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21.
Frequently, Pleistocene climatic cycling has been found to be the diver of genetic structuring in populations, even in areas that did not have continental ice sheets, such as on the Qinghai‐Tibetan Plateau (QTP). Typically, species distributed on the plateau have been hypothesized to re‐treat to south‐eastern refugia, especially during the Last Glacial Maximum (LGM). We evaluated sequence variation in the mitochondrial DNA gene Cytb and the nuclear DNA gene RAG‐1 in Rana kukunoris, a species endemic to the QTP. Two major lineages, N and S, were identified, and lineage N was further subdivided into N1 and N2. The geographical distribution and genealogical divergences supported the hypothesis of multiple refugia. However, major lineages and sublineages diverged prior to the LGM. Demographical expansion was detected only in lineage S and sublineage N2. Sublineage N1 might have survived several glacial cycles in situ and did not expand after the LGM because of the absence of suitable habitat; it survived in river islands. Genetic analysis and environment modelling suggested that the north‐eastern edge of QTP contained a major refugium for R. kukunoris. From here, lineage S dispersed southwards after the LGM. Two microrefugia in northern Qilian Mountains greatly contributed to current level of intraspecific genetic diversity. These results were found to have important implications for the habitat conservation in Northwest China.  相似文献   
22.
In vitro slow fluctuations in the level of horseradish peroxidase activity were observed in long-range experiments (72–144 h). Besides random fluctuations, regular slow oscillatory patterns with period lengths ranging from 10.0 to 39.0 h were detected by statistical analysis. The possibility that these oscillations in enzyme activity could have reflected changes in the physical environment of the experimental setup has been thoroughly examined and ruled out. Periodic exposition of the enzyme solution, otherwise kept in darkness, to blue light illumination was shown to influence the period of the oscillations. The changes in enzyme activity were correlated with a modification of the Michaelis constant estimated using guaiacol as substrate. This result was confirmed by the action of chemical modifiers of the enzyme, such as ferulic acid and rutin. It is thought that the observed oscillations in horseradish peroxidase activity are due to spontaneous and specific changes in the tridimensional structure of the enzyme in the thermic reservoir.  相似文献   
23.
Actin filament (F-actin) assembly kinetics determines the locomotion and shape of crawling eukaryotic cells, but the nature of these kinetics and their determining reactions are unclear. Live BHK21 fibroblasts, mouse melanoma cells, and Dictyostelium amoebae, locomoting on glass and expressing Green Fluorescent Protein-actin fusion proteins, were examined by confocal microscopy. The cells demonstrated three-dimensional bands of F-actin, which propagated throughout the cytoplasm at rates usually ranging between 2 and 5 microm/min in each cell type and produced lamellipodia or pseudopodia at the cell boundary. F-actin's dynamic behavior and supramolecular spatial patterns resembled in detail self-organized chemical waves in dissipative, physico-chemical systems. On this basis, the present observations provide the first evidence of self-organized, and probably autocatalytic, chemical reaction-diffusion waves of reversible actin filament assembly in vertebrate cells and a comprehensive record of wave and locomotory dynamics in vegetative-stage Dictyostelium cells. The intensity and frequency of F-actin wavefronts determine locomotory cell projections and the rotating oscillatory waves, which structure the cell surface. F-actin assembly waves thus provide a fundamental, deterministic, and nonlinear mechanism of cell locomotion and shape, which complements mechanisms based exclusively on stochastic molecular reaction kinetics.  相似文献   
24.
25.
Oscillations in the electrostatic energy gap [Velec(t)] for electron transfer from the primary electron donor (P) to the adjacent bacteriochlorophyll (B) in photosynthetic bacterial reaction centers are examined by molecular-dynamics simulations. Autocorrelation functions of Velec in the reactant state (PB) include prominent oscillations with an energy of 17 cm–1. This feature is much weaker if the trajectory is propagated in the product state P+B. The autocorrelation functions also include oscillations in the regions of 5, 80 and 390 cm–1 in both states, and near 25 and 48 cm–1 in P+B. The strong 17-cm–1 oscillation could involve motions that modulate the distance between P and B, because a similar oscillation occurs in the direct electrostatic interactions between the electron carriers.  相似文献   
26.
