ScienceWeek

SCIENCEWEEK

June 29, 2007

Vol. 11 - Number 25

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There is nothing like astronomy to pull the stuff out of man. His stupid dreams and red-rooster importance: let him count the star-swirls.

-- Robinson Jeffers (1887-1962)

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Contents (full text below):

1. Astronomy: Inside a Cosmic Train Wreck

2. Genetics: Evolutionary Insights from Sponges

3. Behavior: A Narrow Road to Cooperation

4. Neurophysiology: Stressful Pacemaking

5. Earth science: Silicon-Enhanced Core

6. Probabilistic Reasoning by Neurons

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1.

Science 29 June 2007: Vol. 316. no. 5833, pp. 1852 - 1854 DOI: 10.1126/science.1139057

Astronomy: Inside a Cosmic Train Wreck

Paolo Coppi

When young galaxies crash into each other, the result is often not a pretty sight. The violent gravitational forces of the encounter rip apart the beautiful galactic spiral arms, and gas and stars shoot out into intergalactic space at high velocity (see the figure). Yet we are only realizing now that the most important result of such an encounter is often not visible to us at all.

Much of the gas in the collision is not flung out but instead cools quickly, collapsing to the center of the system. Eventually tens of billions of solar masses of gas can pile up into a region only a few hundred light-years across. The gas becomes so dense that it blocks most light and so compact that standard ground-based telescopes cannot resolve the details of the collapse due to blurring by Earth's atmosphere. The same density and compactness that make the gas collapse so hard to study observationally also make it hard to study theoretically. Two papers in this issue begin to lift the veil on this unexplored central region. On page 1877, Max et al. (1) report an advance in ground-based imaging that permits us to directly observe black holes in the densest areas of the collapse, and on page 1874, Mayer et al. (2) present high-resolution simulations showing how black holes in the colliding galaxies follow and respond to the collapsing gas.

To penetrate the dense gas, Max et al. used a detector operating at infrared wavelengths. To achieve high spatial resolution, they used an adaptive optics technique in which the shape of the telescope mirror is modified in real time to compensate for jittering of the image due to atmospheric turbulence. This combination enables Max et al. to present one of the highest-resolution observations yet of the central, "nuclear" region of the NGC 6240 galaxy merger, mapping out its distribution of stellar light and unambiguously reconciling the different estimates for the positions of the two supermassive black holes that lurk there.

Mayer et al. present complementary theoretical calculations that are some of the most realistic to date of the gas distribution at the center of a merger. Although several important physical effects, in particular the "feedback" of energy from the luminous central stars and black holes back into the collapsing gas, ultimately require better modeling, the calculation already seems accurate enough to resolve a long-standing puzzle: Rather than wander forever around the center of the merger, two black holes in a system like NGC 6240 should quickly merge to emit a potentially detectable blast of gravitational wave radiation.

Why is so much effort going into understanding what happens when gas-rich galaxies, and in particular massive ones, collide? Comparison of data from experiments such as the Wilkinson Microwave Anisotropy Probe, which tells us what primordial density fluctuations looked like, to data from galaxy surveys like the Sloan Digital Sky Survey, which tells us what those density fluctuations have evolved into today, strongly suggests that we live in a universe where the matter density is dominated by unknown massive particles that interact only gravitationally ("cold dark matter"), i.e., one where today's galaxies assembled hierarchically, from mergers of smaller galaxies (3).

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2.

Science 29 June 2007: Vol. 316. no. 5833, pp. 1854 - 1855 DOI: 10.1126/science.1144387

Genetics: Evolutionary Insights from Sponges

Michael W. Taylor, Robert W. Thacker, Ute Hentschel

Sponges (phylum Porifera) are among the most ancient of the multicellular animals, or Metazoa, with a fossil record dating back at least 580 million years (1). Found both in marine and freshwater environments, they filter-feed by pumping water through their bodies, which can contain a remarkable number of microbial symbionts. Sponges lack many of the characteristics typical of animals, but recent genomic studies--including the report by Jackson et al. on page 1893 of this issue (2)--have shown that they possess many major metazoan gene families. Sponges are thus invaluable systems for studying the evolution of metazoans and their interactions with microorganisms. Furthermore, their highly stable skeletons are of interest to materials scientists.

