This Week in Science

Science  29 Oct 2010:
Vol. 330, Issue 6004, pp. 556
  1. Ancient Innovations


      Pressure flaking is a method of forming points, grooves, and notches on stone tools in which a tool is pressed up against another stone, instead of striking it. It has been thought to be a fairly recent innovation, arising in the Upper Paleolithic 20,000 or so years ago. Mourre et al. (p. 659), show that tools from Blombos Cave, dating to about 75,000 years ago, have grooves and patterns resembling production by heat treatment followed by pressure flaking. Replication experiments were performed using similar source material followed by microscopic study of the tools. Despite the evidence for an early innovation, it seems that pressure flaking was not used widely elsewhere until much later; thus, such early innovations may have been sporadic ephemeral.

    1. Designing Carbon Dioxide Traps

        One widely discussed means of stemming the rise in atmospheric carbon dioxide concentration is to capture the gas prior to its emission and then bury it. The materials currently known to best adsorb CO2 for this purpose tend to involve amine groups; however, the precise molecular details of adsorption often remain murky, and rational improvement of sorbent properties by structural modification has been challenging. Vaidhyanathan et al. (p. 650; see the Perspective by Lastoskie) have crystallographically resolved the binding motifs of CO2 in an amine-bearing metal-organic framework solid. Accompanying theoretical simulations matched the experimental observations.

      1. Closing in on Extraterrestrial Earths

          With close to 500 extrasolar planets discovered to date, researchers are starting to estimate the occurrence of low-mass planets to help our understanding of how planets form and evolve. Based on observations of 166 nearby stars with the Keck Telescope, Howard et al. (p. 653) report the occurrence of short-period planets around Sun-like stars as a function of planet mass. Planet formation models predicted that planet occurrence would increase with decreasing mass, such that satellites with masses similar to that of Neptune, and less, would be more common than gas-giant planets like Jupiter. Contrary to predictions, there is no dearth of planets with masses 5 to 30 times that of Earth, implying that the models may need revision. Nevertheless, observations suggest that 23% of Sun-like stars may be orbited by a close-in, terrestrial mass planet.

        1. Gallium Nitride Grown on Graphene

            CREDIT: CHUNG ET AL.

            Nitride semiconductor materials used in light-emitting diodes and lasers are usually grown on single-crystal sapphire substrates with intermediate buffer layers. Instead, Chung et al. (p. 655) used graphene as a substrate for gallium nitride growth and found that nucleation of the gallium nitride layers was enhanced by first depositing zinc oxide, which grew as vertical nanowalls on the graphene. The gallium nitride layers displayed strong photo- and electroluminescence and, even better, the layers could be transferred to flexible substrates such as plastic.

          1. Self-Recognition and Survival

              Soay sheep are a remnant of an ancient breed of sheep that, although intensively studied for many years, live unmanaged on the remote Scottish island of St. Kilda. Life is harsh on the island, and the numbers of sheep show cycles of winter population crashes and high exposure to infection. Graham et al. (p. 662; see the Perspective by Martin and Coon) measured levels of self-reactive antibodies in the sheep called antinuclear antibodies (ANA). Having high ANA levels was a heritable trait that reflected generally high levels of immunoglobulin in individuals and of specific antibodies to parasitic worms. Female sheep with high levels of ANAs survived better during crash years, but had fewer births. If these sheep did reproduce, although the lambs tended to be small, they tended to have higher rates of early survival. Thus, maintaining high antibody levels apparently reflected investment in immunity and greater survival, but doing so was also associated with reduced reproductive success.

            1. Innate Innit?

                Innate lymphocytes (ILCs) are a recently described population of immune cells that produce cytokines like those associated with T helper cells, but lack the recombined antigen receptors characteristic of T cells. Again, like some T helper cell lineages, a proportion of ILCs express the transcription factor RORγt. These include lymphoid tissue inducer (LTi) cells required for fetal lymphoid tissue organogenesis and a population of natural killer (NK)–like cells that function in gut immune responses. Sawa et al. (p. 665; see the Perspective by Veldhoen and Withers) wondered whether the RORγt-expressing ILCs all develop from the same progenitor population. Indeed, they found a fetal liver progenitor that gave rise to several phenotypically distinct populations. However, the LTi cells were not progenitors for the NK-like cells. It seems the trajectory of different ILC populations is developmentally regulated, and postnatally ILCs are favored that play a role in intestinal defense before the gut is fully colonized by intestinal microbiota.

              1. Ribosome Assembly Pathway

                  The bacterial ribosome is made up of two subunits (50S and 30S) that together comprise 55 proteins and three large ribosomal RNAs. Both subunits self-assemble from their components in vitro, and hence, Mulder et al. (p. 673) were able to use a derivation of time-resolved electron microscopy to monitor assembly of the 30S subunit. This approach identified 14 distinct assembly intermediates and allowed characterization of the population, conformation, and protein composition of the intermediates along the assembly pathway.

