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61.
The murid rodent subfamily Sigmodontinae contains 79 genera which are
distributed throughout the New World. The time of arrival of the first
sigmodontines in South America and the estimated divergence time(s) of the
different lineages of South American sigmodontines have been controversial
due to the lack of a good fossil record and the immense number of extant
species. The "early-arrival hypothesis" states that the sigmodontines must
have arrived in South America no later than the early Miocene, at least 20
MYA, in order to account for their vast present-day diversity, whereas the
"late-arrival hypothesis" includes the sigmodontines as part of the
Plio-Pleistocene Great American Interchange, which occurred approximately
3.5 MYA. The phylogenetic relationships among 33 of these genera were
reconstructed using mitochondrial DNA (mtDNA) sequence data from the ND3,
ND4L, arginine tRNA, and ND4 genes, which we show to be evolving at the
same rate. A molecular clock was calibrated for these genes using published
fossil dates, and the genetic distances were estimated from the DNA
sequences in this study. The molecular clock was used to estimate the dates
of the South American sigmodontine origin and the main sigmodontine
radiation in order to evaluate the "early-" and "late-arrival" scenarios.
We estimate the time of the sigmodontine invasion of South America as
between approximately 5 and 9 MYA, supporting neither of the scenarios but
suggesting two possible models in which the invading lineage was either (1)
ancestral to the oryzomyines, akodonts, and phyllotines or (2) ancestral to
the akodonts and phyllotines and accompanied by the oryzomyines. The
sigmodontine invasion of South America provides an example of the advantage
afforded to a lineage by the fortuitous invasion of a previously
unexploited habitat, in this case an entire continent.
相似文献
62.
Multiple duplications of yeast hexose transport genes in response to selection in a glucose-limited environment 总被引:11,自引:2,他引:9
When microbes evolve in a continuous, nutrient-limited environment, natural
selection can be predicted to favor genetic changes that give cells greater
access to limiting substrate. We analyzed a population of baker's yeast
that underwent 450 generations of glucose-limited growth. Relative to the
strain used as the inoculum, the predominant cell type at the end of this
experiment sustains growth at significantly lower steady-state glucose
concentrations and demonstrates markedly enhanced cell yield per mole
glucose, significantly enhanced high-affinity glucose transport, and
greater relative fitness in pairwise competition. These changes are
correlated with increased levels of mRNA hybridizing to probe generated
from the hexose transport locus HXT6. Further analysis of the evolved
strain reveals the existence of multiple tandem duplications involving two
highly similar, high- affinity hexose transport loci, HXT6 and HXT7.
Selection appears to have favored changes that result in the formation of
more than three chimeric genes derived from the upstream promoter of the
HXT7 gene and the coding sequence of HXT6. We propose a genetic mechanism
to account for these changes and speculate as to their adaptive
significance in the context of gene duplication as a common response of
microorganisms to nutrient limitation.
相似文献
63.
Evidence for an enhanced substrate requirement by marine mesophilic bacterial isolates at minimal growth temperatures 总被引:6,自引:0,他引:6
Bacterial isolates from the subtropical southeastern continental shelf were cultured in a matrix of temperature and substrate concentrations encompassing a range of temperature and substrate concentrations equal to and exceeding natural ones. At the annual minimum temperature, marine heterotrophic bacterial isolates required higher concentrations of dissolved substrates for active growth than are usually found in seawater. We show this to result from a nonlinear interaction of the combined effects of temperature and substrate concentration on bacterial growth and respiratory rate. As a result, bacterial and protozoan utilization of phytoplankton production during winter and early spring is low, permitting greater energy flow to zooplankton and benthic animals, while in late spring, summer, and fall, the microbial loop dominates energy flux and organic carbon utilization. Escherichia coli shows a similar nonlinear response to temperature at minimal substrate concentrations, albeit at a higher range of concentrations than were utilized by the marine isolates. Thus, bacteria from subtropical regions are shown to have a differential growth response near the minimum temperature for growth, depending on the concentration of available substrates.
Offprint requests to: W.J. Wiebe. 相似文献
64.
Changes in Bacterial Numbers and Leucine Assimilation during Estimations of Microbial Respiratory Rates in Seawater by the Precision Winkler Method 总被引:2,自引:1,他引:1
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During incubation of seawater in bottles, the decrease in dissolved oxygen is often nonlinear over time scales frequently used to measure respiration. Numbers of bacteria always increase, and rates of assimilation of dissolved leucine often increase exponentially. This suggests that sample handling disrupts the previously existing food web, leading to shifts of trophic state and unbalanced growth. Potential errors in measuring respiratory rate can be minimized by documenting these variables. 相似文献
65.
The daily locations of socially active male grey seals were recorded during three successive breeding seasons on North Rona (Outer Hebrides, Scotland). Areas of occupancy were defined for 38 individually identified males. Socially active males showed a high degree of site fidelity, determined from the overlap of areas and distance moved by individuals between seasons. The degree of site fidelity displayed by individuals was not related to measures of individual mating success, dominance, date of arrival to the colony, length of stay at the colony or changes in these values between seasons. As both male and female grey seals display site fidelity on N. Rona, individual recognition of potential opponents and mates is possible. There is also potential for the production of full sibling offspring in successive seasons. 相似文献
66.
