Interactions among resource partitioning, sampling effect, and facilitation on the biodiversity effect: a modeling approach

Resource partitioning, facilitation, and sampling effect are the three mechanisms behind the biodiversity effect, which is depicted usually as the effect of plant-species richness on aboveground net primary production. These mechanisms operate simultaneously but their relative importance and interactions are difficult to unravel experimentally. Thus, niche differentiation and facilitation have been lumped together and separated from the sampling effect. Here, we propose three hypotheses about interactions among the three mechanisms and test them using a simulation model. The model simulated water movement through soil and vegetation, and net primary production mimicking the Patagonian steppe. Using the model, we created grass and shrub monocultures and mixtures, controlled root overlap and grass water-use efficiency (WUE) to simulate gradients of biodiversity, resource partitioning and facilitation. The presence of shrubs facilitated grass growth by increasing its WUE and in turn increased the sampling effect, whereas root overlap (resource partitioning) had, on average, no effect on sampling effect. Interestingly, resource partitioning and facilitation interacted so the effect of facilitation on sampling effect decreased as resource partitioning increased. Sampling effect was enhanced by the difference between the two functional groups in their efficiency in using resources. Morphological and physiological differences make one group outperform the other; once these differences were established further differences did not enhance the sampling effect. In addition, grass WUE and root overlap positively influence the biodiversity effect but showed no interactions.     

A global analysis of root distributions for terrestrial biomes

Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient to the data for each biome (Gale and Grigal 1987). is a simple numerical index of rooting distribution based on the asymptotic equation Y=1-^d, where d = depth and Y = the proportion of roots from the surface to depth d. High values of correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (=0.913, 0.943, and 0.943, respectively), with 80–90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (=0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m^-2. Tropical evergreen forests had the highest root biomass (5 kg m^-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m^-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil. (=0.952), while shrubs had only 21% in the same depth increment (=0.978). The rooting distribution of all temperate and tropical trees was =0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was =0.966 (r ²=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.     

The familial adenomatous polyposis region exhibits many different haplotypes

In the present study, we used five different polymorphic markers to construct the haplotype at the adenomatous polyposis coli (APC) locus in families with familial adenomatous polyposis (FAP) and in the normal Italian population. Non-ambiguous haplotypes were reconstructed from 246 normal chromosomes and 65 FAP chromosomes. In the control population, the four polymorphisms intragenic to APC gave rise to 16 haplotypes, the most common of which (II and XV) accounted for over 50% of all chromosomes. In FAP patients, 13 haplotypes were found but their distribution was not statistically different from normal subjects. Eighty complete chromosomal haplotypes (many fewer than the theoretical maximum of 208) for the five polymorphic sites assayed were observed in the control population, 35 being found in the FAP patients. We compared the distribution of these haplotypes within the two groups; no statistically significant differences between normal and FAP chromosomes were found. The elevated heterogeneity of FAP chromosomes was clearly confirmed by the observation that 19 patients who carried one or other of the two most common APC mutations (nt 3183 and nt 3927) showed 18 different haplotypes. On the basis of these results, we were not able to identify a founder FAP chromosome. Various mechanisms are presented to explain this observation. Received: 5 November 1997 / Accepted: 3 February 1998     

Measurement of the villus surface area and its regional variation in the human full-term placenta

The surface/volume ratio and the surface density of the chorionic villi in different cotyledonary regions of the human mature placenta were studied by stereologic methods. The villus surface/volume ratio showed a mean value of 812.3 cm2/cm3 (standard deviation = 89.2 cm2/cm3). There were no significant differences according to the site from which the sample was obtained. The villus surface density in normal mature placentae was (496.3 +/- 49.0) cm2/cm3. The last parameter showed no differences among regions. Despite the absence of significant differences of exchange surface areas among the cotyledonary regions considered, other important parameters, such as trophoblast thickness, frequency of vasculo-syncytial membranes, as well as the maximal gradient of concentration, facilitates the maternal-fetal transfer by simple diffusion mechanism in the central-parabasal region.     

