Previous studies of the Malvales have shown that the Sterculiaceae are not monophyletic. However, members of four traditionally recognized tribes of Sterculiaceae (Byttnerieae, Theobromeae, Hermannieae, Lasiopetaleae) appear to constitute a clade named Byttnerioideae. Here we analyze sequences of the chloroplast gene ndhF for 37 species of Byttnerioideae. The monophyly of this diverse group of plants is supported with the inclusion of Kleinhovia from a fifth tribe of Sterculiaceae (Helictereae). The Hermannieae is the only tribe that appears to be monophyletic as traditionally circumscribed. Rulingia and Commersonia, two genera formerly placed in the Byttnerieae, are in a clade with all members of the Lasiopetaleae. The remaining Byttnerieae, Theobromeae, and Kleinhovia form two clades at the base of the Byttnerioideae. These results imply that the unusual hooded petals of many Byttnerioideae (e.g., Theobroma cacao) are plesiomorphic for the clade, with subsequent transitions to large, flat petals or small, scale-like petals that lack a distinct hood. Stamen number has traditionally been used to distinguish tribes in the Sterculiaceae. However, our analyses indicate that this character shows extensive homoplasy, with both increases and decreases in stamen number occurring within the Byttnerioideae.
Communicating Editor: John V. Freudenstein