Complex Systems and
Networks
Special Online
Collection
Network analysis has emerged as a powerful way of studying phenomena as
diverse as interpersonal interaction, cell circuitry, and the structure
of the Internet. A special section of the 24 Jul 2009 Science
explored how scientists are pushing network analysis to its limits
across disciplinary fields and allowing us to understand how the world
works from new vantage points. News stories looked at the application
of network analyses to societal challenges from traffic jams and the
spread of infection, to terrorism. A Review article explained how
molecular biologists are using networks to analyze basic cellular
circuitry and a series of Perspectives revisited the discovery of
scale-free networks and the foundations of effective network analysis,
and examined what networks can teach us about the interactions that
underlie biodiversity, sustainability, economics, transportation
systems, and epidemiology. Online, Science Careers highlighted
careers in network science, and the 28 Jul issue of Science
Signaling looked at the complexity of cellular signaling, and efforts
to understand both the macroscopic cellular response and the
microscopic signaling network and its constituents and dynamics.
Patterns of Pacific Warming
The El Niño Southern Oscillation (ENSO) is a quasi-periodic
cycle of warming and cooling in the eastern, near-equatorial Pacific
Ocean that is associated with marked ocean temperature changes in the
tropics and with long-range weather connections across the globe
including tropical cyclone activity in the Atlantic Ocean. In the 3 Jul
2009 Science, Kim et
al. report that warming in the eastern tropical Pacific is not the
only recurring pattern of sea-surface temperature variability in the
Pacific. The team reports that extensive warming in the central Pacific
also occurs on a quasi-periodic basis, has a large effect on
atmospheric circulation, and is more predictable than El Niño.
These central Pacific warming events have become more frequent in the
last few decades and are also associated with increased tropical
cyclone activity (in contrast to El Niño). An accompanying
Perspective by G. J.
Holland noted that a better understanding of eastern Pacific and
central Pacific warming modes and how they interact may lead to useful
improvements in predicting seasonal Atlantic tropical cyclones and
other impacts of these coupled atmosphere-ocean phenomena.
Phoenix on Mars
The Mars Phoenix Lander -- a spacecraft outfitted with a robotic arm
for excavation and instruments for chemical and geological analysis --
landed on Mars in May 2008 with the goal of studying the ice-rich soil
of the planet's northern artic region. Four Reports in the 3 Jul 2009 Science
described data that Phoenix returned during its five months of
operation, including evidence of water ice and surface alteration by
liquid water. Smith
et al. described a layer of ice found 5 to 18 centimeters beneath
the surface and alkaline soil containing salts and aqueous minerals,
the formation of which likely required the presence of water. Boyton
et al. reported evidence of calcium carbonate at the Phoenix
landing site, which they surmise formed during past interaction of
atmospheric carbon dioxide with liquid water films on particle
surfaces. Aqueous chemical analyses of the martian soil described by Hecht
et al. revealed that most of the soluble chlorine at the surface is
in the form of perchlorate, which may play a role in controlling soil
and atmospheric water content. Finally, Whiteway
et al. described water-ice clouds, similar to cirrus clouds
on Earth, detected in the atmosphere of Mars by laser remote sensing.
These clouds formed during the day and precipitated to the ground each
night, providing a mechanism to place ice that the red planet's surface.
"Swine" Flu Pathology
Since its identification in April 2009, the swine-origin 2009 A(H1N1)
influenza virus has continued to circulate in humans worldwide, and in
early June, the World Health Organization determined that the criteria
for a pandemic had been met. Now, two studies in the 24 Jul 2009 Science
(published online 2 Jul) have used ferrets, an established model for
human influenza, to evaluate the pathogenesis an transmissibility of a
selection of 2009 A(H1N1) virus isolates compared with seasonal H1N1
viruses.
