GENETIC CHANGES IN NATURAL POPULATIONS OF DROSOPHILA MELANOGASTER

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.