D. n. nasuta (2n=8) andD. n. albomicana (2n=6), the chromosomal races of thenasuta subgroup have mutual open genetic systems. Hybrid populations are completely fertile. By utilising fixed inversion differences in second and third chromosomes, the extent of coassociation of parental chromosomes in the gene pool of the hybrids have been estimated. There is a significant decline in the Inversion polymorphism of the second chromosome, while in the third chromosome both the parental and the hybrid associations are equally frequent. The reduction in the frequency of heterokaryotypes in the second chromosome is due to the establishment of different coadapted systems in the parental chromosomes which has been favoured by selection. This does not occur in the third chromosome. This dochotomy of differential coadaptation within the elements of the same genotype is the first of its kind and the implications of these findings are discussed.