Evolution and Development in the Amorphoid Clade (amorpheae: Papilionoideae: Leguminosae): Petal Loss and Dedifferentiation

We used comparative developmental morphology to study the evolution of nonpapilionaceous corollas in the amorphoid clade of the tribe Amorpheae (Papilionoideae). This clade consists of five genera in which there are no papilionaceous corollas (five petals differentiated into one banner, two wing, and two keel petals). We studied the ontogenies of three nonpapilionaceous forms: corollas consisting of one petal (exemplified by Amorpha canescens), no petals (Parryella filifolia), and five petals in two types (Errazurizia megacarpa). We compared these to the ontogeny of a papilionaceous corolla (exemplified by the closely related Psorothamnus scoparius). In A. canescens, all petals initiated, but four did not grow beyond the primordial stage. In P. filifolia, no distinct petal primordia were visible. The corolla of E. megacarpa, which has only two types of petals, exhibits nonpapilionaceous characteristics at an early ontogenetic stage.Aside from the earliest primordial mounds, the petals ofPsorothamnus andErrazurizia do not resemble each other, indicating that paedomorphosis is not responsible for the nonpapilionaceousness of Errazurizia. Comparing the morphological results to a phylogeny,we infer a single origin of the characteristics that differentiateErrazurizia petals fromPsorothamnus petals, and we infer at least two evolutionary events leading to the reduced corolla in Parryella, Amorpha, and Errazurizia rotundata. When considered in the context of the remaining Amorpheae, in which additional floral diversification has occurred, and in the context of the entire papilionoid group, in which floral form is relatively conserved, our results indicate a relaxation of selective or developmental constraint within the clade Amorpheae.