19. Gamete eliminator detected in the wild progenitor of Oryza sativa

Gamete eliminator which causes abortion of gametes due to allelic interaction was first reported in tomato by Rick (1966). The genic model assumes that the parents have S/S and S^a/S^a and both female and male gametes with S^a abort only in the heterozygote S/S^a while the homozygotes of S/S and S^a/S^a are fertile. Accordingly, the S/S^a plant produces only gametes with S and the selfed progenies all become fertile (S/S). Such sterility genes seem to be of wide occurrence between distantly related taxa and serve as one of the genetic machanisms for hybrid sterility in rice (Sano et al. 1979; Sano 1983) although the modified genic expression was also reported in rice (Kitamura 1962; Ikehashi and Araki 1986; Sano 1986). Two different gamete eliminators (S\1\ and S\2\) were identified in a cross between the two cultivated rice species, Oryza sativa and O. glaberrima (Sano et al. 1979). In addition, a similar sterility gene as gamete eliminator was suggested by Oka (1964) which caused sterility in an Indica-Japonica hybrid. In order to determine if such a sterility gene is widely distributed in rice species, the extraction of sterility genes is now under way through successive backcrosses using various taxa having the AA genome. This report shows that the wild progenitor of O. sativa, O. rufipogon, has a gamete eliminator and, as a result, genes linked to it show severely distorted segregations in the progeny of crosses.

In order to extract a sterility gene from W593 (a perennial wild rice from Malaya), successive backcrosses were made by using T65wx (an isogenic line of Taichung 65 with gene wx) as the recurrent parent. The initial cross was W593 (female)X T65wx (male) and the hybrid was backcrossed as the maternal parent with T65wx. The successive backcross plants were all semi-sterile. The semi-sterile plants showed partial seed fertility although no abnormal pollen grains were detected with iodine staining. The semi-sterile BC\7\F\1\ plants were reciprocally backcrossed with T65wx giving rise to only semi-sterile plants (Table 1). On the other hand, all of the selfed progenies was fertile and the fertile progenies gave only semi-sterile plants when crossed with T65wx. this showed that W593 has

Table 1. Segregation patterns for infertility observed in the backcross generations. The sterility gene (S\6\) was introduced from W593 into T65wx by successive backcrossings.

_____________________________________________________________________________
                                                         No. of plants
                                                        _____________________
Cross                           Genotype                Fertile  Semi-sterile
_____________________________________________________________________________
BC\7\F\1\(f) X T65wx(m) S\6\/S\6\^a X S\6\^a/S\6\^a            0            13
T65wx(f) X BC\7\F\1\(m) S\6\^a/S\6\^a X S\6\/S\6\^a            0            43
BC\7\F\2\                                                  42             0
BC\7\F\2\ X T65wx*                                          0            87
________________________________________________________________
*Ten fertile BC\7\F\2\ plants were testcrossed with T65wx.

Fig. 1. Linkage relations estimated from segregation patterns observed in the BC/8F1 generation. The recombination values are shown in %.

a gamete eliminator and the opposite allele is completely eliminated when heterozygous plants produce gametes. The linkage relations between the sterility gene in question, Se, for photoperiod sensitivity, and wx were estimated based on data from 43 BC\8\F\1\ plants as shown in Fig. 1, and the gamete eliminator was symboled S\6\. The recombination value between Se\1\ and S\6\ was estimated to be 0.16+/-0.015 based on 311 BC\8\F\2\ plants.

References

Ikehashi, H. and H. Araki, 1986. Genetics of F1 sterility in remote crosses of rice. In Rice Genetics, p. 119-130. IRRI, Manila.

Kitamura, E., 1962. Genetic studies on sterility observed in hybrids between distantly related varieties of rice, Oryza sativa L. Bull. Chugoku Agr. Exp. Sta. Ser. A8: 141-205. (Japanese/English)

Oka, H. I., 1964. Considerations on the genetic basis of intervarietal sterility in Oryza sativa.In Rice Genetics and Cytogenetics, pp. 158-178. Ed. by IRRI, Elsevier, Amsterdam.

Rick, C. M., 1966. Abortion of male and female gametes in the tomato determined by allelic interaction. Genetics 53: 85-96.

Sano, Y., 1983. A new gene controlling sterility in F1 hybrids of two cultivated rice species. J. Hered. 74: 435-439.

____, 1986. Sterility barriers between 0. sativa and 0. glaberrima. In Rice Genetics, p. 109-118. IRRI, Manila.

____, Y. E. Chu and H. I. Oka, 1979. Genetic studies on speciation in cultivated rice, 1. Genic analysis for the F1 sterility between O. sativa and O. glabberima. Jpn. J. Genet. 54: 121-132.