This Week in Science

Science  27 Oct 1995:
Vol. 270, Issue 5236, pp. 557
  1. Springing forth together

    The radiation of life near the end of the Precambrian has been thought to have involved roughly an initial emergence of large soft-bodied animals in the Vendian followed by the appearance of shelly faunas in the Cambrian. Grotzinger et al. (p. 598; see news story by Kerr, p. 580) dated volcanic rocks interbedded with sedimentary rocks in Namibia that bracket these events. The dates andcomparison of other geochemical and biostratigraphic data with other rock sections suggest that the Vendian faunas were extant from 549 to as young as 543 million years ago, which is essentially the ageof the Precambrian-Cambrian boundary. There does not seem to be a great temporal hiatus between the two faunas.

  2. Shedding light on membranes

    Lipid monolayers are used to construct two-dimensional arrays and as a model for biological membranes. These monolayers exhibit domain structure that can vary as a function of chemical composition, pressure, and temperature; however, information on the detailed domain structure has beendifficult to obtain. Hwang et al. (p. 610) have used near-field scanning optical microscopy to image domain boundaries, to measure chemical composition, and to follow the partitioning of a probe material into the different phases. Such features were not visible in far-field epifluorescence microscopy.

  3. Arc melting

    Subduction of oceanic crust fuels melting in the overlying mantle, which produces volcanic arcs. In theory, the composition of volcanic rocks sampled across a magmatic arc might be expected to reflect in some way the effect of the systematic dehydration of the subducted slab on the mantle. Ryan et al. (p. 625) examined the variation of several elements that should reflect changes in fluid composition in lavas from the Kurile arc. Elements with strong affinities for water decreased in abundance with distance from the subduction zone.

  4. Coming up to air

    Surface reactions are important in many catalytic processes, and their kinetics are the focus of intensive research. One of the problems in these studies is that many of the techniques used require high vacuum, so reaction conditions, especially pressure, are often unrealistic. Rotermund et al. (p. 608) have developed optical techniques that allow the real-time observation ofreaction kinetics of carbon monoxide oxidation on a platinum surface from high vacuum to atmospheric pressures. They also observed new phenomena in pattern formation.

  5. Mountains in high relief

    What determines the relief of mountains? Locally it would seem that the intact strength of rocks allows weathering to produce steep cliffs. Schmidt and Montgomery (p. 617), however, propose thaton the larger scale of mountains, topography is controlled by mountain-scale material strength and bedrock landsliding. Strength at these scales would reflect the strength of the weakest rock units. Theauthors compared the slope profiles in the Cascades and Santa Cruz mountains of the western United States with models of slope stability and experimental data on rock strength.

  6. Multicellular origins

    The timing of the appearance of multicellular organisms in the Precambrian is uncertain; one problem has been that fossils of these organisms, which lacked hard parts, are scarce. Shixing and Huineng (p. 620) describe fossils resembling multicellular algae from the Tuanshanzi Formation in the Jixian area, north China, dated at approximately 1700 million years ago. The fossils range up to about1 centimeter in length.

  7. What mother will tolerate

    Because of paternal inheritance, the fetus is usually not a histocompatible match to its mother. Why, then, if they are in contact, does the immune system of the mother not attack the fetus? Tafuri et al. (p. 630) show that during pregnancy, maternal T cells reactive to the father's alloantigens are tolerized. This tolerance is lost after birth, and may explain why in some cases autoimmune diseases go into remission during pregnancy.

  8. Working under stress

    In response to stress, such as prey recognizing a predator, the sympathetic nervous system increases cardiovascular activity and releases the hormone epinephrine. Jansen et al. (p. 644) now show, as hypothesized by Cannon years ago, that a set of neurons controls both of these functions. Heart and adrenal gland neurons in rats were injected with two different types of attenuated virus. These viral labels could be traced back to a common set of neurons in the hypothalamus and brainstem.

  9. Separated and unequal

    How cells become specialized is a central question in developmental biology. Two reports examine the mechanism by which an asymmetric cell division in the bacteria Bacillus subtilis generates two progeny cells with different fates, the mother cell and the forespore (see news story by Roush,578). Arigoni et al. (p. 637) have studied the localization of SpoIIE, a protein required to activate spore-specific gene expression during spore formation. SpoIIE is asymmetrically localized during division at the polar septum and resides at the division site when division is complete. Duncan et al. (p. 641) found that SpoIIE is a serine phosphatase. Localized SpoIIE functions to overcome the inhibition ofthe spore-specific transcription factor sigmaF.

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