Transference of function, in which one organ takes over the function usually accomplished by another organ, is a very important phenomenon in plant evolution. Dogwoods provide an example, as some species have white, petaloid bracts that attract pollinators, a function normally done by petals and/or sepals. Dogwoods present an ideal system in which to study the evolution of transference of function, because: a) one entire clade in the genus (ca 10 species) has showy bracts, while the remaining ca 40 species lack showy bracts; and b) all species have flowers with true petals that typically become white, providing a morphological and developmental-genetic control for what constitute petal attributes. Development of petaloid bracts involves a suite of changes including size, pigment production, etc., as compared to inflorescence bud bracts in the paraphyletic outgroup, the small-bracted and bractless dogwoods. A simple evolutionary-developmental explanation for acquisition of petaloidy would be the ectopic expression, in bracts, of genes specifying petal identity, i.e. A and B class, and SEPALLATA-like MADS-box genes. We have cloned cDNA sequences of B-class genes homologous to AP3 and PI of Arabidopsis and to TM6 of Lycopersicon from both a species with petaloid bracts, C. florida, and one without them, C. alba. RT-PCR results show that all three genes are expressed in flowers of both species, but we have not found expression of any of them in the mature petaloid bracts of C. florida. We are now testing expression by other methods and in earlier stages of bract expansion, and we are cloning other genes. If confirmed, absence of B-class gene expression would indicate that determination of petaloid bracts in dogwoods is due to elements other than B class genes.

Key words: Cornus, evolutionary development, petaloid bracts