11. A gamete eliminator reconfirmed in a Japonica-Indica hybrid

Yoshio SANO

National Institute of Genetics, Mishima, 411 Japan

Gamete eliminator which causes abortion of gametes due to allelic interaction is frequently observed in interspecific hybrids in plants. An allele (S) induces abortion of gametes carrying the opposite allele (S^a) only in the heterozygote so that infertility appears following hybridization between them. Since it affects both male and female gametes, seed fertility as well as pollen fertility is affected and severe segregation distortion occurs in later generations. Oka (1964) reported from an Indica-Japonica hybrid that PTB10 (Indica cultivar) has a gamete eliminator. The present study was carried out to reconfirm the presence of gamete eliminator in PTB10. A backcross population was produced using T65wx as the recurrent parent which is a near-isogenic line of Taichung 65 (Japonica type) with wx. In each backcross generation until BC\5\, semi-sterile F\1\ hybrids were backcrossed as the maternal parent. From BC\6\ to BC\8\, semi-sterile plants were crossed repiprocally with T65wx. The reciprocal crosses gave only semi-sterile plants while the selfed plants were mostly fertile (Table 1). This type of segregation is explained by assuming that the semi-sterile plant is S/S^a and all male and female gametes carrying S^a are inviable in the heterozygote (S/S^a). Thus, the selfed BC\8\F\2\ Plants are expected to be homozygous for S. When the selfed plants were test-crossed with T65wx, all the hybrids were semi-sterile as

  Table 1. Segregation patterns for seed fertilities in the backcross
generations between PTB10 and T65wx
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               Cross            Seed fertility (%)              No. plants
Generation  Male  Female  5  15  25  35  45  55  65  75  85  95  examined
==========================================================================
BC6-BC8F1   F1    T65wx               1  13  16   3                 33
BC6-BC8F1   T65wx F1                  3  11  13   5  2              34
BC8F2       (Selfed)                          2   5  2   21  44     74
BC9F1*                                4  27  33  11  2              77
==========================================================================
   *Randomly selected 12 fertile segregants in the BC8F2 were test-crossed
with T65wx

expected (Table 1). The gamete eliminator seemed to be independent of wx and Rc.

References

Oka, H.I., 1964. Considerations on the genetic basis of intervarietal sterility in Oryza sativa. In Rice Genetics and Cytogenetics, p. 158-174. International Rice Research Institute, Los Banos, Philippines.