Attack frequency and the tolerance to herbivory of Neotropical savanna trees

Tolerance is the ability of a plant to regrow or reproduce following damage. While experimental studies typically measure tolerance in response to the intensity of herbivory (i.e., the amount of leaf tissue removed in one attack), the impact of how many times plants are attacked during a growing season (i.e., the frequency of damage) is virtually unexplored. Using experimental defoliations that mimicked patterns of attack by leaf-cutter ants (Atta spp.), we examined how the frequency of herbivory influenced plant tolerance traits in six tree species in Brazil’s Cerrado. For 2 years we quantified how monthly and quarterly damage influenced individual survivorship, relative growth rate, plant architecture, flowering, and foliar chemistry. We found that the content of leaf nitrogen (N) increased among clipped individuals of most species, suggesting that Atta influences the allocation of resources in damaged plants. Furthermore, our clipping treatments affected tree architecture in ways thought to promote tolerance. However, none of our focal species exhibited a compensatory increase in growth (increment in trunk diameter) in response to herbivory as relative growth rates were significantly lower in clipped than in unclipped individuals. In addition, the probability of survival was much lower for clipped plants, and lower for plants clipped monthly than those clipped quarterly. For plants that did survive, simulated herbivory dramatically reduced the probability of flowering. Our results were similar across a phylogenetically distinct suite of species, suggesting a potential extendability of these findings to other plant species in this system.     

Effects of fire disturbance on ant abundance and diversity: a global meta-analysis

We conducted a meta-analysis of the effects of fire on the abundance and alpha diversity of ants based upon data published over the past 70 years. Overall, fire reduced ant diversity by 18 %, but had no effect on ant abundance. However, there was significant variation in the effect of fire on ant diversity amongst different vegetation types. Fire significantly decreased ant diversity in forests—especially in tropical forests—whereas in deserts, grasslands, and savannas it did not. Similarly, fire had a strong negative mean effect on ant diversity in sites where it is uncommon, but did not significantly affect diversity where it is a recurrent phenomenon. There is evidence that, in forests, wildfires have a stronger negative effect on ant diversity than does prescribed burning. In addition, we found marginally significant differences in the effect of fire on the abundance and diversity of forest ants among studies that sampled ants at different times post-fire, or that sampled ants from different soil strata. In contrast, fire did not significantly affect the abundance or diversity of savanna ants, and this was true even after we took into account the geographic location of the study, the ant community sampled, the time since fire, and the fire regime. Overall, the results of our study indicate that habitat type is an important predictor of ant community responses to fire. However, even within a given habitat, reported effects were quite variable among the studies reviewed, evidencing the idiosyncratic nature of fire effects on ants.     

An Overview of Epigenetic Assays

A significant portion of ongoing epigenetic research involves the investigation of DNA methylation and chromatin modification patterns seen throughout many biological processes. Over the last few years, epigenetic research has undergone a gradual shift and recent studies have been directed toward a genome-wide assessment. DNA methylation and chromatin modifications are essential components of the regulation of gene activity. DNA methylation effectively down-regulates gene activity by addition of a methyl group to the five-carbon of a cytosine base. Less specifically, modification of the chromatin structure can be carried out by multiple mechanisms leading to either the upregulation or down-regulation of the associated gene. Of the many assays used to assess the effects of epigenetic modifications, chromatin immunoprecipitation (ChIP), which serves to monitor changes in chromatin structure, and bisulfite modification, which tracks changes in DNA methylation, are the two most commonly used techniques. J. T. DeAngelis and W. J. Farrington are contributed equally.     

Synthesis of bioinorganic antimicrobial peptide nanoparticles with potential therapeutic properties

Amphiphilicity and cationicity are properties shared between antimicrobial peptides and proteins that catalyze biomineralization reactions. Merging these two functionalities, we demonstrate a reaction where a cationic antimicrobial peptide catalyzes self-biomineralization within inorganic matrices. The resultant antimicrobial peptide nanoparticles retain biocidal activity, protect the peptide from proteolytic degradation, and facilitate a continuous release of the antibiotic over time. Taken together, these properties demonstrate the therapeutic potential of self-synthesizing biomaterials that retain the biocidal properties of antimicrobial peptides.     

On vaccine efficacy and reproduction numbers

We consider the impact of a vaccination programme on the transmission potential of the infection in large populations. We define a measure of vaccine efficacy against transmission which combines the possibly random effect of the vaccine on individual susceptibility and infectiousness. This definition extends some previous work in this area to arbitrarily heterogeneous populations with one level of mixing, but leads us to question the usefulness of the concept of vaccine efficacy against infectiousness. We derive relationships between vaccine efficacy against transmission, vaccine coverage and reproduction numbers, which generalize existing results. In particular we show that the projected reproduction number RV does not depend on the details of the vaccine model, only on its overall effect on transmission. Explicit expressions for RV and the basic reproduction number R0 are obtained in a variety of settings. We define a measure of projected effectiveness of a vaccination programme PE=1-(RV/R0) and investigate its relationship with efficacy against transmission and vaccine coverage. We also study the effective reproduction number Re(t) at time t. Monitoring Re(t) over time is an important aspect of programme surveillance. Programme effectiveness PE is less sensitive than RV or the critical vaccination threshold to model assumptions. On the other hand Re(t) depends on the details of the vaccine model.     

