The relationship between nuclear DNA content and chromosome number was investigated in Andropogon gerardii. The distribution of cytotypes in a natural population of this grass was also examined. Nuclear DNA content was determined using flow cytometry rather than the traditional method of Feulgen microphotometry. Our results demonstrate the increased accuracy and speed of this new method in the detection and study of polyploidy. Nuclear DNA content is strongly correlated to chromosome number in Andropogon gerardii (r = 0.971, P <0.01). The natural population of this grass was found to consist of plants with 2N = 60 chromosomes (hexaploid cytotype) and 2N = 80 chromosomes (octoploid cytotype), in equal proportions. Intermediate cytotypes were lacking in the natural population, although three progeny plants grown in the greenhouse from wild-collected seed show intermediate values of nuclear DNA content and have 2N = 70 chromosomes. The two coexisting cytotypes are intermingled and show no difference in microhabitats. The absence of septaploids in the natural population suggests that the two cytotypes are probably reproductively isolated. Key words: polyploidy, DNA contents, flow cytometry, polymorphism.
Species are generally regarded as having a single, typical and characteristic chromosome number and nuclear DNA content. Indeed, the species-specific nuclear DNA content is referred to as the constant value. This chapter investigates polyploid polymorphism in The Great Plains generally, and focuses on a single case. It compares the evidence for polyploid polymorphism in three major families, the Poaceae, Asteraceae and Fabaceae, of North American grasslands. Since macrogeographic patterns must ultimately be produced by forces acting on individual plants, the chapter explains the distribution of intraspecific polyploidy within and between sites across the range of one species, big bluestem, Andropogon gerardii. The results of these two investigations are aimed at determining the function, if any, of intraspecific polyploid polymorphism, and determining whether intraspecific polyploidy is an adaptive strategy of some sort, a transitory state indicating evolution or speciation in progress, or a selectively neutral condition.
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