Artificial hybridization in the Hawaiian endemic genus Labordia (Loganiaceae).

Cross-pollinations were performed within and among eight species of Labordia from Oahu, Molokai, and Hawaii, and one species from Guam of the closely related genus Geniostoma. Detailed floral examination confirmed that the species are functionally dioecious, i.e., a given individual lacks either ovules or pollen grains. Female inflorescences bagged to prevent pollination never produced seed, but intraspecific crosses between male and female individuals nearly always yielded fruits with viable seed (>80%). Interspecific crosses between species from different islands and separate taxonomic sections of the genus also yielded good fruit set (>55%). Interspecific F1 hybrids were vigorous and appeared to be morphologically intermediate to their parents. Intergeneric crosses between Geniostoma and Labordia failed. Chromosome counts from Labordia species were found to be 2n = 80 or 2n =~80, twice the chromosome complement of two collections examined from its postulated ancestor, Geniostoma rupestre. Labordia species are distinct morphologically, ecologically, and geographically but apparently lack genetic barriers to interbreeding. This suggests that geographical and ecological isolation, recentness of colonization, and/or rapid speciation have been important factors in the origin of species of Labordia.