Species interactions can explain Taylor's power law for ecological time series

One of the few generalities in ecology, Taylor's power law, describes the species-specific relationship between the temporal or spatial variance of populations and their mean abundances. For populations experiencing constant per capita environmental variability, the regression of log variance versus log mean abundance gives a line with a slope of 2. Despite this expectation, most species have slopes of less than 2 (refs 2, 3-4), indicating that more abundant populations of a species are relatively less variable than expected on the basis of simple statistical grounds. What causes abundant populations to be less variable has received considerable attention, but an explanation for the generality of this pattern is still lacking. Here we suggest a novel explanation for the scaling of temporal variability in population abundances. Using stochastic simulation and analytical models, we demonstrate how negative interactions among species in a community can produce slopes of Taylor's power law of less than 2, like those observed in real data sets. This result provides an example in which the population dynamics of single species can be understood only in the context of interactions within an ecological community.     

A cell line that can induce thymocyte positive selection.

The thymus positively selects thymocytes that bear T-cell receptors which recognize antigen presented by self major histocompatibility complex (MHC) proteins. Positive selection is usually driven by MHC products on radiation-resistant cortical epithelial cells. It is unknown whether positive selection is mediated by all thymic epithelial cells or by some specialized subsets. Here we introduce an H-2b-expressing thymic epithelial cell line into the thymuses of lethally irradiated H-2k animals reconstituted with H-2b/k F1 BM or fetal liver cells. I-Ab-restricted T cells are found in these animals, demonstrating that selection occurs on the introduced epithelial cells.     

Anaerobic microbial metabolism can proceed close to thermodynamic limits.

Many fermentative bacteria obtain energy for growth by reactions in which the change in free energy (DeltaG') is less than that needed to synthesize ATP. These bacteria couple substrate metabolism directly to ATP synthesis, however, by classical phosphoryl transfer reactions. An explanation for the energy economy of these organisms is that biological systems conserve energy in discrete amounts, with a minimum, biochemically convertible energy value of about -20 kJ mol-1 (refs 1, 2, 3). This concept predicts that anaerobic substrate decay ceases before the minimum free energy value is reached, and several studies support this prediction. Here we show that metabolism by syntrophic associations, in which the degradation of a substrate by one species is thermodynamically possible only through removal of the end product by another species, can occur at values close to thermodynamic equilibrium (DeltaG' approximately 0 kJ mol-1). The free energy remaining when substrate metabolism halts is not constant; it depends on the terminal electron-accepting reaction and the amount of energy required for substrate activation. Syntrophic associations metabolize near thermodynamic equilibrium, indicating that bacteria operate extremely efficient catabolic systems.     

High-Tc superconducting materials for electric power applications.

Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable. This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K. Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form. So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices. The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven. Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds.     

Relative fitness can decrease in evolving asexual populations of S. cerevisiae

It is generally accepted from the darwinian theory of evolution that a progressive increase in population adaptation will occur in populations containing genetic variation in fitness, until a stable equilibrium is reached and/or the additive genetic variation is exhausted. However, the theoretical literature of population genetics documents exceptions where mean population fitness may decrease in response to evolutionary changes in gene frequency, due to varying selective coefficients, sexual selection or to epistatic interactions between loci. Until now, no examples of such exceptions have been documented from fitness estimates in either natural or experimental populations. We present here direct evidence that, as a result of epistatic interactions between adaptive mutations, mean population fitness can decrease in asexual evolving populations of the yeast Saccharomyces cerevisiae.     

Human immunodeficiency virus genetic variation that can escape cytotoxic T cell recognition.

In a longitudinal study of HIV seropositive patients, there were fluctuations in the specificity of cytotoxic T cells for the virus. This was matched by variability in proviral gag DNA epitope sequences in the lymphocytes of these patients. Some of these viral variants are not recognized by autologous T cells. Accumulation of such mutations in T-cell antigenic targets would provide a mechanism for immune escape.