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Twelve local populations of Drosophila sulfurigaster albostrigata from the Philippines, Palawan, Borneo, the Malay Peninsula, Thailand and Sri Lanka, were crossed to D. s. bilimbata and D. s. sulfurigaster. With respect to postmating isolation, the fertility and the sex ratio distortion in F1 hybrids and the frequency of the insemination reaction causing the reaction mass were investigated. In crosses to D. s. sulfurigaster, the extent of postmating isolation, both prezygotic and postzygotic ones, were relatively weak. In crosses to D. s. bilimbata, the extent of both kinds of isolation suggested that local populations of D. s. albostrigata have genetically diverged among the six regions described above. In interpopulational hybridization, hybrids from crosses of marginal populations showed the hybrid breakdown in the F2 or F3 generation for egg-to-adult viability. On the other hand, in premating isolation to D, s. neonasuta estimated by no-choice experiments, significant genetic differentiation among local populations of D. s. albostrigata was not detected. In intersubspecific crosses, between two groups; one consists of D. s. neonasuta and D. s. albostrigata and the other D. s. bilimbata and D. s. sulfurigaster, prezygotic isolation by the insemination reaction causing the reaction mass was developed in either of the reciprocal crosses, and postzygotic isolation by F1 male sterility and the sex ratio distortion in F1 progeny in the other crosses. Strong one sided sexual isolation was observed between D. s. neonasuta and the other group including D. s. bilimbata and D. s. sulfurigaster. It is clear that isolating mechanisms to prevent the gene exchange between subspecies have developed to some extent. On the basis of the degree of genetic and reproductive differentiation estimated by hybridization experiments, the processes of subspecies formation in D. sulfurigaster are discussed.

Genetic and reproductive differentiation in Drosophila sulfurigaster.