Hillebrandia is a monotypic genus of the Begoniaceae, a family that currently includes two other genera, Begonia (1400 species) and Symbegonia (12 species). Hillebrandia is distinguished by its semi-inferior ovary, which is fully inferior in other Begoniaceae, and by the presence of 8-10 perianth segments compared to 2-5(8) perianth segments in other members of the family. In addition, Hillebrandia is the only begonia that is endemic to the Hawaiian Islands. Parsimony-based phylogenetic analyses of nuclear-encoded genes (ribosomal 18S and ITS, the internal transcribed spacer region) and chloroplast-encoded genes (rbcL and the atpB-rbcL spacer region) place Hillebrandia at the base of a clade that includes the remainder of family Begoniaceae. Begoniaceae are globally distributed in the tropics, but lack a fossil record to indicate its age or geographic origin. Molecular studies have indicated that Begonia had its origins in Africa and the African begonias are the closest species to Hillebrandia. By comparing rbcL sequences from Hillebrandia and two African begonias, and by using fossil information from close outgroup species, we have estimated the divergence time of Hillebrandia and Begonia to be approximately 40 million years ago, an age that predates the oldest of the current Hawaiian Islands. Our data suggest that Hillebrandia is an island relict on a relatively young archipelago. Various hypotheses as to Hillebrandiaís origin are considered.

Key words: Begoniaceae, biogeography, Hillebrandia sandwichensis, phylogeny