National Institute of Genetics, Mishima, 411 Japan
Low rates of success of crossing have been experienced occasionally in crosses among wild and cultivated rice strains. In strain W593 from Malaysia, a gene for low cross-compatibility was found, and its chromosomal location was roughly determined as reported in this paper.
A gamete eliminator causing F1 semi-sterility, S6, was found in strain W593, which was linked with Se1 for photoperiod sensitivity (Sano 1989). The recombination value between S6 and Se1 was estimated to be 7%, that between Se1 and wx 34%, and that between S6 and wx about 39% (Sano 1990). In the process of backcrosses to introduce S6 into line T65wx, a marked difference in cross fertility was found between reciprocal crosses, suggesting that the section of chromosome 6 in-between two loci, Wxa and S6, carried a dominant gene which reduced cross fertility when the carrier was used as the maternal parent and pollinated by T65wx.
To check this assumption, three semi-isogenic lines carrying different genes in this segment and line T65wx were tested for cross fertility in a dialiele experiment. The result showed a reduction in cross fertility only when the line with the whole segment carrying Wxa to S6 was pollinated with T65wx pollen (Table 1).
This suggests that a gene for low cross fertility is located between Wxa and Se1. The cross incompatibility seems to be due to zygotic deterioration after fertilization. The gene causing this low cross fertility in a particular cross is tentatively designated as Lcr1(t). Its penetrance was relatively low when pollinated by T65wx, but was near 100% when pollinated by variety Koshihikari. The distribution of this gene and its interaction with other genes are under observation.
Table 1. Percent seed set in cross-pollination among T65 isolines with different genes from W593 =============================================================================== Pollen parent Maternal parent T65Wxa with (Generation) T65wx T65wx with S6 With Se1&S6 Se1&S6 =============================================================================== T65wx(recurrent parent) 73(268)a 84(154) 60 (111) 74(49) T65wx with S6 (B9F5) 55(191) 70(58) 81 (47) 71(52) T65wx with Se1&S6(B10F4) 61(57) 69(102) 71 (52) 67(63) T65Wxa with Se1&S6(B8F5) 13(232) 76(96) 64 (50) 79(68) =============================================================================== a-Number of pollinated spikelets in parentheses.
Sano, Y., 1989. Gametic eliminator detected in the wild progenitor of Oryza sativa. RGN 6: 93-94.
Sano, Y., 1990. Genetic analysis of an alien chromosomal segment introduced from Oryza rufipogon into O. sativa. Jpn. J. Breed. 40 (Suppl. 2): 306-307. (in Japanese)