The direction of pollen flow between two co-occurring rice species, Oryza sativa and O. glaberrima

Abstract

Two cultivated rice species, Oryza sativa L. and O. glaberrima Steud., are often grown mixed in farmers' fields in West Africa. Two sympatric species were collected from two different fields. The frequency of interspecific hybridization between the two species was estimated from the seeds harvested in farmers' fields and in artificial mixtures. The two species were isolated by prezygotic isolating barriers such as autogamy and differences in time of flowering, however, the isolation was incomplete. The frequency of hybridization varied from 0·8 to 4·5 per cent. In addition, all 87 hybrids were found from the seeds which O. glaberrima produced. This shows that pollen flow occurs from O. sativa to O. glaberrima when they hybridize. Generally, O. glaberrima had a higher per cent of exserted stigmas as compared with O. sativa. The direction of pollen flow appeared to be explained by a higher efficiency of pollen capture in O. glaberrima. The genetic consequences were also discussed in relation to gene flow between two distantly related species.