www.sciencemag.org SCIENCE VOL 344 2 MAY 2014 447

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Examining Supramolecular ExchangeMicrotubules are a natural example of a one-dimensional (1D) supramolecular structure. Synthetic examples of 1D fi brils often have monomers linked by weak noncovalent interactions that allow monomers to exchange in and out of the fi brils. Albertazzi et al. (p. 491) used a combination of super-resolution microscopy on individual fi brils and stochastic simulation to study monomer exchange in fi brils formed from stacked 1,3,5-benzenetricarboxamide motifs. Exchange did not require large-scale depolymerization and repolymerization, or reassembly of fragments, but proceeded through individual monomers exchanging homogeneously throughout the fi brils.

Improving Reactions at InterfacesAlloying precious metals such as platinum with more abundant transition metals, such as iron and nickel, can both improve their catalytic reactivity and lower catalyst cost. Chen et

al. (p. 495) explored using coatings of iron oxide–hydroxide layers on supported platinum nanoparticles for CO oxidation. The presence of this layer allowed the reaction to run rapidly at room temperature by bringing together different reaction sites on the two metals. The addition of nickel improved catalyst lifetime,

and an oxidative transformation created a more complex nanoparticle morphology that increased platinum utilization.

Nanotube EngineeringIn general, the reversible self-assembly of nanotubes through noncovalent bonding either gives fully assembled or fully disassembled products. Fukino et al. (p. 499, published online 10 April; see the Perspective by Hudson

and Manners) developed a system with an intermediate possibility. Hollow nanotube structures were assembled from ferrocene-based tetratopic pyridyl ligands mixed with AgBF4. Through oxidation of the ferrocene groups, the tubes could be cut into stable, large rings and then reversibly reassembled into nanotubes by reduction of the ferrocene groups.

Watching Brain MoleculesFunctional neuroimaging using magnetic resonance imaging (MRI) molecular sensors would be useful for mechanistic analysis of neural systems. Lee et al. (p. 533) applied MRI-detectable sensors to measure molecular aspects of brain function in conjunction with noninvasive imaging. The molecular sensor revealed a quantitative spatiotemporal map of dopamine release in the ventral striatum, a brain area involved in processing rewards.

Increasing Turnover Rising concentrations of CO2 in the atmosphere are likely to stimulate plant growth, leading to increased biomass and soil carbon stocks, thereby lessening the rate of the rise in atmospheric CO2 levels. Van Groenigen et al.

(p. 508, published online 24 April) report the results of a meta-analysis and modeling that show that increasing the concentration of atmospheric CO2 also stimulates microbial decomposition of organic carbon in soils, by roughly the same amount that it increases soil organic carbon, leading to lower equilibrium soil carbon inventories and limiting the accumulation of carbon. Thus, soils may not provide as much carbon storage as hoped.

Predicting Responses to Drought The U.S. Corn Belt accounts for a sizeable portion of the world’s maize growth. Various infl uences have increased yields over the years. Lobell

et al. (p. 516; see the Perspective by Ort and

Long) now show that sensitivity to drought has been increasing as well. It seems that as plants have been bred for increased yield under ideal conditions, the plants become more sensitive to non-ideal conditions. A key factor may be the planting density. Although today’s maize varieties are more robust to crowding and the farmer can get more plants in per fi eld, this same crowding takes a toll when water resources are limited.

Methylating the Family TreeDNA sequences show a high level of similarities between humans and ancient hominids but the degree to which there are differences between methylated regions in their genomes that may explain phenotypic differences is unclear. Gokhman et al. (p. 523, published online 17 April) demonstrate that naturally degraded methylated cytosines in ancient DNA are converted to thymines and can be used to reconstruct ancient methylomes. The results suggest differences in methylation in bone tissues between modern humans and ancient hominids in a set of genes important for limb development.

Remodelin Nuclear DefectsDeregulation of A-type lamin proteins leads to disorganization of chromatin structure and misshapen nuclei, which are believed to underlie the pathologies of various human diseases, including the premature aging disorder Hutchinson Gilford progeria syndrome (HGPS) and various cancers. Larrieu et al. (p. 527) developed a small molecule, Remodelin, that not only improved nuclear shape defects of human lamin A/C–depleted cells, HGPS cells, and aged normal cells, but also decreased the levels of a DNA damage marker and improved global cellular fi tness.

