GENETIC STUDIES OF SPECIATION IN CULTIVATED RICE

1. GENIC ANALYSIS FOR THE F₁ STERILITY BETWEEN O. SATIVA L. AND O. GLABERRIMA STEUD

Abstract

The F₁ plants between Oryza sativa L. and O. 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 sporo-gametophytic interaction” model of sterility genes which assumes that the sativa and glaberrima parents have S₁^aS₁^a S₂S₂ and S₁S₁ S₂^aS₂^a, respectively, and that if a S gene is present in the maternal tissue, gametes with S^a deteriorate. Then, the F₁ plants having S/S^a are 50 percent sterile and produce S gametes only. Evidence for this hypothesis was that in an S₁/S₁^a hybrid, a gene controlling apiculus coloration was closely linked with S₁; then, the F₂ produced colored plants only since the gametes carrying S₁^a 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.