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International Rice Research Institute, P. 0. 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
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
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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
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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
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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
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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
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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
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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
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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
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Total # of # of plants with
F3 family# plants spikelet fertility reactiona
scored __________________________________
F PF PS CS
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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 -
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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
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Completely sterile Total# of test- # of plants with pollen
F2 plant test- cross F1 (spikelet) fertility reac-
crossed to IR54 progenies tiona,b
scored _____________________________
F PF PS CS
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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 - -
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aF (fertile)=: 60-100% pollen fertility; PF (partially fertile)=30-60%, PS
(partially sterile)= 1-30%, CS (completely sterile)==O.bIn parenthesis. F (fertile)=80-100% spikelet fertility, PF (partially fertile)=30-80%; PS (partially sterile)= 1-30%; CS (complete sterile)=O.
Table 4. Fertility reaction of five restorers crossed to
IR17492A and its fertile counterpart (B line)
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Total no. of # 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 _
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aF (fertile=80-100% spikelet fertility; PF (partially fertile)=30-80%; PS
(partially sterile)= 1-30%; CS (completely sterile)=0.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.
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).
