Editors' Choice

Science  07 Dec 2018:
Vol. 362, Issue 6419, pp. 1125
  1. Paleoecology

    Fire and grassland evolution

    1. Andrew M. Sugden

    Fire feedback explains the Miocene expansion of the grassland biome.

    PHOTO: RICHARD DU TOIT/MINDEN PICTURES/GETTY IMAGES

    Global grassland underwent a massive expansion in the late Miocene epoch, 5 million to 8 million years ago. Karp et al. examined the role of fire in this expansion, through measurements of fire-derived hydrocarbons and grass-diagnostic carbon isotopes in sediments in Pakistan. They found evidence of a simultaneous increase in seasonality of precipitation and the occurrence of regular fire along with the opening of the landscape and the expansion of grasslands. Their results indicate that a grassland-fire feedback system was a key driver in the expansion of grasslands, a relationship that has remained an integral feature of this ecosystem ever since.

    Proc. Natl. Acad. Sci. U.S.A. 115, 12130 (2018).

  2. Neuroscience

    Inhibition in the fear-learning circuitry

    1. Peter Stern

    Many mental health disorders can be traced to abnormal associative learning. The basolateral amygdala of the brain plays a central role in associative learning and the formation of emotional memories and motivated behaviors. The relevance of the amygdala's anatomical substructure for the acquisition of memories is less clear. Tipps et al. used neuron-specific chemogenetics to systematically probe the circuitry and signaling mechanisms involved in auditory fear learning in mice. Stimulating inhibitory interneurons or inhibiting pyramidal cells was enough to induce an association between a behavior and an auditory cue. This understanding is key to developing therapies for diseases in which associative learning has been disrupted.

    eNeuro 5, ENEURO.0272-18.2018 (2018).

  3. Water Resources

    Managing an unwelcome effect

    1. H. Jesse Smith

    Climate change will have a large impact on the timing and size of the Sierra Nevada snowpack.

    PHOTO: ISTOCK.COM/SIERRARAT

    One of the biggest challenges presented by climate change is water resource management. In the western United States, the storage and release of water by the mountain snowpack is a critical component controlling the summertime flow of headwaters of California's major reservoirs. Rhoades et al. describe how mountain snowpacks will be affected by climate change in that region and how that can be expected to influence peak water volume, peak timing, accumulation rate, and melt rate of water discharge. Among other effects, they project that by the end of the century, peak snowpack timing will occur 4 weeks earlier and peak water volume will be 80% lower under a high–greenhouse gas–emissions scenario.

    Geophys. Res. Lett. 10.1029/2018GL080308 (2018).

  4. Cell Biology

    Keeping a toehold on the matrix

    1. Stella M. Hurtley

    Within our bodies, most tissues are organized in association with an extracellular matrix. The matrix keeps cells where they are supposed to be, and cells adhere to the matrix via integrin-containing cell adhesions. During mitosis, cells round up and release their tight association with the matrix. Lock et al. show that despite this, mitotic cells ensure that they retain their correct location by using a so-called reticular form of cell adhesion. A range of adherent cultured human cells was found to show this property. Reticular adhesions lack several components of classical adhesion complexes, including talin and actin. A key constituent of reticular adhesions is integrin β5, and a lack of it interfered with the retention of spatial localization through normal mitotic cell divisions.

    Nat. Cell Biol. 20, 1290 (2018).

  5. Optoelectronics

    Steering electrons in graphene

    1. Ian S. Osborne

    Guiding and controlling the movement of electrons in solid-state systems is crucial for applications such as ultrafast electronics and the generation of high-harmonic light. Although such electronic control is readily achieved in semiconductors, metals and small-bandgap materials can pose more of a challenge. Heide et al. show that they can control the trajectory of electrons within the two-dimensional plane of graphene using a double pulse from a laser. Tuning the relative polarization between the two pulses by carefully varying the time delay between them allows the direction electron flow to be manipulated on femtosecond time scales. Such an optical technique offers a relatively simple route to study the electronic and topological properties of other two-dimensional materials on ultrafast time scales.

    Phys. Rev. Lett. 121, 207401 (2018).

  6. Physics

    Identifying localization in two dimensions

    1. Jelena Stajic

    Disordered interacting quantum many-body systems can become hopelessly localized. This so-called many-body localization has been studied in one-dimensional systems. In two dimensions, experiments indicate that the localization persists, but because it is difficult to tell the difference between no thermalization and slow thermalization, more theoretical work is needed. Relying on an approximate numerical method, Wahl et al. undertook large-scale simulations of a two-dimensional lattice of bosonic atoms in the presence of interactions and disorder. At intermediate disorder strengths, the on-site entanglement entropies exhibited a bimodal distribution, indicating a coexistence of localized and thermalized states; as disorder was increased, localized states took over. The authors were able to extract a critical disorder strength and set a benchmark for future experiments.

    Nat. Phys. 10.1038/s41567-018-0339-x (2018).

  7. Microbiome

    Global aeroplankton dispersal

    1. Caroline Ash

    Airborne particulates include large numbers of living organisms, as well as dust, pollutants, and other chemicals. Cáliz et al. collected aeroplankton fortnightly for 7 years in the Spanish Pyrenees. High-throughput sequencing of 16S and 18S amplicons identified microbes, including potential pathogens, that had made landfall in rain and snow. Distinct seasonal and climate signals in the data associated with the origin of the air masses. For example, winter microbial fallout originated from as far away as the North American taiga, and summer-occurring organisms contained desert-adapted bacteria from North Africa. Over the collection period, air-mass origins shifted, possibly as a result of climate change. Most atmospheric microbes are cosmopolitan, and it seems the upper atmosphere acts as a global highway for many taxa.

    Proc. Natl. Acad. Sci. U.S.A. 115, 12229 (2018).