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<jats:title>Ironed Out</jats:title> <jats:p> In large regions of the ocean, low levels of the essential nutrient, iron, limits primary productivity. Iron's chemistry and bioavailability are highly dependent on pH. Rising concentrations of atmospheric CO <jats:sub>2</jats:sub> is leading to the acidification of the ocean. <jats:bold> Shi <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="676" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1183517">676</jats:related-article> , published online 14 January; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5966" page="654" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1186151">Sunda</jats:related-article> </jats:bold> ) show that the bioavailable fraction of iron dissolved in the ocean may decline as a result of the decrease in ocean pH, which affects the rate of iron uptake by diatoms and coccolithophores. Unless iron input to the oceans increases, these changes may lead to an increase in the iron stress of phytoplankton. </jats:p>

<jats:title>Evolutionary Hops</jats:title> <jats:p> In order for animals to colonize new environments and disperse, specific traits may be required. Toads form a large and ecologically diverse group and have achieved extremely wide distribution in a very short time frame. To test what traits may be associated with species range in modern toads, <jats:bold> Van Bocxlaer <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="679" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181707">679</jats:related-article> ; see the news story by <jats:bold>Pennisi</jats:bold> ) phylogenetically reconstructed the evolutionary history of morphological and physiological traits linked to the toads' geographic ranges. Toads have evolved an optimal dispersal phenotype comprising many traits, including those affecting reproductive choices and water retention, which preceded their expansion and radiation. </jats:p>

<jats:title>Not at the Mercy of the Wind</jats:title> <jats:p> How can insects that migrate at high altitudes on fast-moving winds influence their direction of migration, when wind speeds typically exceed their self-propelled air speeds by a factor of three or four? Using automated vertical-looking entomological radar systems, <jats:bold> Chapman <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="682" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1182990">682</jats:related-article> ) show that compass-mediated selection of favorable tailwinds, and partial correction for crosswind drift, are widespread phenomena in migrant insect species. Specialized flight behaviors have decisive influence on the migration pathways achieved by insects. Thus, contrary to popular belief, migrant insects are not at the mercy of the wind. </jats:p>

<jats:title>Complex Cooperativity</jats:title> <jats:p> Cooperativity in multisubunit protein complexes is classically understood in terms of either a concerted model, in which all subunits switch conformation simultaneously, or a sequential model, in which a subunit switches conformation whenever a ligand binds. More recently, a “conformational spread” model has suggested that a conformational coupling between subunits and between subunit and ligand is probabilistic. Using high-resolution optical microscopy, <jats:bold> Bai <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="685" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1182105">685</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="5966" page="653" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1186121">Hilser</jats:related-article> </jats:bold> ) observed multistate switching of the bacterial flagellar switch complex that was previously understood in terms of a concerted allosteric model. The conformational spread model gives quantitative agreement with the data. </jats:p>

<jats:title>VSV in 3D</jats:title> <jats:p> Rhabdoviruses are a family of negative-stranded RNA viruses that includes rabies virus, which have a characteristic bullet shape. Though structures of individual rhabdovirus proteins have been reported, how these are organized into a bullet shape has remained unclear. Now, <jats:bold> Ge <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="689" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181766">689</jats:related-article> ) report a cryo-electron microscopy structure of a model rhabdovirus, vesicular stomatitis virus. The structural data and examination of mutants allows modeling of virion assembly. </jats:p>

<jats:title>Protective Abundance</jats:title> <jats:p> Massively repeated sequences are generally dangerous to genomes because they promote recombination and, potentially, genome instability. Eukaryotic ribosomal RNA genes (rDNA), which are highly transcribed, are organized into large arrays of repeats and have a system that actively maintains these large arrays. In getting to the bottom of this apparent contradiction, <jats:bold> Ide <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="693" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1179044">693</jats:related-article> ) found that reducing the number of rDNA repeats in yeast resulted in a marked sensitivity to DNA damage, owing to heavy rDNA transcription preventing repair of compromised DNA replication forks. It appears that the additional copies of rDNA both reduce the ability of transcription to interfere with DNA repair and provide, through the action of condensin, templates for the recombination-based repair of replication-induced damage. </jats:p>

<jats:title>Transmission of Drug Resistance</jats:title> <jats:p> Understanding the dynamics of drug-resistant strains of HIV and the key determinants affecting their evolution and spread is crucial for predicting future effects of drug treatment. Current models can only track one resistant strain, so <jats:bold> Smith? <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="697" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1180556">697</jats:related-article> , published online 14 January) used empirical data from San Francisco to parameterize models that consider the transmission of single-, double-, and triple-resistant HIV strains. Many people who are infected with a resistant strain are capable of infecting more than one other person; a scenario that could trigger an epidemic wave of drug-resistant virus. At a time when the World Health Organization's strategy for universal testing and treatment is being rolled out, the insights gained from this work are not restricted to HIV transmission and treatment in resource-rich countries, but are widely applicable. </jats:p>

<jats:title>Detection Versus Localization</jats:title> <jats:p> Active-sensing systems allow an animal some control over the information acquired from the environment. For example, echolocating bats are known to control many aspects of their sonar signal design. However, their sensory data-acquisition strategies are still not fully understood. <jats:bold> Yovel <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="701" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1183310">701</jats:related-article> ) found that Egyptian fruit bats ( <jats:italic>Rousettus aegyptiacus</jats:italic> ) did not aim their sonar beams directly on target in a standard localization task. Instead, the bats pointed the main lobe of their sonar beam off-axis. Bats approaching the target emitted sounds to the left and right of its localization, in an alternating manner, so that the maximum slope of the change in intensity of the sonar beam was often positioned close to the target. This echolocation strategy is ideal for target localization but occurs at the cost of target detection. </jats:p>

<jats:title>Centrosome-Free Axonal Regeneration</jats:title> <jats:p> Neuronal axon growth is thought to depend on microtubules that are assembled at the centrosome, the classical microtubule organizing center (MTOC), which may even dictate axon growth. However, such a local trigger of neuronal polarization and focal microtubule nucleation in the cell body appears difficult to reconcile with the sophisticated microtubule arrays observed in neurons. Now, <jats:bold> Stiess <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="704" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1182179">704</jats:related-article> , published online 7 January) provide evidence, through physical ablation of the centrosome in mammalian neurons, that axon growth is regulated by acentrosomal microtubule nucleation. The findings suggest that the centrosome loses its function as an MTOC during neuronal development, that axon growth depends on decentralized microtubule assembly, and that neuronal differentiation takes place in the absence of a functional centrosome. </jats:p>

<jats:p> The 2009 Gordon Research Conference schedule was published on pages 708 to 734 of this issue of the print version of <jats:italic>Science</jats:italic> . The current schedule can also be found online at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.grc.org/">www.grc.org/</jats:ext-link> . </jats:p>