Recently, nonavialan dinosaurs with feathers on their fore- and hindlimbs have been described. Zheng et al. (p. 1309) describe eleven basal avialan fossils with clear evidence of feathered hindlimbs. Together these fossils show that early avialans possessed four wings, rather than two. A gradual reduction in hindlimb feathering eventually yielded the two-wing condition in today's birds. Such a transition may have accompanied a locomotory decoupling of the fore- and hindlimbs, which facilitated the development of the forelimbs into flight-capable wings.
A Move to Planar Optics
Metamaterials allow light to be manipulated in ways that cannot be done with naturally available materials. Subwavelength metallic nanoantenna arrays patterned onto a surface can provide the basis for planar optical devices, in which bulk optical elements that are typically thousands of wavelengths in size can be "flattened" into a two-dimensional sheet less than a wavelength thick. Kildishev et al. (10.1126/science.1232009) review progress in the optics of metasurfaces and discuss promising applications for surface-confined planar photonics components.
Glass or metal fibers can show incredible flexibility. Natalio et al. (p. 1298; see the Perspective by Sethmann) used the protein silicatein-α, which is responsible for the biomineralization of silicates in sponges, to guide the formation of spicules made of calcite. These synthetic spicules could be bent to a high degree because of their inherent elasticity, whilst retaining the ability to guide light.
The Thalamus in Fear and Memory
The medial prefrontal cortex (mPFC) mediates the cognitive control of many high-level brain functions. However, it is unclear which synaptic projections from the mPFC to subcortical regions are critical for maintaining the proper balance between retention and generalization of fear memory details. Using an array of behavioral, physiological, and anatomical techniques, Xu and Südhof (p. 1290) describe a neural circuit that controls memory generalization and specificity. This circuit involves the nucleus reuniens (NR), a thalamic nucleus of largely unknown function. Optogenetic activation of NR neurons in awake behaving mice revealed the role of the NR in fear memory generalization.
Although specific genes involved in animal coloration have been identified, the underlying selection for genetic variation in color-specific adaptation is not well understood. Examining the Agouti gene and other loci in the deer mice of Nebraska, where predation selects for light-colored mice in light environments and dark-colored mice in dark environments, Linnen et al. (p. 1312) find evidence of multiple genetic variants under selection affecting coloration. The light color of Sand Hills mice is not the result of a single large-effect mutation, but is because of many accumulated mutations, each with a smaller phenotypic effect.
When a solitary wave travels atop the surface of a fluid, its shape generally changes with time, with some of its components traveling at velocities slightly different than others. In nonlinear media, this spreading effect may be countered by a slimming effect stemming from the non-linearity, which generates an object with perfectly preserved shape, called a soliton. Solitons have been observed in fluids, granular media, and other systems. Mohseni et al. (p. 1295) detected a dissipative soliton (one that also balances gain and dissipation) in a magnetic system, in the form of a magnetic droplet consisting of a core of spins pointing opposite to the external magnetic field. The droplet exhibited peculiar dynamics and could be controlled by electric current.
Growth factors help to coordinate metabolism with growth in part by stimulating the activity of the protein kinase mTORC1 (mechanistic target of rapamycin complex 1). Ben-Sahra et al. (p. 1323, published online 21 February) and Robitaille et al. (p. 1320, published online 21 February) independently identified a key target of mTORC1—carbamolyl-phosphate synthase 2, or CAD, the rate-limiting enzyme for de novo synthesis of pyrimidines. Metabolomic profiling and phosphoproteomic analyses of normal cells and cells lacking signaling by mTORC1 converged on CAD as a key point at which growth-promoting signals also ramp up production of nucleic acids.
Under the Sea Floor
Microorganisms living in basaltic sea floor buried beneath sediments derive energy from inorganic components from the host rocks that interact with infiltrating seawater, which brings dissolved oxygen and other trace nutrients with it. Lever et al. (p. 1305) directly sampled the subseafloor community off the eastern flank of the Juan de Fuca Ridge in the Pacific Ocean and found evidence for ongoing microbial sulfate reduction and methanogenesis. Multiyear incubation experiments with samples of host rock confirmed the microbial activities measured in situ.
