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排序方式: 共有6171条查询结果,搜索用时 31 毫秒
991.
992.
993.
Slik JW Aiba S Bastian M Brearley FQ Cannon CH Eichhorn KA Fredriksson G Kartawinata K Laumonier Y Mansor A Marjokorpi A Meijaard E Morley RJ Nagamasu H Nilus R Nurtjahya E Payne J Permana A Poulsen AD Raes N Riswan S van Schaik CP Sheil D Sidiyasa K Suzuki E van Valkenburg JL Webb CO Wich S Yoneda T Zakaria R Zweifel N 《Proceedings of the National Academy of Sciences of the United States of America》2011,108(30):12343-12347
The marked biogeographic difference between western (Malay Peninsula and Sumatra) and eastern (Borneo) Sundaland is surprising given the long time that these areas have formed a single landmass. A dispersal barrier in the form of a dry savanna corridor during glacial maxima has been proposed to explain this disparity. However, the short duration of these dry savanna conditions make it an unlikely sole cause for the biogeographic pattern. An additional explanation might be related to the coarse sandy soils of central Sundaland. To test these two nonexclusive hypotheses, we performed a floristic cluster analysis based on 111 tree inventories from Peninsular Malaysia, Sumatra, and Borneo. We then identified the indicator genera for clusters that crossed the central Sundaland biogeographic boundary and those that did not cross and tested whether drought and coarse-soil tolerance of the indicator genera differed between them. We found 11 terminal floristic clusters, 10 occurring in Borneo, 5 in Sumatra, and 3 in Peninsular Malaysia. Indicator taxa of clusters that occurred across Sundaland had significantly higher coarse-soil tolerance than did those from clusters that occurred east or west of central Sundaland. For drought tolerance, no such pattern was detected. These results strongly suggest that exposed sandy sea-bed soils acted as a dispersal barrier in central Sundaland. However, we could not confirm the presence of a savanna corridor. This finding makes it clear that proposed biogeographic explanations for plant and animal distributions within Sundaland, including possible migration routes for early humans, need to be reevaluated. 相似文献
994.
Yang L Jelsbak L Marvig RL Damkiær S Workman CT Rau MH Hansen SK Folkesson A Johansen HK Ciofu O Høiby N Sommer MO Molin S 《Proceedings of the National Academy of Sciences of the United States of America》2011,108(18):7481-7486
Laboratory evolution experiments have led to important findings relating organism adaptation and genomic evolution. However, continuous monitoring of long-term evolution has been lacking for natural systems, limiting our understanding of these processes in situ. Here we characterize the evolutionary dynamics of a lineage of a clinically important opportunistic bacterial pathogen, Pseudomonas aeruginosa, as it adapts to the airways of several individual cystic fibrosis patients over 200,000 bacterial generations, and provide estimates of mutation rates of bacteria in a natural environment. In contrast to predictions based on in vitro evolution experiments, we document limited diversification of the evolving lineage despite a highly structured and complex host environment. Notably, the lineage went through an initial period of rapid adaptation caused by a small number of mutations with pleiotropic effects, followed by a period of genetic drift with limited phenotypic change and a genomic signature of negative selection, suggesting that the evolving lineage has reached a major adaptive peak in the fitness landscape. This contrasts with previous findings of continued positive selection from long-term in vitro evolution experiments. The evolved phenotype of the infecting bacteria further suggests that the opportunistic pathogen has transitioned to become a primary pathogen for cystic fibrosis patients. 相似文献
995.
Geerdink N van der Vliet T Rotteveel JJ Feuth T Roeleveld N Mullaart RA 《Child's nervous system》2012,28(7):977-985
Purpose
Brain MR imaging is essential in the assessment of Chiari II malformation in clinical and research settings concerning spina bifida. However, the interpretation of morphological features of the malformation on MR images may not always be straightforward. In an attempt to select those features that unambiguously characterize the Chiari II malformation, we investigated the interobserver reliability of all its well-known MR features.Methods
Brain MR images of 79 children [26 presumed to have Chiari II malformation, 36 presumed to have no cerebral abnormalities, and 17 children in whom some Chiari II malformation features might be present; mean age 10.6 (SD 3.2; range, 6-16) years] were blindly and independently reviewed by three observers. They rated 33 morphological features of the Chiari II malformation as present, absent, or indefinable in three planes (sagittal, axial, and coronal). The interobserver reliability was assessed using κ statistics.Results
Twenty-three of the features studied turned out to be unreliable, whereas the interobserver agreement was almost perfect (κ value?>?0.8) for nine features (eight in the sagittal plane and one in the axial plane, but none in the coronal plane).Conclusions
This study presents essential features of the Chiari II malformation on MR images by ruling out the unreliable features. Using these features may improve the assessment of Chiari II malformation in clinical and research settings. 相似文献996.
