Editors' Choice

Science  03 Oct 2003:
Vol. 302, Issue 5642, pp. 19

    Why Whorl?

    1. Linda Rowan

    Snails, or gastropods, come in a wide variety of shapes, some with many coils of their tube-shaped shells, others with just one or two large coils, and some with irregular or straightened tubes. Is this just some random evolutionary experiment or are there environmental forces that drive the snails to curl? Fossilized gastropod larval shells from the Paleozoic (450 to 250 million years ago) are largely open-coiled, but later larval shells are mostly tightly coiled. Nützel and Fryda have compiled a database of larval shells showing that the number of species with open whorls dropped from 61% in the Ordovician to 2% in the Permian, at the end of the Paleozoic. There are no open-whorled larval shells in the fossil record from the Triassic, the beginning of the Mesozoic. A snail starts life as a planktonic larva in an egg capsule, and a tight coil would be stronger and better protected from predators. Thus gastropods may have evolved from an open coil to a tight coil because of predation. Unfortunately there is no evidence in the fossil record of predator-prey relationships for gastropods, and although healed cracks and other evidence of possible predation are found in extant larval shells, their planktonic predator remains unknown. — LR

    Geology 31, 829 (2003).


    Long-Distance Thermometry

    1. TWIL

    From environmental issues, such as groundwater monitoring, to safety issues, such as fire detection in buildings, the one parameter that is most often measured is the temperature. Although individual temperature sensors can be installed throughout a system, for some applications this might prove prohibitively expensive or may just be unsuitable. What about a temperature sensor that could be unwound from a spindle and cover distances up to 50 km? Cho et al. show that optical fibers can be used for such applications. By detecting and analyzing the backscattered light arising from the interaction between pump and probe pulses launched into the fiber, they could measure temperature differences along the 50-km fiber, with 1°C sensitivity at the front end of the fiber to 13°C at the back end and with a spatial resolution of 15 m. For applications requiring fast continuous monitoring over long periods of time and long distances, such a distributed temperature sensor should be particularly useful. — ISO

    Opt. Lett. 28, 1651 (2003).


    DNA Makes a Move

    1. Julia Fahrenkamp-Uppenbrink

    Because of the specificity of its base pairing, DNA is being tested as a central component in the design of nanomechanical devices. Yan et al. have designed a DNA nanoactuator that can reversibly expand and contract a two-dimensional (2D) array of DNA strands.

    The authors first devised a nanoactuator that has two states. The first state consists of four DNA strands forming a bulged three-arm DNA branch junction with a loop sticking out in the center. Addition of an appropriate “fuel strand” that is complementary to the loop sequence stretches the structure into the linear second state. The nanoactuator is then incorporated into a 2D DNA lattice. The cavities in the 2D lattice can then be switched reversibly from 14 nm × 14 nm to 14 nm × 20 nm, without deterioration of the lattice. This nanoscale motion may be helpful in the assembly of future nanoelectronic devices. — JFU

    Angew. Chem. Int. Ed. 42, 4342 (2003).


    Antarctic and Greenland Climate Changes

    1. H. Jesse Smith

    To investigate the temporal relationship between climate changes in the Northern and Southern Hemispheres, the ages of climate events recorded in ice cores from near the two poles are often compared. However, there is a large inherent uncertainty in this technique because it is difficult to date ice cores absolutely. Caillon et al. describe a possible way around this problem by measuring the isotopic compositions of Ar and N2 and the concentration of methane trapped in the bubbles of ice cores from Antarctica. When the local temperature change is fast and large enough, the resulting thermal gradient in the developing ice causes measurable isotopic fractionation of Ar and N2, thereby recording the temperature change event at the site. Methane is well mixed in the atmosphere on a global scale, and atmospheric methane levels increased when wetlands expanded because of Northern Hemispheric warming. Thus, by comparing the two signals, a robust estimate of the relative timing of warming in the Northern and Southern Hemispheres can be made. This technique, applied to 108,000-year-old ice from Vostok in Antarctica, indicates that the South warmed approximately 2000 years before the North, consistent with the popular idea of a climatological “bipolar seesaw.” — HJS

    Geophys. Res. Lett. 30, 1899 (2003).


    Genomic Recipe

    1. Barbara Jasny

    It is a lot harder to characterize an organism if you can't get it to grow in culture. The microorganism Tropheryma whipplei, which causes gastrointestinal Whipple's disease, has not been able to be grown outside of fibroblast cells. However, with the availability of the complete genome sequence of two strains of T. whipplei, Renesto et al. discovered how to please this fastidious bacterium.

    The <1-megabase genome contains a great deal of functional information, but computer modeling studies revealed that it lacks the machinery to make several amino acids. With this knowledge, the investigators were able to design a cell-free culture medium that supported the growth of three different strains of T. whipplei as well as a new strain isolated from a heart valve. This type of approach should lead to other successes in growing previously unculturable intracellular pathogens. — BJ

    Lancet 362, 447 (2003).


    Loops and Gates

    1. Gilbert J. Chin

    One of the critical concerns applicable to all membrane transport proteins is how to move the target molecule across the cell membrane without creating a nonspecific, potentially lethal, leak. The usual solution is to keep the transporter closed with a gate and to design a binding site to recognize the desired substrate; opening of the gate is then regulated, for example, by changing the transmembrane voltage, in the case of KvAP, or by binding of a proton, in the case of lacY.

    Chen et al. have examined the mechanism for the maltose transporter, which consists of two ATP-binding subunits—the MalK dimer—and two integral membrane subunits—MalF and MalG. Crystal structures of the MalK dimer in open, semi-open, and closed conformations (which are governed by ATP hydrolysis) reveal a tweezers-like movement of the monomers. The reorientation of MalK2 with respect to the cytoplasmic surface of MalFG has the effect of exerting force at the point of contact, which is formed by the Q loops from MalK and the EAA loops from MalFG. Closing the cytoplasmic portions of MalFG would then lever open the external portions, enabling maltose to enter the transport cavity. Finally, hydrolysis of ATP would reverse the tweezers motion and allow maltose access to the cytoplasm. — GJC

    Mol. Cell 12, 651 (2003).


    Meltdown in an Island Forest

    1. Andrew M. Sugden

    Examples of the harmful effects of alien species introductions continue to flood into the ecological literature, but few introduced species are as destructive as ants. O'Dowd et al. report a particularly devastating instance of an invasive ant whose effects cascade through an entire tropical island ecosystem, ultimately leading to the death of rainforest canopy trees. The yellow crazy ant, Anoplolepis gracilipes, was first introduced to Christmas Island in the Indian Ocean about 70 years ago. The ants persisted for decades at low density before their population began to explode in the late 1980s, forming supercolonies infesting one-fifth of the island. The ants eliminate the red land crab, a keystone consumer in the forest floor ecosystem. The elimination of the crab results in vastly increased tree seedling density and decreased litter decomposition on the forest floor. Meanwhile, in the forest canopy, the presence of ants promotes mutualistic population growth of honeydew-secreting scale insects, in turn resulting in increased growth of honeydew-dependent fungi that cause dieback and death of canopy trees. This invasional “meltdown” over a 2-year period shows how an invader can alter the trophic dynamics of an entire ecosystem through direct effects at multiple levels in the food web. — AMS

    Ecol. Lett. 6, 812 (2003).