Floral Evolution in the Detarieae (Leguminosae): Phylogenetic Evidence for Labile Floral Development in an Early-Diverging Legume Lineage

Abstract

Premise of research. We examine floral evolution and phylogenetic relationships in the monophyletic Detarieae and related lineages of Caesalpinioideae. Tribe Detarieae (82 genera) includes nearly half of the genera in subfamily Caesalpinioideae and represents some of the most diverse legumes with respect to floral morphology.Methodology. A total of 67 floral ontogenetic and morphological characters were combined with DNA sequences from the plastid trnL-F and matK regions of 34 Detarieae species and representatives of Cassieae, Cercideae, Caesalpinieae, Papilionoideae, and Mimosoideae, for which we have near-complete ontogenetic series. The morphological and ontogenetic characters were optimized onto the resulting most parsimonious phylogenetic trees and Bayesian topology to study character evolution.Pivotal results. Our study supports previously proposed relationships within the tribe Detarieae and among caesalpinioid lineages and indicates that certain features (bracteole and hypanthium characters, sepal initiation, anther position in bud, overlap in timing of initiation of organ whorls) are phylogenetically informative for particular clades whereas others (reductions in petal and stamen number, sepal and petal initiation patterns) have evolved multiple times in parallel in the Detarieae and other Caesalpinioideae. These analyses suggest that modifications that occur early in ontogeny can be good phylogenetic characters for distinguishing both major taxonomic groups and more closely related taxa and that morphological differences that differentiate species within genera can be caused by changes that occur at all stages of ontogeny. Phylogenetic distribution of character states and ontogenetic evidence suggest that in the Caesalpinioideae, loss or suppression of organs within a whorl, both of which are very common, usually does not affect development in subsequent whorls.Conclusions. Our analyses reveal several switches from zygomorphy to actinomorphy (and vice versa), but in Caesalpinioideae (contrary to Papilionoideae), zygomorphy is likely not clearly associated with higher diversification rates. This study suggests that floral initiation patterns are much more variable in Caesalpinioideae than in the other two subfamilies. Although particular patterns may be canalized in certain lineages of the subfamily (Cercideae, Caesalpinieae), in other clades (Detarieae, Dialiinae) floral development is more labile, explaining the high diversity in floral morphology encountered overall in the paraphyletic Caesalpinioideae.