Clermontia, endemic to the Hawaiian Islands, comprises 22 species, 15 of which have a second whorl of petals in place of the sepals typical of the outer whorl of lobelioid flowers. These outer whorl organs are identical to petals both in their anatomy and cell surface morphology. We have been investigating the molecular basis for this homeotic phenotype using both molecular phylogenetic and in situ gene expression approaches. Ongoing phylogenetic analysis of 5S-NTS sequences suggests the possibility of a single origin for the double-corolla trait on the island of Oahu no more than 3.7 million years ago. The phenotypic diversity (including flowers with rotate, bilabiate, and tubular corollas) and genetic similarity observed among double-corolla Clermontia species suggests that morphological radiation can follow rapid fixation of a new floral groundplan. This process would be enhanced by inbreeding (accompanying pollinator extinction or disturbance) and founder effect in island environments with rapidly evolving ecological niches. As sepal/petal organ identity is affected in double-corolla clermontias, MADS box gene expression patterns are expected to be altered. Specifically, ectopic B-function MADS box gene activity is predicted in the sepal whorl. In Arabidopsis, overexpression of PI results in partial conversion of sepals to petals without other organ identity changes (Krizek and Meyerowitz, 1996). Petalization of the first whorl is more complete in transgenic plants overexpressing both PI and AP3, but such flowers also have stamens replaced by carpels. New data from in situ hybridization experiments with B- and C-function MADS box gene probes demonstrate that ectopic expression of the Clermontia PI orthologue is responsible for the double-corolla phenotype. This research is supported by the Lewis B. and Dorothy Cullman Foundation.

Key words: Campanulaceae, Clermontia, development, Hawaiian Islands, homeosis, MADS box genes