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<jats:title>Surveying Savannas</jats:title> <jats:p> Savannas are structurally similar across the three major continents where they occur, leading to the assumption that the factors controlling vegetation structure and function are broadly similar, too. <jats:bold> Lehmann <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6170" page="548" related-article-type="in-this-issue" vol="343" xlink:href="10.1126/science.1247355">548</jats:related-article> ) report the results of an extensive analysis of ground-based tree abundance in savannas, sampled at more than 2000 sites in Africa, Australia, and South America. All savannas, independent of region, shared a common functional property in the way that moisture and fire regulated tree abundance. However, despite qualitative similarity in the moisture–fire–tree-biomass relationships among continents, key quantitative differences exist among the three regions, presumably as a result of unique evolutionary histories and climatic domains. </jats:p>

<jats:title>The Makings of a Choosy Pathogen</jats:title> <jats:p> The oomycete <jats:italic>Phytophthora infestans</jats:italic> is responsible for potato blight. A closely related pathogen afflicts the 4 o'clock flower. To assess why such similar pathogens are restricted to one host or the other, <jats:bold>Dong et al.</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6170" page="552" related-article-type="in-this-issue" vol="343" xlink:href="10.1126/science.1246300">552</jats:related-article> ; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6170" page="496" related-article-type="in-this-issue" vol="343" xlink:href="10.1126/science.1250171"> <jats:bold>Coaker</jats:bold> </jats:related-article> ) analyzed similar effectors from both pathogens. The results suggest that the host specialization that led to evolutionary divergence depends on reciprocal single–amino acid changes that tailor the pathogen effector to a specific host protease that is being disabled. Thus, small changes can open the door for a pathogen to jump to another species of host and, itself, diversify into another species of pathogen. </jats:p>

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<jats:title>The Message in the Medium</jats:title> <jats:p> What is the point of autocrine signaling in which a cell produces a signal that activates receptors on its own cell surface? An internal signal seems simpler, unless there is value to allowing neighboring cells to know what other cells are up to. <jats:bold>Youk and Lim</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" related-article-type="in-this-issue" xlink:href="10.1126/science.1242782">10.1126/science.1242782</jats:related-article> ; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6171" page="624" related-article-type="in-this-issue" vol="343" xlink:href="10.1126/science.1250244"> <jats:bold>Lee and You</jats:bold> </jats:related-article> ) explored the broad range of signaling outcomes that can result in a system in which some yeast cells could secrete and sense a signal whereas others could only sense signals from their neighbors. The cells were engineered so that the response of the two cell types could be distinguished from one another. Experiments and mathematical modeling showed that depending on how circuits were constructed—for example, how much receptor was present, how the signal molecule was degraded, the presence of feedback, the density of the cell culture, and so on—a range of behaviors was possible: Some conditions favored activation of one type of cell over another. Others altered the timing or consistency of the response within a population. The principles revealed could also be used in other biological contexts or in the design of synthetic biological cell systems with desired regulatory properties. </jats:p>