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31.
Gametophytic competition among pollen grains has been proposed as an important mechanism of sexual selection in plants. The purpose of this paper is to examine the contribution of pollen source on in vivo pollen tube growth in Chamaecrista fasciculata. We addressed two questions: 1) Is pollen tube growth affected by the genetic relatedness between the pollen source and the pollen recipient? 2) Is there significant phenotypic variation among pollen donors for pollen tube growth? We compared pollen tube growth by measuring the number of pollen tubes which germinated, reached quarter length of style, and reached the ovary resulting from self- and outcross-pollinations. The outcross pollinations included three interplant distance classes: near (within genetic neighborhood, ca. 1 m), far (between far neighborhoods and within subpopulation, ca. 20 m), and distant (between neighborhoods and adjacent subpopulations, ca. 50–100 m). Our results show that pollen tube growth was not affected by genetic relatedness, by differences between self and outcross, nor by differences due to phenotypic variation among pollen donors. In contrast, maternal environment had a strong impact on pollen tube growth. These results suggest a lack of gametophytic competition and indicate little opportunity for sexual selection on pollen tube growth in C. fasciculata. 相似文献
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34.
Genetics of high level penicillin resistance in clinical isolates of Streptococcus pneumoniae 总被引:5,自引:0,他引:5
Victoria A Barcus Kiran Ghanekar Maggie Yeo Tracey J Coffey Christopher G Dowson 《FEMS microbiology letters》1995,126(3):299-303
Abstract Mosaic penicillin-binding proteins (PBP) 1A, 2X and 2B genes were cloned from four clinical isolates of Streptococcus pneumoniae with levels of susceptibility to penicillin ranging from 1.5 to 16 μg benzylpenicillin ml−1 . In each instance it was possible to transform either the penicillin-sensitive laboratory strain R6 or a sensitive clinical isolate 110K/70 to the full level of penicillin resistance with these three penicillin-binding proteins alone. Until now it has not been possible to clearly determine whether alterations to PBP1A, 2X and 2B alone were sufficient to attain high level penicillin resistance. 相似文献
35.
Various nonlinear regenerative responses, including plateau potentials and bistable repetitive firing modes, have been observed in motoneurons under certain conditions. Our simulation results support the hypothesis that these responses are due to plateau-generating currents in the dendrites, consistent with a major role for a noninactivating calcium L-type current as suggested by experiments. Bistability as observed in the soma of low- and higher-frequency spiking or, under TTX, of near resting and depolarized plateau potentials, occurs because the dendrites can be in a near resting or depolarized stable steady state. We formulate and study a two-compartment minimal model of a motoneuron that segregates currents for fast spiking into a soma-like compartment and currents responsible for plateau potentials into a dendrite-like compartment. Current flows between compartments through a coupling conductance, mimicking electrotonic spread. We use bifurcation techniques to illuminate how the coupling strength affects somatic behavior. We look closely at the case of weak coupling strength to gain insight into the development of bistable patterns. Robust somatic bistability depends on the electrical separation since it occurs only for weak to moderate coupling conductance. We also illustrate that hysteresis of the two spiking states is a natural consequence of the plateau behavior in the dendrite compartment. 相似文献
36.
Correlation of enzymatic,metabolic, and behavioral deficits in thiamin deficiency and its reversal 总被引:5,自引:0,他引:5
Gary E. Gibson Hanna Ksiezak-Reding Kwan-Fu Rex Sheu Victoria Mykytyn John P. Blass 《Neurochemical research》1984,9(6):803-814
To clarify the enzymatic mechanisms of brain damage inthiamin deficiency, glucose oxidation, acetylcholine synthesis, and the activities of the three major thiamin pyrophosphate (TPP) dependent brain enzymes were compared in untreated controls, in symptomatic pyrithiamin-induced thiamin-deficient rats, and in animals in which the symptoms had been reversed by treatment with thiamin. Although brain slices from symptomatic animals produced14CO2 and14C-acetylcholine from [U-14C]glucose at rates similar to controls under resting conditions, their K+-induced-increase declined by 50 and 75%, respectively. In brain homogenates from these same animals, the activities of two TPP-dependent enzymes transketolase (EC 2.2.1.1) and 2-oxoglutarate dehydrogenase complex (EC 1.2.4.2, EC 2.3.1.61, EC 1.6.4.3) decreased 60–65% and 36%, respectively. The activity of the third TPP-dependent enzyme, pyruvate dehydrogenase complex (EC 1.2.4.1, EC 2.3.1.12, EC 1.6.4.3.) did not change nor did the activity of its activator pyruvate dehydrogenase phosphate phosphatase (EC 3.1.3.43). Although treatment with thiamin for seven days reversed the neurological symptoms and restored glucose oxidation, acetylcholine synthesis and 2-oxoglutarate dehydrogenase activity to normal, transketolase activity remained 30–32% lower than controls. The activities of other TPP-independent enzymes (hexokinase, phosphofructokinase, and glutamate dehydrogenase) were normal in both deficient and reversed animals.Thus, changes in the neurological signs during pyrithiamin-induced thiamin deficiency and in recovery paralleled the reversible damage to a mitochondrial enzyme and impairment of glucose oxidation and acetylcholine synthesis. A more sustained deficit in the pentose pathway enzyme, transketolase, may relate to the anatomical abnormalities that accompany thiamin deficiency.Dedicated to Henry McIlwain. 相似文献
37.
