This Week in Science

Science  13 Dec 2002:
Vol. 298, Issue 5601, pp. 2087
  1. In Brevia

    The apicoplast, an organelle of the malaria parasites and its relatives, has been shown by Funes et al. (p. 2155) to have originated from the secondary endosymbiosis of green algae, with subsequent lateral gene transfer to the host nucleus.

  2. On the Trail of an Impact

    The size, composition, and energy of bolide impacts can be inferred not only from the craters that were left or filled in on Earth's surface, but also from far-flung ejecta deposits. Walkden et al. (p. 2185) have found an ejecta deposit of Triassic age in southwestern Britain that may have come from the Manicouagan crater in northeastern Canada (a distance of about 2000 kilometers), and hence may represent an energetic event that had global repercussions. If further work confirms the 214-million-year age of the ejecta deposit, then this event will have occurred just prior to the species extinctions recognized at the Triassic-Jurassic boundary about 202 million years ago.

  3. Short-Lived Solar System Tracers

    Radiogenic nuclides with short half-lives that were present in the early solar system are now extinct, but their abundances can be extrapolated from decay products that remain in some mineral grains in primitive meteorites. Marhas et al. (p. 2182) found evidence for beryllium-10 but no evidence of aluminum-26 or calcium-41 in hibonite grains in the Murchison meteorite. The 10Be could only have been produced by irradiation from the young Sun, so the absence of 26Al and 41Ca indicates that these two nuclides found in other meteorite grains may have come from a distinct stellar source outside of the protosolar nebula. This source might have helped to trigger solar system formation.

  4. Growing Flow

    Large changes in the discharge of Eurasian rivers can affect the formation of North Atlantic Deep Water and could have an impact on oceanic thermohaline circulation and global climate. Peterson et al. (p. 2171; see the news story by Stokstad) report observations that show increased river discharge to the Arctic Ocean from Eurasia by 7% since 1936. The increase correlates with global and regional warming as well as with the North Atlantic Oscillation, a large-scale atmospheric alternation. If the increase continues to follow predicted temperature trends (such as those of the Intergovermental Panel on Climate Change), the discharge may become great enough to affect the North Atlantic thermohaline circulation by the end of this century.

  5. A Superfluid of Fermions?

    The realization of a fermionic superfluid from a condensed cloud of strongly interacting atoms remains one of the major issues in ultracold gases. However, conventional routes to achieve the fermionic counterpart to that formed from bosons have not been successful. Using a mixture of spin-up and spin-down lithium-6 atoms confined to an optical trap, O'Hara et al. (p. 2179; see the Perspective by Pitaevskii and Stringari) observed an anisotropic expansion of the atomic cloud after release from its trap that may signal the onset of a superfluid phase.

  6. Genomic Clues into Chordate Origins

    The sea squirt Ciona intestinalis, an invertebrate chordate that has been extensively studied in embryology, occupies a critical node on the metazoan and chordate phylogenetic trees. Dehal et al. (p. 2157; see the cover and the news story by Pennisi) generated a draft genome sequence and assembled the euchromatic regions of C. intestinalis. Because ascidians like Ciona apparently diverged prior to the genome-scale expansions that occurred in the vertebrate lineage, they may reflect the chordate genome prior to the emergence of vertebrates. Indeed, many of its genes show stronger similarity to invertebrate proteins than to those found in vertebrates, but nearly one-sixth of the Ciona genes possess a vertebrate counterpart. In addition, Ciona-specific genes were identified, as well as curious innovations as seen in the presence of genes involved in cellulose metabolism.

  7. Stormy Weather

    Global climate models have predicted a shift toward more extreme precipitation patterns. In an effort to understand how increased storm variability would affect a grassland ecosystem, Knapp et al. (p. 2202) performed a 4-year field study in which they experimentally manipulated the variability in growing season rainfall without altering total precipitation. More extreme storm patterns led to enhanced plant species diversity but increased water stress in the dominant plant forms and affected overall carbon cycling by reducing productivity and soil CO2 flux.

  8. Game Birds

    In the Iterated Prisoner's Dilemma game, a “defecting” player who chooses an immediate reward does better for a single throw, but a cooperating player does better in the long run. Repetition of the game combined with reciprocity between individuals can lead to the indefinite maintenance of cooperation. However, it has been suggested that temporal discounting (the decreased value of a larger award if it is delayed) might constrain cooperation in non-human animals. In an experimental study, Stephens et al. (p. 2216; see the Perspective by Mesterton-Gibbons and Adams) offered pairs of captive blue jays immediate food rewards as well as delayed rewards that could be seen by the birds but were not “paid out” until several plays later. Stable cooperation between the birds was seen and was favored when discounting was reduced and when the opponents adopted a reciprocating strategy.

  9. Music in the Brain

    Is there a correlation between the formal geometrical representation of tonal music and the brain activity in a listener trying to detect a wrong note in a melody? In a functional magnetic resonance imaging study, Janata et al. (p. 2167; see the Perspective by Zatorre and Krumhansl) found consistent activity within the rostromedial area of prefrontal cortex across different listening sessions. Representations of tonality were structured within sessions with different subregions representing different parts of the tonality surface, but this structure varied across sessions and subjects. This dynamic organization might reflect the relative nature of musical structure, and its representation in the rostromedial prefrontal cortex in a given session may be influenced by interactions between short-term as well as long-term memory.

