Abstract

Species maintain their identity through reproductive isolating mechanisms, which are broadly classified into prezygotic and postzygotic isolating mechanisms. In the Drosophila bipectinata species complex, investigations were made on the degree of crossability (a prezygotic isolating mechanism) and the causes of hybrid male sterility (a postzygotic isolating mechanism) to analyse the phylogenetic relationship. Among the four species, D. bipectinata crosses with Drosophila parabipectinata freely in one direction and both of them also cross with Drosophila malerkotliana easily but it is difficult to cross all the three species with Drosophila pseudoananassae. In the hybrids involving D. pseudoananassae, no sperm were observed indicating high degree of perturbance during spermatogenesis while in the other hybrids immotile sperm were present indicating comparatively less disturbance during spermatogenesis. Testis size, which is an indicator of degree of perturbance during spermatogenesis and used as a proxy for sterility was measured in the four species and their hybrids. It was of same size in D. bipectinata, D. parabipectinata and D. malerkotliana but larger in D. pseudoananassae. In the hybrids involving D. pseudoananassae, testes were atrophied while in other hybrids it was larger. Since, the size of testis does not exhibit uniform pattern of variation in hybrids, it cannot be used as a good indicator for sterility. Therefore, we also measured the size of seminal vesicles (storing organ of sperm) in the four species and their hybrids. Interestingly, the size of seminal vesicles was reduced uniformly in all the hybrids indicating its use as better proxy for sterility. Further, the seminal vesicle size in D. pseudoananassae was smaller than that in the other three species. These observations provide evidence for phylogenetic proximity of D. bipectinata, D. parabipectinata and D. malerkotliana and their remote relationships with D. pseudoananassae.