ScienceWeek

SCIENCEWEEK

December 8, 2006

Vol. 10 - Number 48

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It is impossible to dissociate language from science or science from language, because every natural science always involves three things: the sequence of phenomena on which the science is based; the abstract concepts which call these phenomena to mind; and the words in which the concepts are expressed. To call forth a concept, a word is needed; to portray a phenomenon, a concept is needed. All three mirror the one and the same reality. Words are thus required to preserve and transmit ideas, so that it is clear that the advancement of a science and the improvement of its technical vocabulary go hand in hand. No matter how certain we are of the phenomena, no matter how adequately our concepts reflect them, we cannot help perpetuating wrong ideas unless we have a precise terminology in which to express ourselves.

-- Antoine Laurent Lavoisier (1743-1794)

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Contents (full text below):

1. Developmental Biology: On the Turing Model and Patterns. 2. Atmosphere Science: On the Formation of Ice Clouds. 3. Evolution: Ecology and Toxic Trace Elements. 4. Science Policy: On Future Science and Technology in China

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Also Noted:

Ocean. The World's Last Wilderness Revealed. Robert Dinwiddie and Louise Thomas. DK, New York, 2006. Hardback: 512 pp. ISBN 0756622050. More information at: http://www.amazon.com/exec/obidos/ASIN/0756622050/scienceweek

Quantum Field Theory I: Basics in Mathematics and Physics. A Bridge Between Mathematicians and Physicists. Eberhard Zeidler. Springer, Berlin, 2006. Hardback: 1044 pp., illus. ISBN 3540347623. More information at: http://www.amazon.com/exec/obidos/ASIN/3540347623/scienceweek

What Sustains Life? Consilient Mechanisms for Protein-Based Machines and Materials. Dan W. Urry. Springer, New York, 2006. Hardback: 650 pp., illus. ISBN 081764346X. More information at: http://www.amazon.com/exec/obidos/ASIN/081764346X/scienceweek

The Creative Brain. The Science of Genius. Nancy C. Andreasen. Plume (Penguin Group USA), New York, 2006. Paperback: 213 pp., illus. ISBN 0452287812. More information at: http://www.amazon.com/exec/obidos/ASIN/0452287812/scienceweek

The Genius Factory. The Curious History of the Nobel Prize Sperm Bank. David Plotz. Random House Trade Paperbacks, New York, 2006. Paperback: 285 pp., illus. ISBN 0812970527. More information at: http://www.amazon.com/exec/obidos/ASIN/0812970527/scienceweek

Hot Thought. Mechanisms and Applications of Emotional Cognition. Paul Thagard, in collaboration with Fred Kroon et al. MIT Press, Cambridge, MA, 2006. Hardback: 313 pp., illus. ISBN 026220164X. More information at: http://www.amazon.com/exec/obidos/ASIN/026220164X/scienceweek

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1. DEVELOPMENTAL BIOLOGY: ON THE TURING MODEL AND PATTERNS

The following points are made by P.K. Maini et al (Science 2006 314:1397):

1) What are the underlying mechanisms that give rise to complex patterns in biology? Despite recent advances in biotechnology and mathematical modeling, this still remains a largely open question. New work (1) has made a major advance toward answering this question by identifying key molecular players in hair follicle growth and by confirming the validity of perhaps the best-known mathematical model for biological pattern formation.

2) In a seminal paper (2), Alan Turing proposed that spatial patterns result from a phenomenon he termed "diffusion-driven instability". He showed mathematically that small spatial fluctuations in an otherwise well-mixed system of reacting and diffusing chemicals could become unstable, and that amplification of these fluctuations could lead to a spatial pattern of chemicals that he termed morphogens (i.e., substances that stimulate the development of form or structure in an organism). He proposed that this spatial arrangement could serve as a prepattern for development. Turing's work was ground-breaking because the mathematical nature of the resulting patterns is wholly counterintuitive; since their discovery, they have motivated much mathematical research. However, the model has been the subject of controversy because it has been deemed too simplistic and the search for real biological examples has been neglected. Moreover, although diffusion-driven instability has been shown to be present in chemistry, there is substantial evidence in the fruit fly Drosophila to refute the model for biology (3). The report by Sick et al (1), by providing the first compelling biological evidence for the Turing model, is thus a landmark publication.

