This Week in Science

Science  14 Feb 2020:
Vol. 367, Issue 6479, pp. 752
  1. Plant Pathology

    A plant pan-genome immunity landscape

    1. Laura M. Zahn

    English laurel afflicted by the bacterial plant pathogen Pseudomonas syringae, subspecies of which cause various diseases in plants.

    PHOTO: NIGEL CATTLIN/MINDEN PICTURES

    Plant pathogens elicit an immune response through effector proteins. In turn, plant genomes encode genes that determine species-specific recognition of these effectors by a process known collectively as effector-triggered immunity (ETI). By examining a range of strains of the pathogen Pseudomonas syringae that infect the model plant Arabidopsis thaliana, Laflamme et al. generated a P. syringae Type III Effector Compendium (PsyTEC) and in turn identified the genes responsible for ETI in Arabidopsis. This pan-genome analysis revealed that relatively few A. thaliana genes are responsible for recognizing the majority of P. syringae effectors. These results provide insight into why most pathogenic microbes only infect specific plant species.

    Science, this issue p. 763

  2. Germ Cell Biology

    Conserved gene specifies germ cell

    1. Beverly A. Purnell

    Germ cells are the exclusive progenitors of gametes. In most studied animals, including humans, germ cells are produced only once during embryogenesis and are not replenished in adult life. DuBuc et al. studied germ cell induction in the clonal cnidarian Hydractinia symbiolongicarpus, an animal that forms germ cells continuously in adult life from stem cells that also generate somatic cells. A single transcription factor is capable of converting the animal's adult stem cells to germ cells. A similar gene also controls germ cell induction in mammalian embryos, but its action there is limited to a single event in early embryogenesis.

    Science, this issue p. 757

  3. Paleontology

    More evidence for a massive turtle

    1. Douglas H. Erwin

    Partial specimens of the extinct, nonmarine turtle Stupendemys geographicus have given paleontologists some indication of its truly massive size, but the biology and systematics of this species remained unclear. Cadena et al. found remains in La Tatacoa Desert in Colombia that set a size record for the largest known complete turtle shell—2.40 meters from neck to tail—and represent the first lower-jaw specimens from the Urumaco region since 1994. These data support the interpretation that there was only one giant erymnochelyin taxon that exhibited sexual dimorphism, with horns in males and hornless females. With this new information, researchers can construct a more complete picture of the predation, diet, geography, and ecology of this species.

    Sci. Adv. 10.1126/sciadv.aay4593 (2020).

  4. Physics

    Understanding fragile topology

    1. Ian S. Osborne

    Exploiting topological features in materials is being pursued as a route to build in robustness of particular properties. Stemming from crystalline symmetries, such topological protection renders the properties robust against defects and provides a platform of rich physics to be studied. Recent developments have revealed the existence of so-called fragile topological phases, where the means of classification due to symmetry is unclear. Z.-D. Song et al. and Peri et al. present a combined theoretical and experimental approach to identify, classify, and measure the properties of fragile topological phases. By invoking twisted boundary conditions, they are able to describe the properties of fragile topological states and verify the expected experimental signature in an acoustic crystal. Understanding how fragile topology arises could be used to develop new materials with exotic properties.

    Science, this issue p. 794, p. 797

  5. Reproductive Biology

    Cellular remodeling of the amnion

    1. Annalisa M. VanHook

    The embryo-surrounding amnion remodels through epithelial-tomesenchymal transition (EMT) and the reverse process, termed MET. Richardson et al. found that in amnions from mice and human term births, EMT was greater after labor compared with before the onset of labor. Oxidative stress and the inflammatory cytokine transforming growth factor–β, which are increased at the end of pregnancy, promoted EMT, whereas the pregnancy maintenance hormone progesterone promoted MET. These data suggest that oxidative stress and inflammatory factors accumulate at parturition to trigger EMT and amnion weakening and rupture.

    Sci. Signal. 13, eaay1486 (2020).

  6. Catalysis

    Overcoming surface defects

    1. Phil Szuromi

    Dry reforming of methane with carbon dioxide creates a mixture of hydrogen and carbon monoxide—synthesis gas—which can be converted into liquid fuels. However, heterogeneous catalysts for this reaction are prone to deactivation through unwanted carbon deposition (coking) and loss of surface area of adsorbed metal nanoparticles through agglomeration (sintering). Y. Song et al. used highly crystalline fumed magnesium oxide to support molybdenumdoped nickel nanoparticle catalysts (see the Perspective by Chen and Xu). On heating, the nanoparticles migrated on the oxide surface to step edges to form larger, highly stable nanoparticles. This process also passivated sites for coking on the oxide to produce a catalyst with high activity and longevity at 800°C.

