| Summary: | Various kinds of reproductive barriers have been reported in intraspecific and interspecific crosses between the AA genome <i>Oryza</i> species, to which Asian rice (<i>O. sativa</i>) and African rice (<i>O. glaberrima</i>) belong. A hybrid seed sterility phenomenon was found in the progeny of the cross between <i>O. sativa</i> and <i>O. meridionalis</i>, which is found in Northern Australia and Indonesia and has diverged from the other AA genome species. This phenomenon could be explained by an egg-killer model. Linkage analysis using DNA markers showed that the causal gene was located on the distal end of chromosome 1. Because no known egg-killer gene was located in that chromosomal region, this gene was named <i>HYBRID SPIKELET STERILITY 57</i> (abbreviated form, <i>S57</i>). In heterozygotes, the eggs carrying the <i>sativa</i> allele are killed, causing semi-sterility. This killer system works incompletely: some eggs carrying the <i>sativa</i> allele survive and can be fertilized. The distribution of alleles in wild populations of <i>O. meridionalis</i> was discussed from the perspective of genetic differentiation of populations.
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