Munster et al. and Maines
et al. found that both seasonal and pandemic H1N1 viruses infect
ferrets' nasal cavities, but that only the pandemic virus penetrates
into the lungs. Maines et al. also found the pandemic virus in the
ferret intestinal track, a finding that may help explain the
gastrointestinal distress and vomiting reported in human patients. By
placing infected and healthy ferrets in cages next to one another, the
teams discovered that the new virus can be transmitted through the air,
but the results were mixed: Munster et al. found that the virus spreads
equally as well as the seasonal strain, whereas Maines et al. found
that it is a less efficient transmitter. Despite the variable results,
ferret models will be important for continued monitoring of 2009
A(H1N1) and its pathogenicity and transmission, as well as for future
testing of drugs or vaccines. A News
story by M. Enserink, published in the 3 Jul issue, highlighted the
studies.
Restoration Ecology
Special
Issue Introduction
Our planet’s future may depend on the maturation of ecological
restoration -- a young discipline focused on the repair and management
of damaged or degraded ecosystems. In a special section of the 31 Jul
2009 issue, Science highlighted restoration projects around the
world and considered the state of the science in this emerging field.
Perspectives discussed paleoecology as a component of restoration
science, limits to remediation posed by invasive species, the roles of
soil microbes and of pollination in restoring ecosystems, and how to
assess restoration effectiveness. News stories, meanwhile, examined
efforts to restore damaged coral reefs, to fight off invasive species
in South Africa, and to balance ecosystem complexity and economic
reality in the forests in China and Borneo and rubber plantations in
Southeast Asia. Finally, three research papers contributed to our
understanding of restoration challenges and successes: a Research
Article evaluated current efforts to restore marine ecosystems and
rebuild global fisheries and two Reports published online, ahead of
print, in Science Express described the successful restoration
of native oysters in the Chesapeake Bay and a meta-analysis of 89
restoration projects worldwide showing overall positive impacts on both
biodiversity and the provision of ecosystem services.
Flow at the Edge
Solid materials have historically been divided into conductors and
insulators. However, researchers have recently described a new state of
matter, called topological insulators, in which the bulk material is an
insulator, but with a metallic surface that can conduct a flow of
current owing to unique spin-orbit interactions in the material. In a
Research Article in the 17 Jul 2009 Science, Roth
et al. showed that this current flow takes place in so-called edge
states around the boundary of the sample without the need for an
applied magnetic field. The team demonstrated this effect using a
voltage probe in a sample consisting of quantum wells of mercury
telluride. The system is analogous to the current transport seen in
two-dimensional electron gases in the presence of high magnetic fields.
An accompanying Perspective
by M. Büttiker noted that edge states can be used to direct
electrons from one place to another, in a manner similar to directing
beams of photons with optics, and therefore hold great promise for
quantum information processing.
Iridescence with a
Twist
The beautiful iridescent colors of beetles, butterflies, sea creatures,
and birds have long fascinated scientists. These animals derive their
color from the interaction of light with the structure or morphology
that is imprinted on their external surfaces. In a Report in the 24 Jul
2009 Science,
Sharma et al. examined the structural basis of iridescence in
the green metallic scarab beetle Chrysina gloriosa, which
selectively reflects color from its exoskeleton in the form of
left-handed circularly polarized light when illuminated with
unpolarized light. Bright field microscopy shows that the beetle
exoskeleton is organized primarily of micrometer-scale hexagonal cells,
though pentagonal and heptagonal cells also appear depending on local
body curvature. Each cell contains a series of upwardly spiraling
concentric layers, which leads to the unusual light polarization. This
arrangement of nearly concentric nested layers is analogous to the
ordering of molecules in chiral nematic liquid crystals, which have
been widely studied for their possible use in photonic devices.
Although the biological function of circularly polarized reflection in
these beetles remains unknown, an accompanying Perspective
by P. Vukusic noted that intraspecific communication is a
possibility, citing recent evidence for such signaling in an iridescent
marine crustacean.
Starved to Life?
