This Week in Science

Science  08 Apr 2005:
Vol. 308, Issue 5719, pp. 161
  1. Triple Stitch

    CREDIT: BINO ET AL.

    During the last decade, alkene metathesis has become a versatile tool in the synthesis of organic compounds and polymers. The reaction is a net exchange, in which metal-catalyzed cleavage of a C=C double bond leads to recoupling of the resulting fragments with a new partner. The analogous reaction of alkynes, or C≡C triple bonds, has been slower to develop and has often required harsher conditions. Bino et al. (p. 234; see the Perspective by Bunz) show that a trimolybdenum cluster, bearing the two separated halves of 2-butyne, reacts in room-temperature aqueous solution to stitch together the alkyne.

  2. A Star Is Reborn

    As they grow old, stars with approximately the mass of our Sun experience explosive flashes just before their nuclear furnaces shut down. These flashes expel the stars' outer shells and leave behind hot dense remnants called white dwarfs. Most of these remnants simply cool down, but some can experience a late explosion that restarts nuclear burning and expands them into giant stars again. Hadjuk et al. (p. 231; see the Perspective by Asplund) report observations and a stellar model of V4334 Sgr, a “born again” giant that reignited in 1992 and that was discovered by amateur astronomer Sakurai. The subsequent temperature drop of V4334 Sgr is 100 times faster than had been expected. The calculated mass ejection rates suggest that reignition events contribute unexpectedly large amounts of carbon and carbonaceous dust to the interstellar medium.

  3. Through Stick and Thin

    Coatings are often used to modify the durability, wettability, or optical properties of a substrate. Typically, one needs to tailor and optimize the chemistry of the material being coated to ensure that it properly adheres to the bulk object. Ryu et al. (p. 236) show that thin cross-linkable polymer films can be made to adhere to a wide range of substrates, thus presenting a uniform surface onto which further materials can be deposited.

  4. A Snowball's Chance

    It has been argued that Earth was covered in ice by glaciers that extended from the poles to the equator as many as four times during the Neoproterozoic (between 750 and 580 million years ago), in what are commonly referred to as “Snowball Earth” periods. Estimates of the length of these episodes (assuming that they actually occurred) range from 100,000 to 30 million years. Bodiselitsch et al. (p. 239; see the news story by Kerr) report independent estimates of the duration of two of these periods, based on the accumulation of extraterrestrial iridium, which indicate that snowball conditions persisted for 3 million to 12 million years.

  5. Reheating the Early Solar Nebula

    CREDIT: YOUNG ET AL.

    An important heat source in the early solar system was the decay of 26Mg to 26Al. It has also been assumed that early solar system condensates had the same initial 26Al/27Al ratio. Because Al is one of the first elements to condense out of the nebular gas into Ca-Al-rich inclusions (CAIs), this decay system has provided perhaps the most important chronometer for dating events between formation of the solar nebula and the first planetesimals. Young et al. (p. 223) now show that this assumption of a homogenous incorporation of Al in early condensates is not correct, and that the initial ratio was likely much higher. This revision implies that many CAIs reflect a history of much episodic reheating in the solar nebula for several hundred thousand years.

  6. Assessing the Brain of Homo floresiensis

    Homo floresiensis is the diminutive hominid dated to about 12,000 years ago that was discovered recently in a cave on the island of Flores. Many aspects of this find are puzzling, including its small stature and brain, which is about the size of those of australopithecines such as Lucy's (dated to 3 million years ago). Falk et al. (p. 242, published online 3 March 2005; see the 4 March news story by Balter) have now provided a view of the shape of the brain obtained from a virtual endocast of the skull and compared it with those of other possibly contemporaneous hominids (Homoerectus and Homo sapiens) and apes. The brain of H. floresiensis, now estimated at 417 cubic centimeters, is most like that of a small H. erectus, but also has some differences, including a derived frontal lobe and an expanded temporal lobe.

