Editors' Choice

Science  18 Oct 2013:
Vol. 342, Issue 6156, pp. 290
  1. Evolution

    Push-Me, Pull-You

    1. Sacha Vignieri

    Natural selection is expected to shape phenotypes around an adaptive optimum. In cases where the trend is expected to be directional—bill lengthening in birds, for example—the trait under selection should move toward this optimum. In reality, however, traits often vacillate around it. One cause for this lack of movement is opposing selection. From a 25-year study of green-rumped parrotlets in Venezuela, Tarwater and Beissinger describe evidence for opposing selection as a causal factor. Females that reproduced early in the year produced more offspring; however, both the females and their offspring had lower survival in the following year, indicating a trade-off between selection for fecundity and that for viability. Further, these patterns were oppositely influenced by environmental conditions. Rainfall strongly selected for early breeding, whereas breeding density favored later breeding. Climate change could induce a declining positive-feedback loop wherein small numbers of offspring would be produced due to low rainfall, and this would lead to increased selection for viability and continued low recruitment.

    Proc. Natl. Acad. Sci. U.S.A. 110, 10.1073/pnas.1303821110 (2013).

  2. Applied Physics

    Ridge-Riding Plasmons

    1. Ian S. Osborne

    Nanophotonics aims to integrate the speed of optics with the nanometer size scale of the electronics industry. The several-orders-of-magnitude difference in size scale of the respective components present a technological challenge to that integration. Surface plasmons, the subwavelength collective light-induced electronic excitations that propagate at the surface of metals, can bridge that size gap and so are an area that is being actively pursued. However, there tends to be a trade-off between how far the plasmons can propagate and the extent of their confinement. Using a silicon-on-insulator platform with a silver overlayer, Mu et al. present a simulation study showing that forming a ridge in the silicon can help relax the restrictions of that trade-off. By varying the geometry of the ridge, they show that the confinement of the plasmons can be enhanced without compromising their propagation length. The compatibility of their structure with conventional electronics processing techniques also lends itself favorably to the development of integrated optoelectronic circuits and devices.

    Appl. Phys. Lett. 103, 131107 (2013).

  3. Cancer

    TAMing Gliomas

    1. Paula A. Kiberstis

    Tumor-associated macrophages (TAMs) facilitate tumor growth by stimulating cancer cell proliferation, by helping cancer cells evade the immune system, and by promoting tumor angiogenesis. In human cancer patients, high levels of TAMs often correlate with poor prognosis. These cells are thus an exciting target for new cancer therapies, most of them designed to destroy TAMs. In a twist on this concept, Pyonteck et al. describe a promising therapy that works not by destroying TAMs but by “reeducating” the cells so that they lose their tumor-promoting functions and instead acquire tumor-suppressive functions. In mouse models of glioma, a brain tumor that is resistant to most therapies, the authors found that administration of a brain-penetrant inhibitor of colony-stimulating factor 1 receptor (CSF-1R), a signaling protein that regulates macrophage differentiation, resulted in tumor regression. In the setting of CSF-1R inhibition, TAM survival was promoted by cytokines secreted by the glioma cells. The inhibitor produced a characteristic gene expression signature in the mouse TAMs; in TAMs found in one subtype of human glioma, the same pattern correlated with increased patient survival.

    Nat. Med. 19, 10.1038/nm.3337 (2013).

  4. Biochemistry

    Stepwise Energetics

    1. Gilbert Chin

    One fact that has been drilled into legions of introductory biochemistry students is that the hydrolysis of adenosine triphosphate (ATP) produces energy that is yoked to other reactions. What is less generally inculcated is that some of this energy can be extracted when ATP binds to proteins, in the form of conformational change, and that this can drive reactions too. In a study of nitrogenase, Duval et al. illustrate these two processes in a series of elegant kinetic measurements. They find that the interaction of the ATP-bound Fe protein and the MoFe protein triggers sequential electron transfer from the P cluster to the M cluster (both in the MoFe protein) and then from the Fe-S cluster in the Fe protein to the P cluster. Only after these transfers take place is the ATP hydrolyzed, and only after Pi is released can the ADP-bound Fe protein let go of the MoFe protein. This cycle repeats until the eight electrons needed to effect dinitrogen reduction (and hydrogen evolution) have been delivered. The distinction between binding and hydrolysis will remind biochemists of a certain age of the actomyosin ATPase.

