skip to main content


Search for: All records

Creators/Authors contains: "Usher, E"

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Genetic divergence along the central Baja California Peninsula, Mexico, has been hypothesized to reflect a Pliocene cross-peninsular seaway that previously isolated northern and southern populations of terrestrial plants and animals. One way to test this hypothesis is through quantitative analysis of relict channels preserved on low-relief paleo-surfaces. Recognition of tidal channels on relict landscapes offers a powerful tool for reconstructing past sea level in tectonically active arid coastal regions where crustal uplift results in relative sea-level fall and preservation of ancient channel networks. This method requires reliable criteria to distinguish fluvial versus tidal channels, which is challenging due to the overlap of standard metrics for the two channel types, and possible inheritance or overprinting of geometries. We improve the utility of existing metrics and explore the potential for identifying paleo-sea-level indicators by analyzing modern and ancient channels to identify unique patterns in planform geometry and to evaluate their applicability for classifying tidal versus fluvial origins. Preliminary measurements of geographically diverse modern systems reveal distinct, quantifiable differences between the two channel types in along-channel curvature, width, and wavelet spectra. Modern tidal channels display a pronounced and systematic down-channel increase in channel width and decrease in curvature. In contrast, modern fluvial channels do not display spatial patterns in channel width and curvature along their lengths. These patterns provide diagnostic criteria that can be paired with wavelet analysis of meander belts to classify the paleoenvironment of ancient channels based on their planform geometry. We apply this approach to evaluate the origin of channels preserved on relict landscapes in the San Ignacio trough in the central Baja California peninsula, a former low-relief embayment of the Pacific Ocean. Early results reveal the presence of ancient tidal channel networks at elevations of ~ 50-300 m above modern sea level on surfaces that are independently dated to be ca. 4-5 Ma. These findings provide evidence for post 4-Ma uplift in the mid-peninsular region and an ancient tidal environment that may have isolated northern and southern terrestrial populations. 
    more » « less
  2. Abstract

    Tardigrades are microscopic animals that survive desiccation by inducing biostasis. To survive drying tardigrades rely on intrinsically disordered CAHS proteins, which also function to prevent perturbations induced by drying in vitro and in heterologous systems. CAHS proteins have been shown to form gels both in vitro and in vivo, which has been speculated to be linked to their protective capacity. However, the sequence features and mechanisms underlying gel formation and the necessity of gelation for protection have not been demonstrated. Here we report a mechanism of fibrillization and gelation for CAHS D similar to that of intermediate filament assembly. We show that in vitro, gelation restricts molecular motion, immobilizing and protecting labile material from the harmful effects of drying. In vivo, we observe that CAHS D forms fibrillar networks during osmotic stress. Fibrillar networking of CAHS D improves survival of osmotically shocked cells. We observe two emergent properties associated with fibrillization; (i) prevention of cell volume change and (ii) reduction of metabolic activity during osmotic shock. We find that there is no significant correlation between maintenance of cell volume and survival, while there is a significant correlation between reduced metabolism and survival. Importantly, CAHS D's fibrillar network formation is reversible and metabolic rates return to control levels after CAHS fibers are resolved. This work provides insights into how tardigrades induce reversible biostasis through the self‐assembly of labile CAHS gels.

     
    more » « less