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<jats:title>Potassium Permeation</jats:title> <jats:p> Two–pore domain potassium (K2P) channels conduct K <jats:sup>+</jats:sup> ions across the plasma membrane of eukaryotic cells. They help to maintain the cellular resting potential and their modulation can tune cellular excitability (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6067" page="416" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1217679">Poulsen and Nissen</jats:related-article> </jats:bold> ). <jats:bold>Miller and Long</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="432" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1213274">432</jats:related-article> ) describe a high-resolution crystal structure of the human K2P channel K2P1 (TWIK-1) and <jats:bold> Brohawn <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="436" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1213808">436</jats:related-article> ) present a high-resolution structure of the lipid and mechanosensitive human channel TRAAK. In both structures an extracellular domain constricts the channel entrance so that K <jats:sup>+</jats:sup> ions reach the selectivity filter through side portals. Openings in the transmembrane region expose the central cavity to the lipid bilayer and a helix is kinked so that its C-terminal part lies in the cytosol-membrane interface. The structural features explain K2P conductance and gating and give insight into how the channels are regulated by diverse stimuli. </jats:p>

<jats:title>Potassium Permeation</jats:title> <jats:p> Two–pore domain potassium (K2P) channels conduct K <jats:sup>+</jats:sup> ions across the plasma membrane of eukaryotic cells. They help to maintain the cellular resting potential and their modulation can tune cellular excitability (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6067" page="416" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1217679">Poulsen and Nissen</jats:related-article> </jats:bold> ). <jats:bold>Miller and Long</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="432" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1213274">432</jats:related-article> ) describe a high-resolution crystal structure of the human K2P channel K2P1 (TWIK-1) and <jats:bold> Brohawn <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="436" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1213808">436</jats:related-article> ) present a high-resolution structure of the lipid and mechanosensitive human channel TRAAK. In both structures an extracellular domain constricts the channel entrance so that K <jats:sup>+</jats:sup> ions reach the selectivity filter through side portals. Openings in the transmembrane region expose the central cavity to the lipid bilayer and a helix is kinked so that its C-terminal part lies in the cytosol-membrane interface. The structural features explain K2P conductance and gating and give insight into how the channels are regulated by diverse stimuli. </jats:p>

<jats:title>Porous Membranes</jats:title> <jats:p> Thin semi-permeable membranes are commonly used as chemical barriers or for filtration purposes. While the size of the pores will influence which molecules are able to pass, other factors—including the surface chemistry of the pore walls, electrostatic interactions, and differences in solubility—can also affect the diffusion rates. There is also a trade-off between the thickness of the membrane regarding strength and permeation rates (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6067" page="413" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216923">Paul</jats:related-article> </jats:bold> ). <jats:bold> Karan <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="444" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1212101">444</jats:related-article> ) fabricated membranes from amorphous carbon, which showed excellent strength and could be used for filtrations involving organic solvents. <jats:bold> Nair <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="442" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1211694">442</jats:related-article> ) observed unusual behavior in graphene-based membranes which were able to prevent the diffusion of many small-molecule gases, including helium, but showed almost barrier-free movement of water. </jats:p>

<jats:title>Porous Membranes</jats:title> <jats:p> Thin semi-permeable membranes are commonly used as chemical barriers or for filtration purposes. While the size of the pores will influence which molecules are able to pass, other factors—including the surface chemistry of the pore walls, electrostatic interactions, and differences in solubility—can also affect the diffusion rates. There is also a trade-off between the thickness of the membrane regarding strength and permeation rates (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6067" page="413" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216923">Paul</jats:related-article> </jats:bold> ). <jats:bold> Karan <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="444" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1212101">444</jats:related-article> ) fabricated membranes from amorphous carbon, which showed excellent strength and could be used for filtrations involving organic solvents. <jats:bold> Nair <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="442" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1211694">442</jats:related-article> ) observed unusual behavior in graphene-based membranes which were able to prevent the diffusion of many small-molecule gases, including helium, but showed almost barrier-free movement of water. </jats:p>

