Phyogenetic differentiation of cultivated rice. XXI. The sporophytic pollen sterility: its genetic basis and intervarietal relationships as shown by F2 sterility.

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Abstract
The partial pollen sterility of true-breeding lines derived from a varietal hybrid of rice (Oryza sativa L.) was attributable to duplicate genes causing sporophytic sterility in certain homozygous combinations, in the manner previously described (Oka and Doida) for seed sterility. The sporophytic pollen sterility was characterized by an instability of pollen development resulting in a large variance of pollen fertility among spikelets of the same plant. To examine the distributions of gametophytic and sporophytic sterilities in varietal hybrids, the F1 and F2 plants from crosses of 38 rice strains with 3 test-strains (an Indica, a Japonica, and an Asian annual strain of O. perennis) were observed for pollen fertility. The cytoplasmic male sterility did not seem to be involved in the material. The magnitude of sporophytic F2 sterility was estimated by subtracting the amount of gametophytic F1 sterility possibly transmitted to the F2 population from the mean F2 sterility. Both the gametophytic and sporophytic sterilities of hybrids were involved in the Indica-Japonica differentiation of rice varieties, but they were not correlated among individual crosses. The crosses with the test-strain of O. perennis showed little sterility in both the F1 and F2 generations. This suggests that the wild progenitor-the Asian form of O. perennis would have dominant genes at many of duplicated loci and recessive mutations accumulated with domestication might have resulted in the differentiation of varietal groups like the Indica and Japonica types.