On Coadaptation in Drosophila

The coadaptation of the gene complexes carried by different gene arrangements within a population depends upon the integrity of the genic contents of the inverted segments. Certain combinations of inversions-triads, in the above account-destroy this integrity by facilitating serial transfer of genes from one arrangement to another by crossing-over. Since the different inversions of a population are generally coadapted, populations should possess with high frequency, at most, two of the three inversions of any one triad. In general, the data from populations of D. robusta and D. pseudoobscura support this hypothesis; one member of each triad is usually absent or rare within any one locality. Three inversions that are not members of a triad should not be opposed by selection and, in agreement with the hypothesis, three such inversions are frequently present in populations with high frequency. We conclude that coadaptation frequently outweighs aneuploidy as a selective force in determining inversion frequencies within populations. The possible role of triads in the geographical distribution of inversions and in speciation are also discussed.