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Science Roundup - a monthly newsletter for AAAS
Members - 30 January 2004
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This month in Science Roundup:
A
Survey of Salmon Safety
Multiple Ebola
Epidemics Devastated Apes
Imprinting Across Plant
Generations
The Dynamic X
Chromosome
Running Energetics
Consciously Forgetting
Monkey Language Limits
Humans Braved the
Arctic Early
Mesoporous
Silicate Films from Supercritical Fluids
Reliable Radiocarbon
Dating
The Galactic Habitable
Zone
Stardust Clues to
Stellar Evolution
Remaking NASA
A Survey of Salmon Safety
Global salmon consumption has increased annually during the last two
decades, undoubtedly spurred by the fish's noted benefits in reducing
cardiovascular disease and providing a healthy source of protein and vitamin D.
But should fishmongers beware? More than half the salmon sold annually is
farm-raised, and previous small-scale reports have hinted that these fatty
fish, which feed on other fish, may also be "bioaccumulating"
pollutants. In the 9 Jan 2004 Science, Hites
et al. (http://www.sciencemag.org/cgi/content/short/303/5655/226)
reported on the largest survey yet of contaminants found in salmon. The team
sampled some 700 salmon from around the world for more than 50 pollutants
including polychlorinated biphenyls (PCBs), which have been shown to cause
cancer in some animals. They found that farmed fish have higher levels of toxic
pollutants than wild salmon caught in the ocean and that the source, as many suspected, is the feed. Salmon farmed in Europe had the
highest levels, followed by those in North America, whereas Chilean salmon were
the cleanest. As noted in an accompanying News story by E. Stokstad
(http://www.sciencemag.org/cgi/content/short/303/5655/154a),
some researchers are taking the results as evidence that eating the wrong kind
of fish has real dangers, while other experts believe the risk is outweighed by
the health benefits of eating farmed salmon.
Multiple Ebola Epidemics Devastated Apes
The Ebola virus is a truly horrific killer, inducing raging fevers and
widespread hemorrhaging, and -- in it most lethal guise -- killing more than 80%
of its victims. Since the 1995 outbreak in Zaire that grabbed worldwide
attention, central Africa has suffered nearly a dozen more deadly outbreaks. In
addition to the obvious human toll, many researchers estimate that Ebola has
killed thousands of great apes in recent years and may push them close to
extinction within the next decade. As a result, public health experts and
conservationists alike are urgently trying to pinpoint the source of the
continuing epidemic. In a study reported in the 16 Jan 2004 Science,
Leroy et al. (http://www.sciencemag.org/cgi/content/short/303/5656/387)
tested tissue samples from human and animal victims of five outbreaks in
western central Africa (between 2001 and 2003). They found that each outbreak
was caused by a genetically distinct virus, and that many localized epidemic
chains could be distinguished. Thus, a large proportion of the ape populations
in this region have probably died as a result of multiple rounds of Ebola virus
infection in the past four years. As noted in an accompanying News story by G.
Vogel (http://www.sciencemag.org/cgi/content/short/303/5656/298a),
some think the geographic pattern of outbreaks suggests that apes are catching
the disease primarily from other apes, while others argue that an unidentified
natural carrier like bats may be a source of new infections. Nevertheless, both
sides seem to agree that surveillance of animal mortality could help to predict
and prevent future human Ebola outbreaks.
Imprinting Across Plant Generations
Genetic imprinting is an intriguing mechanism for controlling gene
expression. whereby one of the two copies (alleles) of
a gene received from the mother and father is silenced in the embryo. DNA methylation -- the "tagging" of certain
nucleotides with methyl groups -- plays a crucial role in determining this
"epigenetic" state in both animals and plants. In animals, imprinting
is erased and re-established in each generation; in flowering plants, however,
epigenetic states can be inherited over many generations. In a report in the 23
Jan 2004 Science, Kinoshita et al. (http://www.sciencemag.org/cgi/content/short/303/5657/521)
described a mechanism that explains how this "one-way" control is
established. In plants, two identical male gametes fertilize two distinct
female gametes, an egg cell and the central cell -- which give rise to the
embryo and nurturing endosperm, respectively. The FWA gene, which
encodes a transcription factor involved in flowering in Arabidopsis, is
not expressed in adult tissues. However, the team showed that it is imprinted
in the endosperm of the seed specifically, and is expressed only from the
maternal allele. Because the endosperm does not contribute to the next
generation (it degenerates as the seed matures), the activated FWA gene
does need not be silenced again. Thus, in contrast to the situation in animals,
double fertilization enables plants to use "one-way" control of DNA
imprinting and DNA methylation in the endosperm. An
accompanying Perspective by F. Berger (http://www.sciencemag.org/cgi/content/short/303/5657/483)
highlighted the new work.
