Editors' Choice

Science  10 Oct 2008:
Vol. 322, Issue 5899, pp. 167

    Staggered Starts

    If you are a small animal with many predators in your environment, you'd better react quickly to the earliest sign of danger. Startle reflexes have evolved as one way to deal with such situations. Input from sensory organs is rapidly transmitted to efferent systems, usually the muscles that are involved in flight responses. One such reflex is the backward tailflip response in crayfish. The coincident arrival of synaptic inputs is crucial because only when enough synapses fire simultaneously will they be able to activate the giant fiber system; however, the medial giant fibers receive input from widely differing distances along the crayfish antennules. Mellon and Christison-Lagay found that sensillar axons have precisely calibrated conduction velocities. Axon diameters near the flagellum tip were very small, and axon diameters increased continuously toward more proximal locations along the flagellar axis. In nonmyelinated nerve fibers, conduction velocity is proportional to the square root of the axon diameter. This linear adjustment of axonal conduction velocity thus ensures that a train of action potentials from an array of feathered sensilla on the first antennae arrives simultaneously in the brain and can drive the medial giant fibers to produce the tailflip response. — PRS

    Proc. Natl. Acad. Sci. U.S.A. 105, 14626 (2008).


    What Have We Done?

    Global climate is warming rapidly, and has been for more than 150 years, since around the start of the Industrial Revolution. Climate models suggest that most of the rise is due to greenhouse gas emissions, but the accuracy of the models is not entirely certain, and there have been numerous high-profile disagreements about their credibility. An alternate way to estimate the magnitude of human influence on global temperatures is to look at the observational record. Lean and Rind found that only four factors—ENSO (El Niño-Southern Oscillation), volcanic activity, solar activity, and anthropogenic forcing by greenhouse gases—are required to explain 76% of the variance in the temperature records. Furthermore 90% or more of the warming trend of the past 100 years can be explained by invoking anthropogenic effects, and solar forcing can explain a negligible percentage of the rise in temperature over the past 25 years. Finally, the zonal temperature response to natural and anthropogenic forcing does not increase rapidly with latitude from mid- to high latitudes, as it does in models, and anthropogenic warming effects are more pronounced in the latitudes between 45°S and 50°N than at higher latitudes. — HJS

    Geophys. Res. Lett. 35, L18701 (2008).


    Decisions, Decisions

    Precursor cells begin life faced with some important decisions: when and where to divide, and what sort of cell to become. These decisions can be influenced chemically by growth factors that induce signal transduction pathways within the cell and regulate the transcription of specific genes. Oligodendrocytes make myelin that insulates axons; they arise from precursor cells that migrate and proliferate along the axons before differentiating into mature myelin-forming cells. Rosenberg et al. show that the physical environment of the oligodendrocyte precursor cell acts as a differentiation factor and influences these cell fate decisions. Differentiation was dependent on the packing density of the precursor cells, suggesting that lateral compression of the cells can act as a mechanical signal that activates gene expression programs. — HP*

    Proc. Natl. Acad. Sci. U.S.A. 105, 14662 (2008).

    • * Helen Pickersgill and Chris Surridge are locum editors in Science's editorial department.


    Spinning Soy into Gold

    Many routes exist for making metal nanoparticles but often involve non-water-soluble and/or potentially toxic compounds, a particular concern in preparations for in vivo applications. Shukla et al. prepared gold nanoparticles from aqueous solutions of sodium tetrachloroaurate mixed with soybean extracts. Soybeans contain a wealth of phytochemicals, including proteins, carbohydrates, fats, and isoflavones. Gold nanoparticles formed over the course of a day by direct interaction with soybeans in solution, but better results were achieved using extracts from presoaked soybeans to afford time for the proteins to dissolve. Both low- and high-molecular-weight protein fractions were able to reduce the gold ions, but stabilization of the particles was observed only with the higher-molecular-weight fraction, which effectively coated the particles and prevented aggregation. Particles were stable in salt, histidine, and bovine and human serum albumin solutions and did not show cytotoxicity when tested with human fibroblast cells. Thus, plant matter may be an effective route to making biomedically friendly metal nanoparticles. — MSL

    Small 4, 1425 (2008).


    The Me Generation

    Despite conservation efforts to ensure its survival, an island bird species has remained endangered because of its own social behaviors. The Seychelles magpie robin, whose population dwindled to just seven breeding pairs in 1988, has been the subject of intense conservation efforts for more than three decades. Success has been modest, with reintroductions within the Seychelles archipelago boosting the population size to almost 150 individuals in all. López-Sepulcre et al. show that recovery has been slower than expected because of competition within territorial social groups consisting of a dominant breeding pair and several subordinate individuals, whose reproduction is postponed until one of the dominant pair dies or is competitively ousted. Competitive interactions for dominance within the group reduce the group's reproductive output. Thus, behavior that is advantageous for an individual's fitness (and evolutionary success) can be detrimental to that of the group. Simulations suggest that recovery of the population would have been at least one-third more rapid in the absence of competitive interactions, highlighting the need to take social behavior into account in species recovery plans.— AMS

    J. Anim. Ecol., 10.1111/j.1365-2656.2008.01475.x (2008).


    Hard Graft Made Easy

    Type 1 diabetes is an autoimmune disease that destroys insulin-producing β cells, situated in the pancreas within the islets of Langerhans, leaving sufferers dependent on regular injections of insulin to control their blood glucose levels. An attractive treatment would be the transplantation of islets from healthy donors but, as with all organ transplants, there is the risk of rejection and a need for long-term suppression of the recipient's immune system, leaving the person prey to opportunistic infections. Luo et al. have developed a method to make diabetic mice tolerant to islet grafts by injecting them once 1 week before transplantation and again 1 day afterward with donor spleen cells, which were first treated with the chemical crosslinker 1-ethyl-3-(3'-dimethylaminopropyl)-carbodiimide. Antigen-presenting cells from the donor spleen induced the down-regulation of the host effector T cells that would otherwise orchestrate graft rejection, and encouraged regulatory T cells to provide long-term tolerance to the transplants. Islet cells grafted into diabetic mice produced insulin for several months, and grafts could be replaced without additional treatment, as long as the new islets came from the same original donor. This approach depended on the exact timing and size of fixed-cell injections but, if a similar protocol can be established for humans, it could provide a simple and effective therapy for a very common condition.— CS*

    Proc. Natl. Acad. Sci. U.S.A. 105, 14527 (2008).

    • * Helen Pickersgill and Chris Surridge are locum editors in Science's editorial department.


    Alternative Route to Male Killing

    Wildly distorted sex ratios in certain insects—where as few as 1 male per 100 offspring are produced, dramatically altering patterns of mate competition—are caused by “male-killing” bacteria. Male killing in the parasitic wasp Nasonia vitripennis, in which males develop from unfertilized eggs by parthenogenesis, is caused by the bacterium Arsenophonus nasoniae.

    Ferree et al. show that in infected Nasonia females, unfertilized eggs die during early embryogenesis because of a lack of centrosome activity and the consequent disarray of cell division. In diploid species, centrosomes are generally provided by the sperm. In Nasonia, which have haploid eggs, the centrosomes are derived from “accessory nuclei.” Accessory nuclei, vesicular organelles that are formed from the oocyte nuclear membrane that sequester centrosome components, are intact in both infected and uninfected wasps, suggesting that Arsenophonus interferes with events downstream of their formation. Indeed, haploid male eggs can be rescued by the delivery of paternal centrosomes. Thus, Arsenophonus interferes with the development of unfertilized eggs rather than targeting maleness. — GR

    Curr. Biol. 18, 1409 (2008).