11. Completely sterile populations of wild rice found in the suburb of Bangkok M. akimoto', Y. shimamoto' and H. morishima^2

1) Fac. Agr., Hokkaido Univ.. Sapporo, 060 Japan

2) National Institute of Genetics, Mishima, 411 Japan

 Completely sterile plants were found in a wild rice population in western fringe of Bangkok (Bangkok Noi) in early 1980s. Since then, this area has been repeatedly visited and several similar populations were found nearby (Morishima et al. 1991; Morishima 1994). Sterile plants are morphologically similar to perennial type of Oryza rufipogon, though they show several peculiar characteristics. Their habitats are distributed within a relatively limited area (about 10km x 5km) along the Bangkok Noi canal. Maximum water level is 1-2 m in the end of rainy season. Most of the populations are found near the houses of local people or at the shady fringe of orchard. In some sites, plants of typical 0. rufipogon (morphologically normal and fertile), 0. officinalis and 0. ridleyi were found to grow sympatrically with these 0. rufipogon-like
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sterile plants. Observation made before 1993 on six such populations was summarized by Morishima (1994).

In 1995 and 1996, the senior author (M. Akimoto) made extensive surveys in this area, and found 9 more similar populations, one 0. rufipogon population consisting entirely of normal plants and one cultivated rice field which is well isolated from wild rice populations (Fig. 1).

Several plants were sampled from each site for further study. From the field observations and preliminary experimental results, the following characteristics were pointed out.

a) In five populations among 15 sterile populations, morphologically normal and fertile plants (perennial type of 0. rufipogon) were found at least once in our repeated visits. In such populations, however, sterile plants were always dominating and the frequencies of the normal fertile plants were very low. Moreover, frequency of normal wild rice in this area seems to decrease year by year.

b) Sterile plants tend to have wider and larger leaves, longer panicles and larger stigmas with two to five branchs than normal 0. rufipogon. Some plants showed abnormal development of inflorescence (Fig. 2).

c) Sterile plants show strong perenniality and vigorously grow even in polluted water.

d) Progeny so far examined which were raised from the seeds of the fertile plants grown with sterile plants showed complete sterility.

Fig. 1. Distribution of wild rices in Bangkok-Noi.

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Fig. 2. Sterile plants (a) and its malformed inflorescence (b).

e) We counted chromosome number in 17 plants. Seven plants had 24 chromosomes and 10 plants had 36 chromosomes. Plants with 36 chromosomes showed complete pollen sterility. Since all plants with 24 chromosomes did not expose panicles in our experimental field, it is not known whether they are sterile or not. Plants of normal 0. rufipogon, 0. officinalis and 0. ridleyi coexisting with these sterile plants had 24, 24 and 48 chromosomes, respectively, as expected.

f) All these sterile plants were found to have the same mitochondrial type as that of the normal 0. rufipogon.

g) We tried to cross these sterile plants with 0. sativa (AA), 0. rufipogon (AA), 0. officinalis (CC) and 0. latifolia (CCDD) as pollen parents. But no F1 seed could be obtained so far. Probably not only pollens but also female gametes do not function.

Origin of these sterile plants is not known. Judging from polymorphism found in apiculus color and node color, they were not derived from a single genet. We are trying to determine genomic constitution of this unidentified wild rice using molecular and cytogenetic techniques. A possibility that the genome other than A took part in the formation of this variant type through pollens should not be ruled out. Another problem to be solved is how they could survive under environmental stresses such as polluted water and partial shade condition, which common 0. rufipogon hardly tolerate, and have become so abundant in this area.

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References 

Morishima, H? 1994. Observations at permanent study-sites of wild rice in the suburb of Bangkok. Tropics

3:227-233.

Morishima, H., Y. Shimamoto, T. Sato, H. Yamagishi and Y.I. Sato, 1991. Observations of wild and cultivated rices in Bhutan, Bangladesh and Thailand: Report of study-tours in 1989/90. Rep. Nat. Inst. Genet. Jpn.