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排序方式: 共有113条查询结果,搜索用时 15 毫秒
51.
目的:比较不同时期颈髓损伤的MRI表现及DTI的应用价值。方法:收集急性颈髓压迫病例15例、慢性颈髓压迫病例23例、颈髓慢性压迫合并急性压迫病例12例。15例健康志愿者作为对照组。进行常规MRI检查,应用DTI检查测量表现扩散系数(ADC)值和各向异性分数(FA)。比较各组间ADC值和FA值,并进行统计学分析。结果:急性颈髓迫病例,常规MRI显示颈髓增粗,呈等T1长T2信号;慢性颈髓压迫病例,9例呈长T1长T2信号,14例呈等T1长T2信号;慢性颈髓压迫并急性压迫病例颈髓明显增粗,呈等、长T1明显长T2信号。与对照组比较:急性颈髓压迫组的ADC值和FA值均明显降低,两组的差异有显著性;慢性颈髓压迫组的FA值降低,ADC值增高,两组的差异有显著性;慢性脊髓压迫合并急性脊髓压迫组ADC值与对照组比较无差异,FA值低于对照组。颈髓压迫各组间ADC值及FA值比较差异显著。结论:不同时期颈髓损伤常规MRI图像缺乏特异性,根据ADC值及FA值可判断颈髓损伤的时期。 相似文献
52.
Variability in responses to thermal stress in parasitoids 总被引:1,自引:0,他引:1
GAËLLE AMICE PHILIPPE VERNON YANNICK OUTREMAN JACQUES VAN ALPHEN JOAN VAN BAAREN 《Ecological Entomology》2008,33(6):701-708
Abstract 1. To study phenotypic effects of stress, a stress is applied to cohorts of organisms with an increasing intensity. In the absence of mortality the response of traits will be a decreasing function of stress intensity because of increasing physiological costs. We call such decreasing functions type A responses. 2. However, when stress caused mortality, some studies have found that for high stress intensities, survivors performed as well as control individuals (type B responses). We proposed that type A responses are caused by the physiological cost of stress whereas type B responses are caused by a mixture of physiological costs and selection. 3. The present study exposed Aphidius picipes wasps to an increasing duration of cold storage (cold stress), and obtained variable responses as predicted when both physiological costs and selection of resistant individuals determine the outcome. 4. When cold storage of parasitoids for biological control is desirable, research should be carried out to find (i) the temperature regime and duration of storage and (ii) the least sensitive stage for storage to minimise losses from mortality and reduction of fitness of survivors. 5. Selection by cold stress as observed in the present study could result in rapid adaptation of populations exposed to such stress. 相似文献
53.
S Franz Bender Faline Plantenga Albrecht Neftel Markus Jocher Hans-Rudolf Oberholzer Luise K?hl Madeline Giles Tim J Daniell Marcel GA van der Heijden 《The ISME journal》2014,8(6):1336-1345
N2O is a potent greenhouse gas involved in the destruction of the protective ozone layer in the stratosphere and contributing to global warming. The ecological processes regulating its emissions from soil are still poorly understood. Here, we show that the presence of arbuscular mycorrhizal fungi (AMF), a dominant group of soil fungi, which form symbiotic associations with the majority of land plants and which influence a range of important ecosystem functions, can induce a reduction in N2O emissions from soil. To test for a functional relationship between AMF and N2O emissions, we manipulated the abundance of AMF in two independent greenhouse experiments using two different approaches (sterilized and re-inoculated soil and non-mycorrhizal tomato mutants) and two different soils. N2O emissions were increased by 42 and 33% in microcosms with reduced AMF abundance compared to microcosms with a well-established AMF community, suggesting that AMF regulate N2O emissions. This could partly be explained by increased N immobilization into microbial or plant biomass, reduced concentrations of mineral soil N as a substrate for N2O emission and altered water relations. Moreover, the abundance of key genes responsible for N2O production (nirK) was negatively and for N2O consumption (nosZ) positively correlated to AMF abundance, indicating that the regulation of N2O emissions is transmitted by AMF-induced changes in the soil microbial community. Our results suggest that the disruption of the AMF symbiosis through intensification of agricultural practices may further contribute to increased N2O emissions. 相似文献
54.
