4. Increasing trend of japonica-derived genes in hybrid populations grown under upland conditions

Yo-Ichiro SATO

National Institute of Genetics, Mishima, 411 Japan

In tropical monsoon Asia, japonica cultivars are distributed in upland fields regardless of altitude (Sato 1986). They appear to be adapted in upland fields. To estimate such differential adaptability in terms of gene frequency, the frequencies of genes at the ph,wx and gl-1 loci in hybrid populations derived from indica and japonica parents in the progeny of a cross Act130 X Ac221, were compared between F5 bulk populations raised in upland and lowland fields at Mishima. Ac130 is a lowland indica ftom Taiwan and Ac221 is an upland japonica from Philippines. They have different alleles at said loci. Under the upland condition, the plants were grown at two seeding densities, 50 and 500 seeds per m (seeded in row).

At the ph and wx loci, the frequencies of recessive homozygotes for japonica-derived alleles were significantly higher in the upland populations at both low and high densities than those in the lowland population (Table 1). The frequency of gl-1 homozygotes from the japonica parent was higher in the high-density upland population that in the lowland population.

The data thus suggest that japonica-derived genes tend to increase under upland conditions. It remains unknown why japonica genes are favored in upland fields, particularly at a high planting density. In an upland field at high density, the plants will be subjected to strong competition for water. Yet, in mixed planting experiments, indica cultivars are generally strong competitors against japonicas (Oka 1960), and indica-derived genes generally tend to increase (Oka 1988, p. 172). Probably, selection in this direction is mitigated in upland conditions.

Table 1. Frequency of homozygotes for japonica-derived genes at loci ph, wx and gl-1 in F5 populations raised in lowland field (Low) and in upland field at low (UpL) and high (UpH) densitities

================================================================
                   Population                X2 for difference
               ==========================
Genotype       Lowland  Upland,    density      between populs
               (Low)a   low(UpL)b  high(UpH)c   Low-UpL Low-UpH
================================================================
# of plants observed 123  165      171
ph/ph              0.398 0.594     0.608        10.8**  12.6**
wx/wx              0.365 0.636     0.584        20.7**  13.7**
gl-l/gl-l          0.307 0.321     0.479         0.05    8.6**
================================================================
a-space-planted (20 x 25 cm), irrigated; b and c-directly seeded in rows, rain-fed. b-50seeds/m, c-500 seeds/m.

** P<0.01 (df=1).

References

Oka, H. I., 1960. Variation in competitive ability among rice varieties. Jpn. J. Breed. 10: 61-68.

Oka, H. I., 1988. Origin of Cultivated Rice. Jpn. Sci. Soc. Rress/Elsevier, Tokyo/Amsterdam.

Sato, Y. I., 1986. The Indica-Japonica differentiation of rice cultivars in Thailand and its neighboring countries. In B. Napompeth and S. Subhadravandhu (eds.), New Frontiers in Breeding Researches (Proc. 5th Intern. Congr. SABRAO), p. 185-193. Kasetsart Univ., Bangkok.