Genetic diversity and structure in wild and cultivated populations of an emblematic African tree species, Garcinia kola (Clusiaceae)

Non-timber forest products (NTFPs) are important components of rural people's livelihood, being used notably for food and medicine. Given their socio-economic importance, NTFP species are often integrated in farmers' fields using wild seed material from adjacent forests. In sub-Saharan Africa, the evolutionary history, and more specifically, the cultivation history of these species (geographical origin, dating), is still largely unknown. This study focuses on an African medicinal tree species, originating from the Guineo-Congolian rain forests and commonly known as the bitter kola tree (Garcinia kola, Clusiaceae). We estimated species genome size and chromosome number. We developed eleven highly polymorphic nuclear microsatellite markers and used seven of them to characterize patterns of distribution of genetic diversity within populations of G. kola in the forest (161 wild individuals coming from nine different populations from Cameroon, Gabon, and the Republic of Congo) and on-farm (82 cultivated individuals from five populations from Cameroon). The species is most probably hexaploid, with a relatively large genome size (2C = 25.5 to 28.5 picogram) and ca. 170 chromosomes. Three intra-specific gene pools were observed along a north-south axis within wild populations. This genetic differentiation pattern presents some similarities with patterns already observed in other tree species from the region, suggesting that a common factor, potentially past-climatic oscillations, has affected the demography of forest trees in the same way. Cultivated populations were not spatially structured, and the genetic differentiation between wild and cultivated populations was low (G(ST) = 0.017). The cultivation history of the species was difficult to interpret. It is actually unclear if cultivated populations from Cameroon are derived from wild Cameroonian populations or if they originate from wild populations of West African countries, notably Nigeria.