| Research Abstract |
The Japan islands possess a very rich fauna for the Drosophila virilis section (Diptera, Drosophilidae), one of the major lineages within the subgenus Drosophila. Geographically, the virilis species group is very dominant in the cool temperate zone, the quadrisetata and the melanica species groups in the warm temperate zone, and the angor and the fluvialis species groups in the subtropical zone. For the robusta species group, both Drosophila neokadai of its okadai subgroup and D. lacertosa of its lacertosa subgroup are widely distributed in mainlands of Japan from Hokkaido to Kyushu throughout Honshu. Drosophila okadai of the okadai subgroup is common in cool districts, whereas D. gani in warm districts. Micro-habitats of these species of the Drosophila virilis section are restricted to riparian environments with high humidity and a relatively constant fluctuation of temperatures, such as streams or lakes in mountain areas. On the other hand, two species of the robusta subgroup, D. sor
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didula and D. pseudosordidula, dwell in temperate forests, but are not so abundant in many places.
The Tsugaru Straight lying between Hokkaido and Honshu and the Tokara Straight between Amami-Ohshima and Yakushima are "great waterfalls" for the geographic distribution of Drosophila gani of the robusta group and the angor group, respectively. The former is known as a major zoogeographical boundary of the Blakiston's Line and the latter as the Watase's Line splitting East Asia into two zoogeographic regions, the Oriental and the Palearctic Regions.
Karyotypes and reproductive isolation are studied in two allopatric populations of Drosophila tsigana (one from Guizhou Province in southern China and the other from Hokkaido in northern Japan) and in one population of a closely related species, D. longiserrata from Guizhou. In metaphase plates of larval brain cells, both geographic strains showed 2n=10 chromosomes, with 2 pairs of metacentric (V-shaped), 2 pairs of acrocentric (R-shaped) and 1 pair of dot-like (D-shaped) chromosomes. Drosophila longiserrata showed the same number, 10 chromosomes, comprising 2V, 1J (sub-metacentric chromosomes), 1R, and 1D. X chromosomes of both species were acrocentic, the presumed ancestral form. Pre-mating isolation was complete between D. tsigana and D. longiserrata, and successful mating was also limited in crosses between the two geographic populations of D. tsigana. These results imply that temperate members of the virilis section, the melanica group and probably the virilis group also, might have emerged in the Yun-Gui Highland and its adjacent mountains in southern China.
In the molecular phylogenetic trees constructed by sequences of nuclear and mitochondrial genes, the polychaeta species group, a tropic member of the Drosophila virilis section, is located in the basal position, and most of temperate species groups such as the melanica group and robusta subgroup in the distal one. Our present results strongly supports the Throckmorton's hypothesis on the process of adaptive radiations of the Drosophila virilis section: it might have emerged in tropic regions of the Old World, probably in Asia, spread to higher latitudes along green belt of East Asia, and then migrated to North America via Beringia and diversified in temperate deciduous forests there Less
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