6. Distribution of bacterial blight resistance genes in wild-rice populations of Thailand


National Institute of Genetics, Mishima, 411 Japan

Two perennial and two annual wild-rice populations (Oryza rufipogon) were examined regarding bacterial blight resistance using four Japanese pathogen races. These populations grow in our permanent study-sites located within a range of about 20 km in the northern suburb of Bangkok, and were repeatedly visited for ecological genetic study.

As we already reported (Hamamatsu et al. 1992), two perennial populations adapted to deepwater conditions consist mostly of the plants resistant to all four pathogen races (91% in NE88, 92% in CP20). While annual populations adapted to drier conditions were highly polymorphic containing the plants susceptible to all four races (44% in NE3, 12% in NE4). This difference in polymorphism between perennial and annual types contrasts strikingly with the diversity pattern found in other genetic traits so far studied (Morishima and Barbier 1990). In morphological characters as well as in isozymes, as shown in Table 1, perennial populations have a larger amount of genetic variability than annual populations, reflecting the difference in their propagating and mating systems.

Of two annual populations which are found 1 km apart from each other along a road, NE3 grows in a flat land completely parched in the dry season, and NE4 grows in a lowlying area surrounding a depression which retains water throughout the year (exceptional habitat for an annual type). NE4 showed higher frequency of resistant plants than NE3. This difference was consistently found in the samples collected in different years. To investigate the distribution pattern of resistance in more detail, wild-rice plants growing in roadside ditch between NE3 and NE4 were sampled at 100 m intervals. Fig. 1 shows frequency distributions of plants resistant to respective pathogen races in each sub-population. Frequency of resistant plants tended to decrease as approaching NE3. In plant height and flowering date, similar microgeographical cline was observed in this set of materials when tested under a uniform condition at Mishima. In isozymes, however, such clinal variation was not detected at any polymorphic loci.

It was also found that, in landraces of cultivated rice, ecotypes grown in deepwater contained resistant individuals within population with higher frequency than those grown in drier conditions (our unpublished data). Judging from these results, one of the selective forces shaping the variation pattern for bacterial blight resistance in natural populations might be the condition of habitats. Deepwater which favors the epidemics of this disease has probably increased frequency of resistant plants. As argued by Parker (1992), stable polymorphism of disease resistance can be explained either by 1) frequency dependent selection due to heterogeneous pathogens with each specialized on different host genotypes, or by 2) a balance of opposing selective forces, namely selection for resistance in disease-stress conditions vs. selection against resistance in disease- free conditions (resistance cost). To understand the polymorphism mechanism of bacterial blight resistance in rice, the genetic structure of pathogen populations should be elucidated. The genetic basis of resistance in these wild-rice plants is now under study.



Morishima, H. and P. Barbier, 1990. Mating system and genetic structure of natural populations in wild rice Oryza rufipogon. Plant Species Biol. 5: 31-39.

Parker, M. A., 1992. Disease and plant population genetic structure. In Plant resistance to herbivores and pathogens-Ecology, evolution, and genetics. (R. S. Fritz and E. L. Simms, ed.), University of Chicago Press, p. 345-362.

Hamamatsu, C., T. Miyabayashi and H. Morishima, 1992. Variation in bacterial blight resistance within natural populations of wild rice and farmers' field. RGN 9: 88-90.