The genetic structure of natural populations of Drosophila melanogaster. VI. Further studies on the optimum heterozygosity hypothesis.

VERDOMINANCE for viability of newly arising mutant polygenes has been reported by several investigators in the past decade in Drosophila melanogaster and Tribolium confusum [WALLACE 1957, 1958, 1959, 1963; BURDICK and MUKAI 1958; MUKAI 1961; MUKAI, YOSHIKAWA, and SANO 1966; CRENSHAW 1965 (in radiation-induced mutations). MUKAI, CHIGUSA, and YOSHIKAWA 1964, 1965; MUKAI and YAMAZAKI 1968 (in spontaneous mutations)]. In all cases mutant polygenes controlling viability (viability and fecundity in Tribolium confusum) showed overdominance on the average when they are induced or accumulated in homozygous genetic background. Thus, it might be said that a majority of mutant polygenes controlling viability are overdominant under some restricted conditions. As has been discussed by several researchers (e.g. KIMURA and CROW 1964), unless selection coefficients are very small the segregation load (or balanced load of DOBZHANSKY, SPASSKY, and TIDWELL 1963) becomes remarkably large if a majority of loci are overdominant in natural or artificial equilibrium random mating populations. In fact, the recent results of the direct analysis of natural or artificial populations of Drosophila suggest that genetic variability revealed by homozygosis has been maintained by the balance between mutation and selection pressures (GREENBERG and CROW 1960; TEMIN 1966; WILLS 1966) and in a minor proportion of loci by heterozygote superiority (METTLER, MOYER, and KOJIMA 1966) , although a different conclusion has been reached by other investigators who have stressed that overdominance is common in natural or cage populations (KRIMBAS 1959; WALLACE and DOBZHANSKY 1962; BAND and IVES 1963; DOBZHANSKY and SPASSKY 1963). If we assume that a majority of spontaneous or induced mutant polygenes controlling viability are potentially overdominant but that this is rarely manifested in natural equilibrium populations, we must find the factors suppressing the manifestation of overdominance. In previous reports ( MUKAI, CHIGUSA, and YOSHIKAWA 1965; MUKAI and YAMAZAKI 1968), we have discussed two factors which might suppress the overdominance in equilibrium populations: position