Linkage disequilibrium in isolated populations of Drosophila melanogaster

In order to test the interaction effect of overdominance and random genetic drift on the formation of linkage disequilibria under the condition of multiplicative gene action, linkage disequilibria between isozyme genes, inter se, and between polymorphic inversions and isozyme genes were tested for the second chromosomes of Drosophila melanogaster sampled from two isolated Pacific islands and one locality of the northern district of the mainlands of Japan. The effective sizes of these populations were known to be approximately 3,000 to 6,000 on the basis of the allelism rate of lethal chromosomes and their frequencies. The following results were obtained: (1) No linkage disequilibrium considered to be induced by epistasis, including the interaction between overdominance and random genetic drift, was detected. (2) Nonrandom association between polymorphic inversions and isozyme genes that are included in the inversions or located in the adjacent region outside the inversions was detected. (3) On the basis of the comparison of chi (2) values, indicating the magnitudes of linkage disequilibrium, between the present isolated populations and the Raleigh, N.C., population (Mukai et al. 1974, 1977), the characteristics of the isolated populations and the nature of these linkage disequilibria are discussed.