3) The formation of skin appendages (hairs, feathers, etc.) is an excellent paradigm for patterning because these systems are amenable to experimental manipulation. Nagorcka (4) was the first to propose the Turing model to explain hair pattern formation, but at that stage the molecular biology was lagging behind the theory. It was only in 1998 that Jung et al (5) made the first efforts to link known molecular morphogens with a reaction-diffusion mechanism for feather germ formation. They showed how the size, number, and distribution of appendages could be modulated by altering morphogen concentrations.

4) Sick et al (1) investigated the regulation of hair follicle patterning in developing murine skin. They propose that the protein WNT and its inhibitor DKK are morphogens in the Turing sense. Expression of the protein Dkk1, which inhibits WNT, is actually controlled by secreted WNTs, and both WNTs and DKKs are secreted into the extracellular space where they diffuse, thereby acting over longer distances. Given that the WNT proteins are substantially larger than the DKKs, one would expect a large difference in their rates of diffusion. This makes possible the classical "short-range activation, long-range inhibition" phenomenon that underlies diffusion-driven instability.

References (abridged):

1. S. Sick, S. Reinker, J. Timmer, T. Schlake, Science 314, 1447 (2006).

2. A. M. Turing, Philos. Trans. R. Soc. London Ser. B 237, 37 (1952).

3. M. Akam, Nature 341, 282 (1989).

4. B. N. Nagorcka, Biosystems 16, 323 (1983-1984).

5. H.-S. Jung et al., Dev. Biol. 196, 11 (1998).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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2. ATMOSPHERE SCIENCE: ON THE FORMATION OF ICE CLOUDS

The following points are made by T. Peter et al (Science 2006 314:1399):

1) As moist air rises to colder regions in the atmosphere, the humidity rises above its equilibrium value over ice. To relax this metastability, the air releases its water vapor via ice cloud formation. Such atmospheric ice clouds form in two steps: First, ice nucleates in or on existing aerosol particles; second, these ice particles grow through condensation of supersaturated water vapor onto the ice surfaces. Recent field observations (1-3) call into question the basic principles underpinning the current understanding of ice cloud formation and alter the assessment of water distribution in the upper troposphere.

2) The governing quantity for nucleation and growth is the excess activity relative to the equilibrium humidity over ice, also called ice supersaturation (expressed as a percentage). The equilibrium humidity decreases strongly with falling temperature. Hence, when an ascending air mass cools, it can become supersaturated with respect to ice. Ice nucleation requires a supersaturation above a critical threshold value. Nucleation can occur homogeneously from aqueous solution droplets, or heterogeneously on particles known as ice nuclei. At upper-troposphere temperatures, homogeneous freezing sets in at a supersaturation of ~60% (4); lower supersaturations are sufficient for heterogeneous nucleation. After nucleation, vapor molecules condense onto the ice particles, causing them to grow and the gas phase to become depleted in water until equilibrium is reached.

3) Large-scale regions of persistent supersaturation up to 60% outside ice clouds are not unexpected in the absence of ice nuclei. Yet values even above 100% have been observed in cloud-free regions (1). These values are far above the critical value for homogeneous ice nucleation (5) or cloud chamber data. At least as puzzling are supersaturations of 30% reported to persist inside ice clouds and contrails (2) for at least 1 hour of aircraft measurement time (3). Such large supersaturations are expected to relax rapidly as a result of fast vapor condensation unless continuous cooling remains sufficiently strong. To achieve such cooling, the clouds would have to rise by several kilometers in the measurement time, which contradicts the observations.

4) Measuring water in the upper troposphere is difficult. A major international effort to assess water vapor measurements in the upper troposphere and stratosphere concluded that, on the basis of laboratory calibrations, typical mean accuracies of aircraft and balloon instruments were on the order of 10%. However, direct comparisons in the upper troposphere suggest that differences between various instruments on aircraft and balloons often exceed 25%, especially when temperatures are very low. Also, balloon-borne instruments appear to yield mostly lower supersaturations than do aircraft instruments. Nonetheless, large supersaturations were observed during all recent aircraft and balloon campaigns; these studies used a range of instruments based on different measurement principles. Hence, only a fraction of the observed supersaturations can be ascribed to instrumental inaccuracies.