    Science, this issue p. 777; see also p. 737

  7. Structural Biology

    Strengths and weaknesses of an HIV drug

    1. Valda Vinson

    Retroviruses replicate by inserting a copy of their RNA, which has been reverse transcribed into DNA, into the host genome. This process involves the intasome, a nucleoprotein complex comprising copies of the viral integrase bound at the ends of the viral DNA. HIV integrase strand-transfer inhibitors (INSTIs) stop HIV from replicating by blocking the viral integrase and are widely used in HIV treatment. Cook et al. describe structures of second-generation inhibitors bound to the simian immunodeficiency virus (SIV) intasome and to an intasome with integrase mutations known to cause drug resistance. Passos et al. describe the structures of the HIV intasome bound to a second-generation inhibitor and to developmental compounds that are promising drug leads. These structures show how mutations can cause subtle changes in the active site that affect drug binding, show the basis for the higher activity of later-generation inhibitors, and may guide development of better drugs.

    Science, this issue p. 806, p. 810

  8. Medicine

    From animal models to humans

    1. Gemma Alderton

    A major challenge in medical research is the translation of results in animal models to humans. Animal models are essential to be able to undertake rigorous experimentation to understand mechanisms of disease and therapeutic opportunities. However, disappointing outcomes of clinical trials in patients emphasize the difficulty in ensuring that the data from animal models are relevant to human disease. In a Perspective, Brubaker and Lauffenburger discuss how machine-learning strategies can aid cross-species translation to improve outcomes in humans, but they emphasize that such approaches also come with their own challenges.

    Science, this issue p. 742

  9. Developmental Biology

    Mapping cell fate during hematopoiesis

    1. Beverly A. Purnell

    Biologists have long attempted to understand how stem and progenitor cells in regenerating and embryonic tissues differentiate into mature cell types. Through the use of recent technical advances to sequence the genes expressed in thousands of individual cells, differentiation mechanisms are being revealed. Weinreb et al. extended these methods to track clones of cells (cell families) across time. Their approach reveals differences in cellular gene expression as cells progress through hematopoiesis, which is the process of blood production. Using machine learning, they tested how well gene expression measurements account for the choices that cells make. This work reveals that a considerable gap still exists in understanding differentiation mechanisms, and future methods are needed to fully understand—and ultimately control—cell differentiation.

    Science, this issue p. eaaw3381

  10. Neuroscience

    Memory suppression can help after trauma

    1. Peter Stern

    Therapists have discussed for a long time whether attempts to voluntarily suppress the intrusion of trauma memories are helpful to combat the distressing impacts of trauma. Mary et al. studied survivors of the 2015 Paris terrorist attacks who developed posttraumatic stress disorder and those who did not (see the Perspective by Ersche). Using functional magnetic resonance imaging, they investigated the neural networks underlying the control and suppression of memory retrieval. The results suggest that the characteristic symptoms of the disorder are not related to the memory itself but to its maladaptive control. These results offer new insights into the development of post-traumatic stress disorder and potential avenues for treatment.

    Science, this issue p. eaay8477; see also p. 734

  11. Solid-State Physics

    An unusual conductance sequence

    1. Jelena Stajic

    Effects of correlations between electrons are enhanced in systems of reduced dimensions. The two-dimensional interface between two oxide materials, lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3), exhibits magnetism and superconductivity. In even lower-dimensional systems fabricated in similar heterostructures, electrons can pair without going superconducting. Briggeman et al. have now observed another exotic effect in LaAlO3/SrTiO3 waveguides: At certain magnetic fields, the conductance in these one-dimensional systems exhibits steps of an unconventional sequence. To understand the experimental data, the researchers used a model that accounted for interactions between electrons and found that the phenomenology was consistent with the formation of a series of correlated phases characterized by bound states of three or more electrons.

    Science, this issue p. 769

  12. Materials Science

    Ionic elastomeric material electronics

    1. Marc S. Lavine

    Wearable devices often need to be soft or flexible, and ideally, these properties would extend beyond packaging material to also include the electronics. Some soft ionic conductors have been made in the form of flexible, stretchable, and transparent devices, but leaks from these materials is a concern. Kim et al. demonstrate ionic elastomeric diodes and transistors that harness ionic double layers to rectify and switch ionic currents (see the Perspective by Gao and Lee). This is achieved without trapped liquids by fixing the anions or cations to an elastomer network while the other species of ions remain mobile.

