This month in Science Roundup:
Special Online Collection
An awareness of one’s position and the relationships of others in a group is a rarity among vertebrate species. In a special section of the 7 Sep 2007 Science explored the adaptive advantages of group life and the accompanying development of social skills in human groups and those of our primate cousins. News stories highlighted studies of apes in their natural environments that are providing clues about the evolution of social cognition, and the use of computer-generated realities to explore human behavior and complex social interactions. A series of Review articles dissected the human capacity for prospection and the links between sociality and brain evolution and fitness. Online, a podcast segment discussed the social abilities of children and chimps, and a special video feature explored the value of online "virtual worlds" to studies of social psychology.
Modeling Ethnic Violence
Ongoing conflict between culturally or ethnically distinct groups has led to the deaths of tens of millions of people over the last 100 years. Policy makers hoping to prevent further bloodshed are faced with the difficult challenge of knowing when or where violence will next erupt. Now, in a Report in the 14 Sep 2007 Science, Lim et al. describe the development of computer model based on the principles of phase separation to study patterns in global populations that could help predict where violence might occur. The model posits that violence arises due to the structure of boundaries between groups rather than as a result of inherent conflicts between the groups themselves. In general, highly "mixed" regions are unlikely to engage in violence because public spaces are not typically identified with one cultural group or another. Violence is also unlikely in regions where groups are well separated because the groups do not impose on each other. However, partial separation with poorly defined boundaries -- which enables groups to impose cultural norms on public spaces -- fosters conflict. The model accurately predicted the locations of reported conflict in the former Yugoslavia and in India, which suggests that science may help inform policy debates about how to stop or prevent ethnic conflict. Dr. Bar-Yam, senior author on the study, discussed the work in a related podcast interview.
Clues to Honeybee Collapses
The sudden disappearance of bee colonies in the United States, a phenomenon termed colony collapse disorder (CCD), was first reported in 2006 and has caused losses of up to 90% of hives in some apiaries. Pathogens, pesticides, and the artificial diets fed to bees have all been blamed, but definitive evidence of a culprit has been lacking. In a study published online Science Express on 6 Sep 2007, Cox-Foster et al. used a metagenomic approach to identify a new suspect: Israel acute paralysis virus, or IAPV. Sequence analysis of sick and healthy bees revealed an array of bacteria, fungi, and viruses in all of them. Notably, IAPV was found in 25 of the 30 sick colonies sampled, but in just one of the 21 healthy colonies, suggesting that it is a good marker of CCD (though it remains unclear whether IAPV infection is a consequence or cause of the disorder). As noted in an accompanying News story by E. Stokstad, the researchers point to Australia as a possible source of IAPV. All of the operations infected with the virus had either imported bees from Australia or stored their hives close to other operations with Australian bees, where as the healthy operations had no Australian bees.
How Platelets Form
Platelets -- irregularly shaped and short-lived blood components -- are essential to the clotting process and therefore act as the body’s first line of defense against uncontrolled bleeding. They are produced by large bone marrow cells called megakaryocytes, but the details of this presumed dynamic process have proven difficult to decipher using in vitro and static imaging approaches. Junt et al. filled in some of the missing blanks in a Report in the 21 Sep 2007 Science. Using live imaging with multiphoton intravital fluorescence microscopy, the researchers were able to illuminate the behavior of active platelet-producing megakaryocytes within the bone marrow of mice. The megakaryocytes were observed in close contact with the sinusoids (small blood vessels) of the bone marrow and routinely extended long "proplatelet" protrusions through the sinusoidal epithelial cells, and into blood vessels. The hydrodynamic force of flowing blood appears to shear these protrusions off, releasing proplatelets and platelets into the bloodstream. An accompanying Perspective by A. E. Geddis and K. Kaushansky highlighted the study.
