Apparent Overdominance in Natural Plant Populations
- Jeffry B. Mitton
Surveys of enzyme polymorphisms in plant species provide data to search for evidence of natural selection. Studies of enzyme kinetics typically reveal differences in the performances of the alternate genotypes at a locus, providing an opportunity for natural selection among genotypes within populations. Comparisons among genotypic distributions from successive stages in the life cycle provide sensitive tests for natural selection. Such tests have revealed consistent patterns of overdominance in annual ryegrass (Lolium multiflorum Lam.) and slender wild oats (Avena barbata Pott ex Link). A survey of selection coefficients has revealed that overdominance is common in plants. Samples of mixed age classes of coniferous forest trees usually have genotypic distributions that fit Hardy Weinberg expectations, for these trees produce almost all of their seed by outcrossing; however, when samples are restricted to old trees, large trees, or trees producing cones, genotypic distributions typically contain excesses of heterozygotes. Such heterozygote excesses cannot be produced by selection against inbred genotypes, but must be produced by selection favoring heterozygous genotypes. Moreover, comparisons of trees selected for seed orchards with trees in natural populations reveal that foresters consistently choose highly heterozygous trees. Many observations suggest that overdominance will be expressed more frequently or to a greater degree during environmental stress. For example, resistance to various forms of airborne pollution increases with allozyme heterozygosity in conifers. In pinyon pine (Pinus edulis Engelm.) trees resistant to herbivorous insects are significantly more heterozygous than susceptible trees.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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