An international study, coordinated by researchers from IRD-France, and involving the CEA and the Africa Rice Centre (AfricaRice), has identified the geographic origin of African rice domestication. By sequencing over 246 African wild and cultivated rice genomes, the researchers have shown that this plant was domesticated 3,000 years ago in the Inner Niger Delta in northern Mali.
These results, published on July 5th 2018 in the journal Current Biology , also demonstrate how past climatic changes led to profound societal transformations, notably the adoption of agriculture.
The third most produced cereal in the world after wheat and corn, rice is the main staple in the diets of nearly half the world’s population. A member of the grass family, it is grown for its starch-rich seed.
Humans cultivate two main species of this plant: African rice (Oryza glaberrima) and Asian rice (Oryza sativa). These species diverged genetically around one million years ago, well before their domestication. Domestication of the two species occurred independently in Asia and Africa over the last 10,000 years. Before the present discovery, scientific hypotheses pointed to West Africa1 as the geographic origin of African rice domestication. However, the precise area and the circumstances leading up to domestication remained hazy.
Under the aegis of France Génomique’s IRIGIN programme, researchers from the IRD, the CEA/Genoscope and AfricaRice studied the full sequences of 246 African rice genomes, 163 domestic varieties and 83 wild varieties, harvested in the Sahel and East Africa. In doing so, the researchers generated the largest genomic database for African rice available to date and analysed the genetic diversity of the cultivated.
Using this exceptional data, the scientists identified the origin of African rice domestication as the Inner Niger Delta in northern Mali, over 3,000 years ago. This discovery coincides with that of archaeological traces of rice domestication in the same area.
Furthermore, the researchers have suggested that the aridification of the Sahara may be behind this domestication. Wild African rice populations, harvested by inhabitants of the Sahara at this time, likely decreased dramatically as the Sahara dried up.
The progressive disappearance of these resources may have led to the cultivated form of the plant and to the growing development of agriculture over 2,000 years ago.
‘Past changes to the climate are thought to have led to the emergence of African agricultural civilizations’, said Yves Vigouroux and François Sabot, the IRD researchers who coordinated the study.
Analysing genetic data made it possible for researchers to document the historic evolution of wild and cultivated rice species. The researchers were able to point to the precise moment of the decline of African rice cultivation in the 16th century, following a period in which it had expanded significantly over 2,000 year ago. The decline corresponds to the introduction of Asian rice in West Africa by the Portuguese. Numerous varieties of Asian rice, with higher yields, were introduced and used by growers between 1870 and 1960.
This study shows for the first time that past environmental changes led to societal changes in practices (domestication) and organization of agricultural systems. It also opens up avenues for reflection on the evolution of agriculture in the years to come.
African rice adapted long ago to the Sahelo-Saharan climates, which could be an advantage for future global rice cultivation in the current context of a warming climate (decreased rainfall, increased temperatures, shortened rainy season) and the increasing world population.
The cultivation of species less dependent on irrigation, such as African rice, which is more resistant to hydric stress and higher temperatures, could be an advantage for African and global agriculture.
Scientists are continuing genetic research to understand the genetic basis of African rice adaption to harsh conditions, to arid climates, and its resistance to pathogens to improve current varieties (through selection of promising genes and hybridization, for example). African Rice data collection created by AfricaRice, composed of 349 different varieties, represents an optimal genetic resource to conduct this research.
1. Along the Niger River, which borders 6 states : Sierra Leone, Guinée, Mali, Niger, Bénin and Nigéria.
Philippe Cubry et al. The rise and fall of African rice cultivation revealed by analysis of 246 new genomes, Current Biology , July 5th 2018.
The study was made possible by support from the French National Research Agency (ANR), by France Génomique’s IRIGIN programme, by Investissements d’avenir, and by the ARCAD programme (Agropolis Resource Centre for Crop Conservation, Adaptation and Diversity).