LITT, AMY* and VIVIAN IRISH. MCDB, Yale University, New Haven, CT 06520. - The A of the ABC model: phylogeny of the AP1-like gene family.
The ABC model of floral development postulates that three classes of
genes are responsible for determining the identity of the four types
of floral organs. The genes overlap in function in adjacent organ
whorls, thus A-gene function alone determines sepal identity, A+B
determine petal, B+C determine stamen, and C alone determines carpel
identity. APETALA1 (AP1) of Arabidopsis thaliana has
been identified as an A-function gene, but it is also involved in the
determination of floral meristem identity. The phenotype of the mutant
has features that can be interpreted as a failure of either function,
thus there is no clear evidence that AP1 is involved in
specifying floral organ identity as a separate function. AP1
homologs from other species have not been shown to affect floral organ
identity, thus there is little documentation of the A component of the
ABC model. AP1 belongs to a large and complex subfamily of the
MADS-box gene family, and it is not clear that homologs have been
correctly identified in other species. This is a requirement for
comparative studies, therefore we are studying the phylogeny of
AP1-like genes to be able to identify homologs for further
expression and function studies and to determine where significant
evolutionary events have occurred. Preliminary results suggest that
during angiosperm evolution there was a duplication that produced the
paralagous AP1 and FRUITFULL lineages present in higher
eudicots. There is some evidence that this duplication may have
occurred within the basal eudicots, and may coincide with a similar
duplication in the AP3 (B-function gene) lineage. These
duplication events may be related to the fixation of basic floral
structure that is seen in the higher eudicots.
Key words: ABC model, APETALA1, floral evolution, floral structure, MADS-box genes