Small mitochondrial DNA molecules of wild abortive cytoplasm in rice are not necessarily associated with CMS

A nuclear gene modifying instability of fertility restoration in cytoplasmic male sterile rice

The present study was carried out to examine the genetic mechanism responsible for reversions to fertile phenotype detected in cytoplasmic male-sterile plants of rice. Thecms-bocytoplasm of Chinsurah boro II gave rise to male-sterility in plants without a gametophytic restorer gene (Rf1). Taichung 65 (T65A) was known to be the maintainer which carries no restorer; however, Taichun 65 preserved in our laboratory (T65B) showed partial fertility (about 8% seed set) when crossed with the male-sterile plants. Unexpectedly, the seed fertility gradually increased with repeated selfings and almost fully fertile plants were obtained in the F6generation. The cytoplasmic substitution lines revealed that reversions to fertile phenotype resulted from mutational events at the nuclear level. The genetic experiments indicated that the partial fertility observed in the F1hybrid was controlled by a dominant gene,Ifr1, which was carried by T65B. The results obtained suggested thatIfr1itself was associated with instability of fertility restoration in the presence ofcms-bocytoplasm since partially fertile plants carryingIfr1always showed a tendency for gradual increase in fertility in the later generations. The results are also discussed in relation to a rapid genetic change through intensified gametic selection combined with instability.

RFLP analysis of nuclear DNAs homologous with mitochondrial plasmid-like DNAs in cultivated rice

B1 and B2 are small, circular, mitochondrial plasmid-like DNAs found in male-sterile cytoplasm (cms-Bo) of rice. In this study, nuclear sequences homologous to these DNAs were investigated among a number of rice cultivars. Several copies of nuclear B1-and B2-homologous sequences were detected in all examined cultivars, regardless of the presence or absence of the B1 and B2 DNAs in mitochondria, indicating that the existence of the B1- and B2-homologous sequences in the rice nuclear genome was widespread. A restriction fragment length polymorphism (RFLP) was detected for both sequences, and we propose that these DNAs could be useful RFLP markers for the rice nuclear genome. To analyze these nuclear homologues genetically, segregation analysis of the RFLP was carried out in the F2 progenies of an Indica-Japonica rice hybrid. Of the B1 homologues, there were two nonallelic fragments, one specific to the Indica parent and the other to the Japonica. These results indicate that the B1 and B2 homologues were dispersed in the nuclear genome. The integration of B1-homologous DNA into the nuclear DNA may have occurred independently after sexual isolation of the Indica and Japonica rice varietal groups, or a intranuclear transposition of these sequences took place during the process of rice differentiation into the varietal groups.

Stability of mitochondrial DNA in tissue-cultured cells of rice

Restriction analysis of mitochondrial (mt) DNA from 3-month-old callus cultures of the cytoplasmic male sterile rice, V41A, which contains S2 or "wild abortive" cytoplasm, and its fertile maintainer, V41B, showed the same BamHI restriction profiles as mtDNA from the corresponding leaf material. Similarly, mtDNA of rice (var. Taipei 309) from leaves, a 2-month-old cell suspension (T3MS2/A), a totipotent suspension (T3MS) and a 19-month-old suspension, which had lost its protoplast regeneration ability (LB3), showed indistinguishable BamHI restriction profiles. However, clear differences in mtDNA restriction profiles were observed between LB3 and a 30-month-old suspension culture of Taipei 309 (LB1), which appeared to reflect substantial changes in the relative abundance of specific DNA sequences. Hybridisation of a maizecoxII gene probe to blots of restricted mtDNA confirmed that, while the relative abundance of certain mtDNA sequences was preserved during long-term tissue culture of rice, major changes in abundance were observed with other sequences.