Kappa-chain constant-region gene sequences in genus Rattus: coding regions are diverging more rapidly than noncoding regions 总被引:2,自引:0,他引:2
We have determined the nucleotide sequence of a 1,200-base pair (bp)
genomic fragment that includes the kappa-chain constant-region gene (C
kappa) from two species of native Australian rodents, Rattus leucopus
cooktownensis and Rattus colletti. Comparison of these sequences with each
other and with other rodent C kappa genes shows three surprising features.
First, the coding regions are diverging at a rate severalfold higher than
that of the nearby noncoding regions. Second, replacement changes within
the coding region are accumulating at a rate at least as great as that of
silent changes. Third, most of the amino acid replacements are localized in
one region of the C kappa domain--namely, the carboxy-terminal "bends" in
the alpha-carbon backbone. These three features have previously been
described from comparisons of the two allelic forms of C kappa genes in R.
norvegicus. These data imply the existence of considerable evolutionary
constraints on the noncoding regions (based on as yet undetermined
functions) or powerful positive selection to diversify a portion of the
constant-region domain (whose physiological significance is not known).
These surprising features of C kappa evolution appear to be characteristic
only of closely related C kappa genes, since comparison of rodent with
human sequences shows the expected greater conservation of coding regions,
as well as a predominance of silent nucleotide substitutions within the
coding regions.
相似文献
55.
SÉBASTIEN VILLÉGER GAËL GRENOUILLET VIRGINIE SUC SÉBASTIEN BROSSE 《Freshwater Biology》2012,57(11):2330-2341
1. We measured N and P excretion rates of 470 individuals belonging to 18 freshwater fish species widespread in Western Europe. We assessed the effect of body mass on excretion rates at both the intra‐ and interspecific levels. 2. The high variability in per capita N and P excretion rates was mainly determined by differences in body mass. The scaling coefficients of allometric relationships for both N and P excretion rates were significantly lower than 1 (mean ± SE, 0.95 ± 0.04 and 0.81 ± 0.05, respectively). 3. The slope of the allometric relationship between fish mass and nutrient excretion rate was significantly different among species. We did not detect any influence of phylogenetic conservatism on fish mass and on excretion rates. Further investigations are needed to understand the biological determinants of these differences. 4. This high intra‐ and interspecific variability in per capita excretion rates, coupled with differences in fish body mass, produce marked differences in biomass‐standardised excretion rates. These results thus indicate the necessity for further experimental and in situ investigations on the consequences of nutrient recycling by fish in freshwater ecosystems. 相似文献
56.
57.
A Hidden Markov Model approach to variation among sites in rate of evolution 总被引:20,自引:20,他引:20
The method of Hidden Markov Models is used to allow for unequal and unknown
evolutionary rates at different sites in molecular sequences. Rates of
evolution at different sites are assumed to be drawn from a set of possible
rates, with a finite number of possibilities. The overall likelihood of
phylogeny is calculated as a sum of terms, each term being the probability
of the data given a particular assignment of rates to sites, times the
prior probability of that particular combination of rates. The
probabilities of different rate combinations are specified by a stationary
Markov chain that assigns rate categories to sites. While there will be a
very large number of possible ways of assigning rates to sites, a simple
recursive algorithm allows the contributions to the likelihood from all
possible combinations of rates to be summed, in a time proportional to the
number of different rates at a single site. Thus with three rates, the
effort involved is no greater than three times that for a single rate. This
"Hidden Markov Model" method allows for rates to differ between sites and
for correlations between the rates of neighboring sites. By summing over
all possibilities it does not require us to know the rates at individual
sites. However, it does not allow for correlation of rates at nonadjacent
sites, nor does it allow for a continuous distribution of rates over sites.
