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61.
Peter A. Abrams Locke Rowe 《Evolution; international journal of organic evolution》1996,50(3):1052-1061
The effects of nonselective predation on the optimal age and size of maturity of their prey are investigated using mathematical models of a simple life history with juvenile and adult stages. Fitness is measured by the product of survival to the adult stage and expected adult reproduction, which is usually an increasing function of size at maturity. Size is determined by both age at maturity and the value of costly traits that increase mean growth rate (growth effort). The analysis includes cases with fixed size but flexible time to maturity, fixed time but flexible size, and adaptively flexible values of both variables. In these analyses, growth effort is flexible. For comparison with previous theory, models with a fixed growth effort are analyzed. In each case, there may be indirect effects of predation on the prey's food supply. The effect of increased predation depends on (1) which variables are flexible; (2) whether increased growth effort requires increased exposure to predators; and (3) how increased predator density affects the abundance of food for juvenile prey. If there is no indirect effect of predators on prey food supply, size at maturity will generally decrease in response to increased predation. However, the indirect effect from increased food has the opposite effect, and the net result of predation is often increased size. Age at maturity may either increase or decrease, depending on functional forms and parameter values; this is true regardless of the presence of indirect effects. The results are compared with those of previous theoretical analyses. Observed shifts in life history in response to predation are reviewed, and the role of size-selective predation is reassessed. 相似文献
62.
Ammonium concentrations were determined in near-bottom water and intertidal surface sediments collected in February and July 1993 at five stations of Ria Formosa, a shallow meso-tidal coastal lagoon in southern Portugal. At each station, samples were taken a few minutes before tidal inundation, and subsequently 2, 10, 15 and 20 minutes thereafter. Ammonium concentration in near-bottom waters increased dramatically in the first 2 minutes followed by a decrease during the 18 minutes of flooding (maximum range 10.3-2.2 M). The highest levels in the flooding water were concomitant with a decrease of extractable ammonium recorded in the upper sediment layer (2 cm). Laboratory experiments indicated that ammonium is easily extracted from the sediment solids by physical perturbation, as one would expect when tidal water flushes over the intertidal area. This perturbation results in the export of ammonium from the sediment, by pulse mechanisms of short time intervals. On a daily scale this amount is two orders of magnitude higher than transport resulting from molecular diffusion. 相似文献
63.
沿海防护林防海煞危害初探 总被引:2,自引:0,他引:2
通过后牧村海岸防护林带附近空气中主要盐离子飘尘的观测分析表明,林带对大气盐尘的吸附和截留作用,无林带地区平均沉降量为4.3μg.m^-3.100m^-1,通过林带时为18.4μm^-3.100m^-1。林带的吸队量与树种、枝叶形状有关,侧柏对Cl^-和Na^+的吸附量分别为6330和3350mg.kg^-1,而黑松为1570和950mg.kg^-1,空气中盐尘受天气条件特别是风速的影响,阴天,风速 相似文献
64.
65.
Summary The P-43 ofEudiplodinium and homologous proteins in three other entodiniomorphid species, free-living ciliates, flagellates, and HeLa cells, were identified at the plasma membrane-cytoskeleton interface. Proteins cross-reacting with MAb B6 were also located at the ciliary inner surface of the plasma membrane. Due to the strong adhesion of the plasma membrane to the underlying cytoskeleton, classical extraction with detergents, urea, NaOH, and PTA, failed to separate the two components completely. However, the extraction properties of P-43, associated with its membrane-cytoskeleton interactive functions, suggest that this unglycosylated protein may present some analogies with proteins of the intermediate filaments. Their ubiquity and localization suggest that P-43 and MAb B6 crossreacting proteins may not be strictly epiplasmic but could be amphitropic proteins, strongly anchored to both the plasma membrane and the underlying microfilament framework, via protein-protein binding or by direct insertion in the lipid bilayer.Abbreviations BSA
bovine serum albumin
- Con A
concanavalin A
- EDTA
ethylene diamine tetraacetic acid
- EM
electron microscopy
- IF
intermediate filaments
- MAb
monoclonal antibody
- MET
2-mercaptoethanol
- MW
molecular weight
- PAb
polyclonal antibody
- PBS
phosphate buffered saline
- PMSF
phenylmethylsulfonyl fluoride
- PTA
phosphotungstic acid
- SDS
sodiumdodecyl sulfate
- TAME
Na-p-tosyl-arginine methyl ester
- TLCK
Na-p-tosyl-lysine chloromethyl ketone 相似文献
66.
During the establishment of vesicular-arbuscular mycorrhizas, fungal hyphae contact the root surface, form appressoria and
initiate the internal colonization phase. Structural changes occur in the cell wall, the cytoplasm and the nucleus as the
fungus progresses from a presymbiotic to a symbiotic phase. Nuclei in spores are in G1 whereas in intraradical hyphae they are in G1 and G2. Changes in nuclear organization are evident in various stages in the colonization process. Dramatic changes in both symbionts
occur as the nutrient exchange interface is established between arbuscules and root cortical cells. An interfacial matrix,
consisting of molecules common to the primary wall of the cortical cell, separates the cortical cell plasma membrane from
the fungal cell wall.
