EVOLUTIONARY CHANGE OF THE CHROMOSOMAL POLYMORPHISM IN DROSOPHILA MELANOGASTER POPULATIONS.

Abstract

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