Restricting caloric intake by about 30% has been shown to delay aging
and increase life span in organisms including yeast, worms, and
rodents. A new Report in the 10 Jul 2009 Science now extends
these findings to primates.
Colman et al. reported that rhesus monkeys subjected to caloric
restriction as adults and followed for the last 20 years, showed
decreased mortality and delayed onset of age-related diseases when
compared to normally fed control animals. Specifically, caloric
restriction (CR) reduced the incidence of diabetes, cancer,
cardiovascular disease, and brain atrophy. The restricted monkeys were
given vitamin and mineral supplements to ensure they did not suffer
malnutrition and were treated if they became sick. At the time of
reporting, 50% of the normally fed animals had survived, compared with
80% of the CR animals. A related ScienceNOW
story by M. Torrice commented that "[t]he results come as close as
any can to proving that calorie restriction could significantly slow
aging in humans -- even if such a lean diet would not appeal to most of
us."
Shrinking Sheep
On the remote Scottish island of Hirta, wild Soay sheep have been
getting smaller, shrinking an average of 5% over the last 25 years. In
a Report in the 24 Jul 2009 Science (published online in ScienceExpress
on 2 Jul), Ozgul
et al. argue that climate change is to blame. The team analyzed
body-weight measurements and life-history data for the female sheep in
Hirta, plugging their data into a numerical model that predicts how a
heritable trait such as body size will change over time due to natural
selection and other factors that influence survival and reproduction in
the wild. Their results suggest that the decrease in body size is
primarily an ecological response to environmental variation and that
evolutionary change has contributed relatively little. Hirta has had
unusually short and mild winters over the past quarter century. Thus,
the researchers propose that as result, grass has become available for
more months of the year, meaning that the sheep need not bulk up as
much as before. In addition, Hirta's harsh winters used to kill small
ewes born to young mothers, but now these small ewes survive -- and
because of their low birth weight, they never get as big as normal
sheep. A ScienceNOW
story by N. Siva highlighted the findings.
Sunspot Structure and
Dynamics
Sunspots are large concentrations of strong magnetic field in the solar
atmosphere. They consist of a dark central umbra and a brighter,
filamentary surrounding penumbra. In a Report in the 10 Jul 2009 Science,
Rempel
et al. (published online 18 Jun) presented comprehensive
numerical simulations that help to address fundamental questions about
the fine structure of sunspots and their surroundings (see the related Perspective
by G. Scharmer). The simulations show the development of an outer
penumbra with systematic radial outflows along channels of nearly
horizontal magnetic field in regions where the average field
inclination is greater than 45 degrees. The outflows result from rising
hot plasma that turns over and is guided outward by the strong and
inclined magnetic field. The simulations are consistent with
observations of global sunspot properties, penumbral structure, and
systematic radial outflows, and show that these features can be
understood in terms of convective flow in a magnetic field with varying
inclination.
Getting at Neandertal DNA
Analysis of Neandertal DNA holds great potential for investigating the
population history of this group of extinct hominins, but progress has
been limited due to the rarity of samples and damaged state of
recovered DNA. Now,
Briggs et al. report on a method of targeted ancient DNA sequence
retrieval that circumvents the high cost and technological difficulties
associated with shotgun sequencing and PCR-based approaches. As
described in the 17 Jul 2009 Science, the team used this method
-- which employs probes that specifically isolate Neandertal
mitochondrial DNA (mtDNA) from complex libraries of degraded DNA
isolated from Neandertal bones -- to reconstruct the complete mtDNA
genomes of five Neandertals. The bone samples ranged in age from 38,000
to 70,000 years old and were collected from 4 sites across Neandertals'
geographic range (from Spain to Russia). Analysis of the genomes shows
that genetic diversity among Neandertals was approximately one-third of
that in modern humans. Calculations based on this diversity estimate
indicate that the effective population size of Neandertals (the number
of breeding individuals) was small and may have included fewer than
3500 breeding females. An accompanying News
story by E. Pennisi highlighted the study.
