Identification of Supernatant and Mitochondrial Isozymes of Malate Dehydrogenase on Electropherograms Applied to the Taxonomic Discrimination of Walleye (Stizostedion vitreum vitreum), Sauger (S. canadense), and Suspected Interspecific Hybrid Fishes

Abstract

Walleye (Stizostedion vitreum vitreum) and sauger (S. canadense) commonly yield four isozymes of malate dehydrogenase (MDH) in electropherograms of white muscle extracts from individual homozygous fish. The heat stability of each isozyme as well as reactivity with nicotinamide-adenine dinucleotide (NAD) and NAD analogues have been investigated by measuring the density of each isozyme directly on the electropherograms, after the usual specific staining procedure that links malate oxidation to tetrazolium dye reduction. At least two kinds of supernatant and one kind of mitochondrial MDH in individual fish of each species was demonstrated.Relative abundance of the various isozymes varies with the tissue of origin. In liver, only one heat stable isozyme is observed in both species, and it is likely constituted mainly of supernatant MDH. In heart tissue four isozymes are observed and the liver supernatant form also predominates. In white muscle the two kinds of supernatant MDH appear to be synthesized in comparable amounts and, including the mitochondrial MDH, four isozymes of nearly equivalent catalytic activity are produced. In walleye the three kinds of MDH homozygotes all showed similar enzymatic properties of their corresponding isozymes.In contrast to the polymorphic nature of walleye MDH isozymes the closely related sauger are monomorphic for MDH, and a number of analogous isozymes have identical electrophoretic mobility in the two species. More significantly, the major mitochondrial MDH isozymes have different mobility in each species. This fact is suggested as a taxonomic criterion to distinguish the two species. Some very rare fishes, evidently interspecific hybrids, produced three mitochondrial MDH isozymes. It was also possible to hybridize walleye and sauger MDH isozymes in vitro to produce phenotypes indistinguishable from presumed wild hybrid fishes.