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  • Author or Editor: Ockyung Bark x
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Restriction fragment length polymorphisms (RFLPs) in the nuclear genome were used to assess genetic diversity among cultivated open-pollinated populations (OPs) of bulb onion (Allium cepa L.). Twenty OPs of contrasting day-length responses [long (LD) and short (SD) day] were examined with 104 random cDNA probes and two to four restriction enzymes. Sixty-one probes detected polymorphisms among the OPs for at least one restriction enzyme. Parsimony and cluster analyses were completed and no distinct grouping was observed between LD and SD OPs. Parsimony analysis generated a consensus tree that grouped all but two LD OPs and their relationships to the SD OPs were not resolved. Cluster analysis grouped all but three LD OPs and the distances from that group to the others were not greater than those among LD or SD OPs. These results suggest that LD and SD onions do not represent distinct germplasm sources and LD OPs possess a more narrow genetic background. The paucity of unique fragments among OPs indicates that phenotypic variation, e.g., day-length response or bulb color or shape, does not reflect diverse sources of germplasm.

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The edible Alliums are economically important world-wide. The bulb onion (Allium cepa) is the most widely grown. The Japanese bunching onion (A. fistulosum) has many desirable characters, e.g., resistance to pink root, Thrips, smut, maggot, and Botrytis. Transfer of pink root resistance from A. fistulosum into A. cepa has been attempted for over 60 years. However, sterility of the F1 hybrid is a barrier and there is little evidence of gene introgression during backcrossing to A. cepa. Dr. Corgan has made crosses between A. fistulosum as the seed parent and A. cepa. He backcrossed the F1 hybrids to A. cepa and generated BC2 progenies which showed excellent pink root resistance. RFLPs in the chloroplast genome showed all BC2 progenies had either the normal or sterile cytoplasm of A. cepa. This may be due to not strictly maternal inheritance of the chloroplast DNA or a seed mixture during backcrossing. Other interspecific hybrids and their BC1 progenies had the cytoplasm of A. fistulosum. Nuclear RFLPs show hybrid patterns in the F1 plants. BC1 progenies possess some A. fistulosum markers as evidence of DNA introgression from A. fistulosum into the backcross progenies.

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Allium fistulosum L. (bunching onion) is resistant to many of the important diseases and pests of Allium cepa L. (bulb onion). Although the first interspecific hybrids were generated more than 50 years ago, there is no conclusive evidence that any desirable trait in bunching onion has been successfully transferred to bulb onion by backcrossing. We identified RFLPs in the chloroplast and nuclear genomes to assess DNA transfer from bunching to bulb onion by backcrossing an interspecific hybrid to a bulb onion. Polymorphisms in the chloroplast genome established that the interspecific hybrid and three putative backcross plants had the cytoplasm of a bunching onion. All 57 random cDNA probes detected polymorphisms between the bulb and bunching onion for at least one of two restriction enzymes. The backcross progenies always possessed the bulb-onion fragments and an excess of probes detected the bunching-onion fragments. Only one plant showed an acceptable fit to the expected 1:1 backcross ratio. Significant deviations from expected segregation ratios may be the result of abnormal meiosis in the interspecific hybrid. However, these observations could also be explained by a previously proposed nuclear-cytoplasmic interaction conditