Polygenic nature of "Gametic-Development Genes" controlling intervarietal hybrid sterility in rrce. : Phylogenetic differentiation of the cultivated rice. XII

Abstract

1. The hybrid sterility between distant varieties of cult{vated rice can be explained by assurning differences in duplicate genes which work in the gamete as; development maintainers, i. e. Gametic Development genes. Segregafion of semisterile and fertile plants due to the G. D. genes is seen when two related varieties A and B, differing in fertility in the hybrids with another variety C, are crossed in the manner : (AXB)XC. However, the outcome of sorne of such crosses showed a discontinuous distribution in good pollen percentage, with several frequency peaks at certain points, such as 75%, 60-55%:, 45-40% and so on. In such cases, the mode of distrihution agreed well with that expected under the assurr)ption of five or more sets of G.D. genes between the varieties A and B. 2. Variation in good pollen percentage in the F2 of distant varieties usually appears to be continuous. An exceptional case was found, however, between two rather closely related varieties, where the variation in good pollen percentage appeared to be discontinuous and to have several frequency peaks. The distribution could be explained by assuming five set:s of G. D. genes independent of one another. Considering that differences in several sets of G.D, genes are thus found between vrieties which are not distantly related, it may be inferred that mucb more sets exist between distant varieties and behave like polygenes. Thus, the continuous variations usually seen in the F2 and later generations may be explained by assuming certain sets of G.D, genes. though other genes modifying fertility may also exist. It is possible that many dominant genes are needed for the development of the gametes, and since those genes were duplicated a long time ago, many sets of G.D. genes have been formed due to recessive changes in different loci. 3. As an approach to the biometrical genetical analysis, the variation in fertillty in hybrids was calculated theoretically. It was pointed out that the sterility due to G. D. genes vanishes rapidy with inbreeding generations and disappears almost entirely by F4.