10. A case study of genetic erosion of wild rice in Thailand M. akimoto', Y. shimamoto' and H. morishima

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

2) National Institute of Genetics, Mishima, 41 I Japan

 In recent years, extinction of genetic variation, namely genetic erosion, of wild rice has become a serious problem. The major factors of genetic erosion are 1) destruction of their habitats by human activity, 2) genetic contamination due to the introgression from cultivar.

Drastic decline of natural populations of Asian wild rice (Oryza rufipogon) was reported based on the observations of a long-term project carried out in Thailand (Sato 1994; Morishima and Oka 1995). To obtain the basic information on the genetic erosion of wild rice, present authors investigated how environmental changes of habitat affected genetic structure of wild rice populations at one of the study-sites monitored in the above mentioned project.

The study-site (site code; CP20) is a roadside swamp in the suburb of Ayutthaya city in the central plane of Thailand and inhabited by perennial or perennial/annual intermediate type of 0. rufipogon. We examined two sets of seed samples collected from the same site in 1985 and 1994, respectively. We divided the whole population into four (A-D) subpopulations in 1985 sample and three (1-111) subpopulations in 1994

Fig. 1. Map of the collection site in 1985 and 1994.
Research Notes 61
sample, respectively, as shown in Fig. 1. Each subpopulation consisted of seed samples of 8-24 plants collected on an individual basis. In 1985, this site was densely covered by wild rice plants facing to the highway in one side and to deep water rice field in the other side. In 1993, a gasoline station was constructed in the site and the wild rice population was seriously damaged on account of destruction of their habitat and water pollution. When the seeds were collected in 1994, wild rice plants were sparsely distributed forming several patches. The allozyme variability at 15 loci of 10 enzymes was analyzed using 7 to 15 seeds for each plant. Genotype of mother plants were estimated from the segregation in each progeny following the method of Brown and Allard (1970). Based on the genotypes thus estimated, gene diversities for overall population and for each subpopulation were calculated.

Gene diversity of overall population (H-T) and that averaged over subpopulations (Hs) tended to decrease from 0.278 to 0.249 and from 0.257 to 0.194, respectively, during the period of 1985 and 1994. In contrast, coefficient of gene differentiation (Gs-T) increased from 0.078 to 0.221 (Table 1). It is clear that decrease in plant number and fragmentation of the original population have occurred. This supposedly forced inbreeding of survived plants decreasing genetic diversity in each subpopulation. Under such condition each subpopulation tends to show its own genotype differentiating from each other.

The outcrossing rate of this population was previously estimated as 56% by Barbier (1989). To assess degree of gene flow from cultivated to wild rice population, we analyzed genotype of cultivar collected from the adjacent rice field. It was found that wild rice plants of this population and adjacent cultivar plants generally carried Pgi-I^2 and Pgi-I', respectively, as in many other wild rice and Indica cultivars of this region. The allele frequencies of this diagnostic locus Pgi-I^1 were calculated in respective subpopulations. In both 1985 and 1994, Pgi-I' allele in the wild rice population was found only in the subpopulations located aside of cultivated rice field (Table 2). Most probably introgression has occurred from cultivated rice to wild rice. Frequency of plants having Pgi-I' allele was higher in 1994 than in 1985 (Table 2), indicating that original features of wild rice have been continually replaced by those of cultivar.

Table 1. Gene diversity parameters calculated for the populations

in 1985 and 1994
Year No. of

subpopulations

 H T^1 Hs 2)  Gst 3)
1985 4 0.278 0.257 0.078
Gst 3) 3 0.249 0.194 0.221
1); Gene diversity for whole population.

2); Gene diversity averaged over subpopulations (intrapopulational gene diversity).

3); Coefficient of gene differentiation calculated by (ht-Hs)/ht *

 62 Rice Genetics Newsletter Vol. 13

Table 2. Gene diversity and frequency of plants with Pgi-1 ' in each .subpopulation


Subpopulation No.1 H 2 P 3) Subpopulation No. H P
1985 - A 24 0.269 0.25 1994 -I 16 0.217 0.38
- B 24 0.263 0 - II 8 0.198 1.00
- C 22 0.260 0 - III 12 0.167 0
- D 16 0.237 0
- III 0.257 0.07 Average 0.194 0.38

1) Number of plants examined.

2) Gene diversity calculated for each subpopulation.

3) Frequency of plants having Pgi-I ' .

We must realize that the genetic erosion of wild rice is rapidly progressing and that action for their conservation in natural environment is urgently needed. (Gene symbol: Old system)

References

Barbier, P., 1989. Genetic variation and ecotypic differentiation in the wild rice species Oryza rufipogon. II. Influence of the mating system and life-history traits on the genetic structure of populations. Jpn. J. Genet. 64:273-285. Brown, A.H.D. and R.W. Allard, 1970. Estimation of the mating system in open-pollinated maiz populations using isozyme polymorphisms. Genetics 66: 113-145. Morishima, H., and H.I. Oka. 1995. Genetic erosion in wild and cultivated rice species. RGN 12: 168-171. Sato, Y.I. (ed.), 1994. Ecological-genetic Studies on Wild and Cultivated Rice in Tropical Asia. Tropics 3: 190-245.