Species, genomes, and section relationships in the genus Arachis (Fabaceae): a molecular phylogeny

The economically important genus Arachis (Fabaceae) comprises 80 species restricted to South America. One monograph on the genus divided it into nine sections and included an intuitive assessment of evolutionary relationships. There is no comprehensive phylogenetic study of the genus. To test the current systematic treatment of the genus, we reconstructed a phylogeny for Arachis using nuclear ITS and plastid trnT–trnF sequences from 46 species representing all nine sections. ITS cloning of the allotetraploid species of section Arachis indicated the presence of A and B genome alleles and chimeric sequences. Our study revealed that species from section Extranervosae were the first emerging lineage in the genus, followed by sections Triseminatae and Caulorrhizae, and two terminal major lineages, which we refer to as erectoides and arachis. The lineage erectoides comprises members of sections Erectoides, Heteranthae, Procumbentes, Rhizomatosae, and Trierectoides. Species in the arachis lineage form two major clades, arachis I (B and D genomes species and the aneuploids) and arachis II (A genome species). Our results substantiated the sectional treatment of Caulorrhizae, Extranervosae, and Triseminatae, but demonstrated that sections Erectoides, Procumbentes, and Trierectoides are not monophyletic. A detailed study of the genus Arachis with denser taxon sampling, additional genomic regions, plus information from morphology and cytogenetics is needed for comprehensive assessment of its systematics.