| Papilionoid legumes are characterized by a relatively conserved floral form, but exceptions to the pea-like flower occur. A high concentration of these exceptions appear in the tribe Amorpheae, a clade in which floral evolution has involved dramatic changes in the number, shapes, and positions of floral organs. To study floral diversification in the tribe, we have compared morphological and developmental data to a phylogeny we reconstructed using DNA sequence data (cptrnK/matK and nrITS/5.8S). Floral diversification in Amorpheae has occurred through a variety of evolutionary modes. Novelty, organ loss, and de-differentiation of organs have occurred multiple times. In one major clade, the daleoids---Dalea, Marina, and Psorothamnus---petals appear perched on the fused staminal column above a novel structure, the stemonozone. Stemonozone lengths vary tremendously; however, applying standard criteria for structural and phylogenetic homology, we have inferred that the stemonozones are homologous across taxa in a clade of Dalea + Marina + some Psorothamnus (currently a paraphyletic genus). In addition to novelty, organ loss has occurred at least once and de-differentiation of petals thrice. The monophyletic section Kuhnistera, or prairie clovers, has lost a whorl of stamens; the polyphyletic subgenus Dalea have unfused corollas of non-differentiated petals. In the other major clade of Amorpheae, the amorphoids---Errazurizia, Parryella, Eysenhardtia, Amorpha, and Apoplanesia---organ loss and petal de-differentiation are primarily responsible for variation, but some novelty also occurs. At least two reductions in petal number have occurred in the lineage involving Parryella, Errazurizia rotundata and Amorpha; whether a re-gain also occurred is unclear. One species has petals slightly perched on a stemonozone, similar but phylogenetically non-homologous to the stemonozone in the daleoids. Diversity in Amorpheae is clearly due to a combination of homoplastic changes in various modes of evolution. |
