Gene flow from cultivated rice (Oryza sativa L.) to wild Oryza species (O-rufipogon Griff. and O-nivara Sharma and Shastry) on the Vientiane plain of Laos

Although Asian cultivated rice (Oryza sativa) and its annual (O. nivara) and perennial (O. rufipogon) wild relatives can freely exchange genes under experimental conditions, little is known about gene flow from the cultivated to these two wild species in natural habitats. The area of study was the Vientiane plain of Laos, at the center of the glutinous rice zone. The objectives of this study were (1) to determine the extent of hybridization between rice and its close wild relatives over a large area on the basis of the O. sativa specific traits glutinous endosperm, colorless apiculus and white pericarp; (2) to evaluate the genetic variation in the hybrid plants based on 13 quantitative traits using principal component analysis. Out of 1576 individual plant samples analyzed two hybrid groups were recognized. One hybrid group consisted of O. rufipogon-O. sativa hybrids and represented 10% of wild individual plants analyzed. The other hybrid group was O. nivara-O. sativa hybrids that represented 14% of wild individuals analyzed. These hybrid groups are distinct based on principal component analysis - this is due to both spatial isolation (> 100 m) and reproductive isolation (i.e. differential flowering time). The results indicate that two kinds of gene flow, between (a) O. sativa and O. rufipogon and (b) O. sativa and O. nivara, are occurring throughout the Vientiane plain of Laos.