    Proc. Natl. Acad. Sci. U.S.A. 110, 10.1073/pnas.1311218110 (2013).

  5. Education

    Going Backwards

    1. Brad Wible

    For decades, many U.S. communities have bused students to schools beyond their local neighborhood, a result of Supreme Court decisions aimed at reducing racial segregation. Recent legal challenges have led to the elimination of busing in many areas, with students' schools again determined by the neighborhoods in which they live. In Charlotte-Mecklenburg, North Carolina, busing was eliminated, but school-assignment boundaries were also redrawn, changing which neighborhoods sent students to these schools. Billings et al. took advantage of this natural experiment to study educational and social outcomes. They tracked over 40,000 students, roughly half of whom changed schools as a result of remapping. The black-white student achievement gap widened, and both black and white students scored lower on exams after assignment to new schools with larger proportions of minority students. White students were less likely to graduate high school or attend a 4-year college when assigned to schools with more minority students. Finally, crime rates among minority males increased. Allocating more resources to high-minority schools may have offset some of the negative academic outcomes at earlier ages, suggesting policy remedies to counter negative peer effects in school.


    Q. J. Econ. 129, 10.1093/qje/qjt026 (2013).

  6. Physics

    Intrinsic Conductor

    1. Jennifer Sills

    The unexpected finding of a conducting interface between two insulators, LaAlO3 (LAO) and SrTiO3 (STO), can be explained by extrinsic mechanisms such as the formation of oxygen vacancies; whether this interface is intrinsically conducting is still an open question. Warusawithana et al. designed an experiment to minimize the influence of extrinsic factors and glean the influence of the stoichiometry of the LAO layer on the conductivity of the interface. Their mosaic sample was formed by cutting several STO substrates into four pieces each and mounting the pieces from the different substrates next to each other; an LAO layer was then grown on top using molecular beam epitaxy where the ratio of the La and Al fluxes was finely varied to accomplish a gradient of LAO stoichiometries across the mosaic. The authors found that the conducting samples were Al-rich. In these conditions, the excess Al substituted for the missing La, and no vacancies were formed; the natural discontinuity in polarization at the interface was relieved by the formation of a two-dimensional conducting layer. In contrast, in La-rich samples the discontinuity was neutralized by the migration of cations from the interface, enabled by vacancies created because of the off-stoichiometric composition. Similar mechanisms may be at work in other interfacial systems.

    Nat. Commun. 4, 2351 (2013).

  7. Astronomy

    An Extrasolar Perspective

    1. Maria Cruz

    White dwarfs represent the end stage in the evolution of Sun-like stars. There is evidence that the atmospheres of some of them contain material from planetesimals shredded after their orbits passed too close to the dying star. Jura et al. examined the iron-to-aluminum abundance ratio in the atmospheres of seven white dwarfs with well-measured abundances and found that this ratio varies by more than a factor of 100 across the sample. The authors attribute this variation to igneous differentiation, which concentrates iron in cores and aluminum in crusts, and to impacts and collisions between planetesimals, which vary the iron-to-aluminum ratio. The heat source for igneous differentiation must have come from the radioactive decay of 26Al—a radioisotope that was an important short-term heat source in the early solar system and which is produced within massive stars and injected into the interstellar medium through stellar explosions or stellar winds. The inferred abundance of 26Al in the extrasolar environments where the shredded planetesimals formed is similar to that inferred for the early solar system. Thus, the conventional view that our solar system was unusual in its initial 26Al abundance may not be justified.

    Astrophys. J. 775, L41 (2013).

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