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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

Genetic changes with time are frequently observed in natural populations of Drosophila. Dobzhansky (1971) demonstrated two types of genetic changes in frequencies of polymorphic third chromosomes in D. pseudoobscura and D. persimilis. One is the seasonal cyclic changes and the other is changes over several years. The former was largely due to natural selection with the populations responding to seasonal changes in their environments. The latter was more or less directional and the causation was rather vague. Dobzhansky speculated that the changes were induced by pesticides such as DDT in the environment or by the emergence of new supergenes in the chromosome inversions. Seasonal or annual changes in gene frequencies, especially for lethal genes, have been reported by many investigators who censused natural populations of D. melanogaster. Dubinin (1946) showed in Russian populations an increase of lethal frequency during each breeding season. Such changes were later referred to as the "Dubinin cycle" and have been confirmed in a Japanese population (Minamori and Saito, 1964). However, most seasonal surveys failed to show a clear "Dubinin cycle" (Ives, 1954; Goldschmidt et al., 1955; Kosuda et al., 1969). The temporal shifts of lethal genes in the Massachusetts natural population for the past 30 years have been reported by Ives (1945, 1954, 1970), Band and Ives (1961, 1963, 1968) and Band (1969, 1972a, 1972b). These authors found a correlation of the frcaniuncv chanP's with certain weather factors; positively with summer rainfall and negatively with the average daily temperature range of the week prior to collection. Hoenigsberg et al. (1969, 1973) observed a niche-dependent change of lethal genes in the American tropics. The periodic use of pesticides was considered as a major cause. Recently Minamori et al. (1973) reported a sudden increase of lethal genes in the Hiroshima, Japan, population. The effects of weather factors proposed by Band (1972a) were not evident. Their interpretation is that the environment of Drosophila became favorable to its overwintering. Genetic variation in the Katsunuma, Japan, population in the 1959-1968 period has been reported by Watanabe (1969b). The frequency of lethal second chromosomes was about 15% in that period. Two polymorphic inversions, In(2L)B and In (2R)C, had been maintained in the population at high levels of about 33% and 25% during the early sixties (T. Watanabe, 1967). The purpose of this paper is to report an increase of lethals accompanied by a decrease of the inversions in the population after 1969.

Drosophila melanogaster is a chromosomally polymorphic species, a fact that has been confirmed in various natural populations by many workers (Dubinin et al., 1937; Warters, 1944; Ives, 1947; Carson, 1965; Yang and Kojima, 1972; Mukai and Yamaguchi, 1974; Ashburner and Lemeunier, 1976; Stalker, 1976; Choi, 1977; Mettleretal., 1977; Inoue and Watanabe, 1979; Paik, 1979; Zacharopoulou and Pelecanos, 1980; Knibb et al., 1981; Knibb, 1982). Chromosome variations in nature are usually restricted to paracentric inversions on the four arms of the two major autosomes. Other variations such as X-chromosome and pericentric inversions, deficiencies, duplications, translocations and transpositions rarely are found in nature. Inoue and Watanabe (1979) classified the inversions into four groups, using a classification partly modified from that of Mettler et al. (1977): (1) Common cosmopolitan inversions which are the most frequent world-wide inversions on each arm of the major autosomes. (2) Rare cosmopolitan inversions which are distributed world wide but in lower frequency than the common cosmopolitan inversions. Some populations often lack one or two of these inversions. (3) Recurrent endemic inversions which show temporal polymorphisms in some populations. (4) Unique endemic inversions which are observed in a single individual or its brood from a single population and never found in other populations. Designations of the common cosmopolitan inversions are In(2L)t, In(2R)NS, In (3L)P and In(3R)P. The rare cosmopolitan inversions are labelled In(2L)A, In(3L)M, In(3R)C, In(3R)Mo and In(3R)K. In Japan two inversions, In(2L) W and In(3L) Y, are categorized as recurrent endemic inversions. The unique endemic inversions are often reported as rare inversions, having differing break points which probably correspond to recent mutations. Mettler et al. (1977), in their studies of eastern United States populations, demonstrated highly significant negative correlations between the frequencies of the common cosmopolitan inversions and latitude. Stalker (1980) obtained similar results in addition to documenting seasonal changes of some inversions in midwestern and eastern U.S. populations. Recently, Knibb et al. (1981) reported a north-south cline from Australian populations with inversion frequencies increasing toward the equator. These studies suggest that the standard arrangementbearing chromosomes were more adaptive in cooler environments than were the inversion-containing chromosomes. Inoue and Watanabe (1979) compared the inversion frequencies of several Japanese populations showing a north-south dline in inversion In(3R)P. Clines were not always confirmed in other inversions which was attributed to the small number of populations examined. Inoue and Watanabe (1979) also compared several natural populations from the viewpoint of frequency order of the four common cosmopolitan inversions: In order of decreasing frequency the relation 2Lt > 2RNS > 3RP > 3LP was observed in the middle part of Japan in the past (1960's) and still (1970's) in the northern part of Japan. However, the present populations in the middle part of Japan showed the relation of 3RP > 2Lt _ 2RNS > 3LP. The order change in the inversion frequencies occurred

Collections of D. melanogaster from Japanese populations were analyzed for enzyme and chromosomal polymorphisms. Allelic frequencies at the Adh and αGpd loci were compared with polymorphic inversion (In(2L)B, In(2R)C) frequencies in the second chromosome. There was a significant positive correlation between the frequencies of AdhS and In(2L)B, caused by linkage. On the other hand, inversion-free cage populations maintained in the laboratory for a long time showed considerably larger variation in the frequencies of these enzyme alleles, which seem very likely to be a consequence of random drift. Two fitness components of these enzyme and chromosomal variants were measured in two different environmental conditions; neither of the two loci showed heterozygote superiority in viability or productivity, while the inversion heterozygotes showed a superior productivity compared to the corresponding homozygotes in the fluctuating environment. These findings are compatible with the hypothesis that polymorphic isozyme genes are maintained by random drift of neutral genes in natural populations, and that association with linked inversions is a historical accident.