SummaryThe karyotype of 4 species namely D. agumbensis, D. anomelani, D. truncata and D. cauverii have been analysed. The heterochromatin content has been characterized by using C-and Q-profiles. The Y-and 4th chromosome have shown a great degree of divergence in their form and size. The heterochromatin has played a very important role in bringing about this high degree of differentiation within the limit of 2nϭ8 among the members of the montium subgroup of Drosophila. Montium subgroup, D. agumbensis, D. truncata, D. anomelani, D. cauverii, Karyotype, heterochromatin. Evolutionary divergence among species involves genetic changes at different levels of the genome. Karyotypic divergence is one such component and it may result from chromosome rearrangements and qualitative as well as quantitative changes in heterochromatin. Chromosome differences that are commonly found between species have played a key role in initiating speciation, and over 90% of all the speciating events are accompanied by karyotypic changes (White 1973). The Drosophila montium subgroup, the largest of the melanogaster species group (Lemeunier et al. 1986) comprise of 84 of 173 known species. Of these, 50 species are known karyotypically and heterochromatin of very few species has been characterized. The wide distribution of subgroup combined with many unique features of individual species, together with wealth of species diversification within subgroup made it an attractive system. The members of this group of Drosophila are highly heterogeneous morphologically (Bock and Wheeler 1972) and karyotypically (Baimai 1980, 1998, Shyamala and Ranganath 1994, Suma and Ranganath 1997. Not more than fifty species of the montium subgroup are subjected to karyotypic analysis (Table 2). In the present study karyotypes of 4 species of this montium subgroup namely D. agumbensis, D. anomelani, D. truncata and D. cauverii have been analysed. Key words Materials and methodsThe species used for the present study were collected from different regions of South India. D. agumbensis and D. anomelani from Mysore. D. truncata from Charmadi Ghats and D. cauverii from Coorg. Their iso-female lines were established and, maintained in the laboratory on wheat cream agar media seeded with yeast at 23Ϯ1°C.Mitotic chromsomes from the third instar were prepared following the technique of Stock et al. (1972) with slight modifications. To localize C-band heterochromatin, the procedure adopted by Clayton (1986) and Hatsumi (1987) was followed. Air dried slides were treated with 0.25% Trypsin