Phylogeny and divergence time estimation of Chamaecrista ser. Rigidulae (Leguminosae: Caesalpinioideae)

Abstract

Chamaecrista is a monophyletic genus, but most of its infrageneric categories have been shown to be paraphyletic, including series Rigidulae which currently comprises 30 species, all endemic to Brazil and most occurring in the cerrado vegetation of the central highlands of the country. This molecular phylogenetic study tests the monophyly of C. ser. Rigidulae based on a broad sampling, considers its relationship with members of C. sect. Absus subsect. Absus, and estimates its divergence time in relation to the age of the genus. For that, individual and combined analyses were performed by the parsimony and Bayesian methods using chloroplast (trnL-F) and nuclear (ITS1-5.8S-ITS2) markers and a matrix containing 75 taxa of Chamaecrista (29 belonging to series Rigidulae), 6 of Senna and 1 of Cassia. The analyses showed that series Rigidulae, as traditionally circumscribed, is polyphyletic. When C. brachyblepharis and C. ciliolata are excluded and C. botryoides and C. sincorana included, the series is resolved as monophyletic, comprising 30 species here designated as the Rigidulae clade. This clade is further subdivided into two geographically and genetically structured subclades, the first containing 23 species, mostly from the highlands of Goiás State, and the second with 6 species from the Espinhaço Range, running north to south through central Bahia and Minas Gerais States. Divergence time analyses suggest that the Rigidulae clade originated about 5 million years ago. The recent radiation of the series repeats that seen in other species-rich genera in the Cerrado Biome, thus corroborating previous hypotheses about the recent age of the biome. The Rigidulae clade, as here circumscribed, has the following morphological synapomorphies: asymmetric flowers with their posterior petals similar to a typical papilionoid standard petal, and leaflets divaricate along the leaf rachis.