23. Causes of incomplete fertility-restoring ability of some established restorer lines of 'WA' cytosterility system in rice

K. GOVINDA RAJ and S. S. VIRMANI

International Rice Research Institute, P. O. Box 933, Manila, Philippines

In a study on genetics of fertility restoration in rice made at IRRI, we concluded that the fertility restoring ability of five established restorers, viz., IR26, IR36, IR54, IR9761 and IR2797 in crosses with'WA'cms lines, V20A and IR46828A was controlled by two independent and dominant genes. The mode of action of the restorer genes was different in different restorers (Govinda Raj and Virmani 1988). However, the segregation pattern of crosses with the same restorers and the cms line IR17492-18-10-2-2-2A (herewith designated as IR17492A) did not conform to any of the established ratios (Table 1). We suspected the role of inhibitory gene(s) and the confounding effect of intervarietal F1 hybrid sterility on fertility restoration of the crosses involving IR17492A. The following evidence was gathered in support of our hypotheses.

Hypothesis 1. Role of inhibitory gene(s) on fertility restoration: The partial fertility of the F1's and the appearance of fully fertile segregants in BC and F2 generations of all the five cross combinations involving IR17492A indicated the role of fertility inhibitor genes in the cms line IR17492A. This role was further probed by studying the cross IR17492A/IR54. We studied F3 families from 15 randomly selected fertile F2 plants. Two of the 15 F3 family progenies (5 and 6) bred true for spikelet fertility (Table 2). It appears that genes for fertility inhibition segregated in F2 and their absence in some of the F2 genotypes resulted in normal spikelet fertility of the corresponding F3 families possessing 'WA' cytoplasm.

We randomly selected ten completely sterile plants in the F2 generation of the cross IR17492A/IR54 and testcrossed these sterile plants to IR54. The testcross F1 progenies were scored for pollen and spikelet fertility. We observed a high proportion of fully fertile plants in all the testcross F1 progenies (Table 3). This could again be due to the segregation of fertility inhibition genes present in the female parent IR17492-18A. The male-sterile F2 plants giving normal fertility in the testcross F1 obviously possessed no inhibitory gene for restoration ability.

Hypothesis 2. Role of intervarietal hybrid sterility: This hypothesis assumes that the hybrid between the B (maintainer) line of the cms line exhibiting partial fertility in crosses with the established restorers should express some degree of F1 sterility. The crosses IR17492B/IR54 and IR17492B/IR26 showed more or less normal spikelet fertility in F1 (Table 4) although combinations involving IR36, IR9761 and IR2797 exhibited partial sterility to partial fertility. These results indicated that probably in crosses with IR54 and IR26, intervarietal hybrid sterility did not play a role. Inhibitor genes present in the cms line IR17492A alone were perhaps responsible for the partial fertility in F1. While in other combinations involving IR36, IR9761 and IR2797, intervarietal F1 sterility genes in addition to fertility inhibitor genes, contributed to the decreased spikelet fertility in F1.

These results indicated that cms line IR17492A possessed genetic factors for fertility inhibition. Genes that condition intervarietal F1 sterility might also be contributing to the differential fertility-restoring ability of some restorers analyzed in the present study.

Table 1. Segregation pattern for fertility restoration reaction of the crosses involving cms line IR17492A carrying 'WA' type cytoplasm and five elite varieties/breeding lines

________________________________________________________________
                       No. of plants 
                       with pollen             No. of plants
Generation  Total no.  spikelet fertility       with spikelet
            of plants  reactiona           fertility reactionb  
            scored   _____________________ _____________________
                       F   PF  PS   CS       F   PF   PS   CS
________________________________________________________________
IR17492A/
IR26 F1        12      -   12   -   -        -   5    7    -
IR17492A/IR26
//IRI7492B     96     11   18  27  40        8  11   21   56
IR17492A/
IR26 F2       305    142   62  38  63      153  51   27   74
________________________________________________________________
IR17492A/
IR36 F1        12      -    -  12   -        -   -   12    -
IRI7492A/IR36
//IR17492B    105      4   10  48  43        3   6   22   74
IR17492A/
IR36 F2       177     52   65  54   6       59  48   59   11
________________________________________________________________
IR17492A/
IR54 F1        12      -    2  10   -        -   2   10    -
IR17492A/IR54
//IRl7492B     82      6   14  44  18       77  12   25   38
IR17492A/
IR54 F2       293     86   99  89  19      103  88   66   36
________________________________________________________________
IRl7492A/
IR9761 F1      12      -    -  12   -        -   -   12    -
IR17492A/IR97
61//IRI7492B   95     17   18  50  10       14  18   33   30
IR17492A
/IR9761 F2    294     55   99 133   7       76  94  111   13
________________________________________________________________
IR17492A/
IR2797 F1      11      -    3   8   -        -   3    8    -
IR17492A/IR27
97//IRI7492B   94      4   16  50  24        8  10   28   48
IR17492A/
IR2797 F2     305     84   94 108  19      122  87   68   28
________________________________________________________________
aF (fertile) = 60-100% pollen fertility; PF (partially fertile) 30-60%; PS (partially sterile)=1-30%; CS (completely sterile) = 0.

