This Week in Science

Science  12 Nov 2010:
Vol. 330, Issue 6006, pp. 889
  1. River Food Chains


      Food chain length, or the number of organisms involved in any eat-or-be-eaten relationship, is a key property of aquatic habitats that regulates nutrient cycling, energy flow, and carbon exchange between aquatic ecosystems and the atmosphere. Investigating river food webs in North American watersheds, Sabo et al. (p. 965, published online 14 October) found discharge variation governed the link between ecosystem size (watershed area) and food chain length. River-drying truncated food chain length because intermittent streams have much higher discharge variation than perennial rivers. These results could be important for the management of rivers as drought and climate change may increase the frequency of river drying and discharge variability.

    1. The Making of Amazonian Diversity

        The biodiversity of the Amazon Basin is legendary, but the processes by which it has been generated have been debated. In the late 20th century the prevalent view was that the engine of diversity was repeated contraction and expansion of forest refugia during the past 3 million years or so. Hoorn et al. (p. 927) analyze findings from a diverse range of disciplines, including molecular phylogeny, ecology, sedimentology, structural geology, and palaeontology, to offer an overview of the entire history of this region during the Cenozoic era (66 million years ago). The uplift of the Andes was a pivotal event in the evolution of Amazonian landscapes because it continually altered river drainage patterns, which in turn put a variety of pressures on organisms to adapt to changing conditions in a multiplicity of ways. Hence, the diversity of the modern biota of the Amazon has more ancient origins than previously thought.

      1. The Power Behind P450

          Drugs, toxins and a range of metabolic substrates are detoxified in the liver by family of iron-containing enzymes called cytochrome P450. The iron component transfers oxygen to compounds that are often highly resistant to chemical reaction, but we know very little about the mechanism of this vital detoxification process. Rittle and Green (p. 933; see the Perspective by Sligar) have managed to capture the P450 reaction intermediate by freezing a solution of the enzyme as it reacts with an oxidant. Spectroscopic techniques and kinetic studies then revealed an iron(IV)oxo intermediate that passes its oxygen along to the substrate with remarkable speed.

        1. Liquid Rock Beginnings


            It has long been known that the lunar farside highlands constitute the highest region on the Moon. Garrick-Bethell et al. (p. 949) show that the topography and crustal thickness variations of this elevated region obey a single, simple mathematical function that overall describes one-quarter of the Moon. The key to explaining this find may lie with a similarity between the hot, ancient Moon and one of the icy moons of Jupiter, Europa. Like today's Europa, the Moon's crust once floated on a subsurface ocean, except that it was made of liquid rock, not water. The same tidal effect that operates on Europa's crust, caused by Jupiter's gravitational force, would have also operated on the early Moon because of Earth's influence, and would have produced a pattern of crustal thickness variations similar to that observed in the farside highlands.

          1. Adding CCA

              Translation of a gene sequence into protein is mediated by transfer RNA (tRNA), which has a specific cytosine-adenine (CCA) tail to which amino acids attach and is recognized by enzymes. The tail, however, does not have a DNA template, and instead CCA-adding enzymes bolt on the additional nucleosides. Crystal structures have shown how these enzymes achieve specificity for cytosine, but we did not know how they select the final adenine until Pan et al. (p. 937) described CCA-adding enzyme structures captured at several stages of the reaction. Crystallized enzymes were complexed with a tRNA mimic and the respective cytosine or adenine triphosphate. The final adenine was discovered to be incorporated by the mediation of a single Mg2+ ion in the enzyme, but no more cytosine could be attached because its triphosphate could not then get into the right position for the reaction.

            1. Fungal Invasion or Pollination?


                When pollen finds a compatible flower, it grows a pollen tube which must find the egg cell and release the sperm it carries. In searching for genes that affect pollen tubes in Arabidopsis, Kessler et al. (p. 968; see the Perspective by Govers and Angenent) found a gene previously implicated in susceptibility to powdery mildew infection (the NTA gene). The NTA gene encodes a seven-pass transmembrane protein, which, in combination with a receptor-like kinase called Fer, is needed for successful pollen tube growth; both sets of proteins are also needed for successful powdery mildew invasion. These processes hence share common mechanisms of cell invasion, but where they diverge is in the outcome: embryogenesis or pathogenesis.