  10. Sobering Results

    How does alcohol intoxication impair human psychomotor activity beyond slowing performance and impairing sensory judgments? In a combined electroencephalography (EEG) and behavioral study, Ridderinkhof et al. (p. 2209) found that the anterior cingulate cortex, an area of the brain crucially involved in action monitoring, shows signs of impairment even for amounts of alcohol well below the legal driving limit in most countries. Subjects drinking alcohol showed a significantly altered EEG signal as well as a reduction of post-error adjustments in performance. Alcohol thus interferes with a critical component of cognitive control, which may explain why people under the influence of alcohol fail to recognize poor performance and persist in it.

  11. Overcoming a Broken Heart

    Injury to the mammalian heart leads to fibrotic scar formation rather than regeneration of the damaged heart muscle. Poss et al. (p. 2188; see the Perspective by Scott and Stainier) show that the zebrafish heart responds very differently. Two months after surgical removal of 20% of the ventricle, zebrafish exhibited complete restoration of the heart, apparently through robust division of cardiomyocytes. Thus, zebrafish may prove to be a valuable model system for genetic dissection of the molecular mechanisms underlying heart muscle regeneration.

  12. Waking Up on a Cloudy Morning

    Several photoreceptors have been proposed to be critical for synchronizing the mammalian circadian clock, but the most promising is melanopsin, which is found in specialized retinal ganglion cells. To test melanopsin's functional relevance, Ruby et al. (p. 2211) and Panda et al. (p. 2213) examined mice in which the gene for melanopsin has been deleted and found that light was still able to entrain the clock, but not as effectively. At low light levels, however, light could not entrain the clock in the mice without melanopsin. Thus, melanopsin is not the only photoreceptor but appears to be the one specialized for ensuring entrainment at low light levels.

  13. Aide-Méoire for Antibody Production

    What persuades a memory lymphocyte to stick around for years after an infection has been cleared? For B cells, the explanations have been that either antigens persist somehow or that some B cells develop into long-lived antibody-secreting plasma cells that need no stimulation. Bernasconi et al. (p. 2199) provide evidence for an intermediate mechanism in which nonspecific stimuli—not limited to antigens from any one pathogen—spur B cells into continued antibody production. In culture, human memory, but not naïve, B cells divided strongly in response to CpG sequences of DNA, which are powerful signals to innate immune cells. The T cell cytokine interleukin-15 evoked the same response and, like CpG, could induce some B cells to become plasma cells. Frequencies of antigen-specific plasma B cells and levels of circulating antibody in individuals more than a decade after vaccination agreed with predictions made from these experiments.

  14. Warming Up, Cooling Down

    Warmer temperatures can stimulate plant growth and carbon storage, therefore removing CO2 from the air, but also can accelerate the decay of organic matter in soils and increase atmospheric CO2. Mellilo et al. (p. 2173) studied the effects of warming on soils in a mid-latitude forest in Massachusetts. They found that higher temperatures caused more rapid decay of organic matter, but also an increase of the availability of nitrogen to plants that could allow potential increases of carbon storage in ecosystems where plant growth is limited by nitrogen. Therefore, in some regions, warming may stimulate carbon storage and help slow the rate of further warming.

  15. Metal Nanocubes

    Controlling the shape of metallic nanoparticles has been particularly challenging. Sun and Xia (p. 2176; see the Perspective by Murphy) have found that with the right capping ligand, they can grow silver nanocubes of uniform and controlled size, such that they look like sets of miniature dice. The nanocubes can also be used as sacrificial templates for the growth of uniform gold nanoboxes.

  16. One Downhill Slide into Shape

    The energy landscape theory of protein folding predicts that in certain cases, protein folding should be all downhill. However, most experimental evidence indicates that proteins must cross at least one free energy barrier between the unfolded and folded state. Garcia-Mira et al. (p. 2191) have experimentally identified a downhill folding protein. They monitored the unfolding of the peripheral subunit-binding domain from the 2-oxoglutarate dehydrogenase multienzyme complex of Escherichia coli (BBL) using techniques that probe different features of protein folding. Although BBL has a defined three-dimensional structure at low temperatures, each technique showed a different unfolding transition. A global fit of a statistical mechanical model to all of the data identified BBL as a downhill-folding protein.

  17. Ozone Consumption

    In addition to their well-defined role in antigen binding in the humoral immune response, antibodies recently have been shown to be capable of catalyzing the oxidation of water by singlet oxygen (1O2*). Wentworth et al. (p. 2195; see the Perspective by Nathan and the 15 November news story by Marx) confirmed that the hydrogen peroxide generated from this reaction can be produced at quantities sufficient for bactericidal activity. More surprising, however, was that another species from the reaction, which displays the tell-tale signs of ozone, was needed to finish the job; this same species was detected at the site of inflammation in an antibody-dependent in vivo model. Antibody-generated oxidants, including ozone, may thus be rudimentary contributors to immune defenses.

  18. The Complexities of Splicing

    Splicing of pre-messenger RNAs (pre-mRNAs) results in the excision of introns (non-coding regions) and the joining of exons (protein-coding sequences) to yield a translatable mRNA. This process requires five large RNA-protein complexes (called U snRNPs) and a host of associated factors. Makarov et al. (p. 2205) have isolated two complexes: one a new intermediate poised to execute the first step of the reaction, and the other likely a post-splicing recycling product. They characterize these complexes by mass spectrometry and use a comparative analysis of the changing complement of proteins to draw inferences about structural rearrangements in the spliceosome.