<jats:title>A Passive Optical Diode</jats:title> <jats:p> Electrical diodes are at the core of microelectronics. The optical equivalent, however, has been difficult to realize owing to the time-reversal symmetry of Maxwell's equations that describe electromagnetic propagation. Usually, a control input in the form of a magnetic field is required that breaks that symmetry. Such inputs are not practical for optical integrated circuits. <jats:bold> Fan <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="447" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1214383">447</jats:related-article> , published online 22 December) developed a silicon-based microresonator device that could control the asymmetric transmission of light through it. The passive optical diode was compatible with current complementary metal-oxide semiconductor processing technology and thus should be readily integrated into optoelectronic circuitry. </jats:p>

<jats:title>Boxing in Peroxide</jats:title> <jats:p> Hydrogen peroxide (H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> ) is a powerful oxidant, and its reactivity is exploited in numerous biological, as well as synthetic, contexts. <jats:bold> Lopez <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="450" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1212678">450</jats:related-article> ) have now managed to capture its dianion (O <jats:sub>2</jats:sub> <jats:sup>2-</jats:sup> ) in a cryptand—essentially a molecular box assembled from benzamide derivatives—keeping the dianion stable in organic solution for days through a net of well-placed internal hydrogen-bond donors. The encapsulated dianion exhibited clean oxidative reactivity back to O <jats:sub>2</jats:sub> either by chemical or by electrochemical means. </jats:p>

<jats:title>Magnetic Moon</jats:title> <jats:p> It has long been suspected that the Moon once had a core-dynamo magnetic field. <jats:bold> Shea <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="453" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1215359">453</jats:related-article> ) describe a lunar basalt brought back by Apollo 11 that records evidence for a strong dynamo on the Moon 3.7 billion years ago. This study, together with a previous study of different lunar rock, implies that a lunar core dynamo existed between 4.2 and 3.7 billion years ago, which extends the known lifetime of the lunar dynamo by 500 million years. </jats:p>

<jats:title>Natural Selection Caught in the Act</jats:title> <jats:p> Understanding how new functions evolve has been of long-standing interest. However, the number of mutations needed to evolve a key innovation is rarely known, or whether other sets of mutations would also suffice, whether the intermediate steps are driven by natural selection, or how contingent the outcome is on steps along the way. <jats:bold> Meyer <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="428" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1214449">428</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6067" page="410" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1217807">Thompson</jats:related-article> </jats:bold> ) answer these questions for a case in which phage lambda evolved the ability to infect its host <jats:italic>Escherichia coli</jats:italic> through a novel receptor. This shift required four mutations, which accumulated under natural selection in concert with coevolution of the host. However, when <jats:bold> Tenaillon <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="457" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1212986">457</jats:related-article> ) exposed 115 lines of <jats:italic>E. coli</jats:italic> to high temperature and sequenced them, adaptation occurred through many different genetic paths, showing parallelism at the level of genes and interacting protein complexes, but only rarely at the nucleotide level. Thus, epistasis—nonadditive genetic interaction—is likely to play an important part in the process of adaptation to this environment. </jats:p>

<jats:title>Centrosome Center Stage?</jats:title> <jats:p> The centrosome is a major organizer of the cytoskeleton in animal cells. The precise duplication of this organelle at each cell cycle ensures proper organization of the mitotic apparatus and chromosome segregation. However, centrosomes are dispensable for cell division during some stages of development in mouse and <jats:italic>Drosophila</jats:italic> . <jats:bold>Bornens</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="422" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1209037">422</jats:related-article> ) reviews the role of the centrosome in a variety of organisms and discusses how and why they can be dispensed with on occasion. In this vein, <jats:bold> Azimzadeh <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="461" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1214457">461</jats:related-article> , published online 5 January) now demonstrate that centrosomes in Planarian flatworms, best known for their remarkable regeneration abilities, are completely dispensable for both cell division and development. </jats:p>

<jats:title>Forest Diversification</jats:title> <jats:p> The relative role of neutral and deterministic processes in generating species richness in ecological communities (particularly tropical trees) has dominated recent discussions about patterns of diversity and abundance of species. <jats:bold>Ricklefs and Renner</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="464" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1215182">464</jats:related-article> ) present a test of random diversification and population change by comparing the numbers of individuals and species in plant families between forest plots on different continents that have diversified independently for millions of years. Stochastic theory, based on random events, predicts that there should be no correlation between the regions; however, the data reveal strong, statistically significant correlations. Thus, patterns of diversity appear to be dominated by deterministic processes. </jats:p>