On Coadaptation in Drosophila

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Abstract
The chromosomes of many Drosophila spp. have undergone rearrangement through the occurrence of inversions. Local populations frequently contain 2, 3, or more different arrangements simultaneously. Although the coexistence of 3 arrangements within a single interbreeding population is a common event, the coexistence of certain sets of 3[long dash]triads[long dash]is extremely rare. Triads are 3 gene arrangements in which I is related to II by a single inversion and II is related to III by another single inversion that overlaps the 1st. The rarity with which these triads coexist within the same locality is explained by the inability of such chromosome combinations to form and maintain coadapted gene complexes; recombination between different pairs of arrangements in successive generations destroys such combinations. On the other hand, any 2 of the 3 arrangements of a triad can maintain a coadapted gene system. Data on the geographic distr. of different gene arrangements in D. pseudoobscura and D. robusta support the coadaptation hypothesis better than the older "aneuploid" hypothesis.