This Week in Science

Science  04 Feb 2005:
Vol. 307, Issue 5710, pp. 641
  1. Fungus Monoculture on the Ant Farm


    Leaf-cutting ants live in obligate ectosymbiosis with clonal fungi that they rear for food. These symbionts are vertically transferred during colony foundation, but fungus gardens are, in principle, open for horizontal symbiont transmission later on. Poulsen and Boomsma (p. 741) show that fungal ectosymbionts prevent competing fungal strains from becoming established by ancient incompatibility mechanisms that have not been lost despite millions of years of domestication and single-strain rearing by ants. These fungal incompatibility compounds travel through the ant gut to make the ant feces incompatible with unrelated strains of symbiont. Thus, the fungi manipulate the symbiosis to their own advantage at the expense of the ants' potential interest in a genetically more diverse agriculture.

  2. Saturnian Hot Spot

    Ground-based infrared observations of Saturn with the Long Wavelength Spectrometer on the Keck I Telescope on Mauna Kea reveal a hot spot in the atmosphere within 3° of the south pole, a warm polar cap, anomalous temperature bands, and oscillations in temperatures in the southern hemisphere that are not correlated with cloud patterns. Orton and Yanamandra-Fisher (p. 696) suggest these features are related to radiative forcing and dynamical forcing that are consistent with 15 years of constant solar illumination of the southern hemisphere as Saturn goes through its southern summer solstice.

  3. Two's Company, Three's a Cloud?

    It has long been thought that the in situ creation of new (secondary) cloud condensation nuclei arises mainly from the reaction of gas phase sulfuric acid and water, but the rate of particle formation observed in laboratory studies has been too slow (by many orders of magnitude) to account for the number concentrations found in nature. A faster, ternary mechanism that includes ammonia has been postulated on the basis of theoretical factors. Berndt et al. (p. 698) now report experimental production of particles from a mixture of sulfuric acid and water at concentrations like those naturally found in the atmosphere, with ammonia at concentrations lower than those normally observed. The measured rate is consistent with that required to explain atmosphere number concentrations.

  4. The End of the Line


    The breaking of the translation symmetry of crystals at their surfaces gives rise to localized surface electronic states, and, in principle, similar effects should be seen at the ends of one-dimensional wires. Crain and Pierce (p. 703) present experimental evidence for such electronic states at the ends of one-dimensional gold chains of gold grown on the stepped Si(553) surface. Scanning tunneling microscopy images show markedly different contrast for the end atoms of chains when the bias voltage is reversed, and differential conductance measurements reveal the details of the electronic states of the end atoms that agree well with the results of tight-binding calculations. The formation of end states helps lower the energy of filled states for atoms within the chain.

  5. Glimpses into the P/T Boundary

    The Permian-Triassic extinction was the most extreme in Earth's history. It has been difficult in part to determine the environmental conditions that may have led to the extinction. Grice et al.. (p. 706, published online 20 January 2005) present a detailed chemical analysis of marine sections obtained by drilling off western Australia and South China. The data suggest that the upper part of the oceans at the time of the extinction were extremely oxygen poor and sulfide rich. Ward et al.. (p. 709, published online 20 January 2005), in contrast, reconstruct a record of the terrestrial vertebrate extinctions in the Karoo Basin, Africa. This area preserves the most detailed vertebrate fossil record from this time, but correlating rocks in different parts of the Basin has been problematic. Using paleomagnetism and carbon isotopes, they show that extinctions were accelerated up to a pulse at the boundary, and that the pattern of appearance of Triassic fauna may imply that some originated even before the final pulse.

  6. Protein Kinase Inhibition Revealed

    An important target of the second-messenger cyclic adenosine monophosphate (cAMP) is protein kinase A (PKA). PKA, which regulates processes as diverse as growth, memory, and metabolism, exists as an inactive complex of two catalytic subunits and a regulatory subunit dimer. cAMP binds to the regulatory subunits and facilitates dissociation and activation of the catalytic subunits. Kim et al.. (p. 690) have determined the 2.0 angstrom resolution structure of the PKA catalytic subunit bound to a deletion mutant of the regulatory subunit (RIα). The complex provides a molecular mechanism for inhibition of PKA and suggests how cAMP binding leads to activation.

  7. A Matter of Scale

    A striking feature of morphological diversity across animal species is the variability in the relative size, or allometry, of different appendages. Virtually nothing is known of the forces that underlie the evolution of scaling relationships. Using the butterfly species Bicyclus anynana, Frankino et al.. (p. 718) tested the roles of developmental constraints and natural selection in determining the size of the wings relative to the body, which as a measure of wing loading has clear functional and ecological importance. Artificial selection experiments on the size of the forewing relative to overall body size resulted in a rapid evolutionary response. In this case, developmental constraints did not limit the evolution of the scaling relationship. Instead, it is the pattern of natural selection imposed by the external environment that determine the wing-body size allometry.