M. Victoria Elorza G. Larriba J. R. Villanueva R. Sentandreu 《Antonie van Leeuwenhoek》1977,43(2):129-142
Assays have been developed for some transfer reactions involved in the synthesis of Saccharomyces cerevisiae wall mannoproteins, both in a particulate preparation in the presence of EDTA or Triton X-100, and after lipid extraction with chloroform-methanol at -20C.The mannosyl transferase activities were also studied in cells made permeable to GDP-mannose by toluene-ethanol treatment (in situ). In these permeabilized cells, the glycosylating reactions dependent on lipid carriers (dolichol derivatives) did not function, but those independent of them were unaffected.The lipid-independent mannosyl transferase activities were partially inhibited by nucleotide diphosphates probably in a competitive manner. Increase of the nucleotide diphosphate pool in vivo might slow down the speed of the transfer reactions carried out by the mannan synthetase system. 相似文献
38.
To increase our understanding of the fate of applied nitrogen inPhaseolus vulgaris crops grown under tropical conditions,15N-labelled urea was applied to bean crops and followed for three consecutive cropping periods. Each crop received 100 kg urea-N ha?1 and 41 kg KCl?K ha?1. At the end of each period we estimated each crop's recovery of the added nitrogen, the residual effects of nitrogen from the previous cropping period, the distribution of nitrogen in the soil profile, and leaching losses of nitrogen. In addition, to evaluate potential effects of added phosphorus on nitrogen cycling in this crop, beans were treated at planting with either 35 kg rock-phosphate-P, 35 kg superphosphate-P, or 0 kg P ha?1. Results showed that 31.2% of the nitrogen in the first crop was derived from the applied urea, which represents a nitrogen utilization efficiency of 38.5%. 6.2% of the nitrogen in the second crop was derived from fertilizer applied to the first crop, and 1.4% of the nitrogen in the third crop. Nitrogen utilization efficiencies for these two crops, with respect to the nitrogen applied to the first crop, were 4.6 and 1.2%, respectively. In total, the three crops recovered 44.3% of the nitrogen applied to the first crop. The remainder of the nitrogen was either still in the soil profile or had been lost by leaching, volatilization or denitrification.15N enrichment of mineral-N(NO3+NH4) suggests that at the end of the second crop, the pulse of fertilizer applied to the first crop had probably passed the 120 cm depth.15N enrichment of organic-N suggests that root activity of beans and weeds transported nitrogen to 90–120 cm (or deeper). We could account for 109 kg fertilizer-N ha?1 in harvested biomass, crop residue, and soil at the end of the first cropping period. This indicates an experimental error of about 10% if no nitrogen was lost by volatilization, denitrification, or leaching below 120 cm. At the end of the second and third crops, 76 and 80 kg N ha?1, respectively, could be accounted for, suggesting that 20 to 25% of the applied-N was lost from the system over a 2-crop period. The two types of added phosphorus did not significantly differ in their effects on bean yields. 相似文献
39.
Victoria Dorovska-Taran Diana Raykova Sava Tishkov Blagoy Blagoev 《Bioorganic chemistry》1980,9(4):482-490
The temperature dependence of the hydrolysis of p-nitrophenyl carboxylates with general formula H(CH2)nCOOC6H4NO2 catalyzed by alkaline mesentericopeptidase has been studied (n varying from 1 to 7, temperature range 2–30°C, pH 8.80, 5 vol% dimethylsulfoxide). The activation parameters of the deacylation step depend on the length of the hydrophobic side chain of the substrate molecule (
,
, and
decrease by 2.0 kcal/mol, 4.9 kcal/mol, and 10 eu, respectively, as the length of the acyl carbon chain increases from n = 1 to n = 4). The following criteria were applied to establish a chemical enthalpy-entropy compensation effect: (a) Exner's plot of log
vs
: (b) Petersen's plot of log, k/T vs 1/T; (c) Exner's statistical treatment in coordinates log k vs 1/T; (d) according to Krug et al. (ΔH≠ vs ΔGThm≠). By use of all the above-mentioned criteria the existence of a chemical enthalpy-entropy compensation effect was proved with an isokinetic temperature β of about 470°K, which is significantly higher than the average experimental temperature. 相似文献
40.
The sympathetic influences on the rabbit unstimulated parotid gland were studied. The experiments were carried out in anaesthetized rabbits with the Stenon aduct cannulated. Direct stimulation of the superior cervical ganglion elicits variable salivary flows. The high amylase content in the saliva points to a sympathetic secretory action upon acinar cells. The administration of alpha-adrenergic blocking agents (dihydroergotamine, phentolamine and phenoxybenzamine) clearly reduces and even abolishes the effect of the sympathetic stimulation upon flow. The administration of a beta-adrenergic blocking agent (propranolol) slightly reduces the sympathetic action. However the amylase activity is greatly reduced. All this suggests that the secretory effects on the fluid fraction should predominantly be alpha-adrenergic while on the secretion of enzymes the beta-receptors should play an important role. 相似文献