Dobzhansky's concept of genetic coadaptation: Drosophila ananassae is an exception to this concept.

Dobzhansky was the first to show that the inversion polymorphism in Drosophila pseudoobscura is subject to natural selection and is a device to cope with the diversity of environments. His extensive work on D. pseudoobscura has revealed interesting phenomena of population genetics. In continuation of his work on this species, he constructed a number of homozygous lines for different gene arrangements in the third chromosome, and while employing these lines in intrapopulation and interpopulation crosses, he quantified the fitness of inversion homokaryotypes and heterokaryotypes. Interestingly, his results showed that heterokaryotypes formed by chromosomes originating from the same geographic area exhibited superiority over the corresponding homokaryotypes. However, superiority of heterokaryotypes was lost in the crosses when chromosomes were derived from different localities. Based on these results, Dobzhansky suggested the concept of genetic coadaptation. According to this concept, 'in each locality, the chromosomes with different gene arrangements aremutually adjusted or coadapted to yield highly fit inversion heterozygotes through long continued natural selection. However, this adaptive superiority of inversion heterozygotes breaks down in interracial hybridization experiments when two gene arrangements are derived from different localities'. This concept has received experimental evidence in its favour on the basis of work done in other species of Drosophila, such as D. willistoni, D. paulistorum, D. pavani and D. bipectinata. In all these species, interracial hybridization led to the loss of superiority of inversion heterozygotes. Further, it has been suggested that coadapted polygenic complexes contained in the chromosomes are disrupted as a result of recombination in interstrain crosses. This concept was also tested in D. ananassae, a cosmopolitan and domestic species commonly found in India, while employing three cosmopolitan inversions exhibiting heterotic buffering. In interstrain crosses involving monomorphic and polymorphic strains due to three cosmopolitan inversions, the persistence of heterosis was observed, which does not support the above-mentioned hypothesis of Dobzhansky. Thus, evidence for coadaptation is lacking in natural populations of D. ananassae, which is considered as an exception to the Dobzhansky's concept of genetic coadaptation. Thus, heterotic buffering associated with the three cosmopolitan inversions in D. ananassae is not populational heterosis; rather, it appears to be simple luxuriance.