Editors' Choice

Science  17 May 2002:
Vol. 296, Issue 5571, pp. 1203

    The Effects of Deglaciation

    Our knowledge of the motion of Earth's major tectonic plates, which move at velocities of up to several centimeters per year, has come primarily from observations and dating of marine magnetic anomalies. As mid-ocean ridge magmas cooled, they locked in magnetic signatures that recorded the orientation of Earth's magnetic field and allowed the spreading and the geometry of ocean crust to be reconstructed. More recently, global positioning system (GPS) measurements, which have been made for about 20 years, have provided comparable resolution and perhaps a better indication of current plate motions.

    Sella et al. provide a synthesis of GPS data from the past decade and compare it with the best estimates from magnetic anomalies. Overall, the agreement is good, although there are differences in the motions across the North American, South American, and Caribbean plate boundaries. The resolution of the GPS data is sufficient to show that the current plate motion for the Antarctic, North American, and Eurasian plates has been perturbed by the melting of the major ice sheets during the past 15,000 years.—BH

    J. Geophys. Res.107, 10.1029/2000JB000033 (2002).


    A Texas-Sized Collision

    The supercontinent Rodinia, an accretion of all the continents, existed between 1200 and 600 million years ago (Ma). One piece of ground truth for this tectonic puzzle is the Grenville Province, a region of metamorphism and deformation that runs from Labrador to the Llano Uplift in Texas and formed the coastline of Laurentia. About 1200 Ma, the Grenville province collided with a cratonic piece (Amazonia) of South America. The rocks in the uplift provide evidence for this long-ago collision; however, it has been difficult to identify which piece of Amazonia collided with this proto-Texas region.

    Tohver et al. studied a large sequence of gabbros and basalts from the Nova Floresta formation of Rondônia in western Brazil. These rocks are part of the Amazon craton, and Ar-Ar dating indicates that they are 1200 million years old. Magnetic characteristics of the rocks were used to determine the paleomagnetic pole and to fix the paleogeographic position of Amazonia. It appears that around 1200 Ma, one corner of this craton (oriented roughly 180 degrees from its present-day position) collided with the proto-Texas region and created the Llano Uplift.—LR

    Earth Planet. Sci. Lett.199, 185 (2002).


    A Northerly Migration

    Many groups of organisms have members in both Africa and South America. These disjunct distributions are commonly attributed either to an origin on the Gondwanan supercontinent before its breakup around 100 million years ago (Ma) or to long-distance dispersal across the widening Atlantic Ocean.

    In a molecular phylogenetic analysis of the plant family Malpighiaceae, Davis et al. provide evidence for a new scenario. The Malpighiaceae are a family of trees, shrubs, and lianas occurring throughout the tropics, with a peak of diversity in the New World. Their analysis indicates that the family originated in what is now northern South America about 64 Ma, well after the breakup of Gondwana; at least six separate transoceanic dispersal events would have to have taken place to produce the current distribution, which seems unlikely. Instead, Davis et al. propose that the early Malpighiaceae spread mostly overland across the Northern Hemisphere Laurasian supercontinent, during the climatically favorable thermal maxima in the Eocene and Miocene epochs.—AMS

    Proc. Natl. Acad. Sci.U.S.A. 10.1073/pnas.102175899 (2002).


    Getting Enough Air to Survive

    After infection by tubercle bacteria, the host battles the invader to a standoff, at least as long as the immune system functions effectively. Upon entry, the pathogen subversively invades the immune system through inactivated macrophages; the distressed and infected host cells then emit signals that recruit activated macrophages to encapsulate the site of infection in a characteristic granuloma, or tubercle. Nitric oxide (NO) produced by these macrophages controls pathogen growth by inhibiting aerobic respiration in the bacteria and contributing to the formation of toxic peroxynitrite. Tubercle bacteria do not have flavohemoglobins (which other pathogens use to detoxify NO) but instead have three types of distinctive truncated hemoglobins (trHbs). Ouellet et al. have found that the trHbs act in concert in mycobacteria to acquire enough oxygen to support the conversion of NO into innocuous nitrate.—CA

    Proc. Natl. Acad. Sci. U.S.A.99, 5902 (2002).


