Determinants of density- and frequency-dependent fitness in competing plant pathogens

ABSTRACT Using mathematical models, we investigated how infection and sporulation characteristics of competing plant pathogens determine the density and frequency dependence of relative fitness. Two models, one for the infection stage and one for the sporulation stage of a pathogen's life cycle, describe reproductive output of pathogen strains in mixture as a function of the strains' population densities. Model parameters include infection and sporulation efficiencies, carrying capacities on leaves for sporulating lesions and spore production, and coefficients of interstrain competitive effects in both life cycle stages. Although the models were originally developed for rust fungi, they are generally applicable to any organism with distinct colonization (e.g., infection) and propagative (e.g., sporulation) life cycle stages. In this work, paired hypothetical strains were assigned equal baseline parameter values. Parameters were then altered one at a time for one or both strains, and relative fitness was calculated over a range of densities and strain frequencies. Except for infection efficiency, the fitness benefit conferred by an advantage in a single parameter was always density dependent. Relative fitness was frequency dependent whenever inter- and intrastrain competitive effects were not equal. These results suggest that the fitness of pathogens in nature is rarely fixed, but, rather, may typically be highly dependent on the densities and frequencies of all coexisting strains in a habitat.     

导管螺旋桨在拖网渔船上的设计与应用

根据目前拖网渔船在节能方面的需要,采用导管螺旋桨以改善渔船的推进性能,并对导管螺旋桨的设计和具体应用的优的缺点作初步的探讨。     

The role of groundwater flow in controlling the spatial distribution of soil salinity and rooted macrophytes in a southeastern salt marsh, USA

Groundwater flow is an important factor in governing botanical zonation in the salt marsh at North Inlet, SC. Areas of the marsh adjacent to upland forest are characterized by upward flow of fresh groundwater. This inhibits the infiltration and evapoconcentration of saline tidal water and the development of a habitat for hypersaline-tolerant fugitive species such as Salicornia europaea. Areas of high marsh that are not adjacent to extensive upland forest are characterized by downward gradients in hydraulic head. This allows the infiltration and evapoconcentration of tidal water and the development of hypersaline conditions that are suitable for salt-tolerant fugitives.     

Assessing Trace Element Uptake by Vegetation on a Coal Fly Ash Landfill

Improved methods are required to assess the risks posed by the uptake of potentially toxic elements such as selenium (Se), boron (B), and molybdenum (Mo) by vegetation on contaminated sites. In order to develop such methods and assess risk, vegetation was collected from two sites on a soil-capped coal fly ash landfill near Dunkirk, New York, during June of 1991 and June and August of 1992. The mean concentrations (μg g-1 dry weight) of Se and Mo in the shoots did not exceed, respectively, 0.12 and 18.7 in bird's-foot trefoil (Lotus corniculatus L.), 0.06 and 12.1 in red clover (Trifolium pratense L.), 0.07 and 5.3 in timothy (Phleum pratense L.), and 0.09 and 2.2 in a mixture of grasses. These concentrations were greater than those in the same species harvested concurrently from a non-landfill site. The mean concentrations of B at the landfill ranged from 29 to 53 μg g-1 in the legumes and from 2 to 11 μg g-1 in the grasses, less than those at one non-landfill site but greater than those at another. Within the landfill, the concentration of Se in grasses was not correlated with the concentration of Se in soil and fly ash. The concentration of Se in grasses on both landfill sites was double that of grasses on the non-landfill site despite higher mean concentrations of Se in the upper soil (0–15 cm) on the non-landfill site. Therefore grass roots seem to be accessing Se from the ash by means of mass flow or other mechanisms. Based on our findings of significant variation in trace element uptake among species, harvests, and locations within sites, we recommend that contemporaneous transect sampling of at least two species be used to assess uptake of potentially toxic trace elements on landfills or other sites where contamination may occur.     

The Current State of Solar Modeling

Data from the Global Oscillation Network Group (GONG) project and other helioseismic experiments provide a test for models of stellar interiors and for the thermodynamic and radiative properties, on which the models depend, of matter under the extreme conditions found in the sun. Current models are in agreement with the helioseismic inferences, which suggests, for example, that the disagreement between the predicted and observed fluxes of neutrinos from the sun is not caused by errors in the models. However, the GONG data reveal subtle errors in the models, such as an excess in sound speed just beneath the convection zone. These discrepancies indicate effects that have so far not been correctly accounted for; for example, it is plausible that the sound-speed differences reflect weak mixing in stellar interiors, of potential importance to the overall evolution of stars and ultimately to estimates of the age of the galaxy based on stellar evolution calculations.     

Modeling the Exchanges of Energy, Water, and Carbon Between Continents and the Atmosphere

Atmospheric general circulation models used for climate simulation and weather forecasting require the fluxes of radiation, heat, water vapor, and momentum across the land-atmosphere interface to be specified. These fluxes are calculated by submodels called land surface parameterizations. Over the last 20 years, these parameterizations have evolved from simple, unrealistic schemes into credible representations of the global soil-vegetation-atmosphere transfer system as advances in plant physiological and hydrological research, advances in satellite data interpretation, and the results of large-scale field experiments have been exploited. Some modern schemes incorporate biogeochemical and ecological knowledge and, when coupled with advanced climate and ocean models, will be capable of modeling the biological and physical responses of the Earth system to global change, for example, increasing atmospheric carbon dioxide.     

Maple syrup urine disease in calves: a clinical, pathological and biochemical study

The clinical, pathological and biochemical findings of a study of 30 Poll Hereford, Hereford, Poll Hereford cross or Hereford cross calves affected with branched chain ketoacid dehydrogenase (BCKAD) complex deficiency or maple syrup urine disease (MSUD) are presented. In breeding studies, 6 of 21 calves from obligate heterozygote matings were affected with MSUD, suggesting the disease is inherited in an autosomal recessive manner. Calves were clinically affected from birth, but there were variations in the subsequent course of progressive deterioration of central nervous system function. Concentrations of the branched chain amino acids and keto acids were elevated in pre-suckle plasma and cerebellar water content was higher in affected calves. Activity of BCKAD complex was minimal in fibroblasts cultured from an affected calf. Spongiform encephalopathy and elevated ratios of the branched to straight chain amino acids in formalin fixed cerebral tissue were found in a stillborn foetus and a 3-month-old Hereford calf. These findings suggest the disease occurs prenatally and that a delayed form may exist.     

Analysis of head and leaf reaction towards <Emphasis Type="Italic">Microdochium nivale</Emphasis>

This research examined the variation in the response of eight commercial wheat cultivars to Microdochium nivale isolates using both in vivo FHB tests (AUDPC and RHW measurements) and in vitro detached leaf assays (LGR). Irrespective of fungal variety, the two Italian cvs Fortore and Norba exhibited the greatest amount of visual disease symptoms (mean AUDPC=2.2 and 2.3, respectively), being significantly more susceptible than the other six cultivars (AUDPC 1.24) (P < 0.05). Irrespective of fungal variety, the Italian cv. Norba and the Irish cv. Falstaff were more susceptible than the other cultivars (except Fatima 2) in terms of RHW (P < 0.05), while the cvs Fortore, GK Othalom and Consort were more resistant than the other five cultivars (P < 0.05). In the detached leaf assay, the Hungarian cv. GK Othalom and the Italian cv. Norba were more susceptible (mean LGR=0.79 and 0.81 mm day^–1, respectively) to M. nivalethan the other six cultivars (mean LGR=0.51–0.72) (P < 0.05). Analysis of the relationship between head and leaf reaction to M. nivaleinfection revealed no significant correlation.