1. Eurasian versus Gondwanian origin of the genus Oryza

The section Eu-Oryza of the genus Oryza comprises two species complexes distributed in the tropics throughout the world. It is assumed by some workers (Stebbins l971; Chang l984) that their distribution could have resulted from the breakdown of the Gondwanian supercontinent.

However, isozyme studies of a world collection of Oryza species have demonstrated that the New World species were very closely related to their Old World relatives in Asia (Second l985a,b). According to the genetic distances calculated between Eu-Oryza species and the ecological differentiation of all Oryza species, the Eurasian origin of this genus can be assumed. Most forest- adapted species are found in Asia. Others may be assumed to have been introduced relatively; recently, or may have originated recently through allotetraploidization in Africa,America and New Guinea. Migration to Australia and Africa could have occurred during the Miocene Epoch by land during an arid environment, but to America in a recent past. The origin of the ancestor of the genus Oryza on the African plate appears, however, to be plausible(Second l985c).

Chang, T.T., 1984. Conservation of rice genetic resources: Luxury or necessity? Science 224:251-256

Second, G., 1985a. A new insight into the genome differentiation in Oryza L. through isozymic studies. In A.K. Sharma and Archana Sharma (eds.), Advances in Chromosomes and Cell Genetics, p. 45-78. Oxford & IBH Publ., New Delhi.

Second, G, 1985b. Evolutionary relationships in the Sativa group of Oryza based on isozyme data. Genetique, Selection, Evolution 17(1):89-114.

Second,G., 1985c. Relations evolutives chez le genre Oryza et processus de domestication des riz. Etudes et Theses, ORSTOM, Paris, 180pp.

Stebbins, G.L., 1971. Chromosomal evolution in higher plants. Edward Arnold, London, 215 pp.