MOLECULAR PHYLOGENY AND DIVERGENCE TIMES OF DROSOPHILID SPECIES

The phylogenetic relationships and divergence times of 39 drosophilid species were studied by using the coding region of the Adh gene, Four genera-Scaptodrosophila, Zaprionus, Drosophila, and Scaptomyza (from H awaii)-and three Drosophila subgenera-Drosophila, Engiscaptomyza, and Sophophora-were included, After conducting statistical analyses of the nucleotide sequences of the Adh, Adhr (Adh-related gene), and nuclear rRNA genes and a 905-bp segment of mitochondrial DNA, we used Scaptod rosophila as the outgroup. The phylogenetic tree obtained showed that the first major division of drosophilid species occurs between subgenu s Sophophora (genus Drosophila) and the group including subgenera Dros ophila and Engiscaptomyza plus the genera Zaprionus and Scaptomyza. Su bgenus Sophophora is then divided into D. willistoni and the clade of D. obscura and D. melanogaster species groups. In the other major dros ophilid group, Zaprionus first separates from the other species, and t hen D. immigrans leaves the remaining group of species. This remaining group then splits into the D. repleta group and the Hawaiian drosophi lid cluster (Hawaiian Drosophila, Engiscaptomyza, and Scaptomyza). Eng iscaptamyza and Scaptomyza are tightly clustered. Each of the D. reple ta, D. obscura, and D. melanogaster groups is monophyletic. The splitt ing of subgenera Drosophila and Sophophora apparently occurred about 4 0 Mya, whereas the D. repleta group and the Hawaiian drosophilid clust er separated about 32 Mya. By contrast, the splitting of Engiscaptomyz a and Scaptomyza occurred only about 11 Mya, suggesting that Scaptomyz a experienced a rapid morphological evolution. The D. obscura and D. m elanogaster groups apparently diverged about 25 Mya. Many of the D. re pleta group species studied here have two functional Adh genes (Adh-1 and Adh-a), and these duplicated genes can be explained by two duplica tion events.