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The Luria–Delbrück experiment (1943) (also called the Fluctuation Test) demonstrates that in bacteria, genetic mutations arise in the absence of selection, rather than being a response to selection. Therefore, Darwin's theory of natural selection acting on random mutations applies to bacteria as well as to more complex organisms. Max Delbrück and Salvador Luria won the 1969 Nobel Prize in Physiology or Medicine in part for this work. ==History== By the 1940s the ideas of inheritance and mutation were generally accepted, though the role of DNA as the hereditary material had not yet been established. It was thought that bacteria were somehow different and could develop heritable genetic mutations depending on the circumstances they found themselves: in short, was the mutation in bacteria pre-adaptive (pre-existent) or post-adaptive (directed adaption)? Luria (James Watson's PhD advisor) in particular was obsessed with this idea and was determined to test it. He conceived the experiment at a faculty dance at Indiana University while watching a slot machine.〔Luria SE (1984) A slot machine, a broken test tube: An autobiography. Harper & Row〕 In their experiment, Luria and Delbrück inoculated a small number of bacteria (''Escherichia coli'') into separate culture tubes. After a period of growth, they plated equal volumes of these separate cultures onto agar containing the T1 phage (virus). If resistance to the virus in bacteria were caused by an induced activation in bacteria i.e. if resistance were not due to heritable genetic components, then each plate should contain roughly the same number of resistant colonies. Assuming a constant rate of mutation, Luria hypothesized that if mutations occurred after and in response to exposure to the selective agent, the number of survivors would be distributed according to a Poisson distribution with the mean equal to the variance. This was not what Delbrück and Luria found: Instead the number of resistant colonies on each plate varied drastically: the variance was considerably greater than the mean. Luria and Delbrück proposed that these results could be explained by the occurrence of a constant rate of random mutations in each generation of bacteria growing in the initial culture tubes. Based on these assumptions Delbrück derived a probability distribution (now called the Luria–Delbrück distribution) that gives a relationship between moments consistent with the experimentally obtained values. The distribution that follows from the directed adaptation hypothesis (the Poisson distribution) predicted moments inconsistent with the data. Therefore, the conclusion was that mutations in bacteria, as in other organisms, are random rather than directed. The results of Luria and Delbrück were confirmed in more graphical, but less quantitative, way by Newcombe. Newcombe incubated bacteria in a Petri dish for a few hours, then replica plated it onto two new Petri dishes treated with phage. The first plate was left unspread, and the second plate was then respread, that is, bacterial cells were moved around allowing single cells in some colony to form their own new colonies. If colonies contained resistant bacterial cells before entering into contact with the phage virus, one would expect that some of these cells would form new resistant colonies on the respread dish and so to find a higher number of surviving bacteria there. When both plates were incubated for growth, there were actually as much as 50 times greater number of bacterial colonies on the respread dish. This showed that bacterial mutations to virus resistance had randomly occurred during the first incubation. Once again, the mutations occurred before selection was applied. More recently, the results of Luria and Delbrück were questioned by Cairns and others, who studied mutations in sugar metabolism as a form of environmental stress. Some scientists suggest that this result may have been caused by selection for gene amplification and/or a higher mutation rate in cells unable to divide. Others have defended the research and propose mechanisms which account for the observed phenomena consistent with adaptive mutagenesis. This distribution appears to have been first determined by Haldane. An unpublished manuscript was discovered in 1991 in at University College London describing this distribution. The derivation is different but the results are difficult to compute without the use of a computer. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Luria–Delbrück experiment」の詳細全文を読む スポンサード リンク
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