Biomineralization is an important feature of metazoan life. Animals including vertebrates, insects, mollusks, and sponges use minerals [such as calcium carbonate, iron, and silica] to form skeletal structures such as bones, seashells, and coral reefs (3). Biocalcification arose among many metazoan lineages during the "Cambrian explosion," between 530 and 520 million years ago, when the ancestors of today's animals first appeared in the fossil record. Did these lineages share the same gene(s) for biocalcification, or did multiple independent evolutionary events give rise to the ability to biocalcify? Recent studies, including that by Jackson et al., are beginning to provide an answer to this question.

Jackson et al. use the Indo- Pacific sponge Astrosclera willeyana to show that the last common ancestor of the metazoans possessed a precursor to the alpha-carbonic anhydrases. This gene family is used by animals today in a range of processes including ion transport, pH regulation, and biomineralization (4). By integrating molecular techniques ranging from protein sequencing to gene expression, the authors identified a group of closely related alpha-carbonic anhydrase sequences in A. willeyana. These sequences are similar to those recovered from a whole-genome project on another sponge, Amphimedon queenslandica (5). Together, the sponge alpha-carbonic anhydrases form a sister group to those of all other metazoans.

Jackson et al. confirm that at least one of the proteins from A. willeyana--the Astrosclerin-3 enzyme--possesses alpha-carbonic anhydrase activity. Expression of this protein in Escherichia coli yielded activity comparable to that of a highly active bovine alpha- carbonic anhydrase. Furthermore, the A. willeyana genes coding for these anhydrases were only expressed in the outer portion of the sponge, where the calcareous skeleton is first deposited. Collectively, these data indicate that the alpha-carbonic anhydrase gene family originated from a single ancestral gene. This gene subsequently underwent multiple independent gene-duplication events in other sponges and eumetazoans, yielding the striking structural complexity and diversity we see today among biocalcifying animals.

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3.

Science 29 June 2007: Vol. 316. no. 5833, pp. 1858 - 1859 DOI: 10.1126/science.1144339

Behavior: A Narrow Road to Cooperation

Robert Boyd and Sarah Mathew

In every human society, from small-scale foraging bands to gigantic modern nation states, people cooperate with each other to solve collective-action problems. They share food to ensure against shortfalls, risk their lives in warfare to protect their group, work together in building canals and fortifications, and punish murderers and thieves to maintain social order. Because collective action benefits everyone in the group, whether or not they contribute, natural selection favors non-contributors. So, why do people contribute? Everyday experience suggests that people contribute to avoid being punished by others.

But this answer raises a second question: Why do people punish? From an evolutionary perspective, this question has two parts: First, how can contributors who punish avoid being replaced by "second-order" free-riders who contribute but do not incur the cost of punishing? There has been much work on this topic lately, and plausible solutions have emerged (1-5). However, these solutions are not much good unless we can solve the second problem: How can punishment become established within populations in the first place? On page 1905 of this issue, Hauert et al. provide the first cogent answer to this question (6). Surprisingly, they find that punishment can become established if there are individuals who neither produce collective benefits nor consume collective benefits produced by others.

In previous models of the evolution of collective action, individuals in a group can either contribute and benefit from the public good (i.e., cooperate), or not contribute and benefit (i.e., defect). In the absence of punishment, defection wins. However, if punishment is possible and punishers are common, it does not pay to defect. But punishment is costly to impose. A rare punisher in a group of defectors suffers an enormous disadvantage from having to punish everyone in the group. This means that in very large populations, punishment can sustain cooperation when punishment is common, but punishing strategies cannot increase in numbers when they are rare (i.e., invade a population of defectors). In a finite population, random chance affects the number of each type that reproduce, and the resulting stochastic fluctuations allow punishers to eventually invade a population of defectors, even though selection favors defectors. However, it can take a very long time for this to occur, and thus, most of the time there is no punishment and no cooperation.

