Plant Breeding Dept., International Rice Research Institute, P.O. Box. 933, Manila, Philippines
The wild Oryza species and related genera are an important source of desirable traits for cultivated rice (0. sativa L.), such as resistance to pests and diseases and tolerance to soil stresses (Table 1). The transfer of genes from wild to cultivated rice can be achieved through hybridization, embryo rescue, backcrossing, and selection processes (Jena and Khush 1986; Khush 1977). Since 1986, IRRI has crossed various rice cultivars with nine wild species, using the cultivar as the female parent. Table 2 shows the seed set and embryo rescue data obtained from these crosses.
Seed set of crosses with AA genome species ranged from 9% to 73%, depend-
Table 1. Wild Oryza species with traits of economic importance found in some of their strains _______________________________________________________________ Species Genome Useful traita _______________________________________________________________ 0. rufipogon AA Tolerance to acid sulphate soils and stag- nant flooding elongation ability;source of CMS 0. rufipogon, AA Resistance to grassy stunt virus and blast ("0. nivara") 0. barthii AgAg Resistance to bacterial blight 0.longistaminata A1A1 Floral characteristics for outcrossing 0.eichingeri CC Resistance to BPH, GLH, and WBPH 0.officinalis CC Resistance to BPH, GLH, and WBPH 0. minuta BBCC Resistance to BPH,GLH,WBPH,blast,and bacterial blight 0.australiensis EE Resistance to BPH and drought 0.brachyantha FF Resistance to stemborer and rice whorl maggot 0.ridleyi Resistance to rice whorl maggot,blast,and bacterial blight 0.longiglumis Resistance to blast and bacterial blight 0.glaberrima AgAg Resistance to GLH (cultivated) ________________________________________________________________ a. BPH: brown planthopper, GLH: green leafhopper, WBPH: white-backed plant hopper, CMS: cytoplasmic male sterility Table 2.Seed set and embryo rescue data obtained from crosses of 0. sativa L. with various wild Oryza species and 0.glaberrima _______________________________________________________________________________ 0.sativa Wild species #of Total #of Seed Embryos Germi. #of cultivar and genom male spikelets F1 set cultured nation hybrids (female) (male) strains pollinated seeds % (no.) % obtained obtained _______________________________________________________________________________ IR64 0. rufipogon AA 5 1,316 303 23.0 - "nivara" IR36 " 5 442 163 36.9 - IR54 0. rufipogon AA 5 613 244 39.8 - (inc. "perennis") IR64 5 746 315 42.2 - IR31917a 0.minuta BBCC 4 3,412 125 3.7 40 50 18 IR36 0. latifolia CCDD 4 1,551 52 3.4 46 69.6 32 IR64 " 4 1,671 119 7.1 93 74.2 69 IR64 0.grandiglumis CCDD 1 329 22 6.7 13 53.8 7 IR56 0. brachyantha FF 4 655 2 0.31 2 50 1 IR64 " 4 1,473 5 0.34 5 80 4 IR42 " 3 522 0 0 IR66 " 3 543 3 0.55 3 33.3 1 IR31917a " 2 1,945 1 0.05 1 0 0 IR54 " 1 690 2 0.03 2 50 1 IR36 " 1 184 2 1.1 2 50 1 IR31917a 0. ridleyi 3 2,842 206 7.2 49 28.6 14 IR36 " 3 509 1 0.2 1 0 0 IR64 " 3 452 35 7.7 1 0 0 IR56 " 2 264 0 0 IR66 " 1 87 1 1.1 1 0 0 IR56 O.glaberrima AgAg 2 335 190 56.7 - (cultivated) ____________________________________________________________________________ a.IR31917-45-3-2ing on the rice cultivars and wild species accessions used. For example, IR64 crossed with five 0. nivara accessions showed limited variation in seed set (13.9% 28.9%); crosses with IR36 gave wide variation (9.1%-62.2%). Similarly, crosses of IR54 with 0. rufipogon IRGC Acc. 100907 gave lower seed sets (32.4%) than that with Acc. 103817 (73.0%). In crosses with IR64, Acc. 100907 gave the highest seed set (53.4%). A similar range in seed set was obtained with all AA genome species, suggesting no difference in crossability among species. These intragenomic crosses produced well-developed seeds; consequently, embryo rescue was not necessary.
The intergenomic crosses were less successful, although hybrid seed was obtained with all species. Seed sets were low (0%-26.3%) and commonly less than 10%. Individual accessions of some species differed noticeably in crossability. For example, O. latifolia (CCDD genome) Acc. 100963 and Acc. 100966 gave good seed sets (4.3%-19.7%) with both IR36 and IR64, while Acc. 100964 and Acc. 100965 gave few seeds (0.1%) with only IR36. Similarly, crosses with 0. ridleyi (genome not named) Acc. 100877 consistently gave higher seed sets than with Acc. 100821 and Acc. 101453. However, uniformly low seed sets were obtained from crosses with 0. minuta (BBCC) (0-4.2%) and 0. brachyantha (FF) (0-1.15), regardless of the accession used.
In most intergenomic crosses, the hybrid seed degenerated 10 to 14 days after pollination. Embryos were rescued from 14-day-old seeds, using the technique described by Jena and Khush (1984). However, in crosses with 0. brachyantha, 14-day-old embryos showed poor viability; enibryos were successfully rescued from 8 to 10-day-old seeds. In most hybrids, 50%, or more of the embryos cultured germinated and produced hybrid plants. However, 0. sativa/O. ridleyi embryos germinated particularly poorly. Hybrid plants were obtained from only IR31917-45-3-2/Acc. 100821 embryos.
Successful gene transfer from the AA genome species is faciliated by their high crossability and good hybrid seed development. Such species should receive priority as donors of genes in wide hybridization activities. The hybrids obtained from crosses with 0. rufipogon (including so-called 0. nivara and 0. perennis) are being used to increase the yield potential of cultivated rice. 0. rufipogon is also being used as a source of internode elongation ability. Although intergenomic transfers are more time-consuming, they can be successful if simple tissue culture facilities are available that allow embryo rescue to be performed. At IRRI, back- crossing of the 0. minuta hybrids is underway and blast and bacterial blight resistant progeny have been identified. 0. brachyantha-derived backcross progeny are being screened for stemborer resistance.
References
Khush, G. S., 1977. Disease and insect resistance in rice. Adv. Agron. 29: 265-341.
Jena, K. K. and G. S. Khush, 1984. Embryo rescue of interspecific hybrids and its scope in rice improvement. RGN 1: 133-134.
____and ____, 1986. Production of monosomic alien addition lines of 0. sativa havinga single chromosome of 0. officinalis. In IRRI (ed.), Rice Genetics, p. 199-208. IRRI, Manila.