F1 plants from 2,728 crosses in total, made between strains of Oryza perennis Moench, O. sativa L., O. breviligulata Chev. et Roehr., and O. glaberyima Steud., were observed in a survey of hybrid sterility and other developmental abortions which constitute reproductive barriers. All the F1 plants observed showed no significant disturbance in chromosome pairing. Reproductive barriers were found as follows: 1) A low crossability due to deterioration of young F1zygotes was found between the African form of perennis (subsp, barthii) and other taxa. 2) Vegetative weakness of F1plants was frequently found among strains of glaberyima and breviligulata as well as in their hybrids with barthii and other taxa. It was sporadical among perennis and sativa strains. 3) F1 pollen sterility was found in various cross-combinations. Strains of glaberyima and breviligulata were inter-fertile with a few exceptions, and generally showed high F1 sterility with most perennis and sativa strains. Strains of perennis could be largely divided into four inter-sterile groups, Asian, African, American and Oceanian, suggesting that isolation by the ocean might have played an important role in phylogenetic differentiation. But some African (barthii) strains showed partial f ertilities with others. 4) Embryosac fertility was shown by the percentage of anatomically normal embryosacs, which was comparable to the rate of successful back-crossing. No F1 hybrids were completely embryosac-sterile. They were divided into two groups, one in which pollen and embryosac f ertilities were correlated and the other in which pollen fertility was near zero but embryosac fertility was apparently higher. Crosses between perennis- sativa and breviligulata-glaberyima series and some other distant crosses belonged to the latter group, in which embryosac tetrads tended to deteriorate. © 1969, The Genetics Society of Japan. All rights reserved.