Ectomycorrhizas are characterized structurally by the presence of a mantle of fungal hyphae enclosing the root and usually
an Hartig net of intercellular hyphae characterized by labyrinthine branching. As hyphae contact the root surface, they may
respond by increasing their diameter and switching from apical growth to precocious branching. The site of initial contact
of hyphae may be either the root cap or the ‘mycorrhiza infection zone’. The mantle varies considerably in structure depending
on both the plant and fungus genome. In some ectomycorrhizas, the mantle may be a barrier to apoplastic transport, and in
most it may store polyphosphate, glycogen, lipids and perhaps protein. 相似文献
67.
P. T. Beernink D. R. Tolan 《Protein science : a publication of the Protein Society》1994,3(9):1383-1391
We report the construction of subunit interface mutants of rabbit muscle aldolase A with altered quaternary structure. A mutation has been described that causes nonspherocytic hemolytic anemia and produces a thermolabile aldolase (Kishi H et al., 1987, Proc Natl Acad Sci USA 84:8623-8627). The disease arises from substitution of Gly for Asp-128, a residue at the subunit interface of human aldolase A. To elucidate the role of this residue in the highly homologous rabbit aldolase A, site-directed mutagenesis is used to replace Asp-128 with Gly, Ala, Asn, Gln, or Val. Rabbit aldolase D128G purified from Escherichia coli is found to be similar to human D128G by kinetic analysis, CD, and thermal inactivation assays. All of the mutant rabbit aldolases are similar to the wild-type rabbit enzyme in secondary structure and kinetic properties. In contrast, whereas the wild-type enzyme is a tetramer, chemical crosslinking and gel filtration indicate that a new dimeric species exists for the mutants. In sedimentation velocity experiments, the mutant enzymes as mixtures of dimer and tetramer at 4 degrees C. Sedimentation at 20 degrees C shows that the mutant enzymes are > 99.5% dimeric and, in the presence of substrate, that the dimeric species is active. Differential scanning calorimetry demonstrates that Tm values of the mutant enzymes are decreased by 12 degrees C compared to wild-type enzyme. The results indicate that Asp-128 is important for interface stability and suggest that 1 role of the quaternary structure of aldolase is to provide thermostability. 相似文献
68.
Frederick C. Wedler Michael C. Vichnin Brenda W. Ley Georges Tholey Marc Ledig Jean-Christoph Copin 《Neurochemical research》1994,19(2):145-151
Previous studies have demonstrated that in glia and astrocytes Mn(II) is distributed with ca. 30–40% in the cytoplasm, 60–70% in mitochondria. Ca(II) ions were observed to alter both the flux rates and distribution of Mn(II) ions in primary cultues of chick glia and rat astrocytes. External (influxing) Ca(II) ions had the greatest effect on Mn(II) uptake and efflux, compared to internal (effluxing) or internal-external equilibrated Ca(II) ions. External (influxing) Ca(II) ions inhibited the net rate and extent of Mn(II) uptake but enhanced Mn(II) efflux from mitochondria. These observations differ from Ca(II)–Mn(II) effects previously reported with brain (neuronal) mitochondria. Overall, increased cytoplasmic Ca(II) acts to block Mn(II) uptake and enhance Mn(II) release by mitochondria, which serve to increase the cytoplasmic concentration of free Mn(II). A hypothesis is presented involving external L-glutamate acting through membrane receptors to mobilize cell Ca(II), which in turn causes mitochondrial Mn(II) to be released. Because the concentration of free cytoplasmic Mn(II) is poised near the Kd for Mn(II) with glutamine synthetase, a slight increase in cytoplasmic Mn(II) will directly enhance the activity of glutamine synthetase, which catalyzes removal of neurotoxic glutamate and ammonia. 相似文献
69.
The object of our study is: a model for root growth through a free-boundary problem and the effects resulting from differences in nutrient availability and transport of only one mobile nutrient between the root surface and the rhizosphere produced by an absorption Michaelis-Menten for low and high concentrations. The model equations are solved by two methods: the quasi-stationary method and the balance integral method. The numerical solutions are used to compute radial root growth. Curves of nutrient concentration at the root-soil interface, curve as a function of root radius as well as curves representing root radius as a function of time are plotted. The parameters which are varied are the root absorption power, flux velocity at the root surface, efflux, rhizosphere radius, diffusion coefficient, buffer power, and maximum influx. The two methods show the theoretical results for radial root growth in the range of low and high concentrations. The balance integral method provides more detailed information. 相似文献
70.
This paper presents a simple model for the evolution of reaction norms for age and size at maturity that predicts reaction norms with a variety of shapes. Using realistic parameter values the model predicts reaction norms close to those observed in Drosophila. The major assumptions of the model are: 1) that net reproductive rate is maximized, 2) that growth is determinate, and 3) that mortality rates are independent of age and size at maturity. If, additionally, juvenile mortality is uncorrelated with a growth coefficient, k, the model predicts that selection favors maturation later at a smaller size when k is reduced by environmental factors and that decreased juvenile mortality leads to delayed maturity. These two predictions conform with those found by previous models using other measures of fitness. Correlations between k and juvenile mortality can change the shape of the predicted reaction norm. Depending on the precise form of the correlation, the model can predict done- or bowl-shaped reaction norms and can predict delayed or earlier maturity as k decreases. These shapes are qualitatively different from those predicted by previous models that used different fitness measures. Systematic estimates of the parameter values for this and for related models are required to determine the appropriate fitness measure for models of reaction norms. 相似文献