                1. Daughter Diversity

                    Asymmetric cell division generates cell diversity and maintains tissue homeostasis. In early Caenorhabditis elegans embryos, the mitotic spindle is pulled toward one side of the cell by the molecular motor, dynein, and the cell divides into two unequally sized daughters. However, other types of asymmetric cell divisions (for example, in Drosophila neuroblasts) start with a centrally localized spindle. In this latter case, the mechanism by which two differently sized daughters are created is not understood. Ou et al. (p. 677, published online 30 September; see the Perspective by Grill) studied asymmetric cell divisions in the Q neuroblast lineage during C. elegans development and found that when the spindle was centred, myosin II accumulated at higher levels on the side of what will become the smaller daughter cell, giving rise to asymmetric myosin-based contractile forces acting on the membrane.

                  1. Controlling Chloride Channels

                      CREDIT: L. FENG ET AL.

                      The CLC proteins are a large family of channels and transporters that transfer chloride ions across cell membranes. While structures of two prokaryotic CLCs have been determined, these do not include the cytoplasmic regulatory domains found in eukaryotic transporters, and the structures do not reveal the mechanism of Cl/H+–coupled transport. L. Feng et al. (p. 635, published online 30 September; see the Perspective by Mindell) describe the structure of a eukaryotic CLC protein and found that the regulatory domains interacted closely with the transmembrane domain so that conformational changes are transmitted to the ion pathway. A gating glutamate in the eukaryote transporter is in a different conformation to prokaryotic structures, explaining the 2:1 stoichiometry of Cl/H+ exchange in eukaryotes.

                    1. Processing Prion Phenotype

                        How misfolding of a prion protein translates into transmissible changes in cellular physiology is unclear. Derdowski et al. (p. 680) integrated a computational model of prion aggregate dynamics with an empirical analysis of the physical and functional dynamics of prion protein in yeast cells. Remarkably, they found that prion phenotypes resulted from fluctuations in the accumulation of aggregates and suggested that it is the process rather than the product of protein misfolding that is crucial in establishing the severity or stability of the resulting phenotype.

                      1. Meeting of Minds

                          The performance of humans across a range of different kinds of cognitive tasks has been encapsulated as a common statistical factor called g or general intelligence factor. What intelligence actually is, is unclear and hotly debated, yet there is a reproducible association of g with performance outcomes, such as income and academic achievement. Woolley et al. (p. 686, published online 30 September) report a psychometric methodology for quantifying a factor termed “collective intelligence” (c), which reflects how well groups perform on a similarly diverse set of group problem-solving tasks. The primary contributors to c appear to be the g factors of the group members, along with a propensity toward social sensitivity—in essence, how well individuals work with others.

                        1. Evolution, Gene Number, and Disease

                            Slight variations in the numbers of copies of genes influence human disease and other characters. Variants can be hard to detect when they lie in heavily duplicated and widely similar regions of sequence known as “dark matter.” Sudmant et al. (p. 641) have methods to tease apart the duplicated regions to reveal singly unique nucleotide identifiers. These have turned out to be among the most variable seen in different human population groups—most notably among genes for neurodevelopment and neurological diseases. Such polymorphisms can be genotyped with specificity and may help us understand how variation in copy number may affect human evolution and disease.

                          1. Honing in on HONO2

                              Modeling air pollution requires knowledge of all the interrelated reactions occurring in the atmosphere. Among the most significant is the formation of nitric acid (HONO2) from OH and NO2 radicals. One sticking point in the study of this reaction has been the uncertainty in how often radicals link through an O-O rather than an O-N bond. Mollner et al. (p. 646) measured the partitioning coefficient, as well as the overall consumption rate of the radicals, with an array of highly sensitive spectroscopic techniques in the laboratory. The measurements yielded a well-defined rate constant for nitric acid formation, which was applied to the prediction of ozone levels in atmospheric simulations of the Los Angeles basin.

                            1. Lost Details

                                Changes in ocean circulation are commonly inferred by differences between the distribution of carbon isotopes in the past and now, but making such comparisons neglects the fact that modern fossil fuel burning has modified the carbon isotopic composition of the ocean. This in turn obscures details about recent mass movement of water. Olsen and Ninnemann (p. 658) correct for this effect in the North Atlantic and show that the natural distribution of carbon isotopes has more detail and is clearly related to water mass distributions. The results change some important ideas about glacial-interglacial ocean variations within the context of modern climate variability.

                              1. Small Is Attractive

                                  The optic tectum of zebrafish larvae is required for the detection, tracking, and capture of small, highly motile prey. Del Bene et al. (p. 669) applied a combination of optical, genetic, and pharmacological tools to investigate how neural circuits in the optic tectum filter out low-frequency visual information. Most tectal neurons were tuned to respond selectively to small, moving objects in the fish's visual environment and responded very poorly to large stimuli. This spatial filtering mechanism depended on the activity of a small population of GABAergic, inhibitory interneurons at the tectal surface. Inactivation or destruction of these interneurons removed the size selectivity of deeper neurons and the zebrafish lost their ability to catch prey.

                                1. Touchy Typing

                                    Even the most able typist makes errors, and Logan and Crump (p. 683) have used this real-world task to probe for the existence of two error-detection mechanisms. They inserted errors into words that had been typed correctly by the subjects, and they corrected errors that had been made. By measuring implicit error detection as the slowing of movement just after an error had been committed and by eliciting explicit monitoring of errors by the output shown on the screen, they uncovered a double dissociation. Inserted errors did not lengthen the interval until the next letter was typed, but they were reported by the typist as errors; on the other hand, corrected errors did increase the interval, but were nevertheless claimed by the subjects as having been typed correctly.