    Mind the Gap

    Cells that are subjected to a variety of stresses, such as heat shock, make fundamental changes in the way they process misfolded proteins in order to retrieve, where possible, proteins that have not been damaged fatally. In looking at the stress response of cells, VanSlyke and Musil noticed changes in the way that misfolded proteins within the endoplasmic reticulum (ER) are processed. Generally, misfolded or incompletely assembled proteins are transported out of the ER and into the cytosol for degradation by proteasomes. However, in stressed cells, this dislocation reaction appears to be less efficient, allowing proteins that might previously have been removed from the ER to complete their folding and assembly and subsequently to be transported to the cell surface. The authors specifically examined the folding and transport of components of gap junctions, and the net effect of stress was the assembly of excess functional gap junctions at the cell surface.—SMH

    J. Cell Biol. 157, 381 (2002).


    Modeling Atmospheric Circulation

    The Quasi-Biennial Oscillation (QBO) is a cyclic atmospheric process with a period slightly longer than 2 years; it involves reversals of the easterly and westerly phases of equatorial zonal stratospheric winds. The QBO dominates the interannual variability in the tropical stratosphere and causes a secondary circulation with an associated response in the temperature distribution and global effects on trace gas distributions. Thus, general circulation models must simulate the QBO accurately if they are to predict climate more reliably. Giorgetta et al. have developed a model that successfully simulates the QBO by incorporating improved spatial resolution, a realistic simulation of tropical convection, and the effects of gravity waves. With these improvements, their model yields a QBO very similar to the observed one, forced from a broad spectrum of atmospheric waves (as expected from observations) and without the need for specific ad hoc adjustments.—HJS

    Geophys. Res. Lett.29, 10.1029/2002GL014756 (2002).


    Clear Reflections

    Optical components such as mirrors can be made to very high specification. However, because of stresses arising during operation, mirrors can warp, sag, or bend. For very high-specification systems, adaptive optics uses feedback to iron out the deformations by making tiny mechanical adjustments via a network of actuators attached to the underside of the mirror. Vdovin et al. describe an actuation system that is relatively inexpensive and may be suitable for applications that are not extremely demanding. Their demonstrator system comprises 19 resistors placed in a hexagonal array underneath a flexible mirror. The resistor length is temperature-dependent, and each resistor is controlled and adjusted by a current source. With a response time of 5 seconds and actuator motion of about 6 micrometers, this simple setup is ideally suited to correcting slowly varying deformations and aberrations in mirror systems.—ISO

    Opt. Lett.27, 677 (2002).


    Mixing and Matching, Maybe

    The central position of ribosomal RNA (rRNA) in molecular phylogenetics has recently been reinforced, both by its demonstrated importance in translation and by the evolutionary proximity of nucleic acid machinery (mediating replication and transcription) in eukaryotic and archaeal lineages. Nevertheless, the heated discussion concerning metabolic enzymes has been stoked by the provision of new genome sequences. Now, Canback et al. suggest that the eukaryotic glycolytic enzymes cannot have been obtained en masse from the mitochondrial symbiont, because they do not cluster with the enzymes of α-proteobacteria. On the other hand, these protein trees and those of other metabolic pathways are not simply isomorphic with that of rRNA, which may in part be due to lateral gene transfer among the ancestors of extant α-proteobacteria and even between eukaryotic hosts and pathogenic bacteria. For a historical view of the continuing debate about the origin of the mitochondrion, see Martin et al.GJC

    Proc. Natl. Acad. Sci. U.S.A.99, 6097 (2002); Biol. Chem.382, 1521 (2001).