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