Untangling the taxonomy of the Cladrastis clade (Leguminosae: Papilionoideae) by integrating phylogenetics and ecological evidence

Abstract

The Cladrastis clade is an early-diverged lineage within the subfamily Papilionoideae of Leguminosae. Several taxonomic uncertainties limit a robust understanding of its taxonomy: (1) Should the genus Platyosprion be resurrected? (2) How should Cladrastis be delimited to maintain the generic monophyly? (3) Is Styphnolobium best classified into two or three sections? We assessed specific/sectional/generic boundaries within this clade by integrating evidence from phylogenetic analyses, morphology, and multivariate ecological methods. With an extensive sampling of the clade, we reconstructed the phylogenies using nrDNA (ITS) and plastid DNA regions (matK, psbA-trnH, trnL-F). Based on environmental data for 841 occurrences, clusters of species were sought according to their environmental preferences with the principal component analysis (PCA), and explicit hypotheses on species/sectional boundaries were tested using ecological niche modeling (ENM). A new tribe, Cladrastideae, is established based on the Cladrastis clade with four monophyletic groups: the Cladrastis platycarpa complex, Cladrastis s.str., Pickeringia and Styphnolobium. The C. platycarpa complex constitutes the resurrected genus Platyosprion. The putative species C. chingii, C. parvifolia, C. scandens and C. yungchunii are suggested to be treated as synonyms of Platyosprion platycarpum. Cladrastis is recircumscribed as equal to Cladrastis s.str., with three species from eastern Asia, and one species, C. kentukea, from eastern North America. Styphnolobium is herein classified into three sections: St. sect. Oresbios, St. sect. Styphnolobium and a new section, St. sect. Affine.