Laboratory of Plant Genetic Diversity, National Institute of Agrobiological Resources, Kannondai 2-11-2, Tsukuba, Ibaraki 305, Japan

Partial cross-incompatibility was first found between IR58, a cultivar developed at IRRI, and Akihikari, a Japanese cultivar. These two cultivars were emasculated in hot water at 43°C for 7 minutes and were pollinated to one another. Crossability was only 1.1% in the cross between IR58 as female and Akihikari as male, whereas it was 65.3% in the reciprocal cross (Table 1). The difference of crossability in reciprocal crosses may be caused by cross-incompatibility between the two cultivars or sensitivity to hot water in stigmas of IR58. To test the latter possibility, IR58 was emasculated with hot water treatment at 43°C for 4 to 8 minutes and then was pollinated by Himenomochi, a Japanese cultivar having a glutinous endosperm, and IR58 as the control. Crossability in the cross between IR58 and Himeno-mochi varied from 2.6% to 8.6%, averaging 5.4% (79 seeds obtained/1451 florets pollinated). Crossability was not related to the period of emasculation in hot water. Almost all the seeds (99.6%) obtained were heterozygous for glutinous endosperm, confirming that crossing occurred. On the other hand, crossability using pollen of IR58 was 35.1% to 70.9%, averaging 50.8% (734/1444). This suggested the difference of crossability was not due to the sensitivity to hot water in stigma of IR58

118 Rice Genetics Newsletter Vol. 13

Table 1. Crossability in crosses between indica and japonica cultivars

and was due to cross-incompatibility between IR58 and Japanese cultivars.

Crossability was further investigated using parental cultivars of IR58 such as IR28, IR36, CR94-13 and Kwang Chang Ai (KCA). Since heading time differed among these cultivars, they were crossed reciprocally to different Japanese cultivars with similar heading time in the field. As shown in Table 1, low crossability was found in crosses between IR28 and Akihikari, between IR36 and Nipponbare, and between CR94-13, an Indian cultivar, and two Japanese cultivars, Shiranui and Nishihomare. Decreased crossability in the crosses between IRRI and Japanese cultivars was more remarkable than in the crosses between CR94-13 and Japanese cultivars. Crossability in the crosses between Japanese cultivars as female and indica cultivars as male was comparable to the selfed cross of IR36. Based on these results, it was concluded that partial crossincompatibility occurred among Asian cultivated rices.