This Week in Science

Science  02 Apr 2010:
Vol. 328, Issue 5974, pp. 13
  1. Tailor-Made Tumor


      The biotrophic smut pathogen Ustilago maydis specifically infects the important crop plant, maize. The pathogen elicits large tumors on all aerial maize organs by redirecting primordia into a tumor pathway, and maize developmental mutants can disrupt tumor progression. Skibbe et al. (p. 89) examined gene expression in parallel in both the host plant and the smut pathogen and found that organ-specific gene expression patterns were required in both for tumor formation. Thus, fungal pathogens may exert distinct effects on different organs and tissues in plants, perhaps explaining the diverse pathologies that can be produced in diseased plants.

    1. On the Origins of Magnetism

        The magnetic fields in galaxies and galaxy clusters are thought to result from the amplification of weak primordial magnetic fields, which, according to one class of theories, should exist in the voids between galaxies and galaxy clusters. Neronov and Vovk (p. 73) present evidence for the existence of intergalactic magnetic fields and derive a lower limit for their strength, based on an analysis of data from the Fermi Large Area Telescope. The results place constraints on magnetogenesis models and suggest that magnetic fields originated in the early universe before galaxy formation took place.

      1. Cytomegalovirus Immune Evasion Strategy

          Cytomegalovirus (CMV) infects a large percentage of the world's population. Most of those infected are asymptomatic; however, CMV is a substantial public health concern for immunocompromised individuals and neonates. CMV is unusual in that it can superinfect: it re-infects hosts who are already infected with the virus, even in the presence of a strong, specific immune response. Hansen et al. (p. 102; see the Perspective by Hengel and Koszinowski) now find that in rhesus macaques, a good model for human CMV superinfection, CMV establishes superinfections by evading the immune response mediated by CD8+ T cells. A series of viral mutants deficient in expression of the US2-11 glycoproteins, which regulate antigen presentation to CD8+ T cells, revealed that, although able to establish the initial infection, these viral mutants were unable to superinfect. Depletion of CD8+ T cells from the monkeys allowed infection by the mutant viruses. These results highlight the difficulties in developing an effective protective vaccine against CMV itself, but suggest that CMV-based vectors may be useful in other vaccine efforts such as those against HIV.

        1. Thin Friction

            The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? Lee et al. (p. 76; see the Perspective by Müser and Shakhvorostov) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior.

          1. Oceans Before the Dawn

              Although the fossil record shows a clear explosion of animal diversity in the oceans at the beginning of the Cambrian period (∼542 million years ago), the evolutionary pressures driving this increase in diversity remain unclear. The likely scenario involves drastic changes in the distribution of oxygen in ocean basins, but global glaciations and poor preservation of rocks from this time often prevent a clear picture of global ecology before the dawn of animal life. Li et al. (p. 80, published online 11 February; see the Perspective by Narbonne) characterized the geochemical makeup of sedimentary rocks from south China that indicate alternating layers of sulfide- and iron-rich (i.e., sulfate-limited) anoxic waters in the several million years leading up to the Cambrian. These conditions may have set the stage for an eventual increase in productivity of photosynthetic life that oxygenated the oceans and allowed for rapid animal evolution.

            1. Forming Antimatter Nuclei

                CREDIT: CHEN ET AL.

                Atomic nuclei are everywhere and form all the matter visible to us in the universe. Their counterparts, however—antinuclei or antimatter—are relatively shy about making an appearance. Chen et al. (p. 58 published online 4 March; see the Perspective by Cohen) used the Relativistic Heavy-Ion Collider to coax them into existence by colliding high-energy beams of gold ions head-on. Within the debris from the collisions, evidence was found in the decay paths and particle tracks that suggested the formation of nuclei from antimatter. The ability to form these exotic particles in abundance should help to probe fundamental aspects of nuclear physics, astrophysics, and cosmology.

              1. The Size of the Father

                  Evolutionary biologists have recently discovered an intriguing phenomenon: High-fitness males often sire low-fitness daughters. The emerging consensus is that this sexual conflict overwhelms the benefits of mate choice for “good genes,” thus challenging a central tenet of evolutionary theory. Cox and Calsbeek (p. 92, published online 4 March) provide a counterpoint by showing that female brown anole lizards can obtain genetic benefits by biasing offspring sex to produce disproportionately more sons with high-fitness (large) sires. The advantage of this sex-ratio bias was confirmed by tracking the fitness of sons and daughters in their natural environment, illustrating the importance of sexual conflict in the wild. Thus, because maximal fitness payoffs can be achieved by shifting offspring production from daughters to sons as sire size increases, mate choice can overcome antagonistic fitness effects.