Hauert et al. provide a way out of this dilemma. They introduce a strategy that simply opts out of collective action. These "nonparticipants" neither contribute to the collective good nor consume the benefits, but instead pursue some solitary activity. Surprisingly, this innovation allows punishment to increase when rare. To see why, consider a population of defectors. Hauert et al. assume that nonparticipants get a higher payoff than defectors who attempt to free-ride when there are no cooperators in their group. Therefore, nonparticipants invade the defectors. Now, consider a population of all nonparticipants. Hauert et al. assume that two contributors working together can produce a higher payoff than a nonparticipant working alone. This means that rare contributors invade nonparticipants. Once contributors are common, defectors invade, and the cycle continues. The three strategies oscillate endlessly (7).

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4.

Nature 447, 1059-1060 (28 June 2007) | doi:10.1038/4471059a; Published online 27 June 2007

Neurophysiology: Stressful Pacemaking

Bruce P. Bean

In Parkinson's disease, dopamine-secreting neurons die — perhaps because unrelenting calcium entry during spontaneous electrical activity puts them under unusual pressure.

At a conference on calcium-channel-blocking drugs in the 1980s, a neurologist friend told me of colleagues who kept pills of the calcium-channel blocker nimodipine handy, planning to ingest one immediately should they suffer a stroke. Showing even more faith in the drug, an executive from the pharmaceutical company sponsoring the conference said that he took an unprescribed nimodipine pill every morning with his cereal, on the assumption that it was better not to wait.

Nimodipine and other dihydropyridine drugs were originally developed to treat high blood pressure; they relax vascular smooth muscle by blocking calcium entry. Subsequent off-label use to treat stroke was based partly on the idea that these drugs might also minimize neuronal death resulting from excessive calcium entry following oxygen deprivation. But despite the enthusiasm of my colleagues, controlled clinical trials1 failed to show any beneficial effects of nimodipine for treating ischaemic stroke. Now, however, a study by Chan et al.2 (page 1081 of this issue) has raised the exciting possibility that these or similar calcium-channel-blocking drugs might provide a strategy to treat Parkinson's disease by their effect on particular neurons — those that act by secreting the neurotransmitter dopamine (dopaminergic neurons).

The critical event in Parkinson's disease is death of dopaminergic neurons in a region of the brain known as the substantia nigra (SNc). Progressive loss of these neurons produces devastating symptoms, including tremors and loss of voluntary movement. Despite intensive research, it is not known what the causative pathological events are, nor why they selectively damage dopaminergic neurons. Moreover, current treatment cannot slow disease progression, only ameliorating its symptoms3.

Most commonly, activity in a neuron is triggered by neurotransmitter released by other neurons and diffusing across the gap, or synapse, between the neurons. But dopaminergic neurons in the SNc are spontaneously active even without synaptic input, firing action potentials at about 1–2 hertz (Fig. 1a). Such autonomous 'pacemaking' activity is seen in many types of neuron and requires ion channels that can open at membrane potentials lower than the threshold for firing action potentials. The electrical current entering the cell through these channels then depolarizes the membrane to the threshold for action potentials.

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5.

Nature 447, 1060-1061 (28 June 2007) | doi:10.1038/4471060a; Published online 27 June 2007

Earth science: Silicon-Enhanced Core

Tim Elliott

What elements, besides iron, make up Earth's core? Discrepancies in the isotopic ratios found in rocks from Earth's mantle and in undisturbed meteoritic material indicate strongly that one answer is silicon.

Earth's core is its most inaccessible part. New information on its composition — such as that revealed by Georg et al. on page 1102 of this issue1 — is therefore a hard-won prize. The core is known to be made predominantly of iron, but geophysical estimates of its density require that a lighter element makes up some 10% of its mass. Hydrogen, carbon, oxygen, sulphur and silicon have all been fingered as culprits, but which elements are actually involved remains controversial.

