INTERRACIAL HYBRIDIZATION AND BREAKDOWN OF COADAPTED GENE COMPLEXES IN DROSOPHILA PAULISTORUM AND DROSOPHILA WILLISTONI

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Abstract
The incidence of individuals heterozygous for certain chromosomal inversions exceeds 50% in some local populations or races of D. tropicalis, P. paulistorum, and D. willistoni. In other populations of the same species the frequencies of the same inversion heterozygotes are below 50%. Experimental populations made with flies of a given geographic origin generally retain the characteristics of the local race from which the founders of the population were derived. The chromosomal polymorphism is maintained in the populations because the inversion heterozygotes are superior in fitness to the homozygotes. Experimental populations were set up, the founders of which were hybrids between races having more and fewer than 50% of heterozygotes for certain inversions. In these experimental populations of mixed geographic origins the frequencies of the heterozygotes fell to or below the frequency levels in the parental races having fewest inversions. The superior fitness of the inversion heterozygotes upon coadaptation of the gene complexes which the chromosomes carry. In hybrid populations these gene complexes are broken up by crossing ever, with a consequent loss of the heterosis.