Toxicity of Anaerobic Metabolites Accumulating in Winter Wheat Seedlings during Ice Encasement 总被引:10,自引:9,他引:1
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Ice encasement damages cold-hardened winter wheat without major disruption of cellular organelles. CO2 accumulates during total ice encasement to higher levels in Kharkov than in less hardy Fredrick wheat. Partial ice encasement and exposure to a nitrogen atmosphere at -1 C allows greater CO2 accumulation but neither treatment is as damaging as total ice encasement. Lactic acid accumulates to low levels only during the 1st day of encasement and thereafter remains constant. Exposure of plants to a combination of 50% CO2 and 5% ethanol reduces survival, with a cultivar difference similar to that found in ice-encased plants. Plants in CO2 and ethanol show a proliferation of membranes and nuclear condensation similar to that in cells of ice-encased plants. Permeability increases markedly in the presence of CO2 and ethanol, to levels similar to or greater than those of iced plants. Ethanol alone does not increase permeability but in combination with CO2 raises permeability of the less hardy Fredrick, although not of Kharkov, but reduces survival of both cultivars. A comparison of the endogenous levels of ethanol, CO2, and lactic acid at the 50% kill point of plants due to ice encasement or due to externally supplied metabolite indicates that only CO2 accumulates to independently toxic levels. Permeability and ultrastructural evidence suggest that CO2 and ethanol in combination are the agents reducing plant viability during ice encasement. 相似文献
67.
A comparison of the properties of ATPase associated with wheat and cauliflower plasma membranes 总被引:1,自引:0,他引:1
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Plasma membrane-associated ATPase obtained from cauliflower (Brassica oleraceae L.) florets isolated and assayed by several different procedures was stimulated 150 to 400% by K+. In contrast, winter wheat (Triticum aestivum L. cv. Kharkov 22 MC) shoot and root ATPase obtained by the same methods exhibited only 10 to 25% stimulation by K+. The level of K+-stimulation of the wheat enzyme was not significantly increased by purifying the crude microsomal membrane fraction using sucrose density gradients. ATPase associated with density gradient-purified cauliflower membranes was inhibited by Ca2+, high ATP concentration in the presence of low Mg2+, and by several metabolic inhibitors. In contrast, the wheat enzyme was largely unaffected by all of these treatments. The plasma membranes of intact wheat and cauliflower cells gave a positive reaction with the plasma membrane-specific, phosphotungstic acid-chromic acid stain (PACP). A high proportion of the cauliflower membrane vesicles in the putative plasma membrane-enriched fraction stained with PACP, whereas only a small proportion of the wheat membrane vesicles reacted positively with PACP. These results indicate that a plasma membrane-enriched fraction has been isolated successfully from cauliflower floret tissue, but that none of the procedures used effectively separate plasma membranes from homogenates of wheat shoots and roots. 相似文献
68.
A plasma-membrane fraction rich in ion-stimulated ATPase activity was isolated from cauliflower (Brassica oleracea L.) buds. The activity of the ATPase was dependent on Mg(2+) and stimulated 4-fold by K(+). The lipids of the membrane fraction contained 57% by weight of phospholipid, 16% glycolipid including sterol glycosides, and 27% neutral lipids. Sterols and sterol esters comprised 9% by weight of the total lipid fraction, and the m ratio of total sterol to phospholipid was 0.5. Fatty acid unsaturation of the membrane lipids was 75%. Arrhenius plots of the Mg(2+) and Mg(2+) + K(+) stimulated ATPase activity were biphasic with an increase in activation energy occurring below about 12 degrees C, a response typical of some membrane-associated enzymes of chilling-sensitive plants. No thermal transitions were detected in the membranes or membrane lipids between 0 and 30 degrees C using differential scanning calorimetry and electron spin resonance spectroscopy. This type of thermal behavior is typical of membranes of chilling-resistant plants. It was concluded that the low temperature increase in activation energy of the ion-stimulated, membrane-associated ATPase is an intrinsic property of the enzyme system and not the result of a transition in the bulk membrane lipid. 相似文献
69.
McCafferty DJ Moss S Bennett K Pomeroy PP 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2005,175(6):423-431
The aim of this study was to examine the variation in body surface temperature of grey seal (Halichoerus grypus) pups throughout lactation in response to different environmental conditions. Radiative surface temperatures (T
r, °C) of pups were measured on the Isle of May (56°11′N, 02°33′W), southeast Scotland from 29 October to 25 November 2003.
Records were obtained from a total of 60 pups (32 female and 28 male) from three different pupping sites during early and
late lactation. Pups were sheltered from high wind speeds but air temperature, humidity and solar radiation at pupping sites
were similar to general meteorological conditions. The mean T
r of all pups was 15.8°C (range 7.7–29.7°C) at an average air temperature of 10.2°C (range 6.5–13.8°C). There was no difference
in the mean T
r of pups between early and late lactation. However, the T
r varied between different regions of the body with hind flippers on average 2–6°C warmer than all other areas measured. There
was no difference in mean T
r of male and female pups and pup body mass did not account for the variation in T
r during early or late lactation. Throughout the day there was an increase in the T
r of pups and this explained 20–28% of the variation in T
r depending on stage of lactation. There was no difference in the mean T
r of pups between pupping sites or associated with different substrate types. Wind speed and substrate temperature had no effect
on the T
r of pups. However, solar radiation, air temperature and relative humidity accounted for 48% of the variation in mean T
r of pups during early lactation. During late lactation air temperature and solar radiation alone accounted for 43% of the
variation in T
r. These results indicate that environmental conditions explain only some of the variation in T
r of grey seal pups in natural conditions. Differences in T
r however indicate that the cost of thermoregulation for pups will vary throughout lactation. Further studies examining intrinsic
factors such as blubber thickness and activity levels are necessary before developing reliable biophysical models for grey
seals. 相似文献
70.
Pomeroy SL 《Neuron glia biology》2004,1(2):127-133
Recent evidence argues that the oncogenesis and growth of CNS tumors occurs through dysregulated molecular and cellular mechanisms of neural development. New insights have emerged that have had a significant impact on both research and treatment of these cancers. 相似文献