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A number of spraying experiments showed that the virus cannot enter a plant unless some of the cells are injured. It is not essential that such injury should be brought about in the presence of the virus. The chances of infection fall off rapidly in the first few minutes after injury, but infection occurs occasionally as long as half an hour after the cell is damaged.
Inoculations by micropipette into single cells of the host plant yielded only about one-tenth of the expected number of infections. This suggests differences in the susceptibility of the cells to virus attack. 相似文献
Inoculations by micropipette into single cells of the host plant yielded only about one-tenth of the expected number of infections. This suggests differences in the susceptibility of the cells to virus attack. 相似文献
35.
W. F. LOOMIS C. KLEIN PH. BRACKET 《Differentiation; research in biological diversity》1979,12(1-3):83-89
Following the initiation of development, amoebae of Dictyostelium discoideum aggregate chemotactically toward cyclic AMP (cAMP). Adenyl cyclase, cAMP phosphodiesterase, and cAMP binding sites all increase 20–40 fold during the first few hours of development. It has been shown that addition of 1 mM EDTA and 5 mM MgCl2 accelerates the aggregation process. Likewise, the calcium ionophore, A23187, leads to precocious aggregation while 4 × 10−5 M progesterone considerably delays it These treatments have now been shown to result in increased accumulation of adenyl cyclase in the case of EDTA and Mg2+ or the ionophore and greatly decreased accumulation in the case of the steroid.
Treatment with EDTA and Mg2+ or the ionophore has been shown not only to accelerate aggregation in wild-type amoebae but to overcome complete blocks to aggregation in certain mutant strains. We have found that addition of Mn2+ will also permit aggregation of mutant cells otherwise unable to aggregate. This divalent ion, unlike EDTA and Mg2+ or the ionophore, was shown to directly stimulate adenyl cyclase. Calcium ions were also found to affect the enzyme such that at Ca2+ concentrations found within the cells the great majority of the activity is inhibited. Manganese ions can overcome the inhibition by Ca2+ .
These findings show that conditions which stimulate aggregation result in increased activity of adenyl cyclase either by increased accumulation of the enzyme or by increased activity of the available enzyme, and support the proposed central role of adenyl cyclase in aggregation. 相似文献
Treatment with EDTA and Mg
These findings show that conditions which stimulate aggregation result in increased activity of adenyl cyclase either by increased accumulation of the enzyme or by increased activity of the available enzyme, and support the proposed central role of adenyl cyclase in aggregation. 相似文献
36.
Climate change poses an unprecedented threat to biodiversity worldwide. Consequently, unrecognised taxa may not receive adequate conservation attention to survive. We used molecular and morphological data to address the challenge of species delimitation within the genus Schilbe. The presence or absence of an adipose fin and distribution based on east-flowing, conceivably faster-flowing, or west-flowing, probably more slow-flowing, river systems were considered. Distinctive geographic patterns in genetic variation within southern, eastern, and western African populations were revealed. Particularly, the South African population is distinct from those of Namibia, Botswana and Nigeria. No individuals with rudimentary adipose fins were found at any locality, but specimens from three localities either had or did not have adipose fins. These mixed occurrences are suspected to be a result of human interventions, and that the presence of rudimentary adipose fins in the east African species could be an adaptive feature that serves to stabilise these fish in faster currents. In addition, the genetic divergence observed among African silver catfish from geographically isolated river systems is conceivably the result of micro-evolutionary adaptive responses to different environmental conditions. Collectively, these results distinguish S. depressirostris from S. intermedius. 相似文献
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Sequence variations in small-subunit ribosomal RNAs of Hartmannella vermiformis and their phylogenetic implications 总被引:1,自引:0,他引:1
Evidence of associations between free-living amoebas and human disease has
been increasing in recent years. Knowledge about phylogenetic relationships
that may be important for the understanding of pathogenicity in the genera
involved is very limited at present. Consequently, we have begun to study
these relationships and report here on the phylogeny of Hartmannella
vermiformis, a free-living amoeba that can harbor the etiologic agent of
Legionnaires' disease. Our analysis is based on studies of small-subunit
ribosomal RNA genes (srDNA). Nucleotide sequences were determined for
nuclear srDNA from three strains of H. vermiformis isolated from the United
Kingdom, Germany, and the United States. These sequences then were compared
with a sequence previously obtained for a North American isolate by J. H.
Gunderson and M. L. Sogin. The four genes are 1,840 bp long, with an
average GC content of 49.6%. Sequence differences among the strains range
are 0.38%-0.76%. Variation occurs at 19 positions and includes 2
single-base indels plus 14 monotypic and 3 ditypic single-base
substitutions. Variation is limited to eight helix/loop structures
according to a current model for srRNA secondary structure. Parsimony,
distance, and bootstrap analyses used to examine phylogenetic relationships
between the srDNA sequences of H. vermiformis and other eukaryotes
indicated that Hartmannella sequences were most closely related to those of
Acanthamoeba and the alga Cryptomonas. All ditypic sites were consistent
with a separation between European and North American strains of
Hartmannella, but results of other tests of this relationship were
statistically inconclusive.
相似文献
39.
Susannah M Wyles Garry Browne Gerald PH Gui 《International Seminars in Surgical Oncology : ISSO》2007,4(1):13
A 34 year old woman diagnosed with breast cancer and liver metastases underwent a left subclavian Portacath insertion. During the procedure, the clinical features and the findings of intra-operative investigations provided conflicting evidence of the catheter position. This report highlights the potential difficulties in establishing long-term central venous access, the limitations of common investigations and safety issues relating to the process of subclavian line insertion. 相似文献
40.
FLORAL ANATOMY AND PHYLOGENY IN THE RUTACEAE 总被引:2,自引:0,他引:2