It is shown how to use the Newton-Raphson method to estimate branch lengths
of a phylogeny and to infer from a phylogeny what assignment of rates to
sites has the largest posterior probability. An example is given using
beta-hemoglobin DNA sequences in eight mammal species; the regions of high
and low evolutionary rates are inferred and also the average length of
patches of similar rates.
相似文献
58.
目的比较腺嘌呤诱导与氢化可的松诱导的SD大鼠肾阳虚模型的优劣,从而得到临床症状、生化指标更符合中医“肾阳虚”型的动物模型,以供科研、教学使用。方法雄性SD大鼠100只,随机分成正常组、腺嘌呤造模组及氢化可的松低剂量组、中剂量组、高剂量组各20只,造模成功后检测血肌酐(Scr)、尿素氮(BUN)、尿液中17-羟皮质类固醇(17-OH)和血浆中甲状腺激素(13、T4)、睾丸酮(T)、雌二醇(E2)及模型大鼠骨密度。结果无论从症状主证方面,还是客观指标方面(除骨密度外),腺嘌呤诱导的SD大鼠肾阳虚型动物模型在生化指标研究方面均优于氢化可的松诱导组。结论腺嘌呤诱导的肾阳虚证SD大鼠模型在症状主证方面及其相关的生化研究方面优于氢化可的松组。 相似文献
59.
本文概述了缟蝇科昆虫的生活史、食性、访花习性、栖居环境等生物学特性,并对缟蝇科属、种的地理分布进行了分析。缟蝇科在世界动物地理区的分布统计结果表明,同脉缟蝇亚科为亚世界性分布,其属级阶元在东洋界丰富度最高,其次为澳洲界;其种级阶元在东洋界最为丰富,其次为澳洲界。缟蝇亚科为世界性分布,其属级阶元在新热带界丰富度最高,并且高度特化,其次是东洋界和澳洲界;其种级阶元在新热带界最为丰富,其次为古北界和澳洲界。缟蝇科在中国动物地理区和亚区的分布统计结果显示,其属、种级阶元在华南区最为丰富;同脉缟蝇亚科的种级阶元以海南岛亚区最为丰富,其次为台湾亚区;缟蝇亚科的种级阶元在台湾亚区最为丰富,西部山地高原亚区次之。 相似文献
60.
LAËTITIA BUISSON WILFRIED THUILLER SOVAN LEK PUY LIM GAËL GRENOUILLET 《Global Change Biology》2008,14(10):2232-2248
Stream fish are expected to be significantly influenced by climate change, as they are ectothermic animals whose dispersal is limited within hydrographic networks. Nonetheless, they are also controlled by other physical factors that may prevent them moving to new thermally suitable sites. Using presence–absence records in 655 sites widespread throughout nine French river units, we predicted the potential future distribution of 30 common stream fish species facing temperature warming and change in precipitation regime. We also assessed the potential impacts on fish assemblages' structure and diversity. Only cold-water species, whose diversity is very low in French streams, were predicted to experience a strong reduction in the number of suitable sites. In contrast, most cool-water and warm-water fish species were projected to colonize many newly suitable sites. Considering that cold headwater streams are the most numerous on the Earth's surface, our results suggested that headwater species would undergo a deleterious effect of climate change, whereas downstream species would expand their range by migrating to sites located in intermediate streams or upstream. As a result, local species richness was forecasted to increase greatly and high turnover rates indicated future fundamental changes in assemblages' structure. Changes in assemblage composition were also positively related to the intensity of warming. Overall, these results (1) stressed the importance of accounting for both climatic and topographic factors when assessing the future distribution of riverine fish species and (2) may be viewed as a first estimation of climate change impacts on European freshwater fish assemblages. 相似文献