[Recombination disequilibrium of independent inversions in natural populations of Drosophila subobscura].

Coexistence of linked independent inversions is not infrequent in natural populations of Drosophila subobscura. Assuming that recombination between independent inversions does occur at a certain rate an equilibrium stage can be expected at which the frequencies of the two possible trans-types A StB and AB St and the two possible cis-types AB and A StBSt are in balance. The theoretical equilibrium conditions for any given frequency of independent inversions can easily be calculated and compared to the observed frequency distribution of the four combination types of any local population. Using all available data of 13 different populations of D. subobscura it could be found that linkage equilibrium is very rarely established in nature. Out of 14 independent inversion pairs 12 showed complete or nearly complete linkage disequilibrium. In many populations the deviation from the equilibrium condition is highly significant. This may be due to selective forces acting against one or both recombination types of an association-dissociation group. In the many cases where the linkage disequilibrium is complete one of the recombination types does not occur in nature at all. It probably, then, has a dominant lethal or semilethal effect on its carriers. Another possible explanation is that recombination between independent inversions does not occur at all and that di equilibrium is the result of complete linkage.