This Week in Science

Science  30 May 2008:
Vol. 320, Issue 5880, pp. 1129
  1. Rotifer Gene Scavenging


    Rotifers of the Class Bdelloidea are small freshwater invertebrates known for their unusual ability to withstand desiccation at any life stage and for having evolved for millions of years apparently without sexual reproduction. Gladyshev et al. (p. 1210) find that Bdelloidea have incorporated large numbers of genes from diverse foreign sources, including bacteria, fungi, and plants into their genomes. These foreign genes have accumulated mainly in the telomeric regions at the ends of chromosomes and apparently retain their functional integrity. This ability to incorporate horizontally transferred genes may have been important in the shaping of the Bdelloid genome, which lacks sexual reproduction as a source of variation.

  2. Noun Recognition Software

    Discovering how knowledge is represented within the brain seems an impossibly complicated task, given how even simple nouns, such as celery, might be expected to activate neurons in various areas, such as those involved in eating and tasting. Mitchell et al. (p. 1191) describe a computational model that is based upon a theory that represents nouns as 25-dimensional vectors, where each dimension is a sensory-motor feature such as eating. The vectors are computed from the co-occurrences of words within a data set containing 1012 words. The model is then trained with neural activity data from subjects presented with 60 pictures (for instance, a piece of celery). Previously, a model trained on data from one subject was successful at identifying the noun represented by the pattern of neural activity in another subject. The current model can identify a noun that has not been included in the training set and predict the pattern of activity when a subject is shown a picture of a new noun.

  3. Misguided Good Intentions?

    Large volcanic eruptions that inject massive amounts of sulfate aerosols into the stratosphere, like that of Mount Pinatubo in 1991, cause significant climate cooling. This observation has led to the suggestion that artificial seeding of the stratosphere with sulfur could counteract the warming effects of CO2 and other anthropogenic greenhouse gases. While such a strategy is certainly possible, what unintended consequences might arise? Tilmes et al. (p. 1201, published online 24 April; see the Perspective by Robock) explore one potential consequence: the destruction of stratospheric ozone at high latitudes. The injection of sulfur into the stratosphere would be likely to cause a dramatic increase in the extent of ozone loss in the Arctic, and delay the recovery of the Antarctic ozone hole by 30 to 70 years.

  4. Micro Managing Translational Repression


    MicroRNAs (miRNAs) are small noncoding RNAs that generally repress gene expression. The degree of complementarity between a miRNA and its RNA target is thought to determine the mode of repression. In plants, most characterized miRNAs are highly complementary to their targets, resulting in target cleavage (slicing), whereas those in animals have reduced complementarity and instead down-regulate messenger RNA (mRNA) translation. Brodersen et al. (p. 1185, published online 15 May) conducted a screen in Arabidopsis for factors involved in miRNA-mediated gene regulation and found that, in addition to directing cleavage, highly complementary miRNAs also repressed translation. Furthermore, small interfering RNAs, which are precisely complementary to their mRNA targets and generally thought to repress gene expression by slicing, also have a translational repression component.

  5. Star Light, Star Bright

    Cassiopea A, which is the brightest radio emitter in the sky, is the remnant of a supernova explosion that occurred about 300 years ago. Due to the lack of contemporaneous recorded observations, little information is available about what kind of supernova it was. Krause et al. (p. 1195; see the Perspective by Fabian) now report their observations of an optical echo of the original explosion taken with the Spitzer Space Telescope, three centuries after the direct light from the supernova passed Earth. The spectral signature of the echo suggests that Cassiopea A resulted from the collapse of a red supergiant star.

  6. Probing Martian Rocks

    The Mars rovers and orbiters have shown that many of the oldest rocks on Mars contain a variety of sulfates and other minerals that probably formed due to the evaporation of water. Tosca et al. (p. 1204) present a thermodynamic model to calculate the properties of the desiccating water and explain the sequences of minerals observed. Water in equilibrium with the observed or inferred minerals must have been extremely saline, more so than any known terrestrial fluid, and also have had a low pH. These conditions exceed the known limits of life of Earth and, given that many waters originated in equilibrium with a basaltic crust, waters across Mars may have evolved to be inhospitable.

  7. Cytosolic Iron Chaperone

    Iron is an essential nutrient for virtually every organism because it is required for the activity of iron-dependent enzymes, which control a wide range of essential metabolic processes. Reduced iron is very redox active and can catalyze the formation of damaging reactive oxygen species within cells. For this reason, cells are thought to maintain “free” cytosolic iron at an exceedingly low level. Nevertheless, intracellular iron must be delivered to sites of utilization and storage, and this delivery is thought to be accomplished by iron chaperones, which would be predicted to bind iron and deliver it to other target proteins, such as enzymes or transporters. Shi et al. (p. 1207) now report the discovery of a cytosolic iron chaperone in human cells, Poly r(C)-Binding Protein 1 (PCBP1), which delivers iron to ferritin.

