Catálogo de publicaciones - revistas

<jats:title>Population Limits</jats:title> <jats:p> Extensive pre-Columbian populations inhabited the central and eastern Amazon basin, as evidenced by the clearing and modification of forests. <jats:bold> McMichael <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="1429" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1219982">1429</jats:related-article> ) examined how far inland such activities may have extended by sampling soils across western Amazonia, including river bluffs, which were heavily occupied downstream. Little evidence of human disturbance across a wide region was found by looking for charcoal layers (which would suggest use of fire) and phytoliths, which trace local plants and would indicate the presence of crops. Furthermore, no ceramics or tools were found. Thus, pre-Columbian human populations seem to have been sparse in western Amazonia. </jats:p>

<jats:title>Ancient Associations</jats:title> <jats:p> Submarine seagrass meadows are critical to fisheries and coastline protection and provide feeding grounds for many endangered species, including dugongs and turtles, and serve as a nursery for coral reef fish. The persistence and maintenance of seagrass ecosystems have been mysterious, because accumulation of organic matter in the beds should rapidly lead to toxic sulphide levels in the sediment. Using a meta-analysis and a field study, <jats:bold> van der Heide <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="1432" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1219973">1432</jats:related-article> ) atttribute the 100-million-year success of seagrasses to a three-stage symbiosis. Seagrass beds worldwide contain high densities of small lucinid bivalves that have symbiotic sulphide-oxidizing bacteria in their gills. This association appears to relieve any sulphide stress for seagrasses, while the lucinids and their symbionts profit from the accumulation of degradable organic matter and oxygen release from seagrass roots. </jats:p>

<jats:title>Fear Itself</jats:title> <jats:p> A direct connection exists between aboveground communities and belowground soil microbiota: Soil microbes break down detrital inputs from above. Generally, it has been assumed that this relationship is largely driven by the soil community and the quality of the unconsumed plant-matter that makes up the majority of detritus in most ecosystems. However, <jats:bold> Hawlena <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="1434" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1220097">1434</jats:related-article> ) found that grasshoppers exposed to the threat of spider predation have an altered carbon to nitrogen ratio. When these grasshopper carcasses were subsequently integrated into the plant litter, they significantly slowed the decomposition rate, although there was no impact on the rate of decomposition of the grasshoppers themselves. Thus, the mere presence of predators, and the stress they impose, can have cascading trophic impacts and even influence the process of decomposition. </jats:p>

<jats:title>Reading the Leaves</jats:title> <jats:p> Excess inputs of nutrients—a type of pollution known as eutrophication—threatens biodiversity and water quality in rivers and streams. <jats:bold> Woodward <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="1438" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1219534">1438</jats:related-article> ; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6087" page="1393" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1223250"> <jats:bold>Palmer and Febria</jats:bold> </jats:related-article> ) studied how one key ecosystem process—leaf-litter decomposition—responds to eutrophication across a large nutrient pollution gradient in 100 European streams. Leaf breakdown was stimulated by low to moderate nutrient concentrations but was inhibited at high rates of nutrient loading. </jats:p>

<jats:title>Dynamic Responses</jats:title> <jats:p> Expression of the tumor suppressor p53 is activated in response to cell stress. The dynamics of p53 activation can vary, depending on the stressor, resulting in either pulsatile or constant p53 levels; however, the functional consequence of these different dynamics is unclear. <jats:bold> Purvis <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="1440" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1218351">1440</jats:related-article> ) developed a method to control p53 dynamics in human cells. Pulsing p53 selectively activated genes involved in cell cycle arrest and DNA repair, allowing recovery from DNA damage. In contrast, sustained p53 promoted induction of terminal genes leading to cellular senescence. Thus, protein dynamics can affect cell fate decisions. </jats:p>

<jats:title>To Silence or Not to Silence</jats:title> <jats:p> Repressed genes commonly have methylated DNA, and/or covalent histone modifications associated with silent chromatin, and/or associated small interfering (si)RNAs. All three features are components of gene-silencing systems (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6087" page="1391" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1224272">Jacob and Martienssen</jats:related-article> </jats:bold> ). In a screen for components of DNA methylation gene-silencing systems in the flowering plant, <jats:bold> Moissiard <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="1448" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1221472">1448</jats:related-article> , published online 3 May) identified the genes <jats:italic>AtMoRC1</jats:italic> and <jats:italic>AtMORC6</jats:italic> , which are homologs of the mouse <jats:italic>Microrchidia1</jats:italic> gene. AtMORC1 and AtMORC6 are involved in silencing transposable elements and genes corresponding to DNA-methylated loci, and yet neither gene is required for maintenance of DNA methylation. Instead, <jats:italic>AtMoRC1</jats:italic> and <jats:italic>AtMORC6</jats:italic> are related to proteins that remodel chromatin superstructure, and they seem to control gene-silencing through the higher-order compaction of methylated and silent chromatin. <jats:bold> Qian <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="1445" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1219416">1445</jats:related-article> ) identified an <jats:italic>Arabidopsis</jats:italic> gene, IDM1 (increased DNA methylation 1), that is involved in regulating DNA methylation at loci enriched for repeats and multigene families containing highly homologous genes. IDM1 protects target genes from DNA silencing and recognizes both histone H3 and methylated DNA at target loci and is able to acetylate histone H3. </jats:p>