References (abridged):

1. E. J. Jensen et al., Atmos. Chem. Phys. 5, 851 (2005).

2. R. S. Gao et al., Science 303, [516] (2004).

3. S. H. Lee et al., J. Geophys. Res. 109, D20209, 10.1029/2004JD005033 (2004).

4. T. Koop et al., Nature 406, 611 (2000).

5. In a cooling event, when aerosol particles are exposed to a supersaturation of 60%, the characteristic time for ice nucleation is ~1 min. This drops to less than 1 second for the atmospheric measurements described in (1).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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3. EVOLUTION: ECOLOGY AND TOXIC TRACE ELEMENTS

The following points are made by M. Hartl and I.T. Baldwin (Current Biology 2006 16:R958):

1) The discrepancy between the time scales of a scientist's life (and funding) and evolutionary processes makes the study of evolution in long-lived organisms in real-time a challenge. Consequently, researchers prefer to study extreme selection pressures that force organisms to adapt quickly, such as the adaptation of plants to growth on soils with toxic levels of trace elements. Heavy-metal pollution and the urgent need to develop strategies for cleaning contaminated soils have motivated research into the physiology and ecology of such plants (1,2).

2) How strong selective pressures on contaminated soils can lead to reproductive isolation of adjacent populations in a relatively short time is well documented [3]. Several hypotheses have been proposed to explain the evolutionary advantage of adapting to such unfavorable conditions [4]. The lack of competition from other plants or the absence of attack from pathogens may allow metal-resistant plants to thrive in toxic waste dumps. But the secondary benefits of adapting to cope with toxins may be just as important. The accumulation of toxic trace elements in plant tissue may equip plants with effective defenses against insect herbivores either directly or indirectly by activating defense-related signaling cascades [4]. Several studies have shown that such elemental defenses exist, but their consequences for co-evolving species are unknown.

3) Freeman et al [5] report how higher trophic levels are influenced by strong selective pressures from toxic-element stress. They have shown that the selenium-hyperaccumulating plant Stanleya pinnata, native to the western United States, is well defended against two common generalist pests, the diamondback moth (Plutella xylostella) and the cabbage white butterfly (Pieris rapae). Larvae fed on diets with selenium concentrations as high as those of hyperaccumulating S. pinnata plants die, and adult moths avoid ovipositing on selenium-rich plants. Yet in nature, these selenium-rich plants suffer herbivore damage from a formerly unknown variety of P. xylostella, which has obviously adapted by disarming the elemental defense. These insects thrive on a selenium-rich diet and do not show any oviposition- or feeding-deterrence. Moreover, they can accumulate about four times more selenium in their body tissues as can non-resistant varieties. Such an accumulation may influence the moth's predators or parasitoids: Freeman et al [5] also analyzed the co-occurring parasitic wasp Diadegma insulare and found a correspondingly high amount of selenium, indicating co-evolution at the third trophic level.

4) Although selenium is an essential trace element for many species, it becomes toxic at high levels because of its similarity to sulfur and its consequent assimilation into selenocysteine. Selenocysteine replaces cysteine during protein biosynthesis, which leads to protein misfolding and severe toxicity. One mechanism by which detoxification occurs in selenium-resistant plants is the inactivation of selenocysteine by methylation. Such plant-derived methylselenocysteine is usually demethylated again after ingestion by herbivores, causing severe intoxication. In their analysis of seleno-compounds in all three species, however, Freeman et al [5] found that the selenium-resistant varieties accumulate the inactive methylselenocysteine, whereas the selenium-sensitive varieties accumulate toxic selenocysteine. A decrease in demethylase activity may be the key adaptation; such a loss of activity would in general be a disadvantage, as it prevents the conversion of methyl-cysteine, which occurs in several Brassicaceae species, to cysteine, but with a methylselenocysteine-rich diet, the loss of activity might prove advantageous.

References (abridged):

1 In: A.J. Shaw, Editor, Heavy Metal Tolerance in Plants: Evolutionary Aspects, CRC Press, Boca Raton, Florida (1989).

2 E. Pilon-Smits, Phytoremediation, Annu. Rev. Plant Biol. 56 (2005), pp. 15–39.

3 J. Antonovics, Evolution in closely adjacent plant populations X: long-term persistence of prereproductive isolation at a mine boundary, Heredity 97 (2006), pp. 33–37.