                1. Histone Inheritance

                    Chromatin, the packaging material for eukaryotic genomes, is a potential repository for epigenetic information. The core structure of chromatin is the nucleosome, which consists of an octamer of histone proteins, two dimers each of histones H2A and H2B, and histones 3 and 4. Histones 3 and 4, in particular, carry a series of covalent modifications presumed to be passed on through cell division. Using mass spectrometry of tagged and isotope labeled histones, Xu et al. (p. 94; see the Perspective by Ray-Gallet and Almouzni) followed the inheritance of the histones themselves through mitosis. The H2A-H2B dimers were inherited randomly through cell division, correlating with their lack of major covalent marks. In comparison, replication-deposited H3.1-H4 dimers did not separate through cell division, implying that H3 and H4 histone modifications might be maintained by copying from neighboring preexisting histones. Intriguingly, up to one-quarter of the nonreplication-deposited H3.3-H4 dimers, which mark active chromatin, did split during cell division.

                  1. Sperm Production-Line Maintenance

                      CREDIT: NAKAGAWA ET AL.

                      The average man makes upwards of 1500 sperm per heartbeat. Such a feat requires a robust stem cell system. Using mice, Nakagawa et al. (p. 62, published online 18 March) shed light on some of the properties of the murine germline stem cell system that contribute to its robustness. During steady-state spermatogenesis, the majority of the stem cell population lies within a subset of cells called type A spermatogonia. However, during regeneration (for example, during recovery of the stem cell pool after drug exposure) the system in essence hijacks early-differentiating cells back into the stem cell compartment. Lineage analysis and live-imaging also suggests that during sperm production there is more than one path from a stem cell to differentiation.

                    1. Open Sesame

                        The proteosome plays a key role in cellular homeostasis through catalyzing protein degradation. It is a barrel-shaped nanomachine whose activity is regulated through gating substrate entry. Religa et al. (p. 98) now show that N-terminal gating residues in the proteasome interconvert on a second's time-scale between conformations that place them either outside or inside the proteasomal antechamber. An increase in the number of termini occupying the “in” state decreased rates of hydrolysis. Furthermore, proteasome activators known to increase the proteolysis rate lead to an increase in the number of termini in the “out” state.

                      1. Precise and Efficient Cortical Communication

                          How do thalamic neurons reliably transmit information to the cerebral cortex, despite making few synaptic connections onto their target neurons? Correlated input spikes from the thalamus have been reported, but the number of synchronous inputs and their precision and reliability is unclear. Wang et al. (p. 106) analyzed a unique data set of simultaneous thalamic and cortical recordings in vivo and compared them with simulations of a model cortical neuron. Using the recorded spike trains as inputs to the model, and constraining the model by the measured output spike trains, allowed strong predictions of the degree of convergence of synchronous inputs from the lateral geniculate nucleus. Synchrony, rather than the strength or frequency of synaptic inputs, was key to signaling, and the data suggest that there may be a region of optimal synchronous coding.

                        1. Open and Closed Case

                            Voltage-dependent ion channels are gated by voltage sensors that show a switchlike response to voltage differences across the membrane. Tao et al. (p. 67; see the cover) used mutagenesis, electrophysiology, and x-ray crystallography to gain insight into the molecular basis of this response in voltage-dependent potassium channels. An occluded site was identified that catalyzes translation of positive charges across the membrane. The closed channel appears to be associated with a distribution of conformations, depending on the degree of hyperpolarization of the membrane, whereas the open channel appears to be associated with a specific conformation. Thus, the transition of the ion channel from open to closed occurs over a very small voltage difference.

                          1. Sinking Sea Floors

                              The depths of ocean bottoms are constantly fluctuating at a very slow rate in response to the generation (at mid-ocean ridges) and consumption (at subduction zones) of sea-floor material. Because older sea floor is susceptible to sinking as it cools, it has been assumed that sea-floor depth varies directly with its age. However, Adam and Vidal (p. 83; see the Perspective by Tolstoy) now show that the depth of the Pacific Ocean actually varies in response to the mantle underlying the oceanic crust. This effect is clear when sea-floor depth was measured along lithospheric flow lines—which represent the movement of oceanic crust triggered by mantle convection. Because the ocean bottom does not flatten as predicted by previous models, there is no need to invoke any additional heat supply to sustain old oceanic crust in thermal models of the mantle.

                            1. Flower Power

                                The transcription factor APETALA1 (AP1) controls the transition from vegetative growth to flower production in the plant Arabidopsis. A handful of factors that control AP1 have been identified, as well as some targets that AP1 controls. Kaufmann et al. (p. 85) now apply genome-wide microarray analysis to identify over a thousand genes whose transcription is regulated by AP1. By proximity to AP1 binding sites, over two thousand genes are implicated as putative AP1 targets. Analysis of this network of interactions indicates that AP1 functions first to repress vegetative identity, then to help establish floral primordia, and finally to shape the differentiation of floral parts.