Effects of neutral selection on the evolution of molecular species

We introduce a new model of evolution on a fitness landscape possessing a tunable degree of neutrality. The model allows us to study the general properties of molecular species undergoing neutral evolution. We find that a number of phenomena seen in RNA sequence-structure maps are present also in our general model. Examples are the occurrence of "common" structures which occupy a fraction of the genotype space which tends to unity as the length of the genotype increases, and the formation of percolating neutral networks which cover the genotype space in such a way that a member of such a network can be found within a small radius of any point in the space. We also describe a number of new phenomena which appear to be general properties of neutrally evolving systems. In particular, we show that the maximum fitness attained during the adaptive walk of a population evolving on such a fitness landscape increases with increasing degree of neutrality, and is directly related to the fitness of the most fit percolating network.Comment: 16 pages including 4 postscript figures, typeset in LaTeX2e using the Elsevier macro package elsart.cl

The Curse of Shared Knowledge: Recursive Belief Reasoning in a Coordination Game with Imperfect Information

Common knowledge is a necessary condition for safe group coordination. When common knowledge can not be obtained, humans routinely use their ability to attribute beliefs and intentions in order to infer what is known. But such shared knowledge attributions are limited in depth and therefore prone to coordination failures, because any finite-order knowledge attribution allows for an even higher order attribution that may change what is known by whom. In three separate experiments we investigate to which degree human participants (N=802) are able to recognize the difference between common knowledge and nth-order shared knowledge. We use a new two-person coordination game with imperfect information that is able to cast the recursive game structure and higher-order uncertainties into a simple, everyday-like setting. Our results show that participants have a very hard time accepting the fact that common knowledge is not reducible to shared knowledge. Instead, participants try to coordinate even at the shallowest depths of shared knowledge and in spite of huge payoff penalties

The origin of green icebergs in Antarctica

A comparison of samples from a translucent green iceberg with a core from the Ronne Ice Shelf revealed an excellent agreement in isotopic composition, crystal structure, and incorporated sediment particles. Marine shelf ice which constitutes the basal portion of some ice shelves is considered to be the source of green icebergs. It most likely results from "ice pump" processes which produce large amounts of ice platelets in the water column beneath ice shelves. These subsequently accumulate and become compacted into bubble-free, desalinated ice. Iceberg and drift-buoy trajectories indicate that green icebergs observed in the Weddell Sea originate from the Amery Ice Shelf rather than from the Ronne Ice Shelf, although the latter ice shelf is also a potential source

Mouse Heterochromatin Adopts Digital Compaction States without Showing Hallmarks of HP1-Driven Liquid-Liquid Phase Separation

Mouse cells package heterochromatin into compact foci. Erdel et al. show that these foci lack hallmarks of liquid droplets and rather resemble collapsed polymer globules. Their size, accessibility, and compaction are independent of HP1. They can adopt two distinct folding states that possibly represent the fundamental modes of chromatin compaction

Extracorporeal Photopheresis versus Anticytokine Therapy as a Second-Line Treatment for Steroid-Refractory Acute GVHD: A Multicenter Comparative Analysis

AbstractThe optimal therapy for steroid-refractory (SR) acute graft-versus-host disease (aGVHD) is undefined. We studied patients with SR aGVHD, comparing extracorporeal photopheresis (ECP; n = 57) and anticytokine therapy (n = 41). In multivariate analyses, ECP, adjusted for steroid dose (odds ratio, 3.42; P = .007), and grade >II aGVHD (odds ratio, 68; P < .001) were independent predictors of response. ECP therapy, adjusted for conditioning regimen intensity and steroid dose, was associated with superior survival (hazard ratio [HR], 4.6; P = .016) in patients with SR grade II aGVHD. Grade >II aGVHD at onset of salvage therapy (HR, 9.4; P < .001) and lack of response to therapy (HR, 3.09; P = .011) were associated with inferior survival. These findings require validation in a prospective randomized study

Glacier-bed geomorphic processes and hydrologic conditions relevant to nuclear waste disposal

Characterizing glaciotectonic deformation, glacial erosion and sedimentation, and basal hydrologic conditions of ice sheets is vital for selecting sites for nuclear waste repositories at high latitudes. Glaciotectonic deformation is enhanced by excess pore pressures that commonly persist near ice sheet margins. Depths of such deformation can extend locally to a few tens of meters, with depths up to approximately 300 m in exceptional cases. Rates of glacial erosion are highly variable (0.05–15 mm a−1), but ratesa−1 are expected in tectonically quiescent regions. Total erosion probably not exceeding several tens of meters is expected during a glacial cycle, although locally erosion could be greater. Consolidation of glacial sediments that is less than expected from independent estimates of glacier thickness indicates that heads at the bases of past ice sheets were usually within 30% of the floatation value. This conclusion is reinforced by direct measurements of water pressure beneath portions of the West Antarctic ice sheet, which indicate average headsbed, despite thick ice at subfreezing temperatures. Therefore, in models of subglacial groundwater flow used to assess sites for nuclear waste repositories, a flux upper boundary condition based on water input from only basal melting will be far more uncertain than applying a hydraulic head at the upper boundary set equal to a large fraction of the floatation value