Despite what Hollywood would have us believe, we can't sample Earth's core directly. Instead, we must assess its composition by indirect means. One such method is mass balance: the budget of elements not in the silicate-dominated, outer portions of the Earth must be in the core (Fig. 1). For many elements of interest, the well-mixed, convecting mantle is the only significant reservoir in these outer layers. Thus, if we know the composition of the mantle and of the Earth as a whole, the make-up of the core can be calculated from the difference.

That might sound straightforward. Earth's bulk composition is estimated from analyses of 'undifferentiated' meteorites, thought to be made of the same primordial material from which Earth originally formed. And the composition of the mantle, normally hidden beneath a veneer of crust, can be determined from rare fragments fortuitously exposed at the surface. But significant chemical variability in both the meteorite and mantle samples makes compositional estimates of the bulk Earth and mantle uncertain — and constraints on the make-up of the core poorer still.

Georg et al.1 investigate the possible presence of silicon in Earth's core using a novel isotopic, rather than elemental, mass balance. Different isotopes of an element show the same chemical behaviour, but form bonds of slightly different strengths. This can result in mass differences (fractionations) between reactants and products in chemical reactions. Sometimes, such mass differences are very small. Georg et al. show that this is apparently so for silicon isotopes in silicate melts and minerals. They find that the silicon isotope ratios — expressed as delta30Si, or parts per thousand difference in the ratio 30Si/28Si relative to a reference standard — of a range of mantle-derived silicates are very similar. The silicon isotopic composition of the silicate portion of Earth is thus well defined.

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6.

Nature 447, 1075-1080 (28 June 2007) | doi:10.1038/nature05852

Probabilistic Reasoning by Neurons

Tianming Yang & Michael N. Shadlen

Our brains allow us to reason about alternatives and to make choices that are likely to pay off. Often there is no one correct answer, but instead one that is favoured simply because it is more likely to lead to reward. A variety of probabilistic classification tasks probe the covert strategies that humans use to decide among alternatives based on evidence that bears only probabilistically on outcome. Here we show that rhesus monkeys can also achieve such reasoning. We have trained two monkeys to choose between a pair of coloured targets after viewing four shapes, shown sequentially, that governed the probability that one of the targets would furnish reward. Monkeys learned to combine probabilistic information from the shape combinations. Moreover, neurons in the parietal cortex reveal the addition and subtraction of probabilistic quantities that underlie decision-making on this task.

Decision-making is a complicated process that is often based on more than one source of evidence. The brain needs to combine these sources to maximize the chance of achieving a correct decision or to achieve another related goal. Recent advances in neuroscience are beginning to expose the neurobiological mechanisms that underlie simple decisions1, 2, 3, 4, 5, 6. It has been demonstrated that, when the outcome of a decision is an eye movement, a neural correlate of the evolving decision can be recorded in brain areas associated with high level motor planning and attention allocation7, 8, 9, 10, 11. More specifically, neurons in the lateral intraparietal area (LIP) have been shown to accumulate sensory information provided by earlier visual cortex when a decision is being formed8, 9, 12, 13. The mechanism mimics statistical decision processes that accrue evidence sequentially in the form of a log likelihood ratio (logLR) that favours one outcome over another4, 14, 15. Therefore, it has been hypothesized that a neuronal substrate of probability integration exists in area LIP16.

To test this hypothesis, we trained two monkeys to perform a probabilistic categorization task (Fig. 1a). The task was adapted from the well-known weather-prediction task17, 18 used to study human learning and memory. In each trial the monkey viewed four highly discriminable shapes; these were sampled randomly (with replacement) from a set of ten possible shapes. The shapes were added successively to the display over four half-second epochs. The monkey then made an eye movement to either a red or a green target to receive a reward. Reward was not guaranteed, but was instead governed by a random process based on the combination of preset weights (w) that were assigned to the ten shapes {w1, w2,..., w10}. The sum of the four weights associated with the shapes shown in a trial established the log of the odds that reward would accompany a red or a green choice (see Methods Summary).

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