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121.
Models of Evolution of Reproductive Isolation 总被引:12,自引:3,他引:9
Mathematical models are presented for the evolution of postmating and premating reproductive isolation. In the case of postmating isolation it is assumed that hybrid sterility or inviability is caused by incompatibility of alleles at one or two loci, and evolution of reproductive isolation occurs by random fixation of different incompatibility alleles in different populations. Mutations are assumed to occur following either the stepwise mutation model or the infinite-allele model. Computer simulations by using It?'s stochastic differential equations have shown that in the model used the reproductive isolation mechanism evolves faster in small populations than in large populations when the mutation rate remains the same. In populations of a given size it evolves faster when the number of loci involved is large than when this is small. In general, however, evolution of isolation mechanisms is a very slow process, and it would take thousands to millions of generations if the mutation rate is of the order of 10(-5) per generation. Since gene substitution occurs as a stochastic process, the time required for the establishment of reproductive isolation has a large variance. Although the average time of evolution of isolation mechanisms is very long, substitution of incompatibility genes in a population occurs rather quickly once it starts. The intrapopulational fertility or viability is always very high. In the model of premating isolation it is assumed that mating preference or compatibility is determined by male- and female-limited characters, each of which is controlled by a single locus with multiple alleles, and mating occurs only when the male and female characters are compatible with each other. Computer simulations have shown that the dynamics of evolution of premating isolation mechanism is very similar to that of postmating isolation mechanism, and the mean and variance of the time required for establishment of premating isolation are very large. Theoretical predictions obtained from the present study about the speed of evolution of reproductive isolation are consistent with empirical data available from vertebrate organisms. 相似文献
122.
M H Wiley K R Feingold C Grunfeld V Quesney-Huneeus J M Wu 《The Journal of biological chemistry》1983,258(1):491-496
Two prostaglandins, prostaglandin E1 (PGE1) and prostaglandin B1 (PGB1), block S-phase DNA synthesis in synchronous cultured baby hamster kidney (BHK) cells. The prostaglandin inhibition of DNA synthesis does not appear to require elevated levels of cAMP. In BHK-21 cells that have been "desensitized" to prostaglandin stimulation of adenylate cyclase and, therefore, have control levels of cAMP, PGE1 retains its inhibitory effect on the incorporation of tritiated thymidine into DNA. When BHK cells are exposed to PGB1 (a prostaglandin that does not elicit a cAMP response), DNA synthesis is also blocked. In nonsynchronous cells exposed for 1 h to PGE and then incubated for 1 h with PGE removed, a rebound of DNA synthesis occurs, therefore providing evidence that a transient rise of cAMP in itself is not capable of causing a cascade of reactions that block the synthesis of DNA. In addition, the concentration of PGE required for inhibition of DNA synthesis is significantly less than that required for cAMP generation. Addition of 1 x 10(-8) M PGE to BHK cells can be shown to significantly inhibit DNA synthesis within 30 min, with half-maximal inhibition seen at 3 x 10(-7) M PGE. Cyclic AMP levels for controls were 4.9 +/- 0.2 and 4.6 +/- 0.1 for 1 x 10(-6) M PGE1. These findings suggest that the prostaglandins can act independently of cAMP at physiological concentrations; and, therefore, it is possible that prostaglandins have a physiological role in the control of cell growth during S-phase. 相似文献
123.
A M Wu 《Journal of cellular physiology》1983,117(1):101-108
Formation of human T lymphocyte colonies in semisolid medium from T lymphocyte colony-forming units (TL-CFUs) under stimulation of phytohemagglutinin (PHA) has been reported by several authors. These TL-CFUs were present in unsensitized lymphocyte populations. We report here that such TL-CFUs are capable of renewing themselves. This was observed when colony cells from primary T cell colonies that developed in the presence of PHA were replated in methylcellulose medium containing irradiated autologous leukocytes and PHA. We have also been able to demonstrate serial transfer of TL-CFU for up to six passages. At each passage, colony-forming frequency was determined from the proportional relationship between the number of new colonies obtained and the number of colony cells plated. Examination of the number of new colonies derived from each individual T cell colony ("burst size of TL-CFU") showed that most colonies contained very few new TL-CFU and only a very small number of colonies contained many new TL-CFU. The distribution of burst sizes could be well fitted to a gamma distribution, in agreement with prediction from a stochastic model. We have identified an activity that enhanced the mean TL-CFU burst size three to ten times. This work provides the first evidence in vitro that self-renewal of human T lymphocyte progenitor cells can be stimulated by specific regulatory proteins. 相似文献
124.
Chiral instability at sulfur of S-adenosylmethionine 总被引:1,自引:0,他引:1
S-Adenosylmethionine, generated enzymically in chirally pure form (S configuration at sulfur), undergoes simultaneous irreversible conversion to 5'-deoxy-5'-(methylthio)adenosine and homoserine with a rate constant of 6 X 10(-6) s-1 at pH 7.5 and 37 degrees C and reversible conversion to an enzymically inactive stereoisomer (R configuration at sulfur) with a forward rate constant of 8 X 10(-6) s-1 at pH 7.5 and 37 degrees C. These forms of instability require small turnover times and/or stabilization through macromolecular binding for S-adenosylmethionine, if organisms that utilize it are to avoid losses of metabolic energy. 相似文献
125.
Partial characterization of the phosphotransferase system of human central-nervous-system myelin. 总被引:1,自引:0,他引:1 下载免费PDF全文
The phosphotransferase system of human central-nervous-system myelin was investigated. Evidence obtained indicated the presence of at least two different phosphotransferase systems (cyclic nucleotide-dependent and -independent) in myelin, which were found to be firmly associated with the membrane. The cyclic AMP-dependent kinase of myelin and white-matter cytosol preferentially phosphorylated certain histone fractions and displayed only modest activity with basic protein as substrate. On the other hand, the cyclic nucleotide-independent system showed specificity toward basic protein. Its activity was not only dependent on Mg2+ but it was greatly enhanced by this bivalent cation. Whereas the cyclic nucleotide-dependent kinase could be extracted with buffers containing Triton X-100, the bivalent cation-regulated kinase resisted solubilization from myelin under these conditions. 相似文献
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A simple colorimetric method for determination of protein 总被引:67,自引:0,他引:67