J. Plant Res. 108: 443-450, 1995 Journal of Plant Research (~ by The Botanical Society of Japan 1995 Dioecy in the Endemic Genus Dendrocacalia (Compositae) on the Bonin (Ogasawara) Islands Makoto Kato and Hidetoshi Nagamasu Biological Laboratory, Yoshida College, Kyoto University, Kyoto, 606-01 Japan Dendrocacalia crepidifolia Nakai (Compositae, Senecioneae), the only species of this arboreal genus endemic to Haha Island in the Bonin Islands , was found to be dioecious, Male flowers differ from female ones in having a stunted style (style in female exserted from corolla and deeply bifurcating) and anthers filled with fertile pollen (anthers in the female lacking pollen). The size of the corolla and number of florets per head were similar between male and female flowers. The crown area of this arboreal species was also similar in male and female plants. ~ The sex ratio was 0.55 male, not signifi- cantly different from 0.5. Both sexes produced nectar of similar sugar concentration (ca. 50%). The flowers are pollinated by feral honeybees (Apis mellifera), but they are thought to have been pollinated by small, less- hairy, endemic solitary bees before honeybees were introduced and subsequently became the dominant bee species on the island. The evolution of dioecy of Den- drocacalia on the island is thought to stem from the deleterious effects of inbreeding that are inherent in plants with geitonogamy. The increased geitonogamy on the island has resulted from increased woodiness (i.e., increased number of flowers per plant) and the original dependence on endemic bee pollinators, which are now endangered. Key words: Bonin Islands-- Compositae -- Den- drocacalia ~ Dioecy -- Feral honeybee Adaptive radiation of flowering plants is common on oceanic islands. For example, adaptive radiation is prominent in Hawaii (Carlquist 1974, Helenurm and Gan- ders 1985). Similar adaptive radiation, although on a much smaller scale, occurred in the Bonin (Ogasawara) Islands, 1,000 km south of the mainland of Japan. Char- acteristic speciation in Pittosporum (4 species, Pittospor- aceae), Symplocos (3 spp., Symplocaceae), Callicarpa (3 spp., Verbenaceae) and Crepidiastrum (3 spp., Compositae) are good examples (Ono 1985, Nagamasu 1987, Kawakubo 1990, lto and Ono I990, Soejima et al. 1994). The flora of the Bonin Islands is composed of 260 flowering plant species, containing 112 endemic species and two endemic genera (Ono 1985). One of the endemic genera is Boninia (Rutaceae) which is closely related to Meficope (including Pelea, a genus endemic to Hawaii). The other endemic genus in the Bonin Islands is Den- drocacalia (Compositae, Senecioneae) which is thought to be a member of the subtribe Tussilagininae (Bremer 1994). Dendrocacalia is an example of a genus that evolved woodiness in the islands as have Hedyotis (Rubiaceae), Lobelia (Campanulaceae) and Crepidiastrum (Com- positae). We found that Dendrocacalia, previously known as a hermaphrodite, was actually dioecious. Relatively large proportions of dioecious species in oceanic islands are reported in Hawaii (Carlquist 1974), New Zealand (Godley 1975, Webb and Kelly 1993) and the Bonin Islands (Kawa- kubo 1990). The high prevalence of dioecy on oceanic islands was attributed to correlations between dioecy and the dispersal of seed by birds (Bawa 1980). Den- drocacalia is an exception to this trend because its seeds are wind-dispersed and its putative relatives are not dioecious. There are several genera in which species on islands are dioecious but their relatives on the nearest continent are hermaphrodite (Carlquist 1974, Kawakubo 1990). Thus, we confront the repeatedly discussed prob- lem of why dioecy has evolved in oceanic islands. A correlation between dioecy and pollination by small, generalist insects was pointed out by Bawa and Opler (1975). Are there any characteristics particular to the pollinator faunas of oceanic islands ? Both Hawaii and the Bonin Islands had bizarre bee faunas characterized by small body size, high endemism and non-sociality (Per- kins 1901, Kato 1992), although native pollinator faunas in these islands are now endangered by competition with naturalized honeybees (Messing 1991, Kato 1992) and predation by social wasps and ants (Cuddihy and Stone 1990). To consider the selection pressure favoring dioecy in oceanic islands, we investigated the distribution of males and females and the pollination biology of Dendrocacalia in the Bonin Islands. Unfortunately, the flowers were monopolized by naturalized honeybees everywhere the plant is distributed. We "can only specu- late about the original pollination system by observing native bee visits to other native flowers in adjacent small islands where honeybees have not yet invaded. This paper describes dioecism in Dendrocacalia, reports the distribution and ecological traits of male and female plants, and examines the floral and pollination biology of