Predation,apparent competition,and the structure of prey communities

It is argued that alternate prey species in the diet of a food-limited generalist predator should reduce each other's equilibrial abundances, whether or not they directly compete. Such indirect, interspecific interactions are labeled apparent competition. Two examples are discussed in which an observed pattern of habitat segregation was at first interpreted as evidence for direct competition, but later interpreted as apparent competition resulting from shared predation. In order to study the consequences of predator-mediated apparent competition in isolation from other complicating factors, a model community is analyzed in which there is no direct interspecific competition among the prey. An explicit necessary condition for prey species coexistence is derived for the case of one predator feeding on many prey species. This model community has several interesting properties: (1) Prey species with high relative values for a parameter $\text{r}\text{a}$ are “keystone” species in the community; (2) prey species can be excluded from the community by “diffuse” apparent competition; (3) large changes in the niche breadth of the predator need not correspond to large changes in predator density; (4) the prey trophic level as a whole is regulated by the predator, yet each of its constituent species is regulated by both the predator and available resources; (5) increased productivity may either increase, decrease, or leave unchanged the number of species in the community; (6) a decrease in density-independent mortality may decrease species diversity. These conclusions seem to be robust to changes in the prey growth equations and to the incorporation of predator satiation. By contrast, adding prey refugia or predator switching to the model weakens these conclusions. If the predator can be satiated or switched, the elements a_ij comprising the community matrix may have signs opposite the long-term effect of j upon i. The effect of natural selection upon prey species coexistence is discussed. Unless r_i, K_i, and a_i are tightly coupled, natural selection within prey species i will tend to decrease the equilibrial abundance of species j.     

EXAFS of the type-1 copper site of rusticyanin

Extended X-ray absorption fine structure (EXAFS) spectra at the Cu K-edge have been recorded of the oxidized and reduced form at pH 3.5 of rusticyanin, the type-1 or 'blue'-copper protein from Thiobacillus ferrooxidans. The EXAFS of oxidized rusticyanin is well simulated with models assuming a ligand set of 2 N(His) and 1 S(Cys) at 1.99 and 2.16 A, respectively. Upon reduction, the average Cu-N ligand distance increases by approx. 0.08A. For both redox states studied, the fit by the simulation is significantly improved by including a contribution of an additional sulfur ligand at approx. 2.8 A. From comparison with structural data of other blue-copper proteins, it is concluded that the copper coordination environment is relatively rigid, which may be a clue to its high redox potential.     

Effects of antibiotics on synthesis and persistence of sigma E in sporulating Bacillus subtilis.

A potential regulatory link between the activation of a sporulation-specific sigma factor (sigma E) and forespore septum formation was investigated by treating Bacillus subtilis with inhibitors of protein or peptidoglycan synthesis and monitoring the consequences of these treatments on sigma E activation and septation. Western blot (immunoblot) and electron microscopic analyses revealed that both the formation of sigma E and septation were inhibited to a similar degree when either rifampin or chloramphenicol was added at different times before the second hour into sporulation but that penicillin preferentially blocked septation. We interpret these results as indicating that the syntheses of the gene products for both septation and sigma E activation occur at approximately the same time in development but that synthesis of an intact septum is unlikely to be a prerequisite for the formation of sigma E. We observed that penicillin could not only block septation but, depending on the time of its addition, could also inhibit both the activation of sigma E and the synthesis of its precursor. The basis of this effect is unknown, but it is not due to an overall disruption of protein synthesis. The incorporation of [35S] methionine by the sporulating cultures was unaffected by penicillin treatment. A time course study of the effects of rifampin and chloramphenicol treatments on sigma E levels revealed that both the synthesis of sigma E and its disappearance from sporulating cultures is inhibited by these antibiotics. This suggests that ongoing macromolecular synthesis is required for the turnover of sigma E.     

Elimination of multiple innervation in the developing avian superior oblique muscle (1)

Motor endplates in the developing avian superior oblique muscle first appear on day 18 of incubation. Most of the endplates from this time through hatching (day 27) are innervated by multiple fibers. Each endplate in the post-hatching period is innervated by only one fiber. Time of elimination of multineuronal innervation does not correlate with the time of trochlear neuron loss; the former occurs much later in development. Removal of multiple innervation is therefore, not the cause of the naturally occurring neuron loss.     

Defining essential stem cell characteristics in adipose-derived stromal cells extracted from distinct anatomical sites

The discovery of adipose-derived stromal cells (ASCs) has created many opportunities for the development of patient-specific cell-based replacement therapies. We have isolated multiple cell strains of ASCs from various anatomical sites (abdomen, arms/legs, breast, buttocks), indicating widespread distribution of ASCs throughout the body. Unfortunately, there exists a general lack of agreement in the literature as to their "stem cell" characteristics. We find that telomerase activity and expression of its catalytic subunit in ASCs are both below the levels of detection, independent of age and culturing conditions. ASCs also undergo telomere attrition and eventually senesce, while maintaining a stable karyotype without the development of spontaneous tumor-associated abnormalities. Using a set of cell surface markers that have been promoted to identify ASCs, we find that they failed to distinguish ASCs from normal fibroblasts, as both are positive for CD29, CD73 and CD105 and negative for CD14, CD31 and CD45. All of the ASC isolates are multipotent, capable of differentiating into osteocytes, chondrocytes and adipocytes, while fibroblasts show no differentiation potential. Our ASC strains also show elevated expression of genes associated with pluripotent cells, Oct-4, SOX2 and NANOG, when compared to fibroblasts and bone marrow-derived mesenchymal stem cells (BM-MSCs), although the levels were lower than induced pluripotent stem cells (iPS). Together, our data suggest that, while the cell surface profile of ASCs does not distinguish them from normal fibroblasts, their differentiation capacity and the expression of genes closely linked to pluripotency clearly define ASCs as multipotent stem cells, regardless of tissue isolation location.     

Covalent labelling of histones with aurothiomalate. Gold-labelled H2A-H2B dimers assemble to octamers, which form crystalline helical tubes

Histone octamers were covalently labelled with aurothiomalate at amino groups by the method of carbodiimide activation. The labelling procedure was demonstrated to result in the specific covalent coupling through a single bond of the heavy metal atom label to protein amino groups. Such octamers were dissociated to yield soluble H2A-H2B dimers containing three gold atoms per dimer. The dimers were reconstituted with native H3-H4 tetramers to form labelled octamers, which were crystallized to form helical tubes. This strongly suggests that this procedure resulted in minimal changes of protein conformation.     

Ligand-Induced Translocation of Epidermal Growth Factor Receptor to the Nucleus of NR6/HER Fibroblasts Is Serum Dependent

Ligand-induced translocation of epidermal growth factor receptors (EGF-R) to the nucleus of NR6/HER fibroblasts has been studied by immunoelectron microscopy. Following treatment of NR6/HER cells with epidermal growth factor (EGF) for 1 h, there was a decrease in EGF-R labeling at the plasma membrane and a corresponding increase in EGF-R in the nucleus. This was preceded by a rapid and sustained increase in nuclear phosphotyrosine content, detectable within 2 min of EGF treatment. EGF-R translocation into the nucleus was completely prevented by 18 h serum starvation prior to treatment with EGF. These results indicate that translocation of EGF-R to the nucleus is a controlled process and they suggest theft EGF-R may directly influence nuclear function.