Heterosis and the lack of coadaptation in Drosophila nasuta

Abstract

Natural populations of Drosophila nasuta are polymorphic for several paracentric inversions. Two non-overlapping inversions on the third chromosome (III-2 and III-35) are heterotic in both laboratory and natural populations and display an extreme case of linkage disequilibrium. Intra-and interpopulation crosses involving standard and inverted gene orders of the III-2 and 111-35 inversions were made to test if the polygenic complexes within these inversions are coadapted. The inversion heterozygotes formed by the two chromosomes derived from different localities exhibited heterosis since the viability of the heterozygotes was greater than that of either of the two homozygotes. In intrapopulation crosses, the heterozygotes were also heterotic. The present results thus indicated a lack of coadaptation among polygenic complexes within the inversions of D. nasuta. The data suggest that the heterosis produced by these linked inversions is the result of natural selection and epistasis.