4 C. Poschenrieder, R. Tolra and J. Barcelo, Can metals defend plants against biotic stress?, Trends Plant Sci. 11 (2006), pp. 288–295.

5 J.L. Freeman, C.F. Quinn, M.A. Marcus, S. Fakra and E.A.H. Pilon-Smits, Selenium-tolerant diamondback moth disarms hyperaccumulator plant defense, Curr. Biol. 16 (2006), pp. 2181–2192.

Current Biology http://www.current-biology.com

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4. SCIENCE POLICY: ON FUTURE SCIENCE AND TECHNOLOGY IN CHINA

The following points are made by C. Cao et al (Physics Today 2006 December):

1) In January 2006, China initiated a 15-year "Medium- to Long-Term Plan for the Development of Science and Technology" (MLP). The MLP calls for China to become an "innovation-oriented society" by the year 2020, and a world leader in science and technology (S&T) by 2050. It commits China to developing capabilities for "indigenous innovation" (zizhu chuangxin) and to leapfrog into leading positions in new science-based industries by the end of the plan period. According to the MLP, China will invest 2.5% of its increasing gross domestic product in R&D by 2020, up from 1.34% in 2005; raise the contributions to economic growth from technological advance to more than 60%; and limit its dependence on imported technology to no more than 30%. The plan also calls for China to become one of the top five countries in the world in the number of invention patents granted to Chinese citizens, and for Chinese-authored scientific papers to become among the world's most cited. In all likelihood, the MLP will have an important impact on the trajectory of Chinese development; it thus warrants careful attention from the international community.

2) Preparation for the MLP began in 2003. At that time more than 2000 scientists, engineers, and corporate executives were mobilized into a program of "strategic research" to identify critical problems and research opportunities in 20 areas considered to be of central importance for China's future. The areas include advanced manufacturing, agriculture, basic science, energy, human resources, and national defense. In contrast to earlier planning efforts, the preparations—at least at the outset—were remarkably open. In particular, they included social scientists (mainly economists) and foreign scholars. Eventually, that openness gave way to a more secretive process in which the bureaucracy massaged the reports of the 20 working groups, attempted to reach compromises, and drafted the public version of the MLP. By most accounts, the drafting process was contentious and unusually drawn out. At one point, the onerous process of narrowing the plan's focus and setting priorities required direct intervention by China's premier, Wen Jiabao.

3) The MLP is remarkable in a variety of ways. It builds on important policy initiatives launched in the past 25 years, including the 1995 commitment to strengthen the nation through science, technology, and education and the more recent notion of empowering the nation through talent. Under rubrics such as those, China has made great efforts in the past several years to advance its science and education. Those efforts include increased expenditures on R&D, and they have led to growing numbers of scientists and engineers engaged in R&D and increased enrollments in higher education. Also, as evidenced by new initiatives pertaining to intellectual property law, technology standards, and venture capital, the nation has begun to take seriously the notion of technological innovation as a complex, systemic problem.

4) Despite the many signs of progress in China's S&T, the MLP comes at a time of serious concern about the nation's development. China's leaders have pledged to make the nation an "overall well-off society" (quanmian xiaokang shehui), with a per-capita income of $3000 by 2020, up from $1000 in 2002. Achieving that goal will require continued rapid economic growth. The leadership, however, is aware that the high-speed growth of the past 25 years -- with its overinvestment, inefficient use of resources, and the devastating effect on the environment --cannot be sustained. The path to creating the overall well-off society will necessarily be characterized by technological innovations supporting greater efficiency and productivity, and institutional innovations supporting improvements in governance—greater market discipline and integrity, less government corruption, and greater administrative accountability.(1-5)

References (abridged):

1. R. P. Suttmeier, X. Yao, A. Z. Tan, Standards of Power? Technology, Institutions, and Politics in the Develop-ment of China's National Standards Strategy, National Bureau of Asian Research, Seattle, WA (2006).

2. H. Xin, Science 313, 1721 (2006).

3. Office of Naval Research, The Structure and Infrastructure of Chinese Science and Technology, access no. ADA443315, Defense Technical Information Center, Fort Belvoir, VA (2006).

4. People's Daily online.

5. D. Cyranoski, Nature 430, 495 (2004).

Physics Today http://www.physicstoday.org

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