7. Effect of ozone exposure on F1 gametic series in rice hybrids

Y. I. SATO1, Y. FUJINUMA2and A. AIHARA1

1) National Institute of Genetics, Mishima City, 411 Japan

2) National Institute for Environmental Studies, Onogawa, Tsukuba City, 305 Japan

Gametic competition causing segregation distortion is frequently observed in indica X japonica crosses. The degree of segregation distortion is influenced by both genetic and environmental factors. In this paper, we report a case in which F2 ratios for isozyme markers were influenced by an ozone exposure of F1 plants at the flowering stage in two indica X japonica crosses, Ac435 (from India) X T65

Table 1. Effect of an ozone exposure of flowering F1 plants on the frequencies of F2 homozygotes for japonica-derived alleles at 10 isozyme loci, observed in two indica X japonica crosses

================================================================
          Ac435 x T65                      Ac 143 X Ac521
Locus ============================= ============================
     Control 0/3-treat. Difference Control 0/3-treat. Difference
================================================================
Acp-1 0.18   0.20       -0.02      0.22    0.19       0.03
Amp-1 0.32   0.30        0.02           No segregation
Amp-2 0.24   0.24        0.0       0.25    0.19       0.07*
Amp-3 0.16   0.22       -0.06*          No segregation
Cat-1 0.25   0.21        0.04      0.27    0.25       0.02
Est-2 0.16   0.18       -0.02      0.26    0.21       0.05
Pdg-1 0.21   0.18        0.03      0.24    0.26      -0.02
pgi-1 0.22   0.30       -0.08**    0.16    0.16       0.0
Pgi-2     No segregation           0.27    0.21       0.06
Pox-2 0.25   0.22        0.02      0.23    0.18       0.05
================================================================
* Significant at  5% level,  ** at 1% level.
Control and O\3\ treated populations in both crosses each consisted of 200 plants. For chi-square test, the numbers of homozygotes for japonica-derived allele and of the remainder (heterozygotes plus homozygotes for indica-derived allele) in the control population were taken as the expected numbers, and were compared with those observed in the ozone-treated population.

(from Taiwan) and Ac143 (from Taiwan) x Ac521 (from Japan).

An exposure to ozone (O\3\) at a 0.1 ppm concentration of flowering rice plants (for 24 hrs at ca.30 deg C) was found to affect pollen-tube elongation and to reduce seed fertility significantly. If the sensitivity to ozone and the rate of pollen-tube elongation were controlled by certain gametophytic genes, changes in F2 ratios for markers linked with genes for ozone sensitivity would be observed after an ozone treatment of flowering F1 plants.

In the two crosses used for experiment, distorted F2 ratios were observed at many of isozyme loci in varying degrees (Table 1). In addition, significant differences in the frequency of homozygotes for alleles derived from the japonica parent were found between the control and ozone-treated F2 populations at a few loci, i.e., Amp-3 (Chromosome 6) and Pgi-1 (Chr. 3) in Ac435 X T65 and AMP-2 (Chr.8)in Acl43 X Ac521.The ozone treatment increased japonica-derived alleles in the former cross, but decreased them in the latter cross. This suggests that gametophytic genes modifying ozone reaction are distributed in different chromosomes.