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Summary
An early cradle of agriculture existed around West Africa's Niger River Basin, a flurry of plant genomic studies is showing. Several of the continent's traditional food crops got their start there: a cereal called pearl millet and Africa's own version of rice. Now, a report out this week in Science Advances adds yams to the list of African crops domesticated thousands of years ago in the same region. A drying climate may have spurred the move to farming. The recent findings highlight reservoirs of genes in wild plants that could be exploited to boost the productivity and disease resistance of the domesticated varieties.
Yam genomics supports West Africa as a major cradle of crop domestication
Abstract
While there has been progress in our understanding of the origin and history of agriculture in sub-Saharan Africa, a unified perspective is still lacking on where and how major crops were domesticated in the region. Here, we investigated the domestication of African yam (Dioscorea rotundata), a key crop in early African agriculture. Using whole-genome resequencing and statistical models, we show that cultivated yam was domesticated from a forest species. We infer that the expansion of African yam agriculture started in the Niger River basin. This result, alongside with the origins of African rice and pearl millet, supports the hypothesis that the vicinity of the Niger River was a major cradle of African agriculture.
INTRODUCTION
The emergence of agricultural societies was associated with hotspots of plant domestication (1), often described as domestication centers (2). One of the best known hotspots is the Fertile Crescent in the Middle East, where wheat, barley, oat, lentil, and chickpea, among others, first appeared in the archaeological records (3). The history of crop domestication is much less documented in sub-Saharan Africa, probably because archaeological studies are largely fragmentary (4). One hypothesis about crop domestication in Africa suggests an origin encompassing a large area from Senegal to Somalia (2). This Sahel-wide hypothesis was mainly based on distributions of wild and cultivated African cereals, such as pearl millet (Cenchrus americanus), sorghum (Sorghum bicolor), fonio (Digitaria exilis), and African rice (Oryza glaberrima). Recent studies have challenged this hypothesis and proposed a more restricted area of origin in the western Sahel, near the Niger River basin. Pearl millet was domesticated in a region corresponding today to northern Mali and Mauritania (5), and African rice was also domesticated in Mali (6). To assess whether the vicinity of the Niger River basin could be identified as a major hotspot of domestication, we investigated the domestication of yam, another major staple crop originating from Africa.
Yams (Dioscorea spp.) were domesticated independently at least three times in three different continents: in Asia (Dioscorea alata), in America (Dioscorea trifida), and in Africa (Dioscorea rotundata) (2). In Africa, yam starchy tubers are mainly produced in the "yam belt," a region including the Republic of Côte d'Ivoire, Ghana, Togo, Benin, Nigeria, and Cameroon (7), which accounts for 97% of African yam production (www.fao.org/faostat). Yam production in West Africa is second only to that of cassava, surpassing that of maize, rice, and sorghum. It is therefore a key crop for African food security. The main cultivated yam, D. rotundata, has two close wild relatives (8), the savannah species Dioscorea abyssinica and the forest species Dioscorea praehensilis. Domesticated yam is likely derived from one of these two species (9) or from hybridization between them (7).
In this study, we used whole-genome resequencing of 167 wild and cultivated yams to clarify where and from which species yam was domesticated. Our findings in combination with recent results on pearl millet and African rice (5, 6) suggest that the Niger River vicinity played a major role in the domestication of African crops...…………………
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