This Week in Science

Science  11 Jun 1999:
Vol. 284, Issue 5421, pp. 1733
  1. Fixing the Fossil Tree

    Much of the tree of evolution, and particularly the important details relating to genus and species, has been constructed by cladistics, the analysis of a variety of morphological traits of fossils. Consideration of the temporal occurrence of fossils has been problematic because there may be many gaps in the fossil record and because “living fossils” that preserve primitive traits may persist; often, the temporal data are not weighted or included in the analysis. Fox et al. (p. 1816) constructed a computer model of evolution of several character traits and show that inclusion of the temporal occurrences of fossils greatly improves the likelihood of capturing the correct or a reasonable phylogeny.

  2. Small, Coherent, and Bright

    Smaller optical devices could lead to “all-optical” computing and also find applications in communications and in near-field (subwavelength) optics. Painter et al. (p. 1819) introduce a cavity laser that is only 0.03 cubic micrometer in size. Its operation combines a laser cavity, which consists of a gap between two reflectors in which the photons can build up, and a photonic crystal that has an intentionally placed defect. The defect, a single departure from the otherwise periodic array of holes etched into the material, effectively “locks” or localizes the light output to that particular point.

  3. Assaying Ancient Atmospheres

    Earth's climate was warmer during the middle Eocene, possibly because atmospheric CO2 concentrations were considerably greater than they are today or possibly because of differences in ocean circulation. Pearson and Palmer (p. 1824; see the news story by Kerr) evaluated this hypothesis by constructing a pH profile of the Eocene ocean from boron isotopes in several species of fossil plankton—ocean pH reflects the amount of CO2 dissolved in seawater, which in turn depends on atmospheric CO2. The data imply that atmospheric CO2 concentrations during the middle Eocene were similar to or only slightly greater than modern values.

  4. Directing Hermaphrodite Development

    For the specification of sexual development, many organisms require the action of two different mechanisms, one that determines overt sexual characteristics (sex determination) and one that equalizes gene expression from the sex chromosomes despite the difference in sex chromosome dose (dosage compensation). Dawes et al. (p. 1800; see the cover and the Perspective by Kuroda and Kelley) now demonstrate that a single factor in the nematode Caenorhabditis elegans acts as a “switch” to direct hermaphrodite development. This factor, SDC-2, coordinately regulates both sex determination and dosage compensation by acting as a repressor of the male-specific gene her-1 and by initiating complex formation on the X chromosome to reduce the chromosome's overall expression, respectively. The dual repression functions of SDC-2 in regulating sex determination and dosage compensation in C. elegans exemplifies the elaborate control mechanism necessary for such a complex developmental program.

  5. The Cellular Connection in Hearing

    Hair cells in the inner ear are the crucial first link in a long chain of elements that participate in the hearing process, and the destruction of hair cells is one of the most important causes of hearing loss and deafness. How these cells develop and differentiate during embryogenesis has been largely unknown. Bermingham et al. (p. 1837) now identify a proneural gene, Math1, that is required for the production of hair cells. This homolog of the Drosophila proneural gene atonal is expressed in developing sensory patches. In mutant mice that lacked Math1, no hair cells developed in the embryo.

  6. Holding Both Left and Right

    Although DNA is usually found in a form known as B-DNA, which is a right-handed double helix—meaning that the spiraling directionality of the sugar-phosphate backbone can be mimicked by the thumb and fingers of the right hand—it does transiently adopt a left-handed form, called Z-DNA, in the wake of an actively transcribing RNA polymerase. Schwartz et al. (p. 1841) describe the structure of a complex between Z-DNA and the Z-DNA binding domain of an RNA editing enzyme. They find a familiar motif in DNA-protein interactions, the helix-turn-helix fold used by many proteins in binding to B-DNA. The versatility of this fold is revealed by how the second helix, which normally recognizes specific bases within the major groove when bound to right-handed B-DNA, instead makes polar contacts with the sugar-phosphate backbone of Z-DNA.

  7. Stimulating Synapses

    The underlying mechanisms of the changes in synaptic activity induced by high-frequency stimulation of neurons, such as long-term potentiation (LTP), and their implications for memory, have been controversial; two research articles address these topics (see the news story by Barinaga). The changes induced in AMPA-type glutamate receptor function during repetitive stimulation could be due to greater intrinsic activity or to their recruitment to synapses. Shi et al. (p. 1811) were able to count AMPA receptors in hippocampal slice cultures by tagging a subunit with green fluorescent protein. After tetanic stimulation, a large intracellular pool of AMPA receptors was redistributed into dendritic spines or clustered in dendrites. This phenomenon could be prevented by blockade of NMDA receptors in a manner analogous to synaptic potentiation. How does LTP, an electrophysiological phenomenon, relate to memory? Zamanillo et al. (p. 1805) show that LTP is essentially absent in mutant mice that lack an important glutamate receptor subunit. However, their spatial learning performance in the water maze test is unchanged. These findings indicate that there is a dissociation between LTP and certain forms of spatial memory. The interplay between events on the cellular and synaptic level and the behavior of the organism is more sophisticated than earlier simplistic models assumed.