The Dynamic X Chromosome
The key to sex determination in mammals lies in the number of X chromosomes
-- females have two, males have one. To overcome this genetic imbalance (which
can have severe or even fatal effects), one of the two X chromosomes is
permanently inactivated in every cell in females. As two studies in the 30 Jan
2004 Science described, this inactivation is an unexpectedly dynamic
process. Okamoto et al. (http://www.sciencemag.org/cgi/content/short/303/5658/644)
and Mak et al. (http://www.sciencemag.org/cgi/content/short/303/5658/666)
showed that every paternal X chromosome in the mouse embryo is initially
inactivated by imprinting -- a process whereby the DNA is modified to
subsequently recruit gene-silencing machinery. However, this paternal imprint is
erased at a later developmental stage by a group of primordial undifferentiated
(pluripotent) cells that ultimately give rise to the
fetus. This erasure is then followed by random X inactivation in each cell.
Thus, X chromosomes are initially silenced, reactivated, then
randomly silenced again as their final genetic expression programs are
established in the embryo. As noted in an accompanying Perspective by P. Hajkova and M. A. Surani (http://www.sciencemag.org/cgi/content/short/303/5658/633)
"[T]hese studies provide important insights
concerning not only X inactivation, but also aspects of early mammalian
development and pluripotency."
Another report, in the 23 Jan 2004 Science, suggests that the X
chromosome has been exceptionally active on an evolutionary time scale as well.
Functional genes can be produced during evolution by reverse copying RNA into
DNA (retroposition). Emerson et al. (http://www.sciencemag.org/cgi/content/short/303/5657/537)
examined retroposed genes in the human and mouse
genomes and demonstrated that, over time, the mammalian X chromosome has
generated and recruited an unusually high number of new genes compared with
non-sex chromosomes (autosomes). Gene expression
studies further indicated that most of the autosomal
genes that originated on the X chromosome have male-specific expression, which
is likely the result of natural selection.
Running Energetics
Because it's so tough to directly measure the metabolic energy required by
many individual muscles, physiologists have struggled to explain the energetics of walking and running. Now, using a creative,
indirect approach reported in the 2 Jan 2004 Science, Marsh et al. (http://www.sciencemag.org/cgi/content/short/303/5654/80)
have shown that, contrary to previous predictions, the energy used by muscles
that swing the legs forward is not negligible -- but a surprising one-fourth of
the total energy required for running. The team used colored microspheres, each about the size of a red blood cell, to
measure blood flow as a proxy for energy used by hindlimb
skeletal muscles of running guinea fowl. Although spheres injected into the
birds' hearts could flow through the arterial system, they got stuck in the
capillaries. Because blood flows to muscle tissue according to oxygen demand,
the density of spheres stuck in a capillary bed is proportional to the blood
flow in that volume of muscle, and by inference, to the oxygen consumption
rate. Marsh et al. thus found that regardless of speed, muscles involved in the
swing phase of running consume 26% of the energy used by the limbs and the
stance-phase muscles consume the remaining 74%. The extent to which these
numbers can be generalized to other running animals remains to be seen. An
accompanying Perspective by N. C. Heglund (http://www.sciencemag.org/cgi/content/short/303/5654/47)
highlighted the report and lent historical perspective to the quest to
understand running energetics.
Consciously Forgetting
More than a century ago, Sigmund Freud proposed that unwanted memories can
be excluded from awareness by a process called repression -- but how repression
occurs in the brain has been unknown. Now a brain-imaging study reported by
Anderson et al. (http://www.sciencemag.org/cgi/content/short/303/5655/232)
in the 9 Jan 2004 Science has revealed which brain circuits are active
when we try to forget certain memories. The researchers used functional
magnetic resonance imaging (fMRI) to monitor the
brain activity of volunteers who were taught word pairs and then asked to
either remember certain words or consciously avoid thinking about them. A
subsequent test indicated that the conscious avoidance did actually impair the
subjects' memory. The team found that a network of brain regions, including
several in the prefrontal cortex, were more active during memory suppression
than during retrieval. Furthermore, they identified a novel interaction between
the prefrontal cortex and the hippocampus, which showed reduced activity during
memory suppression. Interestingly, part of this active network is similar to
that involved in stopping overt movement. The results thus support the
existence of an active forgetting process that recruits brain regions known to
be important for executive control functions, and help establish a
neurobiological model of memory control.
