490 Chapter 15 • Large-Scale Chromosomal Changes Another approach is to treat monoploid cells basically as a population of haploid organisms in a mutagenesis-and-selection procedure. A population of monoploid cells is isolated, their walls are removed by enzymatic treatment, and they are exposed to a muta-gen. They are then plated on a medium that selects for some desirable phenotype. This approach has been used to select for resistance to toxic compounds produced by a plant parasite as well as to select for resistance to her-bicides being used by farmers to kill weeds. Resistant plantlets eventually grow into monoploid plants, whose chromosome number can then be doubled using colchicine. This treatment produces diploid tissue and eventually, by taking a cutting or by selﬁng a ﬂower, a fully resistant diploid plant. These powerful techniques can circumvent the normally slow process of meiosis-based plant breeding. They have been successfully ap-plied to important crop plants such as soybeans and tobacco. Figure 15-12 Diploid (left) and tetraploid (right) grapes. [Copyright Leonard Lessin/Peter Arnold Inc.]
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