Rice is the staple food for the world and two rice species are cultivated for food production. Oryza sativa originated in Asia but is widely grown. O. glaberrima originated and is cultivated in West Africa. Both species have same genome AA (Chang, 1976; Morinaga et al., 1957). The high sterility of interspecific hybrids is a serious reproductive barrier (Jones et al., 1997; Morinaga et al., 1957; Morishima et al., 1962, 1963; Sano et al., 1979; Sano, 1983; Tao et al., 1997). Regarding crossability, there is no consensus. Morinaga et al. (1957) and Morishima et al. (1962, 1963) thought that there was no crossing barrier between the two species. Chu et al. (1969) thought also so. The intraspecific hybridization rate was 51% in O. sativa and 62% in O. glaberrima, whereas the interspecific hybridization rate was 39-42%. An extreme example was reported by Jones et al. (1997): from 48 interspecific crosses, only 7 crosses were successful in seed production. Great differences in interspecific crossability were found between different years by Sano et al. (1979), and before BC₃, the cytoplasm played an important role in crossability. Tao et al. (1997) reported that when O. glaberrima was used as maternal parent, higher crossability could be achieved.

Our further studies (Tao et al., 1999) from 1997 to 1998 indicated that, interspecific crossability of O. sativa and O. glaberrima was between the intraspecific hybridization rates of the two species. The latter value was 68.2% to 95.2% for O. sativa, and there was great variation in different seasons (Table 1).

In order to study the factors influencing interspecific crossability, crosses were made in the early season (March-June) of 1998 in Hainan. Three japonica (IRAT104, Mengwanggu, Reimei), one indica (Guichao 2), and five accessions of O. glaberrima (IRGC101942, IRGC103648, IRGC102205, IRGC102277, IRGC102502) were used to make 40 interspecific crosses between O. sativa and O. glaberrima, 6 intersubspecific crosses between japonica and indica, and 6 intraspecific crosses in O. glaberrima. In the winter season (October-March), 1998, the experiment was repeated. Variance analysis indicates that season, genotype of O. sativa, genotype of O. glaberrima, and cytoplasm were significant or highly significant factors for interspecific crossability. All interactions were not significant, whether the model was fixed or free. The F value of season was the biggest, followed by cytoplasm, genotype of O. sativa, and genotype of O. glaberrima (Table 2) Thus, interspecific crossability was a partial barrier, affected by environment (season), cytoplasm, genotype of O. sativa, and genotype of O. glaberrima.

Ladizinsky (1992) classified crossability into both cross-directions complete, successful unilateral, partially successful, and incompatible four kinds. From the above results, interspecific crossability between two cultivated rice species is partially incompatible, and partially

unilateral.

This study was supported in part by Yunnan Natural Science Foundation and West Africa Rice Development Association (WARDA).

References

Chang T. T., 1976. The origin, evolution, cultivation, dissemination, and diversification of Asian and African rices. Euphytica, 25: 425-441.

Chu, Y. E., H., Morishima, and H. I. Oka, 1969. Reproductive barriers distributed incultivated rice species and their wild relatives. Jap. J. of Genetics, 44: 207-223.

Jones M. P., M. Dingkuhn, G. K. Aluko, and M. Semon, 1997. Interspecific Oryza sativa L. X O. glaberrima Steud. progenies in upland rice improvement. Euphytica, 92: 237-246.

Ladizinsky G., 1992. Crossability relations. In: Kalloo G., J.B.Chowdhury (eds.), Distant hybridization of crop plants. Springer-Verlag, Berlin. p.15-31.

Morinaga T., and H. Kuriyama, 1957. Cytogenetical studies on Oryza sativa L.. IX. The F1 hybrid of O. sativa L. and O. glaberrima Steud. Jap. J. Breeding, 7: 57-65 (English with Japanese summary)

Morishima H., K. Hinata, and H. I. Oka, 1962. Comparison between two cultivated rice species, Oryza sativa L. and O. glaberrima Steud. Jap. J. Breeding, 12: 153-165 (English with Japanese summary).

Morishima H., K. Hinata, and H. I. Oka, 1963. Comparison of modes of evolution of cultivated forms from two wild rice species, Oryza breviligulata and O. perennis. Evolution, 17: 170-181.

Sano Y., Y. E. Chu, and H. I. Oka, 1979. Genetic studies of speciation in cultivated rice, 1. Genic analysis for the F1 sterility between O. sativa L. and O. glaberrima Steud. Jap. J. genetics, 54: 121-132.

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

Tao D., F. Hu, G. Yang, J. Yang, H. Tao, 1997. Exploitation and utilization of interspecific hybrid vigor between Oryza sativa and O. glaberrima. In: Jones M. P., M. Dingkuhn, D. E. Johnson, S. O. Fagade (eds.), Interspecific Hybridization: Progress and Prospects. Proceedings of the Workshop: Africa/Asia Joint Research on Interspecific Hybridization between the African and Asian Rice Species (O. glaberrima and O. sativa ), WARDA, M'be, Bouake Cote d'Ivoire, December 16-18, 1996. p. 103-112.

Tao D., F. Hu, Y. Yang, Y. Zhou, H. Chen, 1999. Studies on the interspecific crossability between Oryza sativa L. and O. glaberrima Steud. In: Prospects of rice genetics and breeding for the 21^st Century: paper collection of International Rice Genetics and Breeding Symposium. China Agricultural Scientech Press, Beijing. p. 329-334.