Phylogenetic Differentiation of Cultivated Rice, XX Analysis of the Genetic Basis of Hybrid Breakdown in Rice

Abstract

Distantly related varieties of cultivated rice (Oryza sativa L.) often show partial sterility in their F1 hybrids.The senior writer formerly demonstrated that this sterility could be accounted for by gametic-development genes, i.e., duplicate genes whose double recessive combinations interrupt the development of gametes carrying them (Oka 1953(Oka , 1957b. This hypothesis could plausibly explain segregation in certain ratios of fertile and partially sterile plants in experimental crosses, modification of F2 ratios for various characters and other phenomena characteristic of the hybrid sterility in cultivated rice.However, on its basis, sterility should occur only in heterozygotes for the gametic development genes. Thus, a fertile plant should produce only fertile plants in the progeny, while a semi-sterile plants should produce fertile plants and semi-sterile ones whose fertility is not lower than that of the parent, and a true-breeding semisterile line could not be established.Actually, partially sterile plants have often been found in the F2 and later generations derived from a highly fertile F1 hybrid, and lines true-breeding for sterility appeared in the progeny of such partially sterile plants.The present study showed that besides the gametic-development genes, there is another genic system responsible for the sterility in hybrid derivatives. Materials and MethodsAs mentioned in a previous paper of this series (Oka 1954), many rice strains from various Asian countries were crossed with a certain set of test-strains in order to survey the pattern of hybrid sterility relationships.Using a part of those crosses, fertility variations in F2 and later generations were observed. The parental strains, which were established after repeated self-pollination from a single plant, could be regarded as virtually pure, and were completely self-fertile. The partially sterile lines used for the present work were obtained from the above-mentioned crosses. Detailed accounts for particular strains and methods of analysis of the data will be given together with the results.

Effects of a Major Gene Ur1 Characterized by Undulation of Rachis Branches on Yield and its Related Traits in Rice.

“…However, it is not easy to introduce genes with increasing effects on spikelet number per panicle from an indica variety into Japanese japonica varieties, since the hybrid sterility and hybrid lethality between indica and japonica cause reproductive isolation (Kato et al 1928, Terao and Midushima 1939, Oka 1954and 1957, Oka and Doida 1962. So, if a major gene known to increase spikelet number per panicle can enlarge sink size per unit area and is contained in a japonica variety or line, we could utilize it for developing high yielding japonica varieties.…”

The hybrid breakdown 1(t) locus induces interspecific hybrid breakdown between rice Oryza sativa cv. Koshihikari and its wild relative O. nivara

“…Many reports are available on the genetic analysis of hybrid breakdown in rice (Oka 1957, 1978, Oka and Doida 1962, Yokoo 1984, Sato and Morishima 1988, Wu et al 1995, Li et al 1997, Kubo and Yoshimura 2002, Fukuoka et al 2005, Yamamoto et al 2007, Matsubara et al 2007a, 2007b. These studies demonstrated that hybrid breakdown is induced by the interaction of two or three genes derived from each parent.…”

Nonrandom association of genes and characters found in indica × japonica hybrids of rice

“…This indicates that the associations found among the varietal sample were partly caused by natural or artificial selections at the zygotic stage. Random drift may not be a causal factor because indica-japonica differentiation was also obseved in African cultivers of 0. sativa that have been established apart from Asian cultivers for a long time (de Kochko, 1987 In indica x japonica crosses, partial sterility of hybrids is frequently observed (e.g., Oka, 1953Oka, , 1974Oka and Doida, 1962;Yokoo, 1984;Ikehashi and Araki, 1986) and results in gametic selection. Competition between pollen grains during pollen tube elongation which is caused by particular genes also results in gametic selection.…”