Environment versus dispersal in the assembly of western Amazonian palm communities

Aim It is a central issue in ecology and biogeography to understand what governs community assembly and the maintenance of biodiversity in tropical rain forest ecosystems. A key question is the relative importance of environmental species sorting (niche assembly) and dispersal limitation (dispersal assembly), which we investigate using a large dataset from diverse palm communities. Location Lowland rain forest, western Amazon River Basin, Peru. Methods We inventoried palm communities, registering all palm individuals and recording environmental conditions in 149 transects of 5 m × 500 m. We used ordination, Mantel tests and indicator species analysis (ISA) to assess compositional patterns, species responses to geographical location and environmental factors. Mantel tests were used to assess the relative importance of geographical distance (as a proxy for dispersal limitation) and environmental differences as possible drivers of dissimilarity in palm species composition. We repeated the Mantel tests for subsets of species that differ in traits of likely importance for habitat specialization and dispersal (height and range size). Results We found a strong relationship between compositional dissimilarity and environmental distance and a weaker but also significant relationship between compositional dissimilarity and geographical distance. Consistent with expectations, relationships with environmental and geographical distance were stronger for understorey species than for canopy species. Geographical distance had a higher correlation with compositional dissimilarity for small‐ranged species compared with large‐ranged species, whereas the opposite was true for environmental distance. The main environmental correlates were inundation and soil nutrient levels. Main conclusions The assembly of palm communities in the western Amazon appears to be driven primarily by species sorting according to hydrology and soil, but with dispersal limitation also playing an important role. The importance of environmental characteristics and geographical distance varies depending on plant height and geographical range size in agreement with functional predictions, increasing our confidence in the inferred assembly mechanisms.     

Characterization of the inhibitory effect of glucocorticoids on the DNA replication of adult rat hepatocytes growing at various cell densities

Dexamethasone inhibited the basal and EGF-stimulated DNA synthesis of adult rat hepatocytes in primary culture. The inhibition was glucocorticoid-specific: It was shown by dexamethasone and hydrocortisone, but not by progesterone, testosterone, or estradiol; and was counteracted by the glucocorticoid antagonist RU-38486 in a concentration-dependent manner. Dexamethasone acted by decreasing the rate of entry into S-phase (kG1/S), while cell cycle parameters were unaffected. The steroid was able to decrease the kG1/S severalfold even when added more than 20 hr after EGF, half-maximal effect occurring 11 hr after the addition of dexamethasone. Densely populated areas were much more sensitive to the inhibition by dexamethasone than sparsely populated areas within the same culture dish: A moderate (10 nM) concentration of dexamethasone nearly abolished the DNA synthesis in densely populated areas of hepatocyte cultures with only marginal effect on sparsely populated cells.     

The effect of medium composition on the acetone-butanol fermentation in continuous culture

The effects of growing Clostridium acetobutylicum NCIB 8052 in a chemostat under conditions of glucose, NH(4) (+), PO(4) (3-), Mg(2+), and Fe(2+) limitation were examined. It was noted that limitation of any major nutrient resulted in the same fermentation pattern. Conditions where minor nutrient levels were reduced appeared to stimulate solvent production. Under conditions of Mg(2+) restriction, a productive solventogenic culture was produced, with a yield of total solvents on glucose of 35.5%.     

Unusual ciliate-specific codons in Tetrahymena mRNAs are translated correctly in a rabbit reticulocyte lysate supplemented with a subcellular fraction from Tetrahymena.

The codon usage of Tetrahymena thermophila and other ciliates deviates from the 'universal genetic code' in that UAA and probably UAG are not translational termination signals but code for glutamine. Therefore, translation in vitro of mRNA from Tetrahymena in a reticulocyte lysate is prematurely terminated if a UAA or UAG triplet is present in the reading frame of the mRNA. We show that the addition of a subcellular fraction from Tetrahymena thermophila enables a rabbit reticulocyte lysate to translate Tetrahymena mRNAs into full-sized proteins. The activity of the subcellular fraction is shown to depend on the combined function of a protein component(s) and a tRNA(s). The subcellular fraction is easily prepared and its usefulness for the identification of isolated mRNAs from Tetrahymena by their translation products in vitro is demonstrated.     

Coexpression of cytokeratin and vimentin in Rathke's cysts of the human pituitary gland

Summary Immunohistochemistry with monoclonal and polyclonal antibodies revealed the presence of cytokeratins in epithelial cells of Rathke's cysts in the pars intermedia of the human pituitary gland. With monoclonal antibodies specific for individual cytokeratins, the expression of CK 18, CK 8, CK 7, and CK 19 could be shown in these cells. Within the hypophysis, CK 19 and CK 7 were restricted to Rathke's cysts and a few epithelial cell clusters in the pars tuberalis, whereas other cytokeratins were also present in endocrine cells of the pars distalis. Furthermore, vimentin and, focally, glial fibrillary acidic protein (GFAP) were detected in the cystic epithelia. By double labelling, coexpression of cytokeratin and vimentin, GFAP and cytokeratin, and GFAP and vimentin could be demonstrated. Compiled data of all known cases of coexpression of cytokeratin and vimentin in normal cells reveal physiological correlations and suggest a functional significance of this rare type of coexpression of intermediate filament proteins.     

Variations in the cytoplasmic region account for the heterogeneity of the chicken MHC class I (B-F) molecules

Molecular variation among major histocompatibility complex (MHC) class I (B-F) proteins from B-homozygous chickens is apparently caused by C-terminal variation. Analysis of the total B-F protein pool revealed substantial heterogeneity with two or three molecular mass constituents, each being comprised by several isoelectric focusing variants. This heterogeneity could not be reduced by enzymatic deglycosylation. By contrast, proteolytic removal of a small (M _r 1000–4000) fragment from the chain resulted in the generation of a M _r 36 000 fragment, common to all the molecular mass variants. Unlike the parent proteins, the M _r 36 000 fragment derived from isolated variants yielded identical, simple patterns in two-dimensional gel electrophoresis and identical finger prints in peptide mapping. This, together with N-terminal amino acid sequencing, as well as comparison of hydrophobicity properties of fragments obtained by gradual proteolytic digestion, indicated that the small peptide responsible for the major B-F heterogeneity was situated in the intracellular, C-terminal part.