Monkey Language Limits
The capacity to construct an infinite variety of expressions (words) from a
finite set of elements (vocabulary) distinguishes human language from other
animal communication systems. In a report in the 16 Jan 2004 Science, Fitch and Hauser (http://www.sciencemag.org/cgi/content/short/303/5656/377)
showed that tamarin monkeys are unable to master the
more sophisticated grammars that are central human language. Previous studies
have shown that these monkeys are capable of grasping simple "finite state
grammars" -- which control the types of words that go next to each other
in a sentence. However, when asked to match grammars for length, composition,
loudness, and other acoustic features in the new study, the monkeys could not
understand the more complex and hierarchical rules of "phase structure
grammars", which involve words that are dependent upon each other yet not
near each other in a sentence (as in the "if...then" construction,
for example). Previous results suggest that this computational limitation does
not result from limitations in memory, attention, or number discrimination. An
accompanying Perspective by D. Premack (http://www.sciencemag.org/cgi/content/short/303/5656/318)
used the Fitch and Hauser findings as a springboard to discuss various aspects
of human language evolution, intelligence, and uniqueness.
Humans Braved the Arctic Early
Who the earliest inhabitants of the Americas
were and how they got there have befuddled anthropologists and archaeologists
alike for nearly a century. Many believe that the Clovis
people were the first to settle North America -- by
crossing the Bering Land Bridge that once connected Arctic Siberia with
present-day Alaska -- beginning
about 13,600 years ago. But a surprising report by Pitulko
et al. (http://www.sciencemag.org/cgi/content/short/303/5654/52)
in the 2 Jan 2004 Science
now suggests that humans occupied the Siberian Arctic some 16,000 years earlier
than previously thought. The researchers analyzed a trove a
artifacts uncovered from the Yana River Valley 500
kilometers above the Arctic Circle and dated them to
about 30,000 years ago -- pushing human inhabitance of the region to the height
of the last Ice Age. The discovery thus poses a host of new questions about
where these robust individuals came from and how harsh their environment really
was. Furthermore, although some artifacts, like a spear foreshaft
made of rhino horn, resemble those of the Clovis culture,
other stone tools from the site are vastly different -- which leaves room for
debate about the relationships between these early populations. An accompanying
News Focus by R. Stone (http://www.sciencemag.org/cgi/content/short/303/5654/33)
highlighted the intriguing possibilities posed by the new Yana findings.
Mesoporous Silicate Films from Supercritical
Fluids
As new generations of electronic circuits become ever smaller, the demand
for materials with unique nanostructures and electric properties continues to
grow. Mesoporous metal oxide films are ideal for
applications including sensors and microelectronics because of their large
surface areas, ordered structures, and high mechanical strength. But
controlling the final morphology of these films, and synthesizing them quickly,
have proved troublesome using current methods. In a
report in the 23 Jan 2004 Science,
Pai et al. (http://www.sciencemag.org/cgi/content/short/303/5657/507)
demonstrated that formation on a rigid, preordered copolymer template helps
create durable silica films with high porosity and excellent mechanical properties, that are also good insulators. The team used
"supercritical" carbon dioxide to swell the polymer enough to allow
in the reactants (in this case, ingredients to make silicate)
without deforming the film during the reaction. Supercritical fluids exist
above their critical temperatures and pressures such that they can be tuned
from "gas-like" to "liquid-like" by simply varying
pressure, temperature, or both. Using this technique enabled the silicate to
form within the template, resulting in well-ordered pores over a large length scale.
As noted in an accompanying Perspective by K. P. Johnston and P. S. Shah (http://www.sciencemag.org/cgi/content/short/303/5657/482)
the synthesis rates obtained by Pai et al. were fast
enough for use in a microelectronics fabrication facility, which suggests that
the new method offers great promise for a variety of nanoscale
applications.
