Abstract
Weakness of F(1) plants is frequently found in hybrids between strains of Oryza breviligulata (wild) and O. glaberrima (cultivated rice) endemic to West Africa. A set of two complementary dominant weakness genes, W(1) and W(2), was found to control the observed F(1) weakness. Many breviligulata strains had W(1), while most of the glaberrima and semi-wild strains had W(2) or were free of both. In the weak F(1) plants, tissue differentiation in adventitious roots seemed to be disturbed. Modifier genes affecting the expression of the weakness genes appear to be present also.