<jats:title>To Silence or Not to Silence</jats:title> <jats:p> Repressed genes commonly have methylated DNA, and/or covalent histone modifications associated with silent chromatin, and/or associated small interfering (si)RNAs. All three features are components of gene-silencing systems (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6087" page="1391" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1224272">Jacob and Martienssen</jats:related-article> </jats:bold> ). In a screen for components of DNA methylation gene-silencing systems in the flowering plant, <jats:bold> Moissiard <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="1448" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1221472">1448</jats:related-article> , published online 3 May) identified the genes <jats:italic>AtMoRC1</jats:italic> and <jats:italic>AtMORC6</jats:italic> , which are homologs of the mouse <jats:italic>Microrchidia1</jats:italic> gene. AtMORC1 and AtMORC6 are involved in silencing transposable elements and genes corresponding to DNA-methylated loci, and yet neither gene is required for maintenance of DNA methylation. Instead, <jats:italic>AtMoRC1</jats:italic> and <jats:italic>AtMORC6</jats:italic> are related to proteins that remodel chromatin superstructure, and they seem to control gene-silencing through the higher-order compaction of methylated and silent chromatin. <jats:bold> Qian <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="1445" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1219416">1445</jats:related-article> ) identified an <jats:italic>Arabidopsis</jats:italic> gene, IDM1 (increased DNA methylation 1), that is involved in regulating DNA methylation at loci enriched for repeats and multigene families containing highly homologous genes. IDM1 protects target genes from DNA silencing and recognizes both histone H3 and methylated DNA at target loci and is able to acetylate histone H3. </jats:p>

<jats:title>COPy Coat</jats:title> <jats:p> COPI-coated vesicles are responsible for intracellular vesicular transport both within the Golgi and between the Golgi and endoplasmic reticulum. By applying subtomogram averaging from cryoelectron tomography data, <jats:bold> Faini <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="1451" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1221443">1451</jats:related-article> , published online 24 May) were able to describe the complete three-dimensional structure for COPI-coated vesicles generated in a cell-free, membrane-budding reaction. The structures of multiple individual vesicles reveal assembly principles based on interactions that, unlike those for clathrin-coated vesicles, are not regular: The basic subunit can undergo significant conformational changes and assemble with different stoichiometries. This variability may allow the regulation of membrane curvature and vesicle size. Furthermore, forming a complete closed coat was not required to produce budded vesicles. </jats:p>

<jats:title>Spatial Memory Perturbation</jats:title> <jats:p> The hippocampus is important for learning and memory. However, it is not clear which patterns of neural activity in the hippocampus support specific mnemonic functions. <jats:bold> Jadhav <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="1454" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1217230">1454</jats:related-article> , published online 3 May) developed a real-time analysis system to detect and selectively interrupt a certain type of hippocampal neuronal network event—sharp-wave ripples—during learning. In awake animals, loss of sharp-wave ripples and associated memory replay activity caused a learning deficit specific to spatial working memory but had no effect on reference memory. This learning deficit was present despite the preservation of place-field representations and replay activity during rest. </jats:p>

<jats:title>Controlling the Axon</jats:title> <jats:p> The cellular mechanisms and circuits involved in gamma oscillations in the brain are not fully understood. <jats:bold> Dugladze <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="1458" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1222017">1458</jats:related-article> ) simultaneously performed patch-clamp recordings in the soma and axon of hippocampal pyramidal neurons during gamma oscillations in brain slices. Under these conditions, pyramidal cells were divided into two electrogenic compartments: the soma fired at low frequency, whereas, in the axon, ectopic action potentials were generated at higher frequencies. This functional separation was maintained by highly active axoaxonic interneurons. Powerful inhibition of the axon initial segment by these axoaxonic cells prevented the backpropagation of ectopic action potentials to the somatodendritic compartment. However, when the overall excitatory drive to pyramidal cells was high, normal orthodromic action potentials were generated. </jats:p>