35. Identifying potential sources of cytoplasmic male sterility in rice

More than 95% of the hybrid rice grown in China has been derived from 'wild abortive' (WA) male cytosterile lines (Yuan and Virmani 1988); the WA cytoplasm originated from Oryza sativa f. spontanea (Virmani and Shinjyo 1988). Diversification of the type of cytoplasmic male sterility (cms) available is essential to reduce genetic uniformity and resulting vulnerability of hybrids to pests and diseases.

We crossed 33 accessions of "O. perennis" and two accessions of O. rufipogon with two restorer lines (IR54R and IR64R using the wild species as the female parent. IR54R and IR64R restore fertility in cms lines possessing the WA cytoplasm. Hence, any accession that showed sterility of the hybrid and subsequent backcross progenies should possess a type of cms different from that of WA cytoplasm.

Fertile hybrids (those with pollen fertilities of 31% or higher) were obtained from the following accessions: "O. perennis" 100599, 100639, 100692, 104430, 104446, 104453, 104460, 104725, 104754, 104769A, 104781, 104805, 104833, 104870; O. rufipogon 103817. These accessions were not studied further.

Sterile hybrids (those with pollen fertilities of 30% or less) were obtained from 20 accessions (Table 1). The percentage of sterile plants varied with cross combination; the range was 55% to 100%. Hybrids of 100969 with both restorer lines were 100% sterile.

Selected sterile hybrids were backcrossed with the appropriate restorer line as the recurrent parent. Pollen fertility of the BC1 progenies was tested and progeny with pollen fertilities of 10% or less were backcrossed on a plant-to-plant basis.

The proportion of sterile BC1 progeny obtained in crosses of 15 accessions with either IR54R or IR64R ranged from 2.9% in 104800/IR54R to 43.670 in 100969/IR64R. Accessions 100969 and 103846 produced sterile BC1 progenies with both restorer lines.

In the BC2 generation, fertility was restored in crosses of accessions 103848 and 104778. Sterile BC2 progeny obtained in crosses with six accessions (100969,103846, 104491, 104764, 104859, and 104866) ranged from 1.2% in 103846/IR64R to 30% in 103846/IR54R.

Table 1. Pollen fertility of F1 and backcross progenies derived from crosses of O. rufipogon strains with O. sativa restorer lines IR54R and IR64R

*Pollen fertility classes: CS=completely sterile, 0% pollen fertility, S=sterile, 1-10% pollen fertility; PS=partially sterile, 11-30% pollen fertility; PF=partially fertile, 31-60% pollen fertility; F=fertile, 61-100% pollen fertility.

In most cross combinations, the backcross progenies segregated for sterility, with fertility being gradually restored. This implies that the sterility shown by the F1 hybrids is either under nuclear control, with backcross progeny segregating for factors conferring sterility, or under cytoplasmic control, with backcross progeny segregating for nuclear fertility restoring genes.

In crosses with six accessions, sterility was maintained until the second backcross. These accessions represent potential new sources of cms. Backcrossing of the sterile BC2 progeny will continue, in the hope that sterility will be maintained. These plants also will be crossed with selected maintainer lines to clarify the nature of the sterility.

Virmani, S. S. and C. Shinjyo, 1988. Current status of analysis and symbols for male sterile cytoplasms and fertility restoring genes. RGN 5: 9-15.

Yuan, L. P. and S. S. Virmani, 1988. Status of hybrid rice research and development, In IRRI (ed.), Hybrid Rice, pp. 7-24. IRRI, Manila.