Swimming Through Sand
Although composed of solid particles, sand can behave like a fluid. If you had to swim through sand, how would you do it? Would you use your arms and legs for propulsion or would you make your body as compact as possible and try to wiggle and slither your way through? Maladen et al. (p. 314) used x-ray imaging to study the motion of sandfish lizards as they burrowed into sand. The sandfish lizard does not use its limbs, but instead flattens them against its body and uses large-amplitude traveling wave oscillation of its body to propel itself. Modeling can explain the motion the lizard uses to propel itself through a medium that is neither liquid nor solid.
A Multiple Photon Pileup
The field of quantum optics began with the observation that two independent photons emitted from a thermal source tend to bunch together. The same is true for any number of bosons, but how do the statistics and correlations evolve experimentally as the number increases? Aβmann et al. (p. 297) have developed a streak-camera technique that can distinguish the photon number and measure the higher-order correlations between the photons at the detector. The results confirm the predicted “n factorial” dependence, showing that the tendency to bunch gets stronger as the number of independent photons is increased.
Living on the Edge
Topological insulators are a recently described state of matter in which the bulk material is an insulator but with a metallic surface state that is protected by the topology of the Fermi surface. Roth et al. (p. 294; see the Perspective by Büttiker) now show that the current flow on the surface takes place in edge states around the boundary of the sample. These are similar to the current transport in high-quality two-dimensional electron gases in high magnetic field, which confirms theoretical work on these materials.
Shaking Prevents Breaking
Intuition suggests that vibrational excitation of a molecular bond ought to increase the likelihood of its breaking in an ensuing chemical reaction. W. Zhang et al. (p. 303) observe precisely the opposite outcome in a spectroscopic study of the F + CHD3 reaction. Exciting the C-H stretch leads exclusively to the formation of DF and CHD2 products, in contrast to the more abundant yields of both HF and DF observed in the absence of CH vibration. Though the mechanism underlying this effect remains unclear, the result highlights unanticipated complexity in the reaction dynamics of a relatively simple molecule.
Dissecting Dyslexia and Learning
Difficulties in learning to read, despite reasonable effort and instruction, form the basis of dyslexia. Gabrieli (p. 280; see the cover) now reviews the latest research into the causes of dyslexia. Neuroimaging studies may give early notice of impending dyslexia, and it is hoped that early interventions may lessen the impact of dyslexia. Learning occurs in many settings. Humans uniquely use the formalized settings of schools and curriculum. Infants and children also do plenty of learning outside these settings, often intermingling social interactions. Meltzoff et al. (p. 284) survey the variety of learning contexts that people experience and discuss how recent advances in neuroscience and robotics are driving a new synthesis of learning.
Earth's environment changed markedly over the past 5.2 million years, when a permanent ice sheet has developed in the Northern Hemisphere and the glacial cycle has changed its period from roughly every 40,000 years to the dominantly 100,000-year duration of the past half-million years. One of the biggest questions about these changes is whether they were “threshold” responses to a gradual, uniform cooling trend or whether they represent reactions to discrete episodes of cooling. Sosdian and Rosenthal (p. 306) present deep-ocean temperature records from the North Atlantic that show that the cooling happened in distinct steps, at 3 to 2.5 million years ago and at 1.2 to 0.85 million years ago. Combining their record with that of deep ocean water oxygen isotopes allowed the distinction between effects due to global cooling and ice-sheet dynamics.
Economic Ancient DNA Sequencing
Analysis of ancient DNA is often limited by the availability of ancient material for sequencing. Briggs et al. (p. 318; see the news story by Pennisi) describe a method of ancient DNA sequence retrieval that greatly reduces shotgun sequencing costs while avoiding the many difficulties associated with direct PCR-based approaches. They generated five complete and one near-complete Neandertal mitochondrial DNA genomes, which would have been economically impossible with a simple shotgun approach. Analysis of these genomes shows that Neandertal populations had a much smaller effective population size than modern humans or great apes.
Moths Battling Bats
Many night-flying insects hear the sonar sounds of attacking bats and take evasive action. Among moths, evasive flight is often accompanied by the production of ultrasonic sounds. Three functions of these sounds have been proposed: to startle the bat, to warn of distastefulness, or to “jam” the bat's sonar system. Corcoran et al. (p. 325) studied a species of tiger moth (Bertholdia trigona) that emits a particularly dense series of ultrasonic clicks and the interception behavior of big brown bats (Eptesicus fuscus) presented with silenced or sound-producing tethered moths. If the moth sounds evoke startle, naïve bats should initially break off their attacks, but then the bats should habituate to the sounds and complete subsequent attacks. In contrast, if the moth sounds have a warning effect, naïve bats should initially complete their attacks on sound-emitting moths, discover that the moths are distasteful, and refuse to capture them in future trials. Most of the bats in the tests reliably caught the silenced moths but avoided completing attacks on sound-producing moths, with no evidence of increasing or decreasing probability of capture from the first to the last trial, which suggests that the moths effectively jammed the bats' sonar.
