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.
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