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Geographic Distribution and Enzymatic Properties of Lactate Dehydrogenase Allozymes in the Fathead Minnow, Pimephales promelas

A two-allele Ldha locus polymorphism is described in Pimephales promelas. A north-south cline in gene frequency at the A locus is documented, and kinetic studies on the three LDH genotypes are described which search for a functional basis for maintenance of the polymorphism and the cline. The LDHs of the homozygote for the northern allele possess significantly higher Michaelis constants for pyruvate (lower substrate affinities) than LDHs of the other two genotypes at 25⚬ C and above. Frequency of occurrence of the northern allele in nature fits the distribution expected from observed Michaelis constants. Results indicate that selection is acting at this locus and that substrate affinity is an adaptively significant enzymatic property.