Landgren S von Otter M Palmér MS Zetterström C Nilsson S Skoog I Gustafson DR Minthon L Wallin A Andreasen N Bogdanovic N Marcusson J Blennow K Zetterberg H Kettunen P 《Journal of neural transmission (Vienna, Austria : 1996)》2012,119(7):833-842
Alzheimer's disease (AD) is the most common neurodegenerative disease, and is clinically characterized by cognitive disturbances and the accumulation of the amyloid β (Aβ) peptides in plaques in the brain. Recent studies have shown the links between AD and the immediate-early gene Arc (activity-regulated cytoskeleton-associated protein), involved in synaptic plasticity and memory consolidation. For example, AD mouse models show a decreased expression of Arc mRNA in the brain. In additional, acute Aβ application to brain slices leads to a widespread ARC protein diffusion, unlike the normal defined localization to synapses. In this study, we investigated genetic variation in human ARC and the risk of developing AD. To this end, we genotyped 713 subjects diagnosed with AD and 841 controls without dementia. ARC was sequenced in a group of healthy individuals, and seven previously known SNPs and three novel SNPs were identified. Two of the newly found SNPs were intronic and one, +2852(G/A), was located in the 3'UTR. Three tag SNPs were selected, including the novel SNP +2852(G/A), to relate to risk of AD, Mini Mental State Examination (MMSE) scores and cerebrospinal fluid (CSF) biomarker levels of total tau (T-tau), hyperphosphorylated tau181 (P-tau(181)) and Aβ(1-42). The AA genotype of the newly found 3'-UTR SNP +2852(A/G), was associated with a decreased risk of AD (p (c)?=?0.005; OR?=?0.74; 95?% CI: 0.61-0.89). No associations of single SNPs or haplotypes with MMSE score or CSF biomarkers were found. Here we report a novel ARC SNP associated with a reduced risk of developing AD. To our knowledge, this is the first study associating a gene variant of ARC with any disease. The location of the SNP within the 3'UTR indicates that dendritic targeting of ARC mRNA could be involved in the molecular mechanisms underlying this protective function. However, further investigation of the importance of this SNP for ARC function, ARC processing and the pathology of AD is needed. 相似文献
997.
998.
Batista S Zivadinov R Hoogs M Bergsland N Heininen-Brown M Dwyer MG Weinstock-Guttman B Benedict RH 《Journal of neurology》2012,259(1):139-146
Information-processing speed (IPS) slowing is a primary cognitive deficit in multiple sclerosis (MS). Basal ganglia, thalamus
and neocortex are thought to have a key role for efficient information-processing, yet the specific relative contribution
of these structures for MS-related IPS impairment is poorly understood. To determine if basal ganglia and thalamus atrophy
independently contribute to visual and auditory IPS impairment in MS, after controlling for the influence of neocortical volume,
we enrolled 86 consecutive MS patients and 25 normal controls undergoing 3T brain MRI and neuropsychological testing. Using
Sienax and FIRST software, neocortical and deep gray matter (DGM) volumes were calculated. Neuropsychological testing contributed
measures of auditory and visual IPS using the Paced Auditory Serial Addition Test (PASAT) and the Symbol Digit Modalities
Test (SDMT), respectively. MS patients exhibited significantly slower IPS relative to controls and showed reduction in neocortex,
caudate, putamen, globus pallidus, thalamus and nucleus accumbens volume. SDMT and PASAT were significantly correlated with
all DGM regions. These effects were mitigated by controlling for the effects of neocortical volume, but all DGM volumes remained
significantly correlated with SDMT, putamen (r = 0.409, p < 0.001) and thalamus (r = 0.362, p < 0.001) having the strongest effects, whereas for PASAT, the correlation was significant for putamen (r = 0.313, p < 0.01) but not for thalamus. We confirm the significant role of thalamus atrophy in MS-related IPS slowing and find that
putamen atrophy is also a significant contributor to this disorder. These DGM structures have independent, significant roles,
after controlling for the influence of neocortex atrophy. 相似文献
999.
1000.
de Zubicaray GI Miozzo M Johnson K Schiller NO McMahon KL 《Journal of cognitive neuroscience》2012,24(2):482-495
In two fMRI experiments, participants named pictures with superimposed distractors that were high or low in frequency or varied in terms of age of acquisition. Pictures superimposed with low-frequency words were named more slowly than those superimposed with high-frequency words, and late-acquired words interfered with picture naming to a greater extent than early-acquired words. The distractor frequency effect (Experiment 1) was associated with increased activity in left premotor and posterior superior temporal cortices, consistent with the operation of an articulatory response buffer and verbal self-monitoring system. Conversely, the distractor age-of-acquisition effect (Experiment 2) was associated with increased activity in the left middle and posterior middle temporal cortex, consistent with the operation of lexical level processes such as lemma and phonological word form retrieval. The spatially dissociated patterns of activity across the two experiments indicate that distractor effects in picture-word interference may occur at lexical or postlexical levels of processing in speech production. 相似文献