This Week in Science

Science  14 Mar 2003:
Vol. 299, Issue 5613, pp. 1621
  1. Chronicles of a Meltdown

    Approximately 14,600 years ago, a large pulse of fresh water from melting continental ice sheets, called meltwater pulse 1A, caused sea level to rise by roughly 20 meters in less than 500 years. This event has been attributed to the eastern North American ice sheets and has been associated with a warm interval of climate called the Bølling-Allerød. Weaver et al. (p. 1709; see the news story by Kerr) present results from a climate model of intermediate complexity that show how an Antarctic ice-sheet source for meltwater pulse 1A could have caused both an increase in the intensity of the North Atlantic thermohaline circulation and the resulting shift to warmer climate. This southern melting was a consequence of the warming that began 19,000 years ago in Antarctica. The cold period that followed the Bølling-Allerød is consistent with freshwater forcing by the melting of continental ice sheets in the Northern Hemisphere as overall climate warming continued.

  2. Living on a Curve

    More than 100 years ago, J. J. Thomson worked on how to pack electrons on the surface of a sphere, a problem that arose in his consideration of a model of the atom with rigid electron shells. The generalized problem, the arrangement of any set of repulsive particles on spherical surface, has yielded some theoretical solutions but is still very much an open problem. Working with colloidal particles, Bausch et al. (p. 1716; see the Perspective by Kamien) have developed an experimental system that tackles this problem. They find that above a critical sphere radius, new defects are required to stabilize the packing.

  3. Cloudy Outlook

    Although the effects of cloudiness on climate change are still poorly understood, clouds have likely played an important role in regions like the Arctic that are most sensitive to climate change. Wang and Key (p. 1725) used satellite data to estimate trends in surface temperature, surface albedo, cloud properties, radiation fluxes, and cloud forcing in the Arctic during the last two decades. Cloudiness has been increasing during the sunlit portion of the year and damping the increase in surface temperature. The interaction between the surface and clouds has resulted in very little change to the surface radiation budget during this period, implying that the change in Arctic surface temperatures is due more to the variability of large-scale atmospheric circulation patterns than to local radiative effects.

  4. Taming Turbulent Excitation Waves

    Excitation waves that travel in three dimensions, such as the electrical excitation that propagates through heart muscle, can travel as “scroll waves” that radiate outward from a line of excitation called a vortex filament. Winfree showed about a decade ago that such scroll waves differ from spiral waves in two dimensions because the vortex filaments can move about and launch waves that tend to cancel one another out. Such “turbulence” may underlie cardiac defibrillation. Alonso et al. (p. 1722) have studied this turbulence theoretically and find that it can be controlled by the application of weak periodic forcing at a nonresonant frequency. The change induced in the excitability of the medium can cause the filaments to contract, thus suppressing turbulence.

  5. Less Doubt About Drought

    The collapse of Classic Maya civilization during the 9th and 10th centuries A.D. has often been attributed to drought, despite the lack of detailed environmental records. Haug et al. (p. 1731) now present a record of riverine titanium input into the Cariaco Basin of the southern Caribbean, a proxy for the strength of the hydrological cycle. Three periods of drought, each lasting a decade or less, were coincident with the three phases of Maya city abandonment around 810, 860, and 910 A.D., that have been proposed on the basis of archaeological evidence.

  6. Viral Synapse

    Cell-cell contact is essential for transmission of the retrovirus, human T cell leukemia virus-type 1 (HTLV-I), because naturally infected cells produce few if any viral particles. Like HIV, HTLV-I replicates in the T cells of the immune system and causes serious diseases, including leukemia and tropical spastic paraparesis. Igakura et al. (p. 1713; see the Perspective by Derse and Heidecker) found that unstimulated T cells from HTLV-I-infected people spontaneously transmit HTLV-I Gag protein and the HTLV-I genome to uninfected cells. Such cell-cell transfer required the host cell's microtubule cytoskeleton. The retroviral components were transferred to the uninfected cell through a structure, controlled by the virus, that resembles the “immunological synapse” that forms between T cells.

  7. Better on Film

    Multiferroic materials simultaneously exhibiting ferroelectric, ferromagnetic, and ferroelastic coupling and may find application as actuators, sensors, and in information storage. Wang et al. (p. 1719) show that the heteroepitaxial growth of thin films of BiFeO3 results in a crystalline structure not obtainable in the bulk form and that exhibits enhanced electrical and magnetic properties.

