5. Crossability of wild Oryza species and their potential use for improvement of cultivated rice

L. A. SITCH, R. D. DALMACIO and G. 0. ROMERO

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.IR31917-45-3-2
ing 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.