Synchronizing Photosynthetic Capacity
Coordination of photosynthetic activity with sunlight benefits plant productivity. Noordally et al. (p. 1316) analyzed how the Arabidopsis circadian clock keeps the chloroplasts working in tune with the Sun. SIGMA FACTOR5 (SIG5) is encoded in the cell nucleus and reflects circadian cycles with changes in its own transcript abundance. SIG5 acts, however, in the chloroplast, where it supports photosystem II production.
Surface Molecules Not Quite Desorbing
The dynamics of molecules desorbing from or adsorbing on surfaces requires that molecules rapidly gain or lose a large amount or translational and rotational energy to enter or leave the gas phase. An intermediate precursor state has long been invoked in which molecules interact weakly with the surface but translate along it and exchange energy without forming localized surface bonds. Dell'Angela et al. (p. 1302) found evidence for such a state in changes in x-ray absorption and emission spectra of CO molecules adsorbed on a ruthenium surface after optical excitation rapidly heated the surface. The use of a free electron laser provided high time resolution for x-ray spectroscopy studies. Density function theory and modeling of high temperature states revealed a state that forms from molecules that have not overcome the desorption barrier during heating and that are bonded less strongly than the chemisorbed state.
Mitochondrial Makeup Mapped
Because mass spectrometry (MS) cannot be performed on living cells, biologists currently recover spatial information indirectly, by purifying organelles or protein complexes prior to MS analysis. These purifications often yield false positives because of sample contamination and false negatives because of material loss. Rhee et al. (p. 1328, published online 31 January) present an approach that bridges microscopy and proteomics to produce a spatially and temporally resolved proteomic map of mitochondria from living cells. A nonspecific labeling enzyme (peroxidase) was genetically targeted to the mitochondria within live cells, where it tagged endogenous proteins in a spatially restricted manner within a 1-minute window, for subsequent identification and analysis by MS. This rapid and straightforward technology provides the ability to access otherwise inaccessible cellular regions and requires a very small amount of starting material.
Mercury Methylating Microbes
Mercury (Hg) most commonly becomes bioavailable and enters the food web as the organic form methylmercury, where it induces acute toxicity effects that can be magnified up the food chain. But most natural and anthropogenic Hg exists as inorganic Hg2+ and is only transformed into methylmercury by anaerobic microorganisms—typically sulfur-reducing bacteria. Using comparative genomics, Parks et al. (p. 1332, published online 7 February; see the Perspective by Poulain and Barkay) identified two genes that encode a corrinoid and iron-sulfur proteins in six known Hg-methylating bacteria but were absent in nonmethylating bacteria. In two distantly related model Hg-methylating bacteria, deletion of either gene—or both genes simultaneously—reduced the ability for the bacteria to produce methylmercury but did not impair cellular growth. The presence of this two-gene cluster in several other bacterial and lineages for which genome sequences are available suggests the ability to produce methylmercury may be more broadly distributed in the microbial world than previously recognized.
Several recent papers have revealed the unexpected genetic and pathological overlap between frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). The most common genetic cause is the GGGGCC hexanucleotide repeat expansion upstream of the C9orf72 coding region affecting about 10% of all patients. It is currently unknown how repeat expansion might lead to neurodegeneration. C9orf72 patients show two distinct types of ubiquitinated inclusions in the central nervous system, one of which was identified as phosphorylated TDP-43 protein. However, all inclusions in the cerebellum and most inclusions in the hippocampus and neocortex lack TDP-43, and the actual disease protein is unknown. Mori et al. (p. 1335, published online 7 February; see the Perspective by Taylor) discovered that most of these characteristic inclusions contain poly-(Gly-Ala) and, to a lesser extent, poly-(Gly-Pro) and poly-(Gly-Arg) dipeptide-repeat proteins that are generated by non-ATG–initiated translation from the expanded GGGGCC repeats in three reading frames. The findings yield mechanistic insight into the pathogenesis of FTLD/ALS with C9orf72 repeat expansions and directly link this common mutation to the characteristic pathology.