A gamete eliminator which causes abortion of gametes due to an allelic interaction is widely distributed in interspecific hybrids in plants (Sano 1993a). An allele (S) induces abortion of gametes carrying the alternate allele (Sa) only in the heterozygotes. It is easy to detect a gamete eliminator by backcrosses using a strain with Sa as the reccurent parent. However, infertility caused from the heterozygote (S/Sa) disappears in later generations when a strain with S was used as the reccurent parent since only gametes having S transmit into the progeny. The heterozygote (S/Sa) gives female and male infertilities and they are not separable in the progeny. Thus, a gamete eliminator acts gametophytically, and as a result, it causes a strong distortion in segregation manners for genes linked to it.

Oka (1964) First reported a gamete eliminator in rice from an Indica-Japonica hybrid. The F1 between Indica (PTB10 from India) and Japonica (T65, Taichung 65 from Taiwan) types was backcrossed by using T65 as the pollen parent. The backcrosses always gave semi-sterile plants and the all selfed progeny were fertile. PTB10 was assumed to carry a gamete eliminator and its presence was reconfirmed by test-crosses (Sano 1993b). The gene is now registered as S11/(t). The present study was carried out to examine its location by means of linkage analysis with marker genes.

A near-isogenic line of T65 wx with S11(t) was established, derived from BC8 (Sano 1993). This line was crossed with near-isogenic lines of T65 with various markers such as Rc (red pericarp), bc1 (brittle culm), gl1 (glabrous), nl1 (neck leaf), d2 (daikoku dwarf) and la (lazy habit). All the markers except for la gave a ratio of 3:1 in the F2

Table 1. Segregation distortions for la (lazy habit) caused by gamete elimination of S11(t)

Research Notes 71

populations. In the cross with the near-isogenic line with la (T65la), only one plant was homozygous for la out of 301 plants of the F2 (Table 1). The reduced number of la segregants was explained by assuming a linkage between S11'a (t) and la. The genotypes are considered to be S11(t) S11(t) la+la+ in the near-isogenic line with S11(t) and S11'a(t) S11'a(t) lala in T65la. When they are linked, the frequency of segregants homozygous for la is expected as p^2 since both female and male gametes having la tend to be eliminated together with S11'a (t) in the heterozygote. The observed distortion for la was well explained when the recombination values was assumed as 0.06 ± 0.03 (Table 1 ).

The assumption mentioned above was further examined by using the recombinant homozygous for la. The genotype should be S11(t)S11(t) lala according to the genic model, Therefore, gametes with la are expected to transmit predominantly into the progeny of S11(t)la / S11a(t)la+. The F1 between the recombinant and T65 was semi-sterile in both sexes confirming that it was heterozygous for S11(t). Out of 176 plants of the F2, 104 were homozygous for la, showing an excess in number for la comparing to that expected from a 3:1 ratio. The recombination value was estimated to be 0.23 ± 0.02 based on the segregation for la. It should be noticed that the estimated recombination values differ significantly. This suggests that an alien segment flanking to la+ suppresses recombination around it. Suppression of recombination was also reported in an alien segment near S1 carried by Oryza glaberrima (Sano 1990).

The present results confirmed that PTB10 carries a gamete eliminator of S11(t) which is linked to la on chromosome 11. Another gamete eliminator of S10(t) was reported to be involved in the hybrid between the same parents, showing that it is tightly linked to Wx on chromosome 6 (Sano 1994). PTB10 carries S10'a(t) and S11(t) while T65wx carries S10(t) and S11'a(t). This implies that the parents have different gamete eliminators which kill gametes with the alternate alleles as found in the hybrid between 0. sativa and (9. glaberrima (Sano et al. 1979). (Gene symbol: Old system)

Oka, H.I., 1964. Considerations on the genetic bases of intervarietal sterility in Oryza sativa. In Rice

Sano,Y.,1990. The genic nature of gamete eliminator in rice. Genetics l25: 183-191.

Sano, Y., 1993a. A gamete eliminator reconfirmed in a Japonica-indica hybrid. RGN 10: 76-77.

Sano, Y., 1993b. Constraints in using wild relatives in breeding: Lack of basic knowledge on crop

genepools. In Internal. Crop Sci. I, (ed. D.R. Buxton), p.437-443. Crop Sci. Soc. Amer. Madison, Wl. Sano, Y., R. Sano, M. Eiguchi and H.-Y. Hirano, 1994. Gamete eliminator adjacent to the wx locus as revealed

Sano, Y, Y. E. Chu and H.I. Oka, 1979. Genetic studies of speciation in cultivated rice. 1. Genic analysis for