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Fertility, the component of selection due to female fecundity and male mating success, differed significantly among the ST/ST, ST/AR, and AR/AR karyotypes in experimental populations and varied with karyotypic frequency. In relation to ST/AR, ST/ST females and males had higher fertilities at low frequency; AR/AR males and females were at a significant fertility disadvantage at intermediate frequency, while at low and at high frequencies their fertilities matched or exceeded that of the heterokaryotype. These fertility differences were comparable in size to viability differences previously reported for D. pseudoobscura karyotypes. Differential fertility seems likely to be an important element, perhaps just as important as differential viability, in the balancing selection that maintains the chromosomal polymorphism in this species.

Chromosomal polymorphism in natural populations is largely maintained by the adaptive superiority of inversion heterozygotes. Dobzhansky and his co-workers have carried out many studies on heterosis in Drosophila pseudoobscura populations (summarized in Dobzhansky, 1970). The superior fitness of heterokaryotypes has also been reported in other Drosophila species: D. persimilis (Spiess, 1958; Spiess and Langer, 1964); D. robusta (Carson, 1958; Prakash, 1967); D. willistoni (da Cunha et al., 1950; Pavan et al., 1957); D. ananassae (Tobari, 1962) and D. pavani (Brncic et al., 1969). Extensive chromosomal polymorphism in Japanese populations of D. melanogaster was demonstrated by Watanabe (1967). Two inversions on the second chromosome and four inversions on the third chromosome which had been found all over the world were highly polymorphic in Japanese natural populations. The second chromosome inversion In(2L)B was maintained in the population year after year at a frequency of about 35%. A higher adaptive value of the inversion heterozygotes was expected from estimates of the pre-adult viability. However, superiority was found only when In(2L)B was combined with another inversion, In(2R)C. The viability of the heterozygotes for In(2L)B and standard chromosomes was the same as that of the standard heterozygotes (Watanabe et al., 1964). A significant superiority of this inversion heterozygote in female

Three or four percent of the wild flies in natural populations of D. melanogaster have been found to be sterile. An analysis of sterility associated with the second chromosome revealed a much lower frequency of genetically sterile flies. The accumulation of sterility genes in a cage population was proportional to that of lethal genes, as were their equilibrium frequencies in several natural populations. Many sterile chromosomes were associated with low viability due to pleiotropic effects. The number of chromosomes leading to sterility in both sexes was larger than the expectation based on random combination of male and female sterility genes. This suggests that there is some linkage disequilibrium between male and female sterility genes, as well as a pleiotropic effect of single sterility genes. Some sterility genes were maintained in natural and cage populations, and the patterns of persistence of the sterility genes were very similar to those of lethal genes.

Genetic variabilities in Japanese natural populations of D. melanogaster distributed at Kofu-Katsunuma locality have been analyzed. The frequency of lethal and semilethal chromosomes was 21.5 per cent in coadapted genetic background and 36 per cent in Samarkand isogenic background during the past seven years. The effect of the genetic background on the viability of homozygotes was remarkable for homozygous semilethal and subvital chromosomes. This means that a coadaptive interaction may exist between chromosomes.A clear negative correlation between the frequency of deletelious chromosomes and ‘relative’ viability of quasinormal homozygotes was observed in Drosophila populations including Kofu-Katsunuma.Allelic rates between lethal genes extracted from the natural populations fluctuated during the past several years. The average allelic rate was 3.48 par cent. The low frequency of deleterious genes and high frequency of allelism was assumed to be characteristic for Far East and Russian populations of D. melanogaster.The frequency of elimination (IQ2) caused by deleterious genes in homozygotes was estimated to be 0.0016 in average Kofu-Katsunuma population which was comparatively higher than in other natural populations. On the other hand, the mutation rate (UL+SL) of lethal and semilethal genes on the second chromosome was assumed to be 0.0075, so that the fitness of heterozygotes for deleterious genes should be, on the average, inferior to that of normal heterozygotes and they would be selected out from the natural populations in Japan.

1. Segregation distorter genes (SD) were found in Kofu and Katsunuma natural populations of Drosophila melanogaster.2. The nature of SD gene extracted from Kofu and Katsunuma populations was almost the same as in Madison; segregation distortion occurred only in SD heterozygous males and it did not occur either in SD homozygous males or in females.3. SD gene was mostly found in association with a right arm inversion, In(2R)C, and some SD chromosomes simultaneously carried the same persistent lethal gene which was frequently found and maintained in Kofu and Katsunuma natural populations.4. The distance between the persistent lethal gene and SD gene is so short that its retention in the population may be effectively due to its association with SD. Heterotic gene complexes including the lethal and SD genes were discussed as a persistent mechanism.5. The sensitivity of the second chromosomes to SD action varied from complete sensitiveness (k=1.0) to resistance (k=0.5). The mean k value of many chromosomes isolated from Kofu and Katsunuma populations was 0.77.

Two hundred and ninety second chromosomes extracted from a natural population of Drosophila melanogaster were analyzed to estimate the genetic variance of viability and its components by means of a partial diallel cross (Design II of Comstock and Robinson 1952). The additive and dominance variances are estimated to be 0.009 and 0.0012. Using the dominance variance and the inbreeding depression, the effective number of overdominant loci contributing to the variance in viability is estimated to be very small, a dozen or less. Either the actual number of loci is small, or the distribution of viabilities is strongly skewed with a large majority of very weakly selected loci. The additive variance in viability appears to be too large to be accounted for by recurrent harmful mutants or by overdominant loci at equilibrium with various genetic parameters estimated independently. The excess might be due to frequency-dependent selection, to negative correlations between viability and fertility, or possibly to the presence of a mutator. The selection for viability and fertility, or possibly to the presence of a mutator. The selection for viability at the average polymorphic locus must be very slight, of the order of 10-3 or less.