The realignment of Acosmium sensu stricto with the Dalbergioid clade (Leguminosae: Papilionoideae) reveals a proneness for independent evolution of radial floral symmetry among early-branching papilionoid legumes

The genus Acosmium s.l. (Leguminosae: Papilionoideae) traditionally included up to 20 tree species mostly from South America. Due mainly to radial floral symmetry, the taxonomy of Acosmium has been historically confounded with four other Papilionoideae genera: Dicraeopetalum, Guianodendron, Leptolobium, and Sweetia. We used comprehensively sampled molecular data from the nuclear ribosomal internal transcribed spacer (ITS/5.8S) and two plastid DNA (matK, trnL intron) regions in order to evaluate the monophyly of Acosmium s.l. and to shed light on the relationships of its distinct lineages in the context of the Papilionoideae phylogeny. Parsimony and Bayesian analyses of all data concurred in resolving a polyphyletic Acosmium s.l. Our data agreed with previous molecular phylogenetic analyses that resolved Dicraeopetalum and Sweetia apart from Acosmium s.l. Unexpectedly, Acosmium s.str., thought to belong to the Genistoid clade, is here strongly resolved within the Dalbergioid Pterocarpus clade. Despite being classified until recently as different sections of Acosmium s.l., Leptolobium and Guianodendron were resolved within the Genistoid clade as part of a Bowdichia clade with the genera Diplotropis and Bowdichia. Our results reveal that radial floral symmetry has evolved independently in all Acosmium segregates and strongly suggest that the primacy given to floral characters involving corolla symmetry and petal differentiation in traditional legume classification has often obscured phylogenetic relationships.