Magnetic Brain Stimulation
In recent years, neuroscientists and psychiatrists have touted the potential uses of a noninvasive brain stimulation technique called transcranial magnetic stimulation (TMS). It is an increasingly common technique used to selectively modify neural processing, and it has shown promise in clinical trials for treating psychiatric disorders such as depression, yet little is actually known about how it works and its physiological effects. In a study reported in the 28 Sep 2007 Science, Allen et al. applied TMS to the visual cortex of anesthetized cats while simultaneously measuring neural activity and tissue oxygen (an indicator of energy consumption). Among other results, the researchers found that a train of short TMS pulses caused an immediate increase in neuronal firing that lasted for about a minute, followed by suppression of neuronal responses for several minutes. TMS also disrupted the temporal structure of activity by altering phase relationships between neural signals. The neural changes were faithfully reflected in hemodynamic signals (changes in blood flow and oxygenation), which can be detected by standard neuroimaging techniques such as functional magnetic resonance imaging (fMRI). As noted in an accompanying News story by G. Miller , the new findings, and future investigations of this type will have important implications for how TMS is used in people.
Imaging Nitrogen Fixation
Biological nitrogen fixation -- the conversion of atmospheric nitrogen to ammonia for biosynthesis -- is performed by free-living bacteria and by symbionts of algae, higher plants, and some animals. Despite the essential importance of this process to human nutrition and global ecosystems, researchers have been unable to quantify the incorporation of nitrogen by individual bacteria or to map the fate of fixed nitrogen -- until now. In a Report in the 14 Sep 2007 Science, Lechene et al. used a technique called multi-isotope imaging mass spectrometry (MIMS) to directly image and measure nitrogen fixation by individual bacteria that inhabit the gills of the shipworm *Lyrodus pedicellatus*, a wood-eating marine bivalve. Growing the shipworms in seawater enriched in the rare stable isotope 15-N enabled the researchers to measure nitrogen incorporation, in both bacterial cells and in bacteria-free host tissue, by comparing quantitative mass images and determining the increase in 15-N/14-N ratios relative to the natural abundance ratio. Their data provide strong evidence that newly fixed nitrogen is used by shipworm cells for biosynthesis. In an accompanying Perspective highlighting the Report, M.M.M. Kuypers noted that MIMS technology "is poised to reveal the metabolic diversity of the planet’s microorganisms, 99% of which has eluded cultivation."
A Drier Mars
Special Online Collection
The story of water on Mars has a checkered past. The arrival of the first spacecraft showed that the red planet was bone dry, but subsequent missions have found evidence of ancient salty seas and gushing gullies. Or have they? As described in a special collection of Reports in the 21 Sep 2007 issue, new details afforded by the Mars Reconnaissance Orbiter (MRO) -- the latest arrival on Mars -- are bringing into question many earlier geologic interpretations involving surface water (see the related News Focus by R. A. Kerr. MRO’s High Resolution Imaging Science Experiment (HiRISE), which can image features as small as a half-meter in size, has revealed a plethora of boulders ranging up to about 2 meters in diameter in the middle to high latitudes, which include deposits previously interpreted as fine-grained ocean sediments or dusty snow. Other images of supposed ancient ocean floors and riverbeds show no obvious signs that liquid water was ever present, and reexamination of some landforms implies that they have been formed by flowing lava, not water. The mission has not been devoid of aqueous evidence, however. Features on the rims of impact craters and some gullies indicate the presence of liquid water in the recent past, and radar gravity data show that the cap on Mars’ south pole now holds the largest reservoir of relatively pure water ice on the planet. HiRISE leader Dr. Alfred McEwen discussed the findings in a related podcast interview.
Dark Matter and Early Stars
Standard models of early star formation posit that the first stars were seeded by clumps of slow-moving, cold dark matter that pulled in and condensed nearby clouds of gas by gravitational force. In a Report in the 14 Sep 2007 Science Gao and Theuns presented supercomputer simulations that challenge that view and show that the stars’ properties depend critically on the currently unknown nature of the dark matter itself. The new model includes a "warmer" form of dark matter comprising faster-moving particles. Instead of clumping, this warm dark matter would have first stretched into massive filament-like structures and then fragmented to form stars with a range of masses. The simulations further suggest that coalescence of fragments and stars during the filament’s ultimate collapse may seed the supermassive black holes that lurk in the centers of most massive galaxies. As noted in an accompanying Perspective by V. Bromm, "[t]he macrophysics of early star formation might thus hold important lessons for the microphysics of exotic elementary particles." Dr. Theuns discussed the work in a related podcast interview.