Plethora of Secretory Amyloids
Protein aggregation and the formation of amyloids are associated with several dozen pathological conditions in humans, including Alzheimer's disease, Parkinson's disease, and type II diabetes. In addition, a few functional amyloid systems are known: the prions of fungi, the bacterial protein curli, the protein of chorion of the eggshell of silkworm, and the amyloid protein Pmel-17 involved in mammalian skin pigmentation. Now Maji et al. (p. 328, published online 18 June) propose that endocrine hormone peptides and proteins are stored in an amyloid-like state in secretory granules. Thus, the amyloid fold may represent a fundamental, ancient, and evolutionarily conserved protein structural motif that is capable of performing a wide variety of functions contributing to normal cell and tissue physiology.
The Grim RIPper
Cells can undergo regulated cell death through distinct processes known as apoptosis and necrosis. Regulation of apoptosis is better understood than that of necrosis. In a screen for gene products that participate in control of necrosis in cells treated with TNF (tumor necrosis factor), D.-W. Zhang et al. (p. 332; published online 4 June) identified a protein kinase, RIP3. In cells treated with TNF and a caspase inhibitor that inhibits apoptosis, seven metabolic enzymes interacted with RIP3, some of which are associated with mitochondria. Generation of reactive oxygen species was necessary for TNF-induced necrosis, and depletion of RIP3 reduced the generation of reactive oxygen species. Thus, RIP3 may participate in the mechanisms that link energy metabolism with mechanisms of cell death.
Innate Immunity in the Fly Gut
Drosophila melanogaster is an important model system to study innate immunity, being both easy to manipulate and lacking an adaptive immune system. In order to identify genes that regulate innate immunity, Cronin et al. (p. 340; published online 11 June) performed an RNA interference screen on flies infected with the oral bacterial pathogen, Serratia marcescens. Genes involved in intestinal immunity and regulation of hemocytes, macrophage-like cells critical for phagocytosis and killing of the bacteria, were identified. Several hundred genes conferred either enhanced susceptibility or resistance to bacterial infection. Furthermore, the JAK/STAT signaling pathway was activated in intestinal stem cells after bacterial infection, resulting in enhanced susceptibility to infection, most likely through regulation of intestinal stem cell homeostasis.
A set of diseases, including myotonic dystrophy, are caused by the expansion of a simple repeat in genomic DNA, which, when transcribed into RNA, can be toxic to other cellular processes. Ameliorating the effects of this toxic, repeat-laden RNA may also relieve the symptoms of the disease. Wheeler et al. (p. 336; see the Perspective by Cooper) developed an antisense morpholino oligonucleotide complementary to the expanded repeats found in the myotonic dystrophy protein kinase messenger RNA (mRNA). The morpholino bound the repeats in vitro and displaced the inappropriately bound and sequestered RNA splicing factor, Muscleblind-like 1. In an in vivo mouse model for myotonic dystrophy, local injection of the morpholino corrected a number of cellular defects in muscle, including the alternative mRNA splicing of several genes, among them the muscle-specific chloride channel, CIC1, leading to a marked reduction in the myotonia.
Ecology of Diarrhea
Rotavirus is an important cause of morbidity and mortality globally, and, although the infection takes a terrible toll on infant lives, its epidemiology is rather poorly known. New vaccines have become available and are being introduced in the United States prior to global rollout, but they may have some unexpected effects on disease dynamics. Pitzer et al. (p. 290; see the Perspective by Medley and Nokes) analyzed data and developed models describing the epidemiology of rotavirus before and during adoption of the vaccine. Ecological analysis showed that the birth rate predicted the timing of epidemics much better than climatic variables and that shifts in birth rates explained changes over the years. But as increasing numbers of infants are vaccinated, the pool of susceptible individuals in the population will be reduced, which will affect the annual waves of geographic spread of rotavirus.
Coupled Copper Surface States
Periodic arrays of quantum dots can create new electronic states that arise from coupling of the states created by confinement. Lobo-Checa et al. (p. 300) show that the electronic surface-state of copper can be converted into a regular array of quantum dots. An organic overlayer that is created on the copper surface has pores 1.6 nanometers in diameter that trap the surface states. The coupling of these trapped states is revealed in photoemission experiments, in which a shallow dispersive electronic band is formed.
The initial pulse of warming during the last deglaciation, which defined the start of an interval called the Bølling-Allerød, occurred abruptly about 14,500 years ago. To date, the most detailed simulations used models of intermediate complexity, not with more sophisticated Coupled Global Climate Models (CGCMs) that can synchronously couple both oceanic and the atmospheric components. Overcoming practical and technical challenges, Liu et al. (p. 310; see the Perspective by Timmermann and Menviel) performed such a simulation using CCSM3, a state-of-the-art ocean-atmosphere CGCM. In contrast to previous studies, which indicated that the Bølling-Allerød was triggered by a nonlinear bifurcation between modes of deep ocean circulation in the Atlantic, the results suggest that the event was a transient response caused by the cessation of meltwater input into the surface ocean in the North Atlantic region.
In humans, selenocysteine is the only amino acid that lacks its own transfer RNA (tRNA) synthetase and is synthesized on its cognate tRNA. The process involves mischarging of tRNAsec with serine, phosphorylation of the serine, and then conversion of the phosphoserine into selenocysteine by the enzyme SepSecS using selenophosphate as the selenium donor. Palioura et al. (p. 321) now provide insight into the mechanism of selenocysteine formation, based on the crystal structure of human tRNAsec complexed with SepSecS, phosphoserine, and thiophosphate, together with in vivo and in vitro activity assays. Binding of tRNAsec to SepSecS is required to properly orient phosphoserine attached to tRNAsec for pyroxidal phosphate–based catalysis.