The plant group dealt with in this paper comprises two cultivated species, Oryza sativa L, and 0. glaberrima Steud. and their respective wild relatives, 0. perennis Moench and 0. breviligulata Chev. et Roehr. On circumstantial evidence, sativa must have arisen from the Asian form of perennis (Oka and Chang 1962;Oka 1964a), and glaberrima from breviligulata independently of sativa (Morishima et al. 1963). 0. glaberrima and 0. breviligulata (Series Glaberrima) are endemic to West Africa, while 0. sativa and 0.

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The F1 plants between Oryza sativa L, and 0. glaberrima Steud. are highly pollen-sterile although the chromosomes normally pair in meiosis.They can be backcrossed as some embryosacs remain functional. Isogenic F1-sterile lines having the genetic background of sativa and glaberrima parents were isolated from B8F2 plants, respectively. They were self-fertile and showed semi-sterility in the F1 plants when crossed with the parental strains.But the F2 plants were fully fertile. This F1 sterility was most favorably explained by a "one locus sporogametophytic interaction" model of sterility genes which assumes that the sativa and glaberrima parents have S1aS1a S2S2 and S1S1 S2aS2a, respectively, and that if a S gene is present in the maternal tissue, gametes with Sa deteriorate.Then, the F1 plants having S/Sa are 50 percent sterile and produce S gametes only. Evidence for this hypothesis was that in an S1/S1a hybrid, a gene controlling apiculus coloration was closely linked with S1i then, the F2 produced colored plants only since the gametes carrying S1a and the colorless allele were eliminated.The presence of other genic systems was also suggested. For instance, a true-breeding partly sterile line with sativa background was obtained, which seemed to be homozygous for some complementary or duplicate recessive genes causing sporophytic sterility.Possibly, the F1 sterility is controlled by a complex of genic systems, and the backcrossing to derive isogenic F1-sterile lines in this work resulted in extraction of one of them.