The Genetic Structure of Natural Populations of DROSOPHILA MELANOGASTER. IX:a Prediction of Genetic Equilibrium.

N spite of the fact that radiation-induced and spontaneous mildly deleterious mutant genes (or mutant viability polygenes) show overdominance in an otherwise homozygous genetic background (WALLACE 1957, 1958, 1963; BURDICK and MUKAI 1958; MUKAI, CHIGUSA and YOSHIKAWA 1964; MUKAI, YOSHIKAWA and SANO 1966), the results of recent investigations have indicated that the manifestation of overdominance with respect to fitness (or viability) is rare in equilibrium populations of both Drosophila melunoguster ( GREENBERG and CROW 1960; MUKAI and YAMAZAKI 1964, 1968; TEMIN 1966; CHIGUSA, MUKAI and METTLER, in preparation) and Drosophila pseudoobscuru (WILLS 1966). This conclusion was reached from studies of the detrimental load to lethal load ratio ( GREENBERG and CROW 1960) and the correlation between heterozygote viabilities and the corresponding homozygote viabilities. KIMURA and CROW (1964) reached the above conclusion theoretically, since the magnitude of the segregation load becomes very large if overdominance is common in equilibrium populations. However, the discovery of a large number of isozyme polymorphisms in Drosophila (HUBBY and LEWONTIN 1966; JOHNSON et al. 1966; O'BRIEN and MACINTYRE 1969) has made investigators reconsider overdominance as a mechanism for maintaining genetic variability in populations. For example, SVED, REED and BODMER (1967), KING (1967) , and MILKMAN (1967) have proposed genetic models in which the magnitude of genetic loads does not become extremely large even if overdominance exists. Unfortunately, these models were not set up on the basis of experimental data. MUKAI and his associates have accumulated spontaneous mutant viability polygenes in the second chromosomes of Drosophila mlanoguster, and have estimated the frequency of occurrence of mutant viability polygenes and recessive lethal genes and also the degrees of dominance and epistasis. Applying these values in the present computer simulation study, both the existence of equilibrium in populations of D. melunoguster and the frequency of individuals revealing overdominance with respect to viability were predicted. The results are