Rice is the world’s foremost cereal crop as a human food source. Today’s cultivated varieties derive from the species Oryza sativa and Oryza glaberrina, domesticated respectively in Asia and West Africa. Besides these two domestic taxa, there are around 20 species of wild rice of the genus Oryza, all located in the tropics.
Their common ancestor appears to have emerged from Eurasia about 50 million years B.P. For several decades, scientists have been using molecular biology techniques to explore the evolutionary path which has led to the current diversity of wild rice species. A study published by an IRD research team has shed light on the origin of the settlement of wild rice on the American continent.
Three colonization theories for evolution of rice in the Neotropical region have been confronting each other for many years. The first one takes continental drift as the basis for explaining the current distribution of American rice in the tropical zone, considering that their ancestors already existed before Africa separated from South America. The second postulates the major role of natural seed dispersion across extremely long distances, with migrating birds as agents. The third links the arrival of rice in America to the great exploratory voyages and to the slave trade run by Europeans between West Africa and America.
Recent dating methods using the “molecular clock” were applied to the species present in the American tropical zone. This method, founded on the principle that genetic mutations accumulate in a genome at a rate globally proportional to the time that has elapsed, arrived at an estimated date for the appearance of American wild rice of 300 000 years. However, a relative genetic proximity for certain fractions of the genome of these cereals suggests a much more recent origin by the interaction of crossings between different species. To overcome the constraints imposed by molecular study of a hybrid species grouping together several genomes in one, the IRD team took advantage of available historical documentation. Investigation of the main European herbarium collections compiled by XVIIth and XIXth Century naturalist explorers who travelled around the American continent revealed that almost all the rice specimens listed during this period belong to the same hybrid species.
Moreover, contrary to what the first flora describe concerning a number of neighbouring species, the distribution area of American rice is in fact clearly delimited by the activity of Europeans. Examination of the historical literature also shows that the triangular trade that became established from the XVIth Century between Europe, West Africa and America was the decisive factor in the appearance of the first species of the genus Oryza on the American continent. At that time, seeds were first transported inadvertently in the holds of slave-ships. It was also established that sailors deliberately took with them seeds of a range of exotic plant species, including wild rice, on their many transoceanic voyages.
The hybrid rice first emerged among crops cultivated on the Caribbean islands, were then transported to the mainland by sea, mixed with stocks of cultivated rice seed. The presence of the hybrid rice species inland was for a long time limited to a perimeter around the first Brazilian gold mines that had attracted the colonists with their slaves. The location of this mine on the watershed between the River Amazon and the River Paraguay then enabled new rice species, born of crosses between wild rice of the Old World and domestic rice, to be conveyed by the current of the two rivers and gradually to invade a large area of South America.
The particularly vigorous American rice therefore seems to be the fruit of human migrations across the oceans. They appear to have the special feature of combining in the same genome the genetic characters of many different wild species of the Old World and rice that was already being grown there. This arrangement provided them with a number of qualities, including abundant seed production, ability to be pollinated by cultivated rice varieties, or high resistance to disease.
Confirmation of this theory at molecular level could make these Neotropical species potential candidates for development of a new type of domestic rice. In contrast to a hybrid rice currently being developed by China, the “super-hybrid” rice that could be derived from domestication of American species would offer the advantage that rice-growers could multiply it empirically, in the same way as for the traditional varieties they grow already.
Materials provided by Institut de Recherche pour le Développement, Paris (IRD). Note: Content may be edited for style and length.
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