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排序方式: 共有135条查询结果,搜索用时 15 毫秒
41.
Giannone G Dubin-Thaler BJ Rossier O Cai Y Chaga O Jiang G Beaver W Döbereiner HG Freund Y Borisy G Sheetz MP 《Cell》2007,128(3):561-575
Cell motility proceeds by cycles of edge protrusion, adhesion, and retraction. Whether these functions are coordinated by biochemical or biomechanical processes is unknown. We find that myosin II pulls the rear of the lamellipodial actin network, causing upward bending, edge retraction, and initiation of new adhesion sites. The network then separates from the edge and condenses over the myosin. Protrusion resumes as lamellipodial actin regenerates from the front and extends rearward until it reaches newly assembled myosin, initiating the next cycle. Upward bending, observed by evanescence and electron microscopy, results in ruffle formation when adhesion strength is low. Correlative fluorescence and electron microscopy shows that the regenerating lamellipodium forms a cohesive, separable layer of actin above the lamellum. Thus, actin polymerization periodically builds a mechanical link, the lamellipodium, connecting myosin motors with the initiation of adhesion sites, suggesting that the major functions driving motility are coordinated by a biomechanical process. 相似文献
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J. A. Beaver A. F. Iezzoni C. W. Ramm 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1995,90(6):847-852
Thirty-six sour (Prunus cerasus L.), sweet (P. avium L.), and ground cherry (P. fruticosa Pall.) selections were evaluated for seven enzyme systems and principal coordinate analysis was used to examine isozyme divergence among these cherry species. The enzyme systems studied were phosphoglucose isomerase (PGI), isocitrate dehydrogenase (IDH), phosphoglucomutase (PGM), 6-phosphogluconate dehydrogenase (6-PGD), leucine aminopeptidase (LAP), shikimate dehydrogenase (SKDH), and malate dehydrogenase (MDH). The first principal coordinate, which accounted for 41% of the total variation, separated the diploid sweet cherry selections from the sour, ground, and sour x ground cherry tetraploids. An additional 86 selections were evaluated for up to six of the enzyme systems to determine the polymorphisms at the enzyme loci and the level of heterozygosity between the diploid sweet cherry and the tetraploid species and interspecific hybrids. 6-PGD was the most polymorphic enzyme exhibiting 16 patterns. The tetraploid cherry species were more heterozygous than the diploid sweet cherry with an average heterozygosity of 78% compared to 19% for the diploids. 相似文献
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Seasonal patterns in primary productivity and algal biomass in subtropical Florida lakes along increasing gradients of both dissolved organic color and phytoplankton biomass are presented. Chlorophyll a concentrations and gross primary productivity generally reached maxima during the summer and were most depressed in winter months, regardless of color or trophic classification. Primary productivity was more strongly correlated with chlorophyll a, nutrient concentrations and water clarity in clearwater (< 75 Pt units) than in colored (> 75 Pt units) systems. Amplitudes in algal biomass were considerably smaller than temperate lakes. Variability in primary production in Florida lakes was intermediate to patterns in the temperate zone and tropics, but was more closely aligned to northern latitudes. Within the Florida peninsula, variability of primary productivity decreased from north to south and corresponded to latitudinal gradients in climatic regimes. 相似文献
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The objective of this study was to characterize the zooplankton and phytoplankton assemblages of four different types of wetlands
and to evaluate their use as environmental indicators. Total abundances, community composition, and species diversity were
evaluated for zooplankton and phytoplankton assemblages from 24 wetlands and related to water quality variables. During August
1995, six representative sites were sampled from four types of wetlands designated as constructed, impacted, non-impacted,
or temporary. The plankton assemblages of all wetlands were dominated by cosmopolitan crustacean, rotifer, and phytoplankton
taxa typical of lake plankton communities. Species diversity, richness, and evenness of zooplankton and phytoplankton assemblages
did not differ significantly among the wetland types. Total zooplankton abundance was significantly (p < 0.01) related to
chlorophyll a and total phosphorus concentrations over the range of trophic conditions. Mean zooplankton densities and phytoplankton
biovolumes were similar among the wetlands, however, the relative abundances of major zooplankton groups differed among the
wetland types. Cyanophytes, primarily Oscillatoria spp., were a major component of the phytoplankton across all four wetland
types, and were significantly more abundant within the constructed and temporary sites. On average, rotifers accounted for
79% of total zooplankton abundance within the constructed wetlands and were much less dominant in the non-impacted and temporary
wetlands. Cladoceran, copepodite, and adult copepod concentrations were low in the constructed and impacted wetlands and increased
in the non-impacted and temporary wetlands in conjunction with increased chlorophytes and cryptophytes. Our preliminary survey
suggests that abiotic factors which are known to directly affect phytoplankton may indirectly affect zooplankton composition
in such a way as to use zooplankton assemblages as indicators of water quality. However, further study incorporating seasonal
dynamics and the influence of predators on zooplankton assemblages is needed to fully assess the use of zooplankton community
composition as an environmental indicator for wetland systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
46.
Applicability of planktonic biomanipulation for managing eutrophication in the subtropics 总被引:5,自引:3,他引:2
Although large-bodied cladocerans such asDaphnia can reduce algal biomass significantly in temperate lakes if freed from fish predation, the applicability of such biomanipulation
techniques for eutrophication management in the subtropics and tropics has been examined only recently. Subtropical cladoceran
assemblages differ from those of temperate lakes by their low species richness, early summer gameogenesis, and greatly reduced
body size. Eutrophic Florida lakes are dominated by pump-filter feeding fish rather than by size selective planktivores as
a temperate lakes. Cladocerans in Florida lakes can increase in abundance significantly if freed from fish but fail to have
an impact on algal biomass or composition. The greatest potential for using biomanipulation to manage phytoplankton-dominated
lakes in the subtropics and tropics lies with phytophagous fish. Future research should concentrate on defining the role of
individual fish taxa on phytoplankton composition and community structure, nutrient cycling, and planktonic productivity before
embarking on whole lake manipulation projects. 相似文献
47.
N.S. Doherty P. Poubelle P. Borgeat T.H. Beaver G.L. Westrich N.L. Schrader 《Prostaglandins & other lipid mediators》1985,30(5):769-789
Intraperitoneal injection of zymosan in mice induced rapid extravasation and accumulation of plasma proteins in the peritoneal cavity. Neutrophils began to appear in the peritoneal cavity after a lag period of approximately 3 hours. The injected mice exhibited a pain response (writhing) during the first 30 minutes after injection, but writhing ceased before protein or cell accumulation had reached maximum levels. The injection of zymosan induced synthesis of PGE2 (measured by RIA) which reached maximum levels of 30 minutes, then declined slowly. Peptido-leukotriene levels (detected by bioassay, RIA and HPLC) increased rapidly after injection, reached a peak within an hour of injection and declined to undetectable levels within 4 hours. The early peptido-LT was predominantly LTC4, while later, LTE4 was the major component. LTD4 levels remained low throughout and no LTB4 was detected at any time. Indomethacin treatment elevated levels of peptido-LTs, recued PGE2 levels and inhibited writhing. Phenidone reduced peptido-LT levels. studies demonstrated that zymosan stimulates LTC4 synthesis by peritoneal cells whereas LTE4, LTD4, LTB4 or monoHETES were not detectable (using HPLC methods). The source of enzymes responsible for the metabolism of LTC4 to LTD4 and LTE4 could not be identified. 相似文献
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