  8. Viral Vigilantes

    The first response by a mammal to viral infection is to produce the cell-protective type 1 interferons α and β. The major source of the interferon is not known, but natural interferon-producing cells (IPCs) are known to express CD4 and major histocompatibility complex class II proteins. Siegal et al. (p. 1835; see the news story by Hagmann) have now determined that these cells are the same as the recently identified precursors of type 2 dendritic cells (pDC2s), which can induce a cell-mediated T helper cell response. As these cells mature, they work on various levels to quell viral attacks and they may provide an interesting therapeutic target for boosting antiviral responses.

  9. Fire and Ecosystems

    Fire has a well-recognized nature role in shaping ecosystems, but the impact of human activity on fire dynamics needs to be better understood (see the Perspective by Goldammer). Cochrane et al. (p. 1832) report that in tropical forests, fires caused accidentally by humans have become much more common. A positive feedback occurs that results in the accumulation of combustable biomass, which leads both to an increased incidence and an increased severity of re-burning. Using the location and patterns of fires and subsequent land use as a guide, the authors estimate that these accidental fires are currently responsible for more Amazon deforestation than intentional timber harvesting and agricultural clearing. Keeley et al. (p. 1829) analyzed fire frequency in California brushland using almost a century's worth of data. It has been widely believed that brushlands are similar to woodlands—fire suppression management is needed because fuel accumulation leads to less frequent but more catastrophic fires. In fact, no Californian brushland county appears to have suffered an increase in fire size since 1910; instead, fire frequency is positively correlated with human population density. These results have implications for other fire-prone shrublands in areas of high human population density.

  10. PSD-95 and Neuronal Cell Death

    Excessive Ca2+ influx causes cell death in neurons, but the underlying molecular mechanisms are not fully understood. Sattler et al. (p. 1845) investigated the specific link between NMDA receptor signaling and the intracellular scaffolding molecule PSD-95. They showed that PSD-95 links NMDA neurotoxicity to the production of nitric oxide (NO). Suppressing PSD-95 by antisense oligodeoxynucleotides selectively blocked Ca2+-activated NO generation by NMDA receptors but not by other glutamate or Ca2+ channels.

  11. How the Hillside Got Its Stripes

    The regular and irregular vegetation patterns found in semiarid vegetation can be understood qualitatively. Klausmeier (p. 1826) now provides a model that explains these patterns in mechanistic terms by combining Turing-like instability and spatiotemporal chaos. The model may be useful for predicting the ecological impact of increased grazing or decreased rainfall in semiarid regions of the world.

  12. From Rails to Reels

    During the processes of replication or transcription, the polymerase enzyme is usually envisioned moving progressively down the template molecule. This progressive movement of the polymerase can be compared to the tracking of a locomotive along its rails. However, increasing amounts of evidence now indicate that this polymerase tracking action may be incorrect. Cook (p. 1790) reviews data supporting the idea that the polymerase apparati are attached to a cellular substructure and that it is, in fact, the nucleic acid that is moved, or reeled into the polymerase, like film through a projector. It is also suggested that the fixed polymerase molecules are concentrated within the cell in discrete “factories” that contain multiple polymerase molecules acting on multiple templates at the same time.

  13. Artificial Ionic Crystals

    The interaction of electromagnetic waves with spatially periodic dielectric media lends itself to many applications in opto-communications, such as sensors, waveguides, and microcavity lasers. Lu et al. (p. 1822) propose that the development of novel devices based on this interaction will require a better understanding of the processes. They investigate a simple one-dimensional material composed of a superlattice of oppositely poled ferroelectric layers (sheets of charge with opposite charge polarization). They find good agreement between experiment and theory when the coupling of the electromagnetic waves to lattice vibrations within this artificial ionic crystal is taken into account.

  14. Heat Content Changes in the Pacific Ocean

    The Acoustic Thermometry of Ocean Climate (ATOC) Consortium (Reports, 28 Aug. p. 1327) compared measurements of the heat content of the Pacific Ocean and sea surface height with results from a general circulation model. They concluded that only about half of the changes in sea level “are attributable to thermal expansion.”

    K. A. Kelly et al. comment “that the ATOC estimates [of heat content changes as made by the numerical model] are too small by a factor of two.” Estimates of seasonal heat flux can be reconciled with temperature and sea-level data “after accounting for adiabatic terms, without resorting to large advective contributions.”

    In response, ATOC agrees “that the model may be underestimating the annual cycle of the heat flux” and states that ATOC has “no conflict with the numbers provided by Kelly et al.

    The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/284/5421/1735a

  15. Regional Carbon Imbalances in the Oceans

    C. M. Duarte and S. Augustí (Reports, 10 July, p. 234) compiled data from many studies to model the relation between “community respiration” and “gross primary production” of ocean ecosystems. They found that, in total, the ocean “biota can act as CO2 sinks at the global scale.”

    P. J. le B. Williams and D. G. Bowers comment that the apparent CO2 deficit calculated in the report is a result of “the form of analysis.” They conclude that there is insufficient evidence to suggest “that the open oceans, either as a whole or regionally, are substantially out of organic balance.”

    In response, Duarte and Augustí discuss choices of data sets and analysis and the challenges of making comparisons across scales and studies. They maintain that “the bulk of available empirical evidence” shows a “pattern toward heterotrophy in the oligotrophic ocean….”

    The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/284/5421/1735b

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