47. Production of advanced backcross progenies and monosomic alien addition lines from O. sativa x O. ridleyi
  T. RAM, D. MAHAPATRA, J. RAMOS, R. MCNALLY and D.S. BRAR

International Rice Research Institute, Los Banos, Philippines

Oryza ridleyi (2n = 48), a tetraploid wild species, has several useful genes for resistance to bacterial blight, tungro, yellow stemborer and leaf folder. It is one of the most divergent species of Oryza and has been assigned the HHJJ genome (Aggarwal et al.,1997). Earlier, we produced only a few F1 plants from crosses of O. sativa with O. ridleyi (Brar et al.,1991). These F1's were completely male sterile, necrotic and had poor growth. We continued our efforts to produce backcross progenies by crossing F1 plants with rice cultivars IR24, IR29, IR56 and IR74 as recurrent parents. In general, crossability was low in F1, BC1 and BC2 generations (Table 1). As many as 9,648 florets of one F1 hybrid (IR24 x O. ridleyi) were pollinated with IR24 pollen. However 99.8% of seeds were without embryo (Table 1). After culturing four embryos, one germinated but died at the two-leaf stage. Similarly in BC1 (IR29 x O. ridleyi)/IR29, 3,782 pollinated florets set 1,894 seeds. Out of these seeds, only two had an embryo, but none could germinate after embryo rescue. In BC1, from (IR74 x O. ridleyi)/IR74, 13,935 florets were pollinated with the recurrent parent, and 4,888 seeds were obtained, of which 31 (0.6%) had embryos. After embryo rescue, only 14 embryos germinated and nine plants developeed. All the BC1F1 plants of IR74 x O. ridleyi are necrotic and bushy.

To produce BC1 from the F1 of IR56 x O. ridleyi, 8,386 florets were pollinated with IR56. We obtained 3,798 seed of which 28 had an embryo (0.7%). Thirteen BC1F1 plants could be produced. In BC2 (IR56 x O. ridleyi)// IR56, 99.6% of seeds were without embryo. Out of 158 BC2 embryos cultured, 83 germinated and finally 72 plants were produced. Even in BC3, the crossability was very low (0.4%); out of 33,250 spikelets pollinated only 132 plants could be produced.

The low crossability followed by seedling death at the two-leaf stage was common in both BC1 and BC2 generations. Majority of the BC1 and BC2 seeds were without embryo (99%) and had watery fluid (Table 1). Low crossability among O. sativa and O. ridleyi and necrotic nature of the seedlings were found to be the major barriers to produce advanced backcross progenies (Table1).

The chromosome number of F1 plants (IR56 x O. ridleyi) was 2n = 36. It ranged from 42

to 48 in BC1 plants and from 32 to 42 in BC2 plants. In BC2 only 4 of the disomic plants (2n = 24) obtained were fertile. All other BC1 and BC2 plants were sterile. In BC3, out of 133 plants, 76 were disomics (2n = 24) and 57 had 2n = 25. These putative monosomic alien addition lines, MAAL (2n = 25) are being characterized into different groups based on the plant morphology and through molecular marker analysis. The fertile lines with stable chrosome number would be evaluated for introgression of useful genes from O. ridleyi into rice.

Acknowledgment

The financial support from the Rockefeller Foundation is gratefully acknowledged.

References

Aggarwal, R.K., D.S. Brar and G.S. Khush, 1997. Two new genomes in the Oryza complex identified on the basis of molecular divergence analysis using total genomic DNA hybridization. Mol. Gen. Genet. 254: 1-12.

Brar, D.S., R. Elloran and G.S. Khush. 1991. Interspecific hybrids produced through embryo rescue between cultivated and eight wild species of rice. Rice Genet. Newsl. 8: 91-93.