Developmental morphology and structural homology of corolla-androecium synorganization in the tribe Amorpheae (Fabaceae: Papilionoideae)

Comparative developmental morphology was used to assess structural homology of flowers in Dalea, Marina, and Psorothamnus of the tribe Amorpheae (Fabaceae: Papilionoideae). Dalea, Marina, and some species of Psorothamnus have an unusual petal-stamen synorganization (stemonozone) in which free petals are inserted on a region that is continuous with fused stamen filaments. Developmental studies of these three genera demonstrated similarity during organogenesis. Zonal growth results in several synorganized regions, including the stemonozone of Dalea, Marina, and some Psorothamnus. Psorothamnus species that lack a stemonozone have fused stamens and free petals inserted on the hypanthium, as in most other papilionoid legumes. We concluded that the stemonozone is not strictly homologous to either androecium or receptacle, but that it is the product of a modified androecial developmental program. In the prairie clover daleas, petaloid structures positioned between the stamens have been variously interpreted as petals or as staminodes; we infer that they have an extreme form of the daleoid stemonozone, on which five petals (no staminodes) and five stamens are inserted. Assessing structural homology of these flowers allows us to characterize accurately daleoid morphology for evolutionary studies in the tribe Amorpheae.