  8. Counting Counts

    Concepts of number and of space seem so fundamental to human experience as to be deeply embedded in cognitive structures. In general, young, preschool children intuitively map quantities onto a line where, in most cases, small numbers are placed to the left and large numbers to the right. The scale used in this mapping is logarithmic and not linear, meaning that 10 is placed in the middle of a line spanning 1 to 100. With the advent of schooling, along with exposure to cultural instantiations of number words such as rulers, the mapping shifts to a linear representation. From the latest in a series of visits to an Amazonian indigenous people, the Mundurucu, who lack formal number systems, Dehaene et al. (p. 1217) have obtained evidence that in these people the logarithmic mapping seen in youngsters persists in adults. Thus the log-to-linear shift depends upon culture—the existence of integer words separated by unit increments (for instance, twenty-three and twenty-four) and/or education in linear arithmetical operations such as addition and subtraction.

  9. Lovely Bubbly


    Trapped air bubbles influence the taste, smell, and feel of foods and cosmetics, but keeping air bubbles stable within a liquid for long periods of time can be tricky. When the bubbles get to be micrometers in size, they rapidly expand at the expense of smaller bubbles. Dressaire et al. (p. 1198) show that using a mixture of mono- and di-ester surfactants can stabilize bubbles for more than a year. The surfactants assemble on the surface of the bubbles in a hexagonal pattern, and it is this packing that leads to the long-term stability.

  10. Right Time, Right Place

    Regulatory T cells are highly immunosuppressive lymphocytes that help the body avoid autoimmunity and overzealous immune reactions. However, their existence also presents a dilemma for the immune system, because they might inadvertently shut down useful pathogen-specific immune responses. Lund et al. (p. 1220, published online 24 April; see the Perspective by Kassiotis and O'Garra) present evidence that suggests that regulatory T cells can in fact optimize immune responses during the early stages of infection. Using a mouse model of herpes simplex virus infection, depleting regulatory T cells delayed the arrival of the relevant immune cells at the site of infection. At the same time, inflammatory chemokines became elevated in the lymph nodes. Thus, under normal circumstances, regulatory T cells may minimize the expression of these soluble factors in the lymph nodes in order to redirect immune cells for a timely arrival at the site of infection.

  11. Transcription Factor's Double Life Exposed

    Mutations in the transcription factor MeCP2 (transcriptional repressor methyl-CpG binding protein 2) cause a broad range of neurobehavioral abnormalities, including autism, mild learning disabilities, and mental retardation. MeCP2 has been widely believed to regulate a small number of target genes in the brain by repressing their expression. By applying microarray technology to mouse models that either lack or overexpress MeCP2, Chahrour et al. (p. 1224; see the Perspective by Cohen et al.) now find that this transcription factor regulates more than 2000 genes in the hypothalamus alone and that MeCP2 in fact appears to activate the expression of about 85% of these genes. The discovery that MeCP2 regulates such a large number of genes suggests that therapeutic strategies for MeCP2-related disorders should focus on restoring neuronal function rather than correcting the function of individual gene targets.

  12. People Like Us

    We invariably see ourselves quite differently from how others see us. Pronin (p. 1177) reviews the reasons for this perceptual asymmetry. Perceptual asymmetry arises from differences in the type of information acquired by the individual versus the observer (i.e., introspective awareness of feelings and intentions versus inferences drawn from observed behaviors) and in how these types of information are prioritized and used to weight judgments of our own behaviors versus those of others. A greater appreciation for this intrinsic incongruity may lead to an enhanced understanding, both of ourselves and of others.

  13. Probing the Martian Ice Cap

    The North Pole of Mars has a large polar cap of ice and dust, thought to have accumulated over the past 5 million years, which overlies older sediments. Phillips et al. (p. 1182, published online 15 May; cover; see the Perspective by Grott), using radar mapping from the Mars Reconnaissance Orbiter, have now been able to resolve small layers within the ice cap to deduce its history. The deeper part of the cap contains of a series of four intervals, each of which contain finer layers of reflections probably produced by dust in ice layers. These four packets are separated by clearer ice, which may be related to variations in Mars' orbit. The weight of the ice cap does not deform the underlying sediments, a finding that constrains the depth of the strong crustal layer and implies that the abundance of heat-producing elements in Mars, which affect its geothermal gradient, are the same as in chondritic meteorites.

  14. From Monogamy to Eusociality

    Eusociality as demonstrated by a variety of insects means that only a subset of individuals within a species is reproductively active. A high degree of relatedness may either be the consequence or cause of eusociality. In a comparative analysis of 267 species of bees, wasps, and ants, Hughes et al. (p. 1213) show that independent origins of eusociality are associated with strict monogamy. Because this mating system maximizes relatedness, it suggests that ancestrally eusocial species may be monogamous rather than polyandrous and that kin selection, rather than ecological drivers, may have been the chief force behind the evolution of eusociality.

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