Barriers and prospects for wild crop relative research in Brassica rapa

Wild forms of crop species are an important source of breeding material and insight into the process of domestication. Wild or feral crop relatives can also become undesirable weeds that compete with crops or alter ecosystems. Little is known about spontaneously occurring forms of the economically important species Brassica rapa, which includes napa cabbage, turnips, and other morphologically diverse crops. This lack of knowledge compromises our access to germplasm for breeding, understanding of the centre of domestication, and control of weedy forms. To identify current knowledge gaps and research priorities, we summarized the current state of research on field mustard (B. rapa subsp. sylvestris). To aid future germplasm collection in B. rapa and detect geographic regions susceptible to invasion by weedy B. rapa we modeled climatic habitat suitability of field mustard worldwide. We outline seven priority areas for field mustard research: 1) genetic studies of domestication incorporating a broad sampling of spontaneously occurring populations and crop subspecies, 2) investigation of non-native populations, 3) clarification of feral or wild status of spontaneous populations, 4) ethnobotanical studies of B. rapa in Latin America and elsewhere, 5) collection of spontaneous germplasm and protection of wild populations, 6) correction of mislabelled germplasm in seed banks and databases, and 7) taxonomic recircumscription of B. rapa subspecies. Our species distribution model indicates high habitat suitability for wild B. rapa across the putative native range of temperate Europe, Asia, and North Africa and potential for invasion in Greenland, East Africa, and central Eurasia.