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15.1 Changes in chromosome number
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AGRICULTURAL APPLICATIONS Variations in chromosome number have been exploited to create new plant lines with desirable features. Some examples follow.
Cultivated as Einkorn wheat AA A wild diploid wheat T. monococcum AA A wild diploid wheat possibly T. searsii BB
AB ( 2)
A wild tetrapoloid wheat T. turgidum AA BB
A wild diploid wheat T. tauschii DD
Cultivated 10,000 yr B.P. as Emmer wheat AA BB
A BD ( 2; ~ 8000 yr ago)
Hexaploid wheat T. aestivum AA BB DD
Figure 15-10 The proposed
evolutionary history of modern hexaploid wheat. Amphidiploids were produced at two points in this history. A, B, and D are different chromosome sets.
Monoploids Diploidy is an inherent nuisance for plant breeders. When they want to induce and select new recessive mutations that are favorable for agricultural purposes, the new mutations cannot be detected unless they are homozygous. Breeders may also want to nd favorable new combinations of alleles at different loci, but such favorable allele combinations in heterozygotes will broken be up by recombination at meiosis. Monoploids provide a way around some of these problems. Monoploids can be articially derived from the products of meiosis in the plants anthers. A cell destined to become a pollen grain can instead be induced by cold treatment to grow into an embryoid, a small dividing mass of monoploid cells. The embryoid can be grown on agar to form a monoploid plantlet, which can then be potted in soil and allowed to mature (Figure 15-11). Plant monoploids can be exploited in several ways. In one approach, they are rst examined for favorable allelic combinations that have arisen from the recombination of alleles already present in a heterozygous diploid parent. Hence from a parent that is A/a ; B/b might come a favorable monoploid combination a ; b. The monoploid can then be subjected to chromosome doubling to produce homozygous diploid cells, a/a ; b/b, that are capable of normal reproduction.
Anthers
Figure 15-11 Generating
Diploid plant Immature pollen cells plated Monoploid embryoids grow Monoploid plantlet Monoploid plant
monoploid plants by tissue culture.