bF (fertile) = 80-100% spikelet fertility; PF (partially fertile) 30-80%; PS (partially sterile) 1-30%; CS (completely sterile)= 0.

Table 2.  Fertility reaction of fertile F2-derived F3 progenies 
          of the cross IR17492A/IR54
________________________________________________________________
           Total no.of    no.of plants with
F3 family no.  plants         spikelet fertility reactiona  
            scored       __________________________________
                           F       PF       PS       CS
________________________________________________________________
1              64         51        5        3        5
2              70         56       13        1        -
3              57         51        5        1        -
4              66         60        4        1        1
5              59         59        -        -        -
6              74         74        -        -        -
7              65         48       13        4        -
8              67         41       13        9        4
9              60         59        1        -        -
10             61         58        3        -        -
11             57         47       10        -        -
12             54         52        2        -        -
13             51         49        1        -        1
14             70         63        6        1        -
15             70         62        7        1        -
________________________________________________________________
aF (fertile)=80-100% spikelet fertility; PF (partially fertile)= 30-80%; PS (partially sterile) = 1-30%; CS (completely sterile)=0.

Table 3.Fertility reaction of testcross progenies of completely 
sterile F2 plants(derived from the cross IR17497A/IR54)and IR54
________________________________________________________________
Completely sterile Total no.of test-  No.of plants with pollen 
F2 plant test-     cross F1        (spikelet) fertility reac-
crossed to IR54    progenies       tiona,b    
                   scored          _____________________________
                                   F     PF      PS      CS
________________________________________________________________
1                     32           16    7       0       -
                                  (11) (11)     (1)
2                     35           23   11       1       -
                                  (21) (14)     (0)      
3                     20            7   12       1       -
                                   (8) (11)     (1)
4                     23           17    6       0       -
                                  (15)  (6)     (2)
5                     24           14    8       2       -
                                  (11) (13)     (0)
6                      7            6    0       0       -
                                   (6)  (1)     (0)
7                     10            4    6       -       -
                                   (2)  (8)
8                     44           37    7       -       -
                                  (38)  (6)
9                      5            3    2       -       -
                                   (2)  (3)
10                    12           10    2       -       -
________________________________________________________________
aF (fertile)=: 60-100% pollen fertility; PF (partially fertile)=30-60%, PS (partially sterile)= 1-30%, CS (completely sterile)=0.

bIn parenthesis. F (fertile)=80-100% spikelet fertility, PF (partially fertile)=30-80%; PS (partially sterile)= 1-30%; CS (complete sterile)=0.

Table 4.  Fertility reaction of five restorers crossed to 
          IR17492A and its fertile counterpart (B line)
________________________________________________________________
              Total no. of        No.of plants with spikelet 
F1 hybrid     plants scored       fertility reactiona  
                                  ______________________________
                                  F       PF       PS       CS
________________________________________________________________
IR17492A/IR26       12            -       12       _        _
IR17492B/IR26       11            9        2       -        _
IR17492A/IR36       12            -        -       12       _
IR17492B/IR36       12            _       12       -        _
IR17492A/IR54       11            -        2        9       _
IR17492B/IR54       12            9        3       -        _
IR17492A/IR9761     12            -        -       12       _
IR1749IB/IR9761     12            -        3        9       _
IR17492A/IR2797     12            -        -       12       _
IR17492B/IR2797      6            -        3        3       _
________________________________________________________________
aF (fertile=80-100% spikelet fertility; PF (partially fertile)=30-80%; PS (partially sterile)= 1-30%; CS (completely sterile)=0.

 

Reference

Govinda Raj, K. and S. S. Virmani, 1988. Genetics of fertility restoration of 'WA' type cytoplasmic male sterility in rice. Crop Sci. 28: (Accepted).