Reliable Radiocarbon Dating
Accurate dating is essential in many fields, from archaeology and
geochemistry to paleoclimatology. The main timekeeper
used by researchers studying the past 50,000 years is radioactive carbon
(carbon-14), but its use is not entirely without caveats. Environmental
processes move carbon of varying ages between geochemical reservoirs like the
ocean and the atmosphere, thus perturbing the carbon record in samples like
tree rings and sediments. To obtain accurate dates, these fluctuations must be
accounted for with a calibration curve. In research published in the 9 Jan 2004
Science, Hughen et al. (http://www.sciencemag.org/cgi/content/short/303/5655/202)
showed that a 50,000-year record of carbon-14 dates from sediments in the Cariaco Basin (off the coast of Venezuela) can be tied to
calendar dates from a Greenland ice core, extending the calibration of the
radiocarbon time scale. These data, in conjunction with age data from corals
and records from cave formations and other sediments, should help produce a
detailed and accurate carbon-14 age calibration back to ~50,000 years ago. The
refined radiocarbon record will have a number of important implications, from
the ability to more reliably date historic artifacts to better resolving the
boundaries of millennium-scale climate events. An accompanying Perspective by
Bard et al. (http://www.sciencemag.org/cgi/content/short/303/5655/178)
highlighted the new results.
The Galactic Habitable Zone
In 2001, astronomer Guillermo Gonzalez and his colleagues coined the term
"galactic habitable zone" (GHZ) to describe the place in our galaxy
where complex life can exist. In a report in the 2 Jan 2004 Science, Lineweaver et al. (http://www.sciencemag.org/cgi/content/short/303/5654/59)
calculated the size and position of the GHZ by evaluating four prerequisites
for supporting life: the presence of a host star, enough heavy elements to form
terrestrial planets, sufficient time for biological evolution, and an
environment free of deadly supernovae. Based on these criteria, the researchers
identified the GHZ as a ring-like region about 25,000 light years from the
galaxy's core that widens with time and that is composed of stars that are 4 to
8 billion years old. They further determined that the GHZ has embraced less
than 10% of the stars ever formed in the Milky Way, and that 75% of the stars
in the zone today are older than the Sun, by an average of a billion years. As
noted in an accompanying News story by R. Irion (http://www.sciencemag.org/cgi/content/short/303/5654/27a),
the new analysis intrigues those curious about the existence of complex life
elsewhere -- but others argue that we know too little about the conditions
needed to nurture life to draw any firm conclusions.
Stardust Clues to Stellar Evolution
Discovery of the radioactive element technetium (Tc)
around giant stars about 50 years ago proved that heavy elements are produced
by nuclear reactions within stars, and inspired theories of nucleosynthesis
(the process by which elements are formed) and stellar evolution. Stardust grains,
remnants of stars that lived and died before the solar system, are a rich
source of presolar astrophysical information: Each
grain carries an isotopic record of the initial composition and nuclear
processing in its parent star. Silicon carbide (SiC)
grains are the best studied because of their relatively large size (up to
several micrometers) and relatively high abundance. In a report in the 30 Jan
2004 Science, Savina et al. (http://www.sciencemag.org/cgi/content/short/303/5658/649)
reported evidence for now-extinct technetium in SiC
grains extracted from a meteorite. They used highly sensitive mass spectroscopy
to measure the isotopic abundance of ruthenium, a technetium decay product, and
found that the measured isotopic ratios agree very well with models of these
stars if radioactive technetium is assumed to have been a component of the dust
grains during their formation. As noted in an accompanying Perspective by L. R.
Nittler (http://www.sciencemag.org/cgi/content/short/303/5658/636),
modern stellar models can be quantitatively tested with high accuracy and
precision as a result of these new measurements.
Remaking NASA
On 14 January, President Bush unveiled an ambitious plan for the future of
American space exploration (see News story by A. Lawler in the 23 Jan issue; http://www.sciencemag.org/cgi/content/short/303/5657/444).
Although White House and NASA officials insist that researchers will prosper
under the new proposal, many fear that the plan could jeopardize the future of
any activity that doesn't directly serve the exploration effort. A special News
Focus in the 30 Jan 2004 Science (http://www.sciencemag.org/cgi/content/short/303/5658/610)
examined NASA's future and what it means for space science in light of the new
vision. Included in the controversial proposal are plans to retire the aging
shuttle, cancel flights to the celebrated Hubble Space Telescope, redirect
efforts on the half-built space station to create a human base rather than a
research institute , and design and build a vehicle to take humans to the moon
and then to Mars. Scientists bothered by the perception that science is taking
a back seat to human flight hope that their dreams for robotic space
exploration are also in the picture. Among the endeavors at the forefront of
their minds are missions to explore the moon, construction of larger and more
powerful space telescopes, and design of a nuclear propulsion system to power a
trip to Jupiter and its icy moons. Finally, amidst the emotional debate over
NASA's future, eyes are sure to turn to chief Sean
O'Keefe -- who, two years after his arrival and one year after the Columbia
tragedy, is trying to take NASA back to where it's been -- and beyond.
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