Turtle Body Planning
The turtle shell offers a fascinating case study of vertebrate
evolution based on the modification of a common body plan. The turtle
body plan is unusual in that the ribs are transformed into the dorsal
part of the animal's shell (carapace), and the scapulae or shoulder
blades that are situated outside the ribs in other animals, are found
inside the carapace. In a Report in the 10 Jul 2009 Science, Nagashima
et al. offered insights into how turtle shell evolution might
have occurred. Using whole-mount immunostaining, three-dimensional
reconstructions, and markers for early skeletal precursors, the team
compared the muscular and skeletal changes that take place during
development in chicken, mouse, and Chinese soft-shelled turtle embryos.
Initially, embryos of the three animals share a common developmental
pattern -- one that is likely to have been shared with their last
common ancestor. This pattern, however, is modified in the turtle by a
specific folding of its body wall during embryogenesis such that rib
growth is restricted to the horizontal plane of the carapace and over
the shoulder blades. This folding preserves some of the "ancestral"
connections between skeletal and muscle elements but also produces new
turtle-specific attachments associated with shell formation. An
accompanying Perspective
by O. Rieppel and a ScienceNOW
story by H. Fields highlighted the study.
Tracing Mantle Oxidation
The chemical composition of the Earth’s mantle varies with tectonic
setting. For example, magmas near subduction zones -- convergent
tectonic plate boundaries where the crust is recycled into the mantle
-- are more oxidized than magma near divergent plate boundaries. In a
Report in the 31 Jul 2009 Science, Kelly
and Cottrell used highly sensitive synchrotron-based analytical
methods to investigate this disparity. They found that the oxidation
state of iron increases toward subduction zones and correlates linearly
with water content and the concentration of certain trace elements.
These results suggest that fluids released from wet subducting plates
drive mantle oxidation at subduction zones and help explain the spatial
differences in the mantle’s oxygen content. An accompanying Perspective
by M. M. Hirschmann discussed the findings, noting that "the long-term
consequences for the evolution of Earth remain poorly understood."
In Science Signaling
Focus on Neurodegeneration
Neurodegenerative diseases exact a tremendous toll on afflicted
individuals and their families. Understanding the underlying signaling
cascades that are perturbed by or contribute to neurodegeneration may
identify new therapeutic targets or improve current therapies. A
special
focus issue of Science Signaling published on 21 Jul 2009
brought together research and commentary on the cell signaling aspects
of three common and devastating neurodegenerative disorders. A Research
Article by Santini
et al. and related Perspective
by Klann explained how blocking mTOR signaling may decrease the
unwanted involuntary movements associated with L-DOPA treatment of
Parkinson's disease; a Perspective
by Badtke et al. looked at amyloid formation, which has traditionally
been associated with plaque formation in Alzheimer's disease, but more
recently shown to be an integral part of normal cellular physiology;
and a podcast
segment discussed how a small G protein may limit the toxicity of
mutant huntingtin protein to the brain in Huntington's disease.
Also in Science Signaling
this month:
-- Xie
et al. reported that hypoxia inhibits down-regulation of adrenergic
signaling, which regulates cardiovascular and pulmonary function (7 Jul
2009)
-- Akiyama
et al. demonstrated that asymmetric distribution of IP3 signals is
required for neuronal growth cone navigation (14 Jul 2009)
-- A
focus issue on complexity highlighted computational and
experimental approaches to understanding the complexity of cellular
signaling events (28 Jul 2009)
Image credits (in order of appearance): Science cover, 24 July
2009; NASA/JPL-Caltech/University of Arizona/Texas A&M/M. T.
Lemmon; Grand Tour/Corbis; P. Vukusic/University of Exeter; Rempel et
al., Science 325, 171 (2009); Hiroshi Nagashima
