Lcr (low crossability) results from unidirectional cross-incompatibility in rice (Sano 1992). Reduced seed setting was found when a segment of chromosome 6 was introduced from Oryza rufipogon (W593 from Malaysia) into a Japonica type (T65wx) of rice. When pollinated by T65wx, the plant carrying the alien segment produced aborted and ungerminable seeds while the reciprocal cross between the same parents showed normal seed setting. Location of Lcr was determined by pollinating F2 individuals of Lcr* lLcr with pollen of T65wx. This implies that Lcr controls the cross-incompatibility in the female parent. For simplicity, we propose that Lcr should be renamed as Cinf (cross-incompatibility in the female parent) based on the present results. We report here two more genes involved in the phenomenon, one (cinm) controlling incompatibility in the male parent and the other (Su-Cinf) suppressing the expression of Cinf or Lcr.

Materials used were 3 near-isogenic lines (W593A, 868A and 868F), which were obtained by successive backcrosses using T65wx (Taichung 65 with wx) as the recurrent parent and were derivatives from BC8. W593A carries a segment containing Wxa, En-Se1 (suppressor of photosensitivity), Cinf (cross-incompatibility in the female reaction, =Lcr), Se1 (photosensitivity) and 56 (hybrid sterility) on chromosome 6. 868A and 868F carry a segment from Wx^a to Se1 and a segment with only Se1 derived from an Indica type (Patpaku), respectively.

When 868A and 868F were pollinated to W593A having Cinf, the crosses showed normal seed setting (Table 1). This suggests that the introduced alien segment had a gene (or genes) controlling cross-incompatibility in the male reaction since T65wx without the segment resulted in cross-incompatibility. The gene appeared to inhibit abortion of hybrid seeds and to be located near Se1. In addition, when T65wx x 868A f1 was pollinated to W593A, the cross showed normal crossability again (Table 2). The results imply that the gene responsible for incompatibility is recessive and is expressed sporophytically. Accordingly, it is considered that unidirectional cross-incompatibility

120 Rice Genetics Newsletter Vol. 13

Table 2. Crossabilities in % in test crosses for examining the expressions of genes involved in

Note: * shows significant at 5%. The distortion for wx segregation is due to S6(Sano 1992).

results from an interaction between the recessive gene carried by T65wx and the dominant gene (Cinf) carried by W593A. The recessive gene carried by T65wx was tentatively designated cinm.

Further, an unexpected result was observed when W593A x 868F f1 was pollinated with T65wx (Table 2). The female parent (W593A x 868F f1) gave normal seed setting although W593A x T65wx f1 is expected to show reduced seed setting on pollination with T65wx as Cinf is dominant (Table 2, Sano 1992). The result indicates that the segment with Se1 from Patpaku carries not only cinm+ but also another gene suppressing Cinf in the female organ.

The suppressor seems to be dominant since the heterozygote gave a normal seed setting (55.8%) when pollinated with T65wx, suggesting that the gene is expressed sporophytically. Sporophytic expression of the gene was also confirmed by the observation that wx linked to it segregated into a ratio of 1:1. The suppressor is tentatively designated Su-Cinf.

The present study showed that the unidirectional cross-incompatibility system is controlled by at least 3 genes which are located on chromosome 6 forming a gene complex. Cyto-histological observations revealed that aborted seeds in the cross incompatible system are caused by retardation of the endosperm 4-5 days after fertilization (Sano and Kobayashi 1996). The 3 genes involved in the genetic mechanism all act sporophytically, showing that their gene expressions occur before meiosis. A question arose as to how those genes cause defective seeds after fertilization in spite that they are expressed before meiosis. The present results suggests that the female and male gametes are not genetically equivalent and an interaction among the 3 genes detected brings about reduced seed setting through abortion of seeds. (Gene symbol: Old system)

Sano, Y., 1992. Genetic comparisons of chromosome 6 between wild and cultivated rice. Japan. J. Breed.

Research Notes 121

Sano, Y. and S. Kobayashi, 1996. Genetic dissection of cross-incompatibility in rice. IRRI, Rice Genetics