Abstract
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
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