    Science, this issue p. 773; see also p. 735

  13. Topological Matter

    Controlling the interactions

    1. Jelena Stajic

    Near charge neutrality and subject to perpendicular magnetic fields, graphene is expected to become a ferromagnet with edge states not unlike those in two-dimensional topological insulators. Observing this effect experimentally has proven tricky because very large magnetic fields are needed to overcome the effect of electron-electron interactions, which drive the system to competing states. Instead of amping up the field, Veyrat et al. placed their graphene samples on a substrate made out of strontium titanate, which effectively screened the interactions. Transport measurements confirmed the formation of the characteristic edge states.

    Science, this issue p. 781

  14. Dryland Ecology

    Thresholds of aridity

    1. Andrew M. Sugden

    Increasing aridity due to climate change is expected to affect multiple ecosystem structural and functional attributes in global drylands, which cover ∼45% of the terrestrial globe. Berdugo et al. show that increasing aridity promotes thresholds on the structure and functioning of drylands (see the Perspective by Hirota and Oliveira). Their database includes 20 variables summarizing multiple aspects and levels of ecological organization. They found evidence for a series of abrupt ecological events occurring sequentially in three phases, culminating with a shift to low-cover ecosystems that are nutrient- and species-poor at high aridity values. They estimate that more than 20% of land surface will cross at least one of the thresholds by 2100, which can potentially lead to widespread land degradation and desertification worldwide.

    Science, this issue p. 787; see also p. 739

  15. Carbon Cycle

    Breaking up is easy to do

    1. H. Jesse Smith

    Sinking particles transport carbon to the seafloor, where they are buried in sediments and either provide food for benthic organisms or sequester the carbon they contain. However, only ∼30% of the maximum flux reaches depths of a kilometer. This loss cannot be fully accounted for by current measurements. Briggs et al. used data collected by robotic Biogeochemical-Argo floats to quantify total mesopelagic fragmentation and found that this process accounts for roughly half of the observed flux loss (see the Perspective by Nayak and Twardowski). Fragmentation is thus perhaps the most important process controlling the remineralization of sinking organic carbon.

    Science, this issue p. 791; see also p. 738

  16. Circadian Rhythms

    Redundancy in circadian clocks?

    1. L. Bryan Ray

    The transcription factor BMAL1 is a core component of the mammalian circadian clock; without it, circadian behaviors are abolished. However, Ray et al. found that in animals lacking BMAL1, peripheral tissues synchronized with a brief pulse of the glucocorticoid hormone dexamethasone appear to retain a 24-hour pacemaker that sustains rhythmic gene expression, protein abundance, and protein phosphorylation in excised liver cells and fibroblasts (see the Perspective by Brown and Sato). These oscillations persisted in the absence of cues from changes in light or temperature. The results raise intriguing questions about the possible nature of the oscillator that maintains the observed rhythms.

    Science, this issue p. 800; see also p. 740

  17. Biodiversity Loss

    Cascading impacts of prey loss

    1. Sacha Vignieri

    The global pandemic caused by the amphibian fungal pathogen Batrachochytrium dendrobatidis has decimated frog populations around the world. This decline has been called out as a potential catastrophe for amphibian species. What has been less explored are the impacts of amphibian declines on other members of their ecological communities. Using survey data collected over 13 years, Zipkin et al. looked at diversity and body condition of a tropical snake community after amphibians were decimated by chytridiomycosis. They found that the snake community was less diverse and most species were in decline, except for a few “winning” species.

    Science, this issue p. 814

  18. Neonatal Jaundice

    Targeting acidity in jaundice

    1. Mattia Maroso

    Neonatal hyperbilirubinemia, also called jaundice, is a pediatric condition caused by high bilirubin levels. When associated with acidosis, jaundice can trigger neurotoxicity and lead to neurological impairments. Lai et al. investigated the link between acidosis and jaundice in human samples and animal models. In samples from children with concomitant acidosis and jaundice, neuronal injury was increased compared with children with jaundice and no acidosis. In mice, bilirubin potentiated the activity of acid-sensing ion channels (ASICs) in neurons, increased firing, and caused cell death. Hyperbilirubinemia and acidosis also promoted cognitive impairments in mice, but these were prevented by ASIC deletion. Targeting ASICs could be a promising way to prevent neurological impairments associated with jaundice.

    Sci. Transl. Med. 12, eaax1337 (2020).

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