Editors' Choice

Science  08 Nov 2019:
Vol. 366, Issue 6466, pp. 703
  1. Galaxies

    How massive can a spiral galaxy be?

    1. Keith T. Smith

    NGC 6872, the Condor Galaxy, is a large barred spiral galaxy located 212 million light years from Earth in the constellation Pavo.

    PHOTO: ESO/VLT, JUDY SCHMIDT (CC BY 4.0)

    The most massive galaxies in the Universe are all giant ellipticals, formed in major merger events. Spiral galaxies form more quiescently, by accreting gas from the intergalactic medium. Ogle et al. observed a sample of the most massive spiral galaxies known. For each galaxy, they separately determined the mass of stars and the mass of the surrounding dark matter halo. At the highest masses, the sample diverges from well-known scaling laws that describe lower-mass spiral galaxies, implying a maximum spiral galaxy stellar mass of about 1011.8 solar masses. These galaxies are very efficient at converting gas into stars, which may relate to the apparent mass limit.

    Astrophys. J. Lett. 884, L11 (2019).

  2. Immunology

    Mechanosensing mucosal immunity

    1. Seth Thomas Scanlon

    In lymph nodes, fibroblastic reticular cells (FRCs) and associated collagen fibers assemble into a network of “conduits.” The network is believed to allow the swift and efficient uptake of lymph-borne antigens. The role of these networks in other lymphoid tissues is less well described. Chang et al. characterized a similar conduit system in Peyer's patches (PPs) in the intestine. In this case, the conduits regulate the flow of absorbed luminal fluid. The maintenance of this system by FRCs depends on luminal flow and is needed to sustain lymphocyte recruitment to PPs and for effective humoral immune responses. Intriguingly, FRCs perceive luminal flow by means of the mechanosensitive ion channel Piezo1.

    Nat. Immunol. 20, 1506 (2019).

  3. Behavior

    Avoiding ant jams

    1. Caroline Ash

    Experiments on ants show how they avoid traffic congestion.

    PHOTO: ALLGORD/ISTOCKPHOTO

    Traffic congestion is a daily challenge for humans pursuing individual objectives. Ants also live, work, and commute in crowded circumstances but avoid gridlock. Poisonnier et al. analyzed how the collective foraging activity of Argentine ants never suffers congestion. Ant trails to food sources characteristically show bidirectional movement, which would appear to be vulnerable to collisions. However, why collisions are infrequent is not well understood. Using different sizes of lab colonies, the authors built bridges of different widths between the ants and food. Surprisingly, it was not strict discipline, such as forming lanes, that prevented holdups; rather, individual ants constantly adjusted their behavior to prevent jams. First they avoided leaving the nest at high traffic density; then they restrained themselves from social contact, information sharing, and making U-turns; and finally, they adjusted their speed according to individual perceptions of local crowding. Unlike humans, ants can regulate individual behavior to benefit the smooth functioning of the whole population. Maybe we can program driverless cars to perform as well as ants.

    eLife 8, e48945 (2019).

  4. Cell Biology

    Making a three-way contact

    1. Stella M. Hurtley

    Membrane contact sites between organelles are important in several cellular functions. Within cells, lipids are stored in specialized organelles known as lipid droplets. In some cell types, lipid droplets interact directly with mitochondria. In adipocytes, which are professional fat-storing cells, Freyre et al. found that the outer mitochondrial membrane protein MIGA2 tethers mitochondria to lipid droplets. They identified a lipid droplet–targeting motif and found that MIGA2 also interacts with the endoplasmic reticulum membrane proteins VAP-A and VAP-B. Depleting MIGA2 severely perturbed adipocyte differentiation. MIGA2 is required for the de novo synthesis of lipids and links mitochondrial lipogenesis and endoplasmic reticulum triglyceride synthesis during lipid-droplet loading.

    Mol. Cell 10.1016/j.molcel.2019.09.011 (2019).

  5. Structural Biology

    A hook to understand motility

    1. Valda Vinson

    Bacterial flagella provide the motility required for pathogenicity and biofilm formation. Flagella comprise a motor anchored in the cell membrane, a filament propeller, and a hook region that transmits torque between the motor and the filament. Shibata et al. resolved the complete structure of the hook from Salmonella enterica by cryo–electron microscopy at 2.9-angstrom resolution without imposing helical symmetry. The single protein forms a supercoiled tubular helix composed of 11 protofilaments. Although the hook is built from a single protein, subunits in each protofilament have a slightly different conformation because angles between the three domains are gradually adjusted by interdomain interactions. The conformational tweaks form a curved hook that can rotate by dynamic switching between states. A companion piece by Egelman further discusses the work.

    Nat. Struct. Mol. Biol. 26, 941, 848 (2019).

  6. Materials Science

    Programming programmable materials

    1. Marc S. Lavine

    DNA-decorated nanoparticles can be designed to react like selective atoms where “bonds” form only between certain pairs, depending on the specific DNA sequences attached to each nanoparticle. However, this approach tends either to work best for large, ordered crystals or to fail as particles get trapped in locally bonded pairs. Zornberg et al. show that a locally applied, variable-power laser with a wavelength of 532 nanometers can selectively heat regions of pair-bonded decorated gold nanoparticles, pushing them out of amorphous states and driving either nucleation or growth into larger ordered regions as desired. Particles can be driven away from hot spots, collectively made to pile up in certain regions, or gently annealed through controlled heating.

    Nano Lett. 10.1021/acs.nanolett.9b03258 (2019).

  7. Ice Sheets

    Warm, melt, crack

    1. H. Jesse Smith

    Ice-shelf collapse is one of the most dramatic expressions of the disintegration of ice sheets, yet what controls the speed at which collapse occurs is still not well understood. Robel and Banwell used a cellular automaton model to show that there is an intrinsic speed limit on ice-shelf collapse controlled by cascades of interacting meltpond hydrofracture events. They found that typical flexural length scales of Antarctic ice shelves ensure that hydrofracture interactions remain localized and argue that the speed at which the Antarctic Larsen B Ice Shelf collapsed in 2002 was caused by a season of anomalously high surface meltwater. Rapid ice-shelf collapse can be caused only by a correspondingly rapid increase in meltwater production.

    Geophys. Res. Lett. 10.1029/2019GL084397 (2019).