Oscillations in intracellular free Ca2+ concentration ([Ca2+]i) have been observed in a variety of cell types. In the present study, we constructed a mathematical model to simulate the caffeine-induced [Ca2+]i oscillations based on experimental data obtained from isolated type I horizontal cell of carp retina. The results of model analysis confirm the notion that the caffeine-induced [Ca2+]i oscillations involve a number of cytoplasmic and endoplasmic Ca2+ processes that interact with each other. Using this model, we evaluated the importance of store-operated channel (SOC) in caffeine-induced [Ca2+]i oscillations. The model suggests that store-operated Ca2+ entry (SOCE) is elicited upon depletion of the endoplasmic reticulum (ER). When the SOC conductance is set to 0, caffeine-induced [Ca2+]i oscillations are abolished, which agrees with the experimental observation that [Ca2+]i oscillations were abolished when SOC was blocked pharmacologically, verifying that SOC is necessary for sustained [Ca2+]i oscillations.  相似文献   
27.
The period (in the order of 40 to 80 days) in periodic chronic myelogenous leukemia (PCML) oscillations is quite long compared with the duration of the cell cycle of the hematopoietic stem cells from which the oscillations are presumed to originate (in the order of one or two days). Our objective is to understand the origin of these long-period oscillations using a G0 model for stem cell dynamics. We determine the local stability conditions and show under what conditions the Hopf bifurcation may occur. We interpret the role of each parameter in the loss of stability, and then examine a simpler model to try to deduce possible changes at the stem-cell level that might be responsible for the characteristics PCML.  相似文献   
28.
1 Introduction Dynamic wind loads can excite potentially de-structive oscillations in plants. Without dissipation ofmechanical energy a resonance catastrophe is likely tooccur in dynamic winds with frequency componentsclose to the resonant frequencies of the plant. Four modesof oscillation damping are considered [1] ? Friction with other members of the plant commu-nity as for instance in the dense stand of bamboos. ? Structural damping: other than in the engineeringscien…  相似文献   
29.
Periodic climatic oscillations and species dispersal during the postglacial period are two important causes of plant assemblage and distribution on the Qinghai‐Tibet Plateau (QTP). To improve our understanding of the bio‐geological histories of shrub communities on the QTP, we tested two hypotheses. First, the intensity of climatic oscillations played a filtering role during community structuring. Second, species dispersal during the postglacial period contributed to the recovery of species and phylogenetic diversity and the emergence of phylogenetic overdispersion. To test these hypotheses, we investigated and compared the shrub communities in the alpine and desert habitats of the northeastern QTP. Notably, we observed higher levels of species and phylogenetic diversity in the alpine habitat than in the desert habitat, leading to phylogenetic overdispersion in the alpine shrub communities versus phylogenetic clustering in the desert shrub communities. This phylogenetic overdispersion increased with greater climate anomalies. These results suggest that (a) although climate anomalies strongly affect shrub communities, these phenomena do not act as a filter for shrub community structuring, and (b) species dispersal increases phylogenetic diversity and overdispersion in a community. Moreover, our investigation of the phylogenetic community composition revealed a larger number of plant clades in the alpine shrub communities than in the desert shrub communities, which provided insights into plant clade‐level differences in the phylogenetic structures of alpine and desert shrub communities in the northeastern QTP.  相似文献   
30.
Theory and analyses of fisheries data sets indicate that harvesting can alter population structure and destabilise non-linear processes, which increases population fluctuations. We conducted a factorial experiment on the population dynamics of Daphnia magna in relation to size-selective harvesting and stochasticity of food supply. Harvesting and stochasticity treatments both increased population fluctuations. Timeseries analysis indicated that fluctuations in control populations were non-linear, and non-linearity increased substantially in response to harvesting. Both harvesting and stochasticity induced population juvenescence, but harvesting did so via the depletion of adults, whereas stochasticity increased the abundance of juveniles. A fitted fisheries model indicated that harvesting shifted populations towards higher reproductive rates and larger-magnitude damped oscillations that amplify demographic noise. These findings provide experimental evidence that harvesting increases the non-linearity of population fluctuations and that both harvesting and stochasticity increase population variability and juvenescence.  相似文献   
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