The basal subfamily Caesalpinioideae of the Leguminosae generally is subdivided into four or five tribes, but their monophyly remains questionable. Recent cladistic analyses based on morphological characters and chloroplast rbcL sequences suggest conflicting hypotheses of relationships among tribes and subtribal groupings and of the identification of the basal Caesalpinioideae. Our phylogenetic analysis of the chloroplast trnL intron for 223 Caesalpinioideae, representing 112 genera, plus four Papilionoideae, 12 Mimosoideae and three outgroup taxa, provides some well-supported hypotheses of relationships for the subfamily. Our analysis concurs with the rbcL studies in suggesting that a monophyletic Cercideae is sister to the remainder of the Leguminosae. Among the other tribes of Caesalpinioideae, only the broadly circumscribed Detarieae (including Amherstieae or Macrolobieae) is also supported as monophyletic. The Detarieae s.l. occurs as sister to all Leguminosae, excluding Cercideae. Cassieae subtribes Dialiinae and Labicheinae together are sister to the remaining Leguminosae, which includes a monophyletic Papilionoideae, a paraphyletic Mimosoideae, and several monophyletic groups that correspond to previously defined generic groups or subtribes in the Caesalpinioideae. The trnL intron analysis suggests that basal legumes are extremely diverse in their floral morphology, and that presence of simple, actinomorphic flowers may be a derived feature in a number of lineages in the family.

Communicating Editor: Matt Lavin