A Selective Bottleneck During Host Entry Drives the Evolution of New Legume Symbionts.

During the emergence of new host-microbe symbioses, microbial fitness results from the ability to complete the different steps of symbiotic life cycles, where each step imposes specific selective pressures. However, the relative contribution of these different selective pressures to the adaptive trajectories of microbial symbionts is still poorly known. Here, we characterized the dynamics of phenotypic adaptation to a simplified symbiotic life cycle during the experimental evolution of a plant pathogenic bacterium into a legume symbiont. We observed that fast adaptation was predominantly explained by improved competitiveness for host entry, which outweighed adaptation to within-host proliferation. Whole-population sequencing of bacteria at regular time intervals along this evolution experiment revealed the continuous accumulation of new mutations (fuelled by a transient hypermutagenesis phase occurring at each cycle before host entry, a phenomenon described in previous work) and sequential sweeps of cohorts of mutations with similar temporal trajectories. The identification of adaptive mutations within the fixed mutational cohorts showed that several adaptive mutations can co-occur in the same cohort. Moreover, all adaptive mutations improved competitiveness for host entry, while only a subset of those also improved within-host proliferation. Computer simulations predict that this effect emerges from the presence of a strong selective bottleneck at host entry occurring before within-host proliferation and just after the hypermutagenesis phase in the rhizosphere. Together, these results show how selective bottlenecks can alter the relative influence of selective pressures acting during bacterial adaptation to multistep infection processes.

The Winding Road from Origin to Emergence (of Life).

Humanity's strive to understand why and how life appeared on planet Earth dates back to prehistoric times. At the beginning of the 19th century, empirical biology started to tackle this question yielding both Charles Darwin's Theory of Evolution and the paradigm that the crucial trigger putting life on its tracks was the appearance of organic molecules. In parallel to these developments in the biological sciences, physics and physical chemistry saw the fundamental laws of thermodynamics being unraveled. Towards the end of the 19th century and during the first half of the 20th century, the tensions between thermodynamics and the "organic-molecules-paradigm" became increasingly difficult to ignore, culminating in Erwin Schrödinger's 1944 formulation of a thermodynamics-compliant vision of life and, consequently, the prerequisites for its appearance. We will first review the major milestones over the last 200 years in the biological and the physical sciences, relevant to making sense of life and its origins and then discuss the more recent reappraisal of the relative importance of metal ions vs. organic molecules in performing the essential processes of a living cell. Based on this reassessment and the modern understanding of biological free energy conversion (aka bioenergetics), we consider that scenarios wherein life emerges from an abiotic chemiosmotic process are both thermodynamics-compliant and the most parsimonious proposed so far.

Dissolved metal ions and mineral-liposome hybrid systems: Underlying interactions, synthesis, and characterization.

Liposomes are versatile lipid-based vesicles with interesting physicochemical properties, making them excellent candidates for interdisciplinary applications in the medicinal, biological, and environmental sciences. The synthesis of mineral-liposome hybrid systems lends normally inert vesicles with the catalytic, magnetic, electrical, and optical properties of the integrated mineral species. Such applications require an understanding of the physicochemical interactions between organic molecules and inorganic crystal structures. This review provides an overview on these interactions and details on synthesis and characterization methods for these systems.

Methanol on the rocks: green rust transformation promotes the oxidation of methane.

Shared coordination geometries between metal ions within reactive minerals and enzymatic metal cofactors hints at mechanistic and possibly evolutionary homology between particular abiotic chemical mineralogies and biological metabolism. The octahedral coordination of reactive Fe2+/3+ minerals such as green rusts, endemic to anoxic sediments and the early Earth's oceans, mirrors the di-iron reaction centre of soluble methane monooxygenase (sMMO), responsible for methane oxidation in methanotrophy. We show that methane oxidation occurs in tandem with the oxidation of green rust to lepidocrocite and magnetite, mimicking radical-mediated methane oxidation found in sMMO to yield not only methanol but also halogenated hydrocarbons in the presence of seawater. This naturally occurring geochemical pathway for CH4 oxidation elucidates a previously unidentified carbon cycling mechanism in modern and ancient environments and reveals clues into mineral-mediated reactions in the synthesis of organic compounds necessary for the emergence of life.