Duplication and evolution of floral B-class homeotic genes in eudicots

Although the floral ABC model is generally accepted as ubiquitous in flowering
plants, variation in gene expression has been found in basal angiosperms and basal
eudicots. Accordingly, the basal eudicots and basal core eudicots show highly variable
floral features in terms of flower size and floral organ numbers, for example, the
perianth is absent in Trochodendron aralioides and flowers is generally reduced in
Santalales. These plants are indeed critical to understand the evolutionary
developmental program of perianth evolution in flowering plants. Previous studies
have shown that there is a major duplication event of all ABC class genes along the
branch leading to core eudicot lineages. However, only one of the two B class
homologues (AP3-like) is found duplicated and fixed in this major event, i.e. to euAP3
and TM6 sublineages, but not the PI-like homologues. Such duplication may be
responsible to the diversification of floral features among basal eudicots. Here we
chose taxa from Buxaceae, Trochodendraceae, Dilleniaceae and Santalales to explore
the B class gene duplications among basal core eudicots. Seventeen cDNA clones
encoding B class gene homologues were determined from Buxus microphylla ssp.
sinica, Trochodendron aralioides, Dillenia indica, Thesium chinense, Loranthus kaoi
and Loranthus delavayi. All of these taxa have one to three PI homologues, and
various AP3 homologues. Results from phylogenetic analysis suggest that the first two
taxa have paleoAP3 homologues whereas the later four are most likely having either
TM6 or euAP3-like homologues instead. Based on the phylogenetic analysis, we
proposed the euAP3/TM6 duplication occurred after the separation of the
Trochodendraceae but before the divergence of the core eudicots. Besides, the analysis
for detecting selective constraints also suggests that both PI and AP3 lineages have
been subjected to strong purifying selection, and the switch of selective constraint is
apparently present between basal eudicots and core eudicots. A thorough survey is
necessary to elucidate the correlations between gene duplications and the selective
constraints among different clades of basal and core eudicots. Interestingly, euAP3
homologue was not found in the screening of B class genes from Santalales species,
suggesting the function would be complemented by PI or TM6 homologues if they are
truly absent in Santalales.