ETHOLOGICAL ISOLATION AND PHYLOGENY IN THE GRIMSHAWI SPECIES COMPLEX OF HAWAIIAN DROSOPHILA.

Abstract

The ethological isolation among six allopatric populations, comprised of two closely related species of Hawaiian Drosophila, was measured and used to analyze the phylogenetic relationships among these populations. The two species used in this study are included in the picture-winged species group (Carson and Stalker, 1968). In its present taxonomic status, Drosophila grimshawi Oldenberg is the only species of this group that inhabits all the accessible major islands (i.e. Kauai, Oahu, Maui, Lanai, and Molokai) except Hawaii. On the island of Hawaii a very closely related species, D. pullipes Hardy and Kaneshiro, is found. The only distinguishing morphological character between grimshawi and pullipes is reported to be the coloration of the legs and pleurae (Hardy and Kaneshiro, 1972). A cytological comparison of the six populations showed a strong similarity between them. Carson and Stalker (1968) and Carson and Sato (1969) found grimshawi from the islands of Lanai, Maui, and Molokai (these islands known collectively as the complex of islands) to possess the standard polytene chromosomal sequence as well as one polymorphic inversion on the fourth chromosome (4A). Later studies (Carson et al., 1970) of grimshawi populations from Oahu and Kauai and of pullipes populations showed that they are fixed for the standard chromosomal sequence. That is, they differ from the Maui complex populations only in lacking the polymorphic 4A inversion sequence. For the grimshawi populations the comparison of metaphase karyotypes showed the five rods and one dot haploid chromosomal complement characteristic of most of the picture-winged species (Clayton, 1971). Therefore, contrary to the findings of Craddock (1974), no consistent morphological or cytological differences were detected among the insular populations of grimshawi. Montgomery (1975) reared the adults of both species from field collected substrates and discovered an interesting ecological differentiation. The adults of the Maui complex grimshawi were obtained from the substrates representing 12 different plant families. However, grimshawi from Oahu and Kauai and pullipes from the island of Hawaii were reared exclusively from the rotting bark of trees of the genus Wikstroemia (family Thymeleaceae). On the basis of these studies grimshawi from the Maui complex have been termed ovipositional generalists whereas pullipes from Hawaii and grimshawi from Oahu and Kauai are ovipositional specialists. In addition to this ecological differentiation, Ringo (1974; 1976) observed jousting behavior differences among the males of some of these populations. Using stocks of Maui, Molokai, and Oahu grimshawi and pullipies, Ringo observed a greater difference in jousting displays between generalist and specialist males than between species. That is, although Maui and Molokai grimshawi had similar jousting displays as did Oahu grimshawi and pul1 Published with the approval of the Director of the Hawaii Agricultural Experiment Station as Journal Series Number 2132. 2 Present address: Department of Primary Industries, Entomology Branch, Meiers Road, Indooroopilly, Brisbane, Queensland, Australia 4068.