              1. Odd Magnetar

                  Magnetars are neutron stars that are widely thought to be powered by extremely high magnetic fields. Using data from three different x-ray observatories, Rea et al. (p. 944, published online 14 October) show that a previously known magnetar has a magnetic field that is much smaller than those of other magnetars. A strong magnetic field is thus not a necessary requirement for a neutron star to show magnetar-like behavior, implying that this population is wider than was previously thought.

                1. Few-Body Problem

                    Seemingly simple, quantum mechanical few-body systems are notoriously difficult to describe. Efimov trimers, three-body bound states with interactions tuned to be in close vicinity of the formation of two-body bound states, are the most tractable of these systems, with relevance, for example, in nuclear physics. Observed recently in ultracold atomic gases through their signatures in the rate of inelastic three-body collisions, Efimov trimers are predicted to appear at interaction strengths whose ratios are universally specified. By measuring binding energy as a function of interaction strength, Lompe et al. (p. 940) directly observed the association of three distinguishable atoms into a bound state. This technique may enable more precise studies of the trimer state, potentially revealing the nature of nonuniversal corrections suggested by prior experiments.

                  1. Perfect Imperfections

                      Graphene is composed of six-atom rings, but will include a number of five- and seven-atom rings as defects. Using simulations, Grantab et al. (p. 946) show that more defects do not necessarily lead to greater deterioration of mechanical properties. Mismatches caused by differences in the orientation of neighboring crystals are divided into low- and high-angle grain boundaries, and typically it is the lower-angle boundaries that are stronger. In graphene, by contrast, the larger-angle boundaries, which consist of higher-defect densities, are better able to accommodate the strain and prevent failure that originates in the breakup of the seven-member graphene rings. This suggests ways for synthesizing imperfect graphene sheets that will have mechanical properties that are close to those of perfect graphene.

                    1. Hot Tropical Explosion

                        The Paleocene-Eocene Thermal Maximum (PETM), 55 million years ago, was a unique episode of rapid global warming (∼5°C), often used as an ancient analog for future global climate change. Climate alteration during the PETM has been extensively studied in the marine realm, and from a few temperate to polar terrestrial localities, but little is known about how the tropics responded to the high temperatures and high levels of CO2. Using evidence from pollen analysis, Jaramillo et al. (p. 957) show that rapid tropical forest diversification occurred during the PETM, without plant extinction or regional aridity. Unexpectedly, diversity seemed to increase at higher temperatures, contradicting previous assumptions that tropical flora will succumb if temperatures become excessive.

                      1. A Steady Beat

                          A regular heartbeat depends on steady function of the cardiac pacemaker. The early embryonic heart is neither as steady nor as organized as the mature heart. Arrenberg et al. (p. 971) used zebrafish engineered to express light-sensitive proteins to locate and manipulate the function of the cardiac pacemaker. By controlling the activity of small patches of cells with light beams, the authors monitored the development of the young heart, showing how the cardiac pacemaker develops during embryogenesis.

                        1. Reciprocal Regulation

                            An essential step in many signaling cascades is inositol lipid hydrolysis catalyzed by phospholipase C–β. The latter is activated by the α subunit of the heterotrimeric G protein Gq, and it in turn inactivates Gαq, thus sharpening the signal. Waldo et al. (p. 974, published online 21 October) report structural and biochemical data that explain the basis of this reciprocal regulation. One domain of phospholipase C–β binds to activated Gαq. Though the phospholipase C–β active site remains occluded in the structure, the plug is probably removed upon G protein–dependent orientation of the lipase at the membrane. A second domain of phospholipase C–β accelerates guanosine triphosphate hydrolysis by Gαq causing the signaling complex to dissociate.

