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81.
The classical viewpoint in sperm competition theory, which holds that males evolve sperm allocation strategies in response to a given degree of sperm competition, has recently been challenged by an alternative viewpoint, which holds that the degree of sperm competition is itself a consequence of these same strategies. Here, we present a game theory model that unites these alternative views as the endpoints of a continuum. Based on the recognition that female control over mating may limit the extent to which male strategies affect the degree of sperm competition, we investigate sperm allocation strategies in a setting where females can resist excessive matings more or less successfully. We discuss how conflicting predictions made by previous theory relate to implicit assumptions about female resistance behavior. Moreover, we show that female resistance, while being highly relevant to the predicted relationship between ejaculate size and the degree of sperm competition, has little effect on the predicted positive correlation between relative testis size and the degree of sperm competition. This result strengthens one of the central predictions of sperm competition theory and is in accordance with empirical findings from a wide range of taxa. 相似文献
82.
Excessive glucose production by the liver contributes significantly to diabetic hyperglycemia. The enzyme system glucose-6-phosphatase plays a key role in regulating hepatic glucose production and therefore its inhibition is a potential therapeutic target for the correction of hyperglycemia. It has previously been shown that sulfated steroids, such as estrone sulfate and dehydroepiandrosterone sulfate, inhibit the glucose-6-phosphatase system in vitro, principally through inhibition of endoplasmic reticulum glucose-6-phosphate transport. We report here that in the obese/diabetic ob/ob mouse model, orally administered estrone sulfate reduces the abnormally elevated hepatic glucose-6-phosphatase enzyme activity and enzyme protein levels that are characteristic in the ob/ob mouse, and that this reduction is associated with normalization of blood glucose levels. Other sulfated and non-sulfated steroids also reduced, to a lesser extent, glucose-6-phosphatase enzyme activity - with the exception of dehydroepiandrosterone sulfate, which had no apparent effect on this system in ob/ob mice. Estrone sulfate is therefore an effective antihyperglycemic agent in ob/ob mice, and the glucose-6-phosphatase system can be successfully targeted for the therapeutic management of hyperglycemia in this animal model of non-insulin-dependent diabetes mellitus. 相似文献
83.
Three thermophilic cellulolytic fungi, Chaetomium thermophile var. coprophile, Sporotrichum thermophile, and Thermoascus aurantiacus were studied to determine the conditions for a high rate of cellulose degradation. The range of temperature over which good growth occurred was determined first in a temperature gradient incubator; the optimum temperature was then established in shake flask cultures. T. aurantiacus had the highest optimum growth temperature range (46 to 51 C), whereas S. thermophile had the broadest range over which good growth occurred (36 to 43 C). Optimum temperatures for the three organisms, T. aurantiacus, S. Thermophile, and C. thermophile were 48, 40, and 40 C, respectively. It was found that the addition of an organic carbon and nitrogen source to a cellulose mineral solution medium markedly increased the rate of cellulose degradation. The surfactant, Tween 80, which has been reported to be of value in the production and recovery of the enzyme, cellulase, was shown to be detrimental to the degradation of cellulose in culture. In the medium used, S. thermophile gave the highest rate of substrate utilization; 56% of the cellulose was hydrolyzed in 72 h. The average degree of polymerization of cellulose decreased from 745 to 575. 相似文献
84.
Leung W Iyengar R Leimig T Holladay MS Houston J Handgretinger R 《Cancer immunology, immunotherapy : CII》2005,54(4):389-394
Infection, disease relapse, graft failure, and graft-versus-host disease (GVHD) are significant adverse events associated with allogeneic bone marrow transplantation. Donor natural killer (NK) cells may be an ideal cell type for prevention or treatment of all these adverse events. Therefore, we investigated the phenotype and function of human NK cells purified by using a clinical-scale immunomagnetic method. We found that the NK cell purification procedures did not adversely affect the expression of killer cell immunoglobulin-like receptors, adhesion molecules, intracellular cytokines, perforin, and granzyme B. Purified NK cells had extensive proliferative capacity and potent antitumor activity when assessed using an immunodeficient mouse model. While all mice transplanted with unpurified mononuclear cells developed GVHD, none of the mice transplanted with purified NK cells did. NK cells were highly susceptible to lysis by antithymocyte globulin (ATG), whereas G-CSF had a minimal effect on their natural cytotoxicity. These results support future clinical investigation of the use of purified NK cells for adoptive immunotherapy in the absence of ATG. 相似文献
85.
Control of the retinal circulation at altitude 总被引:1,自引:0,他引:1
86.
L. L. Houston 《Journal of bacteriology》1973,113(1):82-87
Forty-three hisB mutants of Salmonella typhimurium have been screened to determine the molecular size of the resulting histidinol phosphate phosphatase activity, one of the activities of a bifunctional enzyme produced by this gene which also controls imidazole glycerol phosphate dehydrase activity. Mutation in hisB can lead to the loss of both phosphatase and dehydrase activities, or only of dehydrase activity. Through the use of nonsense mutants lacking dehydrase activity, a distinct point of transition was detected near the middle of hisB at which a dramatic change occurs in the size of the phosphatase enzyme that is synthesized. A missense mutant with a lesion in this region has a high-molecular-weight enzyme which is eluted in the void volume of a Sephadex G-200 column. The enzyme from nonsense mutants near the transition point have molecular weights near 40,000. Even though the buffer conditions are designed to favor the stabilization of the high-molecular-weight form, some mutants have both high- and low-molecular-weight forms. The polypeptide chain specified by the operator proximal part of hisB is sufficient to allow the expression of phosphatase activity. The synthesis of substantially less than the complete product of hisB resulted in association into a form similar to the native enzyme which was found in the void volume of a Sephadex G-200 column. 相似文献
87.
88.
Stephen D. Houston 《American anthropologist》1994,96(3):716-718
Mesoamerican Writing Systems: Propaganda, Myth, and History in Four Ancient Civilizations . Joyce Marcus 相似文献
89.
90.