Heterosis and the Maintenance of Enzyme Polymorphism

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Abstract
Single- and multiple-locus heterosis is an important mechanism in maintaining gene-enzyme polymorphism in nature. Several existing models of heterosis are discussed, in both theoretical and molecular terms, and a new model is proposed. This model is based on a greater catalytic efficiency of enzyme in heterozygotes, which leads to a reduction in the amount of metabolic energy required to sustain regulated levels of that enzyme''s activity. The consequence is that a larger proportion of the heterozygote''s energy budget can be channeled into reproduction. The predictions of this model are discussed in light of results from chemostat competition experiments which utilize regulatory mutants in Drosophila and Neurospora and gratuitous induction in bacteria.