  8. Getting Separated

    After nuclear division occurs, the cell itself needs to divide in a process known as cytokinesis. Straight et al. (p. 1743) dissect the process of cytokinesis using a number of inhibitors, including blebbistatin, a novel high specificity inhibitor of myosin II. Blebbistatin inhibits cleavage furrow contraction—the key component of cytokinesis—without affecting mitosis itself or the formation of the actin contractile ring. They find that completing cytokinesis involves ubiquitin-mediated proteolysis, thus providing a mechanistic link between mammalian cytokinesis and that of yeast.

  9. How Breeding in Captivity Can Reduce Fitness

    Captive rearing and release programs designed to supplement populations of endangered or commercially exploited species can actually reduce population fitness because the animals are reared under low-mortality conditions that can favor maladaptive traits. Heath et al. (p. 1738) show that rearing chinook salmon under aquaculture conditions, in which juvenile mortality is intentionally minimized, reduces selection that naturally favors large eggs. The result is that hatchery supplementation of wild populations has substantial negative impacts on chinook salmon by driving nonadaptive traits (small egg size) into the fish populations. The evolutionary change to the maladaptive state occurred in under 20 years.

  10. Is Infectivity a Zero-Sum Game?

    Fungal pathogens can be highly mobile—their spores being wind borne—so why don't super-pathogens emerge that can infect entire host population? Thrall and Burdon (p. 1735; see the Perspective by Brown) have examined a wild flax-rust fungus relationship in Kosciuszko National Park in South Australia and tested a comprehensive set of host and pathogen lines. A trade-off exists between fungus spore production and its infectivity—the more virulent the fungal strain, the fewer spores it can produce.

  11. Damage Control for Daughters

    The fitness of newborn daughter cells of budding yeast may depend in part on the mother cell's ability to retain oxidatively damaged proteins. Aguilaniu et al. (p. 1751) report that although the amount of oxidatively damaged proteins increases with replicative age, they are not passed on to progeny during cytokinesis. This selective partition depends on actin and on the sir2 gene, which encodes a histone deacetylase and is considered a life-span determinant.

  12. Avoiding Retinal Degeneration

    Mutant flies with impairment in the phototransducing signal-transduction cascade undergo a process of retinal degeneration that can be used as a model to understand human degenerative disorders. Acharya et al. (p. 1740; see the Perspective by Ranganathan) show that retinal degeneration in these flies can be inhibited by interfering with the production of sphingomyelin.

  13. The Potential for Protein Insertion

    The so-called TIM22 complex, which inserts proteins into the inner mitochondrial membrane, is powered by the membrane potential. Rehling et al. (p. 1747) have examined the molecular mechanisms involved in inner membrane protein insertion and found that the TIM22 complex acts as a twin-pore translocase. It first binds protein precursors and then uses the membrane potential both to dock precursors with the translocase and then to drive protein insertion.

  14. An Epigenetic Marker for Cancer Screening

    Many deaths caused by colorectal cancer (CRC) could likely be prevented by early detection. Cui et al. (p. 1753; see the Perspective by Ransohoff) investigated the predictive value of an epigenetic marker—loss of imprinting (LOI, a change in DNA methylation) of the gene encoding insulin-like growth factor-II (IGF-2). This gene has been previously linked to CRC and can be assayed in a blood test. In a pilot study involving 172 patients at a colonoscopy clinic, the authors found that LOI was 5 times more likely to be found in individuals with a family history of CRC and 21 times more likely to be found in individuals with a personal history of CRC. These preliminary results suggest that LOI may be a valuable marker for predicting cancer risk.

  15. First Things First

    A fundamental question in paleoclimatology is whether the warmings that mark the end of past glacial cycles were caused by increases in atmospheric CO2 or whether the CO2 increases were the consequence of higher temperatures. The interpretation of ice core data is hampered by the uncertainty of up to 1000 years or more during some periods in the relative ages of the air bubbles that contain trapped CO2 and the older surrounding ice from which temperature is calculated. One way to avoid that complication is to use CO2 and temperature records that both come from gas bubbles contained in the ice. Caillon et al. (p. 1728) present such a method, which uses changes in the isotopic composition of argon, to determine the phase relation between CO2 and climate during Termination III (240,000 years before present). Their paleoatmospheric Ar-isotopic record, from Vostok, Antarctica, suggests that CO2 concentration lagged Antarctic deglacial warming by 800 ± 100 years. This result is consistent with the idea that a significant contribution to glacial-interglacial temperature changes is made by greenhouse gases that amplify the initial orbital forcing.