Importance of passive diffusion in the uptake of polychlorinated biphenyls by phagotrophic protozoa

Unicellular protozoan grazers represent a size class of organisms where a transition in the mechanism of chlorobiphenyl (CB) introduction, from diffusion through surface membranes to ingestion of contaminated prey, could occur. This study compares the relative importance of these two processes in the overall uptake of polychlorinated biphenyls by protists. Uptake rates and steady-state concentrations were compared in laboratory cultures of grazing and nongrazing protozoa. These experiments were conducted with a 10-microm marine scuticociliate (Uronema sp.), bacterial prey (Halomonas halodurans), and a suite of 21 CB congeners spanning a range of aqueous solubilities. The dominant pathway of CB uptake by both grazing and nongrazing protozoa was diffusion. Organic-carbon-normalized CB concentrations (in the protozoan cell) were equivalent in grazing and nongrazing protozoa for all congeners studied. Rate constants for uptake into and loss from the protozoan cell were independently determined by using [3,3',4, 4'-(14)C]tetrachlorobiphenyl (IUPAC no. 77), 0.38 +/- 0.03 min(-1) and (1.1 +/- 0.1) x 10(-5) (g of organic carbon)(-1) min(-1), respectively. Magnitudes of the uptake and loss processes were calculated and compared by using a numerical model. The model result was consistent with data from the bioaccumulation experiment and supported the hypothesis that diffusive uptake is faster than ingestive uptake in phagotrophic unicellular protozoa.     

Plasma Levels of Middle Molecules to Estimate Residual Kidney Function in Haemodialysis without Urine Collection

Background

Residual Kidney Function (RKF) is associated with survival benefits in haemodialysis (HD) but is difficult to measure without urine collection. Middle molecules such as Cystatin C and β2-microglobulin accumulate in renal disease and plasma levels have been used to estimate kidney function early in this condition. We investigated their use to estimate RKF in patients on HD.

Design

Cystatin C, β2-microglobulin, urea and creatinine levels were studied in patients on incremental high-flux HD or hemodiafiltration(HDF). Over sequential HD sessions, blood was sampled pre- and post-session 1 and pre-session 2, for estimation of these parameters. Urine was collected during the whole interdialytic interval, for estimation of residual GFR (GFR_Residual = mean of urea and creatinine clearance). The relationships of plasma Cystatin C and β2-microglobulin levels to GFR_Residual and urea clearance were determined.

Results

Of the 341 patients studied, 64% had urine output>100ml/day, 32.6% were on high-flux HD and 67.4% on HDF. Parameters most closely correlated with GFR_Residual were 1/β2-micoglobulin (r² 0.67) and 1/Cystatin C (r² 0.50). Both these relationships were weaker at low GFR_Residual. The best regression model for GFR_Residual, explaining 67% of the variation, was: GFRResidual=160.3⋅(1β2m)−4.2 Where β2m is the pre-dialysis β2 microglobulin concentration (mg/L). This model was validated in a separate cohort of 50 patients using Bland-Altman analysis. Areas under the curve in Receiver Operating Characteristic analysis aimed at identifying subjects with urea clearance≥2ml/min/1.73m² was 0.91 for β2-microglobulin and 0.86 for Cystatin C. A plasma β2-microglobulin cut-off of ≤19.2mg/L allowed identification of patients with urea clearance ≥2ml/min/1.73m² with 90% specificity and 65% sensitivity.

Conclusion

Plasma pre-dialysis β2-microglobulin levels can provide estimates of RKF which may have clinical utility and appear superior to cystatin C. Use of cut-off levels to identify patients with RKF may provide a simple way to individualise dialysis dose based on RKF.     

Changes in Asymbiotic, Heterotrophic Nitrogen Fixation on Leaf Litter of Metrosideros polymorpha with Long-Term Ecosystem Development in Hawaii

We measured nitrogenase activity (acetylene reduction) of asymbiotic, heterotrophic, nitrogen-fixing bacteria on leaf litter from the tree Metrosideros polymorpha collected from six sites on the Hawaiian archipelago. At all sites M. polymorpha was the dominant tree, and its litter was the most abundant on the forest floor. The sites spanned a soil chronosequence of 300 to 4.1 million y. We estimated potential nitrogen fixation associated with this leaf litter to be highest at the youngest site (1.25 kg ha^-1 y^-1), declining to between 0.05 and 0.22 kg ha^-1 y^-1 at the oldest four sites on the chronosequence. To investigate how the availability of weathered elements influences N fixation rates at different stages of soil development, we sampled M. polymorpha leaf litter from complete, factorial fertilization experiments located at the 300-y, 20,000-y and 4.1 million–y sites. At the youngest and oldest sites, nitrogenase activity on leaf litter increased significantly in the plots fertilized with phosphorus and “total” (all nutrients except N and P); no significant increases in nitrogenase activity were measured in leaf litter from treatments at the middle-aged site. The results suggest that the highest rates of N fixation are sustained during the “building” or early phase of ecosystem development when N is accumulating and inputs of geologically cycled (lithophilic) nutrients from weathering are substantial. Received 4 February 1999; accepted 29 March 2000.