<< Trick and TreatForked-tailed drongos are a particularly intel-ligent type of bird found in Africa. Drongos associate with many other bird and mammal species, which can learn to respond to drongo warning calls. Drongos are also exceptional mimics of the other species’ alarm calls. Though the increased vigilance across these multi species associations is a benefi t to all, drongos some-times use these calls as ploys to scare associated species away from food, which the drongos then steal. However, without some approach to maintain the effectiveness of this deception, the drongos’ ploy would soon be detected. Flower et

al. (p. 513) now show that drongos are able to fool their target species longer by fl exibly varying the type of call they give.

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Interpreting Climate Change MetricsWhile forecasts of climate change effects on biodiversity rely mostly on bioclimatic modeling approaches of varying complexity, an alternative to existing models is to use simple metrics to quantify the exposure of regions to climate changes over time and relate them to different threats and opportunities for biodiversity. It remains poorly understood how existing metrics differ in the information they provide, specifi cally in the context of biodiversity. Garcia et al. (p. 486) review the variety of metrics commonly used to describe climate change in biodiversity-impact assessments covering local changes in climate averages and extremes, regional changes in the availability and position of climates, and the velocity of climate change. While metrics are often arbitrarily chosen in studies of ecology and evolution, and interchangeably used as synonyms of climate change, they capture different dimensions of change and reveal contrasting spatial patterns across the world. Defi ning the links between climate change dimensions and the challenges they represent to species leads to a framework for interpreting climate change metrics.

On-Demand Activity Oligodendroglia ensheath axons in the brain with myelin, which provides the insulation that speeds up transmission of neuronal electrical impulses. The process of myelination in the human brain goes on for decades, concurrent with all manner of brain development and cognitive activity. Gibson et al. (p. 487, published online 10 April; see the Perspective by Bechler and ffrench-Constant) used optogenetics to study myelination in response to neural activity. Electrical activity in the

motor cortex of the brain of awake mice led to proliferation and differentiation of oligodendrocytes and consequently increased myelination and alterations in motor response.

Immunogenetic VariationMany genetic variants have been implicated in disease but their effects in function across tissues and cell-types remain to be resolved. Raj

et al. (p. 519) present an analysis of expression quantative trait loci (eQTL) measuring messenger RNA levels and examined correlations between

genotypes and gene expression in purifi ed monocytes and T cells in healthy individuals of European, African, and Asian descent. Most, but not all, of the eQTLs and their effects on expression were shared between the populations, as well as a

substantial proportion between the cell types. Links were found with disease-associated variants and loci that previous genome-wide analyses have implicated in neurodegenerative and autoimmune diseases.

Optics On the EdgeThe ability to isolate stable materials just one atom thick has provided the impetus for a new generation of thin fi lms technologies. Often, the materials are conceptually imaged in the form of an artist’s impression as single continuous layers—a single crystal sheet of atoms. Scanning probe and transmission electron microscopies reveal the reality, however, where the material resembles a patchwork quilt with polycrystals of different orientation and size separated by grain boundaries forming a mosaic. These structures play an infl uential role in determining the transport and optical

properties of the membrane. Yin et al. (p. 488; see the Perspective by Neshev and Kivshar) demonstrate a simple microscopy technique based on the nonlinear optical response of the materials to probe and characterize atomically thin layers of MoS2. The technique should prove useful during the characterization and optimization of atomically thin membranes.

Vibrating Water ApartThe main route for producing hydrogen for industrial chemical synthesis is steam reforming, in which water and methane react at high temperatures on nickel catalysts to produce hydrogen and carbon dioxide. For both water and methane, the initial dissociation step can be promoted by the translational energy of a molecule as well as its internal vibrational energy, and fundamental studies of these reactions try to determine the relative contributions of these pathways. Although the methane reaction has been well studied, only recently have lasers been available to excite the higher stretching vibrations of water. Hundt

et al. (p. 504) now report a joint experimental and theoretical study of D2O dissociation on the Ni(111) surface. For a given input of energy, vibrational energy was more effective for surmounting the reaction barrier than translational energy.

Specialized EvolutionMany plants make chemical compounds that are potentially of use to humans, but their evolutionary histories are unknown. Chae et al. (p. 510) examined how algae and land plants have been able to evolve secondary metabolism biochemistry—those compounds produced in response to their environment—in the face of purifying evolutionary pressure to maintain primary and necessary metabolic pathways. Genomic data was used to separate the primary from the secondary metabolism pathway genes and to construct the evolutionary trajectories of secondary metabolism. Secondary metabolic pathways tend to be controlled by clustered, co-regulated sets of newly duplicated and maintained genes.

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