Extensive heterozygosity at three enzyme loci in hybrid sunfish populations

Hybrid populations of sunfishes were produced in two different ponds, and the frequencies of allelic isozyme phenotypes were determined for three enzyme systems-malate dehydrogenase (NAD), esterases, and tetrazolium oxidase-in order to estimate the extent of heterozygosity at four different genetic loci. Interspecific F₁ hybrid fry (red-ear male × bluegill female) were produced in vitro. These fry were stocked in ponds at the free-swimming stage. When 1 year old, the F₁ hybrids produced a large F₂ hybrid population. Successful hybrid reproduction occurred each year thereafter. In one pond, a 1-year-old F₂ population exhibited all three isozyme phenotypes (red-ear, F₁, bluegill) at most loci in the approximate ratio of the 1:2:1 expected. In a second pond, 5-year-old individuals of the F₂ generation were morphologically like the F₁ and were all heterozygous for the enzyme loci studied. This unusual degree of heterozygosity in the older F₂ population appeared to be the result of differential survival of mature heterozygous individuals and not the result of early embryonic lethality. The increased heterozygosity at these unlinked loci was assumed to reflect the condition at other genetic loci in the F₂ hybrids. Several possible mechanisms are advanced to explain this apparent heterosis.