Chinese Science Bulletin ©2007 色 Science in China Press 2 Springer-Verlag Cytological mechanisms of interspecific incrossability and hybrid sterility between Oryza sativa L.and O. alta Swallen FU XueLin,LU YongGen'.LIU XiangDong,Li JinQuan Feng JiuHuan Guangdong Provincial Key Laboratory of Plant Molecular Breeding,South China Agriculture University,Guangzhou 510642.China Oryza sativa and O.alta belong to AA and CCDD genomes in Oryza,respectively.Interspecific repro- ductive isolation limits the transfer of favorable genes from O.alta into O.sativa.The cytological mechanisms of interspecific incrossability and hybrid sterility between O.sativa and O.a/ta were studied systematically in this paper.We indentified two cytological causes of interspecific incrossabil- ity.First,we observed embryo sac incompatibility that caused fertilization barriers of variable severity such as non-fertilization,fertilization stagnation and egg cell single-fertilization.Second,we observed hybrid inviability,the major cause for incrossability,apparent from hybrid embryo developmental stagnation and embryo abortion.Hybrid sterility included both embryo sac sterility and pollen sterility. The hybrid embryo sac was completely sterile and exhibited mainly embryo sac degeneration.Hybrid pollen was also sterile and mainly typical abortive.Hybrid sterility was mainly caused by severely ab- normal meioses of megasporocytes and pollen mother cells;it is the most important abnormality,being chromosome sterility.Several methods are suggested to overcome the interspecific reproductive iso- lation between O.sativa and O.alta. Biology of plant reproduction 15