EspI regulates the ESX‐1 secretion system in response to ATP levels in Mycobacterium tuberculosis

The function of EspI, a 70 kDa protein in Mycobacterium tuberculosis, has remained unclear. Although EspI is encoded by a gene within the esx‐1 locus, in this study we clarify previous conflicting results and show that EspI is not essential for ESX‐1‐mediated secretion or virulence in M. tuberculosis. We also provide evidence that reduction of cellular ATP levels in wild‐type M. tuberculosis using the drug bedaquiline completely blocks ESX‐1‐mediated secretion. Remarkably, M. tuberculosis lacking EspI fails to exhibit this phenotype. Furthermore, mutagenesis of a highly conserved ATP‐binding motif in EspI renders M. tuberculosis incapable of shutting down ESX‐1‐mediated secretion during ATP depletion. Collectively these results show that M. tuberculosis EspI negatively regulates the ESX‐1 secretion system in response to low cellular ATP levels and this function requires the ATP‐binding motif. In light of our results the potential significance of EspI in ESX‐1 function during latent tuberculosis infection and reactivation is also discussed.     

The dynamics of bottom–up and top–down control in a New England salt marsh

Traditionally, salt marsh ecosystems were thought to be controlled exclusively by bottom–up processes. Recently, this paradigm has shifted to include top–down control as an additional primary factor regulating salt‐marsh community structure. The most recent research on consumer impacts in southern US marshes has shown that top–down forces often interact with biotic and abiotic factors, such as secondary fungal infection in grazer‐induced wounds, soil nutrients and climatic variation, to influence ecosystem structure. In a more northern salt marsh, located in New England, we examined the separate and interactive effects of nutrient availability, insect herbivory and secondary fungal infection, on growth of the foundation species, Spartina alterniflora. We used a factorial design with two levels of nutrients (control and addition) insects (control and removal) and fungi (control and removal). Nutrient addition increased plant biomass by 131% in the absence of herbivores. When insect consumers were allowed access to fertilized plots, biomass was reduced by nearly 45% when compared with treatments with nutrients and insecticide. In contrast, insect herbivores did not affect plant biomass in unfertilized control treatments. These differences suggest that consumer effects are triggered under high nutrient levels only. We also found that secondary fungal infections in grazer‐induced wounds, in contrast to lower latitude marshes, did not significantly impact primary production. Our results suggest that while New England salt marshes may typically be under bottom–up control, eutrophication can trigger dual control with inclusion of top–down regulation. However, unlike lower latitude marshes, consumer control of plant growth in northern US salt marshes is not dependent on herbivores facilitating fungal infections that then control grass growth, suggesting that the intensity of disease mediated top–down control by small grazers may be regulated by climate and/or grazer identity that co‐vary with latitude.     

Presence of a Chloroplast DNA Sequence in an Autonomous Circular DNA Molecule in Cultured Rice Cells (Oryza sativa L.)

Sequence analysis of twelve DNA fragments, which had previouslybeen found to be extensively amplified in suspension-culturedrice cells, revealed that two of them, isolated on plasmidsdesignated _pE10 and _pE11, have sequences identical to distinctregions of chloroplast DNA (ct-DNA). Both sequences are partof an extrachromosomal circular DNA molecule (ECD). The molecularstructure of the ECD was investigated by a combination of restrictionanalysis, standard and pulsed-field gel electrophoresis, hybridizationwith ct-DNA probes and amplification by the polymerase chainreaction in the presence of oligonucleotide primers homologousto selected regions of rice ct-DNA. The results showed thata continuous and unrearranged stretch of ct-DNA from the longsingle-copy region, of at least 28 kbp in length, is presentin the ECD. It was estimated that the number of copies of theECD in cultured cells was almost equivalent to that of ct-DNAmolecules in rice leaves, while the ratio of ECD to ct-DNA moleculesin the cultured cells was approximately 200: 1. (Received December 12, 1994; Accepted April 4, 1995)     

The effect of chondroitin sulfate molecular weight and degree of sulfation on the activity of a sulfotransferase from chicken embryo epiphyseal cartilages

The transfer of [35S] sulfate from [35S]PAPS, by means of PAPS: chondroitin sulfate sulfotransferase, to various chondroitin sulfates, with different degrees of sulfation and molecular weights is reported. Analyses by digestion with chondroitin AC and specific 4- or 6-sulfatases indicate that the sulfation occurs only in position 6 of the non-sulfated N-acetyl galactosamine moiety. The 50-70% desulfated chondroitin 4/6-sulfates are two times better sulfate acceptors than totally desulfated chondroitin, and the affinity of the sulfotransferase increases markedly from the octa-to the deca-saccharide. These results suggest that sulfation increases sharply only after the growing polysaccharide contains about 10 sugar residues, in the early stages of polymerization, and that the sulfation of chondroitin sulfate may be a process in which the addition of some sulfate groups facilitates further sulfation.