Rice is the world’s most important cereal crop. As a staple food for more than half of the world’s seven billion people, it is of crucial importance in providing food security for an exponentially growing population.....
Like other cereals, rice has been domesticated by humans multiple times independently. Yet, unlike other cereal species, these domestication events have origins on different continents—one in Asia, and one in Africa. Although the exact history of rice in Asia is still disputed, it is clear that Asian rice (Oryza sativa L.) was domesticated from a single wild species (Oryza rufipogon Griff.) approximately 9000 years ago . In contrast, African farmers domesticated rice from another progenitor, Oryza barthii A. Chev., approximately 3000 years ago. This event resulted in a species that is now recognized as Oryza glaberrima Steud.
Asian and African rice have distinct phenotypic characteristics: their grains differ in colour, size, shape and taste. Whereas Asian rice can be milled mechanically, facilitating large-scale production, African rice grains break easily and have to be milled manually with a mortar and pestle. These characteristics have favored the cultivation of Asian rice over African rice in large parts of the world. In Africa, O. glaberrima has largely been replaced by Asian rice, even though African rice is more resistant to abiotic stresses and is often preferred for its taste and its diversity in maturation time. In addition, African rice continues to survive in a ritual context, used in ritual offerings to honor the ancestors, rather than for consumption.
Globalization places these local cultural traditions and the neglected species associated with them under threat. In addition, food demand is rising in many African countries as a result of the growing population, a trend which is reflected in annual rice consumption. Yet, food security is increasingly under pressure from ongoing land use and climate change, limiting the availability of suitable crop land. Both processes have accelerated the shift in cultivation from local African varieties to the more productive Asian varieties. As a result, many traditional landraces of O. glaberrima are disappearing, or have already disappeared.
Even though Asian rice has higher yields, the diminishing genetic diversity of African rice may lead to the loss of other important agronomic traits (such as salt tolerance or blight resistance) that are not represented in O. sativa . Loss of these traits from the gene pool is irreversible and limits the capacity of this species to resist a changing climate—and that of breeders to produce more resilient varieties. An understanding of the evolution of O. glaberrima and its adaptation to different natural environments is therefore an important step in characterizing the agronomic potential of this species, the protection of which will be indispensable for sustaining genetic crop diversity and a food secure future.
Evolutionary origins
Two main competing hypotheses have been proposed concerning the domestication of rice in Africa. One proposes that plant domestication in Africa occurred in a non-centric (geographically diffuse) manner, over a protracted period of time , and has been called the ‘protracted transition model’. According to this hypothesis, rice was domesticated in multiple areas of domestication in West Africa, without a defined moment and centre of origin .
The other proposes a centric (geographically localized) domestication along the Niger River, followed by two secondary diversification events: one along the coast of what are now the countries of Senegal and Gambia, and one in the Guinea Highlands . This has been called the ‘rapid transition model’. According to a particular theory supporting the latter hypothesis, domestication was triggered at an acute time point when climate change started transforming forests into savannah around 4000 years ago .
The sudden drought meant that the increasing population could no longer rely on traditional forest products. However, the nature of hunter-gatherer and pastoral societies in West Africa calls into question whether a definite center of origin is likely ever to be found . Human migration may have assisted the exchange of particular rice varieties between ethnic groups, diminishing the differences between them. In addition, ongoing hybridization with O. barthii and later cultivation alongside O. sativa may have caused interspecific gene flow, which further complicates inferences about domestication origin.
In addition to the centric and non-centric model, an additional theory about crop domestication stipulates multiple (usually two) defined centres, as has been observed in both Asian rice and barley . Such a polycentric origin can explain the existence of two distinct, geographically separated sub-populations or even sub-species, like O. sativa ssp. indica (which originated in India) and O. sativa ssp. japonica (which originated in China). These subspecies of rice have separate origins, although later domestication stages saw extensive gene flow between the two, which has been associated with the transfer of domestication alleles .
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