  7. Amygdala, Neuropeptides, and Fear Behavior

    The neuropeptides vasopressin and oxytocin have opposite effects on fear- and anxiety-related behaviors. At the cellular level, both neuropeptides increase neuronal excitability in different brain regions, including the central amygdala, but the neuronal network underlying these opposite behavioral effects is not yet fully understood. Huber et al. (p. 245) identified discrete, anatomically separate, populations of oxytocin and vasopressin receptors within the central amygdala. They used oxytocin and vasopressin agonists and antagonists to elicit electrophysiologic neuronal changes and were able to construct a hypothetical neural network in which oxytocin and vasopressin exerted opposing effects on anxiety and fear.

  8. A LEAFY Life-Style Change

    The transcription factor LEAFY in flowering plants determines whether key meristems will go on to produce vegetative or floral tissues. In the much more primitive mosses, however, relatives of LEAFY control other aspects of the life cycle. Maizel et al. (p. 260; see the cover) analyzed the changes in LEAFY sequence and function during this span of evolutionary time. The different functions that LEAFY took on through evolution seem to be attributable to alterations in its DNA binding domain.

  9. Maintaining Corn Borer Lines

    In sympatric speciation, two populations diverge from a common ancestor in the same location. Accurate estimates are lacking of the reproductive isolation between sympatric populations that are genetically differentiated from each other. Using an approach based on isotopic characterization of host plants of two sympatric races of the European corn borer, Malausa et al. (p. 258) obtained direct field measurements of assortative mating, the key factor of reproductive isolation between sympatric host races. The reproductive isolation between the two races was almost complete, showing that assortative mating can occur in the absence of spatial and temporal isolation.

  10. Earliest Influences

    In the immune system, developing thymocytes depend critically on an intimate association with the organized stromal microenvironment of the thymus, which is made up primarily of thymic epithelial cells. Akiyama et al. (p. 248, published online 11 February 2005) demonstrate the importance of this association for T cell tolerance. A deficiency in the RING domain ubiquitin ligase TRAF6 caused a loss of organized thymic epithelial architecture. The severe impairment of T cell development then leads to autoimmunity.

  11. A New View of an Old Receptor

    CREDIT: ADAMS ET AL.

    γδ T cells represent a distinct lineage of T cells that undertake a range of specific immune functions. However, compared with their αβ T cell counterparts, the mode of antigen recognition by these T cells is still relatively poorly understood (see the Perspective by Garboczi). By resolving a 3.4 angstrom structure of the complex between a specific mouse γδ T cell receptor (TCR) and its nonclassical class I major histocompatibility complex ligand, Adams et al. (p. 227) have generated a new model of γδ T cell recognition that has features of both innate immune receptor recognition and adaptive recognition through recombination of germline segments. Shin et al. (p. 252) arrive at similar conclusions from a survey of TCR usage at the single-cell level, which leads them to suggest that γδ T cells focus on a relatively narrow range of antigenic ligands.

  12. Mastering Empathy

    Human beings can appreciate that other members of their species think and feel, and that their beliefs and intentions might not correspond with their own. Two decades ago, Wimmer and Perner presented evidence that children do not develop this representational capacity, sometimes referred to as a theory of mind, until ages 3 to 4. Onishi and Baillargeon (p. 255; see the Perspective by Perner and Ruffman) present evidence that 15-month-old infants may be able to master this task, revealing at the very least an understanding of behavior-action rules.

  13. Eat Your Carrots

    The family of carotenoid-synthesizing enzymes includes one that cleaves a 30-carbon precursor (β-carotene) in half to make two molecules of retinal (an essential component of visual pigments) and others that convert β-carotene into the plant hormone abscisic acid and the developmental factor retinoic acid. Kloer et al. (p. 267) present the structure of one of the enzymes in this family and describe how its active site induces an isomerization in the substrate that helps to position the trans-double bond next to the O2 molecule used to effect cleavage.

  14. From Solitary to Social Insects

    Accounting for the reproductive division of labor, or caste differentiation, is central to understanding social evolution in insects. Hunt and Amdam (p. 264) use individual-based modeling to show how the differentiation of polistine wasps (paper wasps) into reproductive “queens” and functionally sterile “workers” could arise through social co-option of regulatory circuits that once controlled a conditional diapause pathway. This developmental pathway would have been a key control element in the life cycle of the solitary ancestor of paper wasps, giving rise to a bivoltine (two-generational) life history pattern. Elucidating how social castes can emerge from specific regulatory elements present in solitary insects, provides a mechanistic explanation for the origins of eusociality.

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