Genic analysis for the sterility of hybrids between distantly related varieties of cultivated rice
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The hybrid sterility between distantly related varieties of cultivated rice can be explained by duplicate genes which work in the gamete as development maintainers (Gametic-Development Genes, or g.d. genes). Hybrids between varieties with those genes in different combinations may produce double recessive gametes which deteriorate at a certain stage of development.
For analysing these g.d. genes, crossing experiments of an ‘(A xB) xC’ type were used, whereA andB are closely related varieties, but differ in fertility in the hybrid with the third varietyC. In one case, it was found that a g.d. gene was linked with the glutinous genegl with a certain recombination value.
When a gene is linked with a g.d. gene, causing sterility in a hybrid, its segregation ratio changes, due to gametic selection in a definite manner. This phenomenon was investigated theoretically and experimentally.
When two independent genes are linked respectively with two pairs of g.d. genes belonging to the same set (the two pairs have the same effect but are located on different chromosomes), recombination of the former genes is restricted to some extent. This phenomenon was demonstrated in theF 2 of some varietal crosses.
It was pointed out that the g.d. gene sets segregating in a hybrid between, distantly related varieties of rice are usually numerous, and produce a continuous array of inter-grades in fertility in theF 2 and later generations. The g.d. gene sets may be said to act as polygenes.
In addition to the g.d. genes, other duplicate genes for fertility seem to be present. The origin of gene duplication was discussed. Since it was found that in tetraploid hybrids between distantly related varieties of rice, the chromosomes derived from the same parent did not tend to pair selectively, the gene duplication in rice was considered to be due more probably to the doubling of chromosomes, as suggested by the hypothesis that rice is a secondary balanced polyploid, than to numerous small translocations.
When varieties with those duplicate genes in different combinations are hybridized, since chromosome recombination tends to bring about unfavourable effects, genotypes similar to the parental strains will increase in the hybrid population. It was pointed out that this tendency might work as an isolating mechanism in mainly self-fertilized plants.
KeywordsRice Variety Segregation Ratio Pollen Fertility Fertile Plant Independent Gene
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