  8. Packaging and Power Combining

    Mitochondrial DNA (mtDNA) is packaged with proteins into a nucleoid. Chen et al. (p. 714) show that one of the mtDNA packaging proteins is the Krebs cycle enzyme, aconitase, that the mitochondrion uses to generate metabolic energy. In this second role, aconitase is required for mtDNA maintenance under particular metabolic conditions. This finding provides a direct link between energy generation and mtDNA stability, mitochondrial disease, and aging.

  9. Fade to Gray


    Aging brings on many changes in the human body, among them the graying of hair. Nishimura et al. (p. 720; published online 23 December 2004) found in a mouse model of hair graying that a deficiency of the gene Bcl-2 caused progressive loss of pigment cells in the bulge of the hair follicle—the hair stem-cell niche. Thus, the physiology of hair graying involves defective self-maintenance of melanocyte stem cells with aging, and may serve as a paradigm for understanding aging mechanisms in other tissues.

  10. Autophagic Arms Race

    One defense against intracellular invaders is to enclose them within autophagic vacuoles that then fuse with degradative lysosomes to destroy the pathogen. Ogawa et al. (p. 727, published online 2 December 2004) show that the invading bacterial pathogen Shigella can be recognized and trapped by autophagy. Generally, the pathogen circumvents the autophagic event by secreting an effector protein called IcsB during multiplication within the host cytoplasm; mutant bacteria lacking IcsB are particularly susceptible to autophagic killing. The Shigella VirG protein acts as the target that stimulates autophagy, but the IscB protein can camouflage it.

  11. Giving Mice the Nod

    The detection of bacteria in the gut by the immune system is regulated, in part, by the Nod proteins, which recognize peptidoglycan motifs from bacteria, and there is a strong association of the inflammatory bowel disorder Crohn's disease with mutations in the Nod2 gene. Nevertheless, questions remain about the normal physiological role of the Nod proteins in maintaining homeostasis in the gut and how impaired Nod function leads to inflammation. Maeda et al.. (p. 734) observed that Nod mutations in mice, corresponding with those carried by Crohn's disease patients, increased susceptibility to intestinal inflammation caused by the bacterial cell wall precursor muramyl dipeptide. Kobayashi et al. (p. 731) generated Nod2-deficient mice. Although these animals did not spontaneously develop intestinal inflammation, they were more susceptible to oral infection with the bacterial pathogen Listeria monocytogenes. Production of a group of mucosal antimicrobial peptides was particularly diminished in Nod2-deficient animals, which suggests that a similar defect may contribute to inflammatory bowel disease in humans.

  12. Cytokine Production and Kaposi's

    When tissues are infected with Kaposi's sarcoma-associated herpesvirus (KSHV), they produce large amounts of proinflammatory cytokines that are linked to disease progression. McCormick and Ganem (p. 739) show that a viral protein, kaposin B interacts with mitogen-activated protein kinase-associated protein kinase 2 and enhances the activity of this host cell protein, serving to block the decay of AU-rich messenger RNAs and increase the level of secreted cytokines. This result explains the association of KSHV-related disease and enhanced cytokine production.

  13. Deconstructing Dislocations

    The most commonly found intermetallic compounds are those with the stoichiometry of AB2, where the A atoms are 1.225 times larger than the B atoms. These structures form Laves phases in which four different atomic planes are parallel to the slip plane. Deformation in these structures is thought to occur through partial dislocations, which may also be responsible for the transformation between cubic and hexagonal structures. Using high-resolution transmission electron microscopy, Chisholm et al.. (p. 701) tracked the motion of columns of atoms under shear, and observed the formation of Shockley partial dislocations, in Cr2Hf. They also observed regions where the structure changed from hexagonal close-packed to face-centered cubic.

  14. A Matter of Timing?

    Understanding how genes function within an organism requires an evaluation of temporal as well as physical interactions. De Lichtenberg et al. (p. 721) have combined confidence-scored protein-protein interaction data with microarray data to provide a detailed view of cell cycle biology in yeast. The approach was used to make predictions about gene function. Almost all cell cycle complexes contained both subunits that showed a dynamic as well as those that had an unchanging pattern of expression. The dynamic subunits were the targets of cyclin-dependent phosphorylation and it was their expression that was associated with the time at which the complex functioned.