                          1. Coughing Back

                              Resurgence of whooping cough (or pertussis) is problematic because of the risk of infant fatality, but it has been occurring despite the availability of a vaccine. Data from Sweden, where national vaccination for whooping cough was discontinued for 17 years, before resumption in 1996, was used by Rohani et al. (p. 982) to build a model that showed that age-specific social contacts were an important influence on the spread of infection. The model helped to explain why resumption of vaccination was so successful in curbing infant whooping cough but had no effect on pertussis in adolescents. The model thus demonstrated how potentially important it is to take account of age-stratification in a population when considering public health policy.

                            1. Lingering Atmospheric Perturbations

                                Nitrous oxide and methane are chemically active greenhouse gases whose atmospheric abundances are greatly influenced by anthropogenic emissions. Prather and Hsu (p. 952) used an atmospheric chemistry model to show how nitrous oxide emissions lower the concentration of tropospheric methane through a chain of chemical reactions that include stratospheric ozone depletion, changes in solar ultraviolet radiation fluxes, altered fluxes of ozone transport from the stratosphere to the troposphere, and increases in the amount of tropospheric hydroxyl radicals. This mechanism acts on a 108-year-long time scale—10 times longer than the atmospheric residence time of methane.

                              1. Feather of the Penguin

                                  Penguins are highly adapted for their cold, aquatic environment. Changes in their wings and feathers have allowed rapid swimming and protection from the near-freezing water. Clarke et al. (p. 954, published online 30 September; see the cover) describe an early penguin, dating to about 35 million years ago, that includes well-preserved feathers. The melanosomes in the feathers, which influence their strength, as well as their color, are like those of many other aquatic birds and unlike those of present-day penguins, even though the morphology of the wings and feathers had already been modified. Thus, in penguins, the shape and form of the feather evolved before microstructural changes occurred. The melanosome arrangement also suggests that the penguin was mostly gray-brown.

                                1. Pulling Together

                                    Human social norms can be explored in experiments that provide insights into the situational conditions and individual preferences that prompt the expression of cooperative behavior and the enforcement of norms. Rustagi et al. (p. 961; see the Perspective by Vollan and Ostrom) analyzed a set of 49 user groups, comprising 679 individuals, who had been given the authority and responsibility to manage forests in the Bale region of Ethiopia. The forests managed by groups with larger proportions of conditional cooperators—that is, individuals who will behave in a cooperative fashion if others do so as well—produced more potential crop trees per hectare. Enforcing such behavior, via patrolling of the forests, which although a costly and time-intensive activity, was the key to productivity

                                  1. Infectious Amyloid?

                                      Patients with Alzheimer's disease have characteristic lesions in the brains associated with masses of polymerized protein called β-amyloid. Recently, evidence from mouse models of Alzheimer's disease shows that brain extracts containing β-amyloid can “infect” otherwise healthy animals when injected directly into their brains. Eisele et al. (p. 980, published online 21 October; see the Perspective by Kim and Holtzman) extend these findings to show that when mice are injected in other parts of their bodies with similar brain extracts after several months, they also develop amyloidosis within their brains.

                                    1. Plumbing in the Brain

                                        Superficial similarities of vasculature in different parts of the body may mask organ-specific developmental nuances. The vasculature of the brain uniquely has to insulate the organ from insults that the rest of the body must tolerate. Kuhnert et al. (p. 985) analyzed the developmental uniqueness of the brain's vasculature through study of a G protein–coupled receptor, GPR124, initially identified by its actions in the vasculature of colon cancer. GPR124 is also involved in normal development of the brain's vasculature. Mice expressing low levels of GPR124 did not develop adequate vasculature in the brain and died from hemorrhages. Mice with too much GPR124 developed a tangled, thin-walled, excessive vasculature in the brain. Although the overexpressing mice survived, they were prone to neurological symptoms such as ataxia. GPR124 seems to control the normal development of the endothelial cells, particularly in the forebrain and ventral neural tube.

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