Cowpea [Vigna unguiculata (L.) Walp.] maternal lineages, chloroplast captures, and wild cowpea evolution

Vigna unguiculata is the major grain legume in Africa and is also important in South-East Asia as a vegetable. The wild relatives, present only in Africa, are currently made of ten subspecies. However, while the different groups are well identified, the relationships between them are not firmly established. The present study of chloroplast diversity through restriction fragment length polymorphism confirms the different groups, as well as the split between savannah subspecies and forest subspecies. It justifies treating subsp. stenophylla as a subspecies independent from subsp. protracta and invalidates the subsp. stenophylla sensu lato concept. It suggests that the species V. unguiculata originated in southern Africa and that the annual subsp. unguiculata var. spontanea originated in eastern Africa. More importantly, it proves that three subspecies are of hybrid origin, between the annual var. spontanea and various perennial subspecies from southern Africa with left-twisted keel. Owing to its annual and inbred properties, var. spontanea spread across all African low- to medium-altitude savannahs, displacing the left-twisted keel savannah perennials from their warmer and drier areas.