Selection in Mutant Populations

摘要

The efficiency of mutation techniques in the improvement of vegetatively propagated crops has been increased by the availability of in vitro systems which miniaturise the target tissues, facilitating mutagenic treatments. Efficiency has also been improved by the development of techniques for the early detection and rejection of gross aberrants. High-frequency polyploids, gross aneuploids and morphological variants can be detected by flow cytometry and image analysis. Cryptic developmental variants can also be detected and eliminated early, using image analysis in potatoes, and this may also apply to other crops. The ability to detect and discard useless variants at an early stage facilitates early selection of elite lines which can be planted in large blocks avoiding the current bottleneck in field evaluation. Traditionally, in successive trials, line numbers were reduced and block sizes were increased. The evaluation of lines as single plants or in small blocks prevents quantitative measurement of environmentally influenced characters. While the above are significant advances, there remains the problem of how to select among populations of elites, that is mutant lines showing the desired character improvement. This is a problem confronting both the mutation breeder and the genetic engineer, where spontaneous background mutation ("somaclonal variation" or multiple mutations) may go undetected. Both are seeking improved lines that are "isogenic" with the parent mutagenised or transformed genotype, and both want to avoid the risk of cryptic defects in the background of the lines selected for release which might, for example, result in crop failure or human health risks. Consumers of genetically modified crops need reassurance that the transformed variety differs only in the claimed character and has not undergone other changes which may adversely affect their health. Here a practical strategy will be discussed which addresses the problems of early detection of useless mutants, the selection of improved lines for "near-isogenicity" with the parent, and screening for background metabolic changes.