GENETIC COADAPTATION IN THE CHROMOSOMAL POLYMORPHISM OF DROSOPHILA SUBOBSCURA. I. SEASONAL CHANGES OF GAMETIC DISEQUILIBRIUM IN A NATURAL POPULATION

Abstract

ABSTRACT Seasonal changes in gene arrangement and allozyme frequencies have been investigated in Drosophila subobscura for several years. Some arrangements (Ost and O  3+4+7) show seasonal variation, which suggests that chromosomal polymorphism is flexible in this species. Seasonal changes in allozyme frequencies for Lap and Pept-1 loci, both located within the same inversions of chromosome O, are significant only inside the Ost arrangement, but not inside O  3+4 arrangement. This arrangement-dependent response of allozyme generates variation in arrangement-allozyme disequilibrium. The historical hypothesis on the maintenance of disequilibria cannot explain these seasonal changes, and some kind of natural selection must be invoked. Association between Lap and Pept-1 is also seasonal inside Ost but not inside O  3+4. We propose that Ost probably consists of a finite array of supergenes that are differentially favored in each season by natural selection. The present evidence on this supergene selection and other genetic, biogeographic and phylogenetic data points to O  3+4 as the most primitive gene order among the present arrangements