Ancient Polynesian Voyaging
Polynesian legends have described epic canoe journeys from Hawai’i to what is now French Polynesia (a group of islands that includes Tahiti and the Tuamotu Archipelago) long before European settlement, but evidence for these long-distance voyages has been lacking. Now, in a Report in 28 Sep 2007 Science, Collerson and Weisler present trace element and isotope analyses of stone wood-working tools called adzes that lend credence to the legendary tales ( listen to a related podcast interview with Dr. Collerson). The team sampled 19 basalt adzes collected more than 70 years ago from the Tuamotu islands and found geochemical data linking many of them to nearly volcanic islands. Notably, isotope and trace element chemistry uniquely links one adze to the Hawaiian island of Kaho’olawe, some 4000 kilometers away. Other adzes collected from the low coral islands of the northwest Tuamotus have sources in the Marquesas, Austral and Society Islands, and the Pitcairn Group, confirming that trade was widespread within East Polynesia. An accompanying Perspective by B. Finney discussed the new findings in light of other efforts to trace ancient Polynesian migration paths over the last two centuries.
Muscular Thin Films
The idea of engineering body parts to replace ones worn out or damaged by aging and disease is no longer the stuff of science fiction. Tissue scientists have already made considerable progress in growing engineered skin and cartilage, but engineering muscle has proven more difficult, in large part because of the complex three-dimensional structure of muscle tissue. In order to mimic muscular action, researchers must align everything from motor proteins within cells on the nanoscale to much large muscle fibers and bundles of fibers on the centimeter scale. Now, in a Report in the 7 Sep 2007 Science, Feinberg et al. have described the creation of muscular thin films consisting of rat heart muscle cells grown on flexible, centimeter-scale plastic that can carry out a wide range of motor functions. The researchers controlled the tissue shape by prepatterning the plastic film with microcontact-printed lines of fibronectin, an extracellular matrix protein to which the heart cells attached. Based on how the team patterned the fibronectin and the thickness of the plastic, they were able to make an array of devices that could perform functions as diverse as gripping, pumping, walking, and swimming. A ScienceNOW story published on 6 Sep by R. F. Service highlighted the study.
Curious Quantum Arithmetic
In ordinary arithmetic, multiplication obeys what is called a commutative law: for any two numbers N and M, the product NM is always equal to MN. In classical physics, measurements of physical properties like a particle’s position and momentum also obey a commutative law. In a Report in the 28 Sep 2007 Science, Parigi et al. presented the results of laboratory experiments demonstrating that quantum mechanical quantities behave rather differently. The researchers showed that adding or subtracting a single photon to or from a light field produces a different result depending on the order in which the two actions are performed, with the final state of light field being different from the initial one. Under certain conditions, the removal of a photon can even lead to an increase in the mean number of photons in the light field -- a result that has previously been predicted. An accompanying Perspective by Boyd et al. noted the ability to "engineer" the quantum state of light may prove useful in areas such as quantum computing and quantum cryptography.
In Science’s STKE
Interferon at 50
Among the body’s arsenal of pathogen fighters are a group of proteins called interferons (IFNs), which were first described 50 years ago, and which play key roles in antiviral defense and immune regulation. In addition to their broad-spectrum antiviral activity, type I IFNs have antiproliferative, antitumor, and immunomodulatory effects. The broad cellular target range of these molecules underlies their therapeutic use as antiviral and antitumor agents, but also elicits side effects that limit clinical use (all type 1 IFNs signal through a common receptor). In a Perspective in the 25 Sep 2007 issue, J. A. Langer discussed how a growing understanding of IFN-receptor interactions is opening the door to enhanced therapeutic possibilities -- including the development of variant IFN proteins with novel activity profiles -- while raising interesting mechanistic questions.
Also in STKE this month:
--J.M.W. Slack highlighted recent work showing that a proton pump is necessary for Xenopus tadpole tail regeneration (25 Sep 2007)
--Agostinis et al. looked at how the aryl hydrocarbon receptor mediates the cellular response to ultraviolet B radiation (11 Sep 2007)
--Larson et al. discussed the role of Toll-like receptors in the development of the central nervous system and in the response to injury (4 Sep 2007)