalists.

  16. Checking the Spindle

    During the process of cell division, if chromosomes fail to align correctly on the bipolar mitotic spindle, completion of mitosis will be delayed until alignment has been corrected. Martin-Lluesma et al. (p. 2267) describe the role of a protein, Hec1, that recruits a pair of proteins, Mad1 and Mad 2, to the kinetochores at the centre of condensed chromosomes. This interaction is important both for the binding of kinetochores to the microtubules of the mitotic spindle and for maintenance of the spindle checkpoint. Mad 1 and Mad 2 were already known to be involved in the spindle checkpoint, but the contribution of Hec1 had been unanticipated and suggests an additional degree of complexity in checkpoint controls in higher eukaryotes versus yeast.

  17. Random Perturbations and Lattice Effects in Chaotic Population Dynamics

    Henson et al. (Reports, 19 Oct. 2001, p. 602) used experimental evidence to illustrate that lattice effects, which can occur in both deterministic and stochastic systems, can dramatically alter the predictions of ecological models of population dynamics, most of which are continuous-state models. They concluded that “a complete understanding of some population systems will require a stochastic blending of both continuous-state and discrete-state models.” Domokos and Scheuring comment that although lattice effects can dominate even in models with a very fine lattice size (i.e., for large habitat sizes in ecological models), there is nonetheless a minimal random noise level that will “destroy all lattice effects” and recover the dynamics of the continuous model. In a response, King et al. comment that although Domokos and Scheuring make interesting inferences, they are based on a model different from the original study model and “a noise structure that appears to be inappropriate for ecological systems.”

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