argystokes
Posts: 766
Joined: Jan. 2006
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This review came out a couple of years ago. It dismisses the concept of directed mutation; that is, the idea that cells sense what area of the genome "needs" to mutate in response to its environment. The main thrust of the paper, however, is that cells can increase their overall mutation rate in response to environmental stressors, leading to an increased chance of achieving beneficial mutations. This kind of hypermutation remains controversial, and rebuttals to this article have been published. Here's the introduction; I don't want to violate any copyright laws:
| Quote | Adaptive Mutation: How Growth under Selection Stimulates Lac+ Reversion by Increasing Target Copy Number
John R. Roth1* and Dan I. Andersson2,3
Microbiology Section, University of California, Davis, Davis, California 95616,1 Department of Bacteriology, Swedish Institute for Infectious Disease Control, S-171 82 Solna,2 Microbiology and Tumour Biology Center, Karolinska Institute, S-171 77, Solna, Sweden3
From the time of Darwin until about 1950, a controversy continued over whether selective stress induces mutations or only affects the relative reproductive success of organisms with different genotypes (30). The controversy was resolved by the classic experiments of Luria and Delbrück (27) and of Lederberg and Lederberg (25), who showed that some bacterial mutants arise prior to application of the selection that allows their detection and thus could not have been caused by selective conditions. However, these experiments used lethal selections and therefore did not eliminate the possibility that another fraction of total mutations might be formed in response to stress and be detected only by nonlethal selection. Shapiro and Cairns et al. reopened the controversy by pointing out this caveat and presenting data that seemed to support stress-induced mutation (7, 45).
Because very few genetic systems behave in ways that suggest stress-induced mutation, the rare cases that seem to exhibit such behavior have attracted close attention. In one case, mutants were later shown to preexist selection (14, 28, 29, 44). For the system devised by Cairns and Foster (5), we suggest that reversion occurs by a multistep process initiated prior to selection and the appearance of stress-induced mutagenesis results from growth under strong selection.
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Proponents of directed mutation, which seems to be a necessity for any kind of front-loading hypothesis, should also check out:
Foster, P. L., and J. Cairns. 1992. Mechanisms of directed mutation. Genetics 131:783-789.
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"Why waste time learning, when ignorance is instantaneous?" -Calvin
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