Phylogenetic systematics of Vigna sensu stricto in the context of Physostigma and allies

Premise: Vigna includes economically vital crops and wild species. Molecular systematic studies of Vigna species resulted in generic segregates of many New World (NW) species. However, limited Old World (OW) sampling left questions regarding inter- and intraspecific relationships in Vigna s.s.

Methods: African species, including the putative sister genus Physostigma, were comprehensively sampled within the context of NW relatives. Maximum likelihood and Bayesian inference analyses of the chloroplast matK-trnK and nuclear ribosomal ITS/5.8 S (ITS) DNA regions were undertaken to resolve OW Vigna taxonomic questions. Divergence dates were estimated using BEAST to date key nodes in the phylogeny.

Results: Analyses of matK and ITS data supported five clades of Vigna s.s.: subg. Lasiospron, a reduced subg. Vigna, subg. Haydonia, subg. Ceratotropis, an enlarged subg. Plectrotropis, and a clade including V. kirkii and V. stenophylla. Genome size estimates of 601 Mb for V. kirkii are near the overall mean of the genus, whereas V. stenophylla had a larger genome (810 Mb), similar to some Vigna subg. Ceratotropis or Plectrotropis species.

Conclusions: Former subg. Vigna is reduced to yellow- and blue-flowered species and subg. Plectrotropis is enlarged to mostly all white-, pink-, and purple-flowered species. The age of the split between NW and OW Vigna lineages is ~6-7 Myr. Genome size estimates cannot rule out a polyploid or hybrid origin for V. stenophylla, potentially involving extinct lineage ancestors of Vigna subg. Ceratotropis or Plectrotropis, as indicated by network and phylogenetic analyses. Taxonomic revisions are suggested based on these results.