Plant Interactions Alter the Predictions of Metabolic Scaling Theory

Metabolic scaling theory (MST) is an attempt to link physiological processes of individual organisms with macroecology. It predicts a power law relationship with an exponent of −4/3 between mean individual biomass and density during density-dependent mortality (self-thinning). Empirical tests have produced variable results, and the validity of MST is intensely debated. MST focuses on organisms’ internal physiological mechanisms but we hypothesize that ecological interactions can be more important in determining plant mass-density relationships induced by density. We employ an individual-based model of plant stand development that includes three elements: a model of individual plant growth based on MST, different modes of local competition (size-symmetric vs. -asymmetric), and different resource levels. Our model is consistent with the observed variation in the slopes of self-thinning trajectories. Slopes were significantly shallower than −4/3 if competition was size-symmetric. We conclude that when the size of survivors is influenced by strong ecological interactions, these can override predictions of MST, whereas when surviving plants are less affected by interactions, individual-level metabolic processes can scale up to the population level. MST, like thermodynamics or biomechanics, sets limits within which organisms can live and function, but there may be stronger limits determined by ecological interactions. In such cases MST will not be predictive.     

The a2m(r-Pa-Pu) Allele of the En-Controlling Element System in Maize

The Enhancer (En) controlling element system in maize includes numerous alleles, one of which, a2m(r-pa-pu), was recovered as a derivative of the a2m(1 1511) allele. The behavior and composition of this allele is described. In the presence of the independently inherited regulatory element, En, the a2m(r-pa-pu) allele mutates and expresses in the aleurone purple, pale and colorless sectors within a colorless background. In the absence of En, the pigmentation is uniformly pale. Colored and colorless germinal mutations of the a2m(r-pa-pu) allele were recovered, although the derivative alleles tested did not respond to En. Uniformly colored derivatives, designated A2', arise only from states of a2m(r-pa-pu) that respond to an active En at relatively early stages in plant development, suggesting that somatic and germinal mutations may arise through similar events. Mutations to A2-expressing alleles appear to occur relatively late in plant development, that is, at or near meiosis, since recognizable mutant sectors do not appear on testcross ears. Kernels exhibiting such mutants are randomly distributed over the ear. The resulting A2' alleles express pigmentation ranging in a continual sequential series from dark pale to intense purple.     

Reversion of the Maize Mutable Allele o2-hf in Plant Ontogeny

Reversions of the mutable allele o2-hfleading to formation of the phenotypically normal kernels or whole endosperm revertants (WER) are studied in the plant ontogeny. The pattern of WER kernel distribution on the ear maps and analysis of their progeny showed that the reversion of the mutable allele o2-hf occurs at the late premeiotic stages of the ear development. Most of whole endosperm revertants on the ears homozygous for both the mutable allele o2-hfand regulatory element Bg-hf are grouped into clusters. The WER kernels are mostly formed during the period from the gamete fusion to the first division of the primary endosperm nucleus and are not embryo revertants. This clustering of revertant kernels seems to be caused by the joint effect of two factors on the early stages of endosperm development. These factors are (1) diffusion of an additional amount of transposase into the nearby Kernels from the developing endosperm, where the level of this enzyme is sufficient to induce excision of the receptor element and (2) the high proportion of the developing kernels with supra- and subthreshold levels of the Bg-hf-encoded transposase.     

Combined Grazing and Drought Stress Alter the Outcome of Nurse: Beneficiary Interactions in a Semi-arid Ecosystem

Ecosystems - Positive interspecific plant–plant interactions in (semi-)arid ecosystems are crucial for supporting ecosystem diversity and stability, but how interactions respond to grazing...     

Exercise-Induced Skeletal Muscle Adaptations Alter the Activity of Adipose Progenitor Cells

Exercise decreases adiposity and improves metabolic health; however, the physiological and molecular underpinnings of these phenomena remain unknown. Here, we investigate the effect of endurance training on adipose progenitor lineage commitment. Using mice with genetically labeled adipose progenitors, we show that these cells react to exercise by decreasing their proliferation and differentiation potential. Analyses of mouse models that mimic the skeletal muscle adaptation to exercise indicate that muscle, in a non-autonomous manner, regulates adipose progenitor homeostasis, highlighting a role for muscle-derived secreted factors. These findings support a humoral link between skeletal muscle and adipose progenitors and indicate that manipulation of adipose stem cell function may help address obesity and diabetes.     

The Environment Affects Epistatic Interactions to Alter the Topology of an Empirical Fitness Landscape

The fitness effect of mutations can be influenced by their interactions with the environment, other mutations, or both. Previously, we constructed 32 ( = 2⁵) genotypes that comprise all possible combinations of the first five beneficial mutations to fix in a laboratory-evolved population of Escherichia coli. We found that (i) all five mutations were beneficial for the background on which they occurred; (ii) interactions between mutations drove a diminishing returns type epistasis, whereby epistasis became increasingly antagonistic as the expected fitness of a genotype increased; and (iii) the adaptive landscape revealed by the mutation combinations was smooth, having a single global fitness peak. Here we examine how the environment influences epistasis by determining the interactions between the same mutations in two alternative environments, selected from among 1,920 screened environments, that produced the largest increase or decrease in fitness of the most derived genotype. Some general features of the interactions were consistent: mutations tended to remain beneficial and the overall pattern of epistasis was of diminishing returns. Other features depended on the environment; in particular, several mutations were deleterious when added to specific genotypes, indicating the presence of antagonistic interactions that were absent in the original selection environment. Antagonism was not caused by consistent pleiotropic effects of individual mutations but rather by changing interactions between mutations. Our results demonstrate that understanding adaptation in changing environments will require consideration of the combined effect of epistasis and pleiotropy across environments.     

Attractive Protein-Polymer Interactions Markedly Alter the Effect of Macromolecular Crowding on Protein Association Equilibria

The dependence of the fluorescence of catalase upon the concentration of added superoxide dismutase (SOD) indicates that SOD binds to saturable sites on catalase. The affinity of SOD for these sites varies with temperature, and with the concentration of each of three nominally inert polymeric additives—dextran 70, Ficoll 70, and polyethylene glycol 2000. At room temperature (25.0°C) and higher, the addition of high concentrations of polymer is found to significantly enhance the affinity of SOD for catalase, but with decreasing temperature the enhancing effect of polymer addition diminishes, and at 8.0°C, addition of polymer has little or no effect on the affinity of SOD for catalase. The results presented here provide the first experimental evidence for the existence of competition between a repulsive excluded volume interaction between protein and polymer, which tends to enhance association of dilute protein, and an attractive interaction between protein and polymer, which tends to inhibit protein association. The net effect of high concentrations of polymer upon protein associations depends upon the relative strength of these two types of interactions at the temperature of measurement, and may vary significantly between different proteins and/or polymers.