Editors' Choice

Science  02 Jun 2006:
Vol. 312, Issue 5778, pp. 1279

    Butterfly Population Dynamics

    Population dynamics of animals are generally considered to be governed by environmental influences and demographic processes. The potential influence of genetic variation on population dynamics, however, has received much less attention. In a study of the Glanville fritillary butterfly in Finland, Hanski and Saccheri provide evidence that allelic variation in the glycolytic enzyme phosphoglucose isomerase (Pgi), which affects metabolic rate and flight performance, also affects population growth. The butterflies inhabit discrete habitat patches, which vary in size and degree of connectivity to other patches. The strength and nature of the Pgi effect on population growth depended on the ecological context. In larger patches, selection favored genotypes with a slower maturation rate, but the opposite was true in smaller patches, where a faster maturation rate would allow efficient exploitation of limited resources. This integration of detailed field study and molecular genetics promises to open new avenues in the study of population dynamics. — AMS

    PLoS Biol. 4, e129 (2006).


    Streams Traced by Speckle

    Particle-imaging velocimetry (PIV), a common technique for studying the flow of fluids, involves seeding a fluid with tracer particles such as dyes or photoluminescent beads, and then tracking their motion over time. In many applications, there is a growing need to understand the flow pattern in all three spatial dimensions. However, the optics involved in PIV generally limit the sampling volume to a thin two-dimensional (2D) sheet within the bulk flowing system.

    Alaimo et al. present a simple technique to address this shortcoming. After directing a coherent probe beam through the flowing particle suspension, they detect and analyze the speckle pattern that results from the interference of the weak portion of light scattered by the seed particles with the intense transmitted portion. Because the speckle pattern arises from particles distributed throughout the whole fluid volume, 3D flow dynamics can be extracted from the 2D velocity mapping data acquired in real time. The authors demonstrate the method using an aqueous suspension of 300-nm-diameter latex spheres. — ISO

    Appl. Phys. Lett. 88, 191101 (2006).


    The Social Life of Bacteria

    We know relatively little about the population biology of bacteria in natural environments such as the soil; it is unclear, in particular, whether strains compete or whether the diversity observed is functional. Vos and Velicer investigated genetic diversity in Myxococcus xanthus, a remarkable social bacterium that indulges in swarming, social predation, and, in the face of starvation, can differentiate to form multi-cellular fruiting bodies. Multilocus sequence typing was used to study the evolutionary relationships among isolates sampled from a 16-x-16-cm patch of soil. More than 20 unique genotypes were found that appeared to have evolved clonally. Most were closely related, but there were rare divergent strains that had perhaps blown in as spores. This type of population structure does not resemble the epidemic populations of pathogens, nor does it resemble the emerging picture for marine bacteria, which seem to accumulate neutral mutations that are not regularly purged. Perhaps the spatially structured soil habitat offers clones protection from selective sweeps, or sympatric genotypes may come into contact when swarming or if the soil is disturbed. In the lab, clone pairs of Myxococcus are known to be highly antagonistic; it will be interesting to see how more closely related strains interact under natural conditions. — CA

    Appl. Environ. Microbiol. 72, 3615 (2006).


    Silicon Seeding

    Why should planets form around some stars but not others? One clue has been that planets are more common around stars rich in iron. However, the role of iron in planetary growth remains unclear. Robinson et al. show that planet-hosting stars are enriched not only with iron but also with silicon and nickel. Silicon, in particular, may be a key player in the process. Initially, silicon is created from the fusion of oxygen nuclei inside the star, perhaps suggesting that planet-ringed stars should also be high in oxygen. Abundant silicon and oxygen could facilitate formation of a disk of solid debris around the star—indeed, silica and silicates are basic building blocks of most large planetary bodies in our own solar system.

    The observed abundance of silicon also supports the core accretion model of the formation of large gas planets, such as Jupiter and Saturn. The hard cores of gas giants must grow rapidly, so that they can sweep in their gas atmospheres before the disk dissipates. To form a planet, the density of solid material in the disk must be high enough for the solids to clump together quickly. High silicon abundance would increase the likelihood of reaching this density threshold, perhaps helped by the presence of nickel and other heavy metals. — JB

    Astrophys. J. 643, 484 (2006).


    Streams Swirled by Dean

    In microfluidic systems, mixing of the low-volume fluid streams is hindered by slow diffusion rates and smooth flow behavior. Although mixing can be enhanced using external energy, passive approaches that rely on the channel geometries are often preferred for sensitive materials. However, such passive strategies can require complex, expensive channel fabrication, such as elaborate three-dimensional (3D) networks and incorporation of groove or ridge features in the channels.

    Sudarsan and Ugaz present an easily fabricated passive design, composed of simple 2D smooth-walled channels. The mixing enhancement arises from Dean flow: the transverse flow field induced in curved channels by the interplay of centrifugal effects and inertial axial motion. A planar split-and-recombine arrangement generated alternating layers of different fluids. When two colored streams moved through the curve, counterrotating Dean vortices caused them to flow through one another and exchange position. In a second device, the authors incorporated an abrupt increase in the channel cross-sectional area, which induced expansion vortices that enhanced mixing in the horizontal dimension. At the same time, vertical mixing occurred through Dean flows brought on by an asymmetric serpentine geometry. — MSL

    Proc. Natl. Acad. Sci. U.S.A. 103, 7228 (2006).


    A Fishy Tale of Diversity

    The lifestyle of the mangrove killifish Kryptolebias marmoratus is a solitary one, in which the fish inhabits areas around red mangrove forests. Populations are generally made up of self-fertilizing hermaphrodites that are homozygous; however, high genetic diversity is observed among lineages. This diversity has been attributed to a high rate of mutation, migration, and genetic drift among populations. Mackiewicz et al. have surveyed 35 microsatellite loci in individual wild-caught fish from Florida. Based on the genotypes of these animals, the authors propose that genotypic diversity results, instead, from outcrossing. This represents a mixed-mating strategy—something that has been observed previously in hermaphroditic plants and invertebrates, but such extensive interspecimen genetic variation in vertebrates with negligible heterozygosity has not been observed. The outcrossing events provide inbred lines with a burst of genetic heterozygosity for subsequent generation of new recombinant inbred lines after self-fertilization resumes. The mixed-mating strategy is likely to provide an adaptive advantage for the harsh environment in which the killifish reside. — BAP

    Proc. R. Soc. London Ser. B 10.1098/rspb.2006.3594 (2006).

  7. STKE

    Women See Friends, Men See Foes

    Gender differences in social behavior are well known. Thompson et al. now show that arginine vasopressin (AVP), which is known to influence the behavior of other mammals, influences social behaviors in humans in a gender-specific manner. AVP or saline was administered intranasally, and various responses to faces of the same sex with happy, neutral, or angry expressions were recorded. Differences in the activity of a muscle in the brow, the contraction of which is associated with anger or threat, were increased in men exposed to AVP and then shown neutral faces, whereas women exposed to AVP showed a decrease in the activity of this muscle in response to happy or angry faces. Although AVP-treated individuals of both sexes exhibited increased anxiety, men reported a decrease in the perceived friendliness or approachability of people with happy expressions, whereas women reported an increase in the approachability or friendliness of people with neutral expressions. The results may provide a molecular mechanism for the evolution of gender-specific responses to stress. — NG

    Proc. Natl. Acad. Sci. U.S.A. 103, 7889 (2006).