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1.
John S. Donaldson 《American journal of botany》1997,84(10):1398-1406
Observations and experiments were carried out over 5 yr to distinguish between wind and insect pollination in the cycad Encephalartos villosus Lemaire (Zamiaceae). They were also designed to determine whether a pollination mutualism exists between E. villosus and Antliarhinus zamiae (Thunberg) (Coleoptera: Brentidae), an obligate ovule parasite that routinely parasitizes a large proportion of the ovules. The percentage of fertilized ovules dropped slightly when wind was excluded from the megastrobilus. However, when insects were excluded by either net bags or insecticide there was a substantial decrease in the proportion of fertilized ovules. Five beetle species belonging to four families were found on the strobili at the time of pollination. Using data on the effectiveness of pollen transfer to the receptive ovule, as well as data on abundance and pollen loads, a pollinator importance value (PIV) was determined for each beetle species and a pollinator importance index (PII) was determined for each population. PII values showed that an undescribed weevil (Porthetes sp., Curculionidae) was consistently the most important pollinator. A. zamiae and an undescribed beetle species within the Xenoscelinae (Languriidae) played a minor role in pollination, and their contributions varied from year to year and between populations. Two additional beetle species, Metacucujus goodei Endrödy-Younga (Boganiidae) and a second species of Xenoscelinae, had very low PII values and probably had little or no effect on pollination. Low PIV scores for A. zamiae were a result of its low numbers on the microstrobilus and the tendency of the beetles to remain on the outside of the megastrobilus. In the interaction between E. villosus and A. zamiae, the cycad does not appear to benefit significantly from a pollination service and I interpret this to mean that the relationship is antagonistic rather than mutualistic. There is, however, a possible mutualism between Porthetes sp. and E. villosus. 相似文献
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Evidence has been presented to show that CD4+ autoreactive T cell lines (ATs)2 in the rat require periodic stimulation with syngeneic spleen cells for in vitro proliferation. This proliferation can be blocked by treatment of the stimulator (spleen) cells with mAb to Ia antigens. Although ATs are Ia+ and can activate the allogeneic MLR, they fail to be autostimulatory. Fractionation of the spleen cells revealed that ATs can be stimulated with B cells and not by macrophages, although the latter were efficient in several accessory cell functions, including antigen presentation, lectin-dependent T cell activation and allogenic MLR response. Moreover, B cells proliferated and differentiated in response to AT cells. These data are compatible with a model in which ATs respond to hitherto undetermined B cell membrane antigen(s) in association with MHC class II antigens. These results may have important implications in understanding autoimmune responses. 相似文献
4.
Peeled mammalian skeletal muscle fibers. Possible stimulation of Ca2+ release via a transverse tubule-sarcoplasmic reticulum mechanism 总被引:4,自引:3,他引:1 下载免费PDF全文
S K Donaldson 《The Journal of general physiology》1985,86(4):501-525
Single muscle fibers from rabbit soleus and adductor magnus and from semitendinosus muscles were peeled to remove the sarcolemma and then stimulated to release Ca2+ by (a) caffeine application or (b) ionic depolarization accomplished via substitution of choline chloride for potassium propionate at constant [K+] X [Cl-] in the bathing solution. Each stimulus, ionic or caffeine, elicited an isometric tension transient that appeared to be due to Ca2+ released from the sarcoplasmic reticulum (SR). The peak magnitude of the ionic (Cl- -induced) tension transient increased with increasing Cl- concentration. The application of ouabain to fibers after peeling had no effect on either type of tension transient. However, soaking the fibers in a ouabain solution before peeling blocked the Cl- -induced but not the caffeine-induced tension transient, which suggests that ouabain's site of action is extracellular, perhaps inside transverse tubules (TTs). Treating the peeled fibers with saponin, which should disrupt TTs to a greater extent than SR membrane, greatly reduced or eliminated the Cl- -induced tension transient without significantly altering the caffeine-induced tension transient. These results suggest that the Cl- -induced tension transient is elicited via stimulation of sealed, polarized TTs rather than via ionic depolarization of the SR. 相似文献
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Plasma beta-lysin and lysozyme following endotoxin administration and the generalized Shwartzman reaction 总被引:3,自引:0,他引:3
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The appropriateness of the Amplified Fragment Length Polymorphism (AFLP) technique for investigating Chondrus crispus Stackhouse populations in the Maritime Provinces of Canada was assessed. The AFLP procedure was first subjected to reproducibility
testing and three shortcomings were noted: 1) failure to reproduce band intensity between replicate runs for the same individual
and primer pair; 2) failure of some bands to replicate; 3) lack of reproducibility for complete replicate runs for some individuals
and primer pairs. In the last-mentioned case, the lack of reproducibility resulted in characteristic electropherograms indicative
of weak reactions. These weak runs can be attributed to poor restriction digest/ligation reactions and/or substandard PCR,
these failures ultimately resulting from low and inconsistent DNA quality. We recommend that reproducibility testing should
be completed routinely in studies using the AFLP technique. In the current work, only fragments and individuals that gave
reproducible results were used in subsequent analyses. The AFLP method resulted in highly variable markers within and between
the populations of C. crispus included in this investigation, which prevented successful resolution of population structure. This situation could result
from a lack of suitability for AFLP markers in population genetic studies, and/or too extensive genetic variation for C. crispus populations to be discerned by the AFLP technique. These two possible explanations are discussed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
L. J. Donaldson 《BMJ (Clinical research ed.)》1992,305(6864):1280-1284
The advent of the Tomlinson inquiry draws attention to the need to strike a balance between market led and planned approaches to health care delivery. This is important not just for hospital rationalisation but also for the preservation and development of services which are provided in a smaller number of hospitals. Specialised services are often in the forefront of raising standards of care and introducing new developments and innovations. They are the only option for a small number of patients with serious illnesses. In the internal market for health care provision created by the 1990 NHS reforms more sophisticated and flexible mechanisms must be found to provide stability for specialised services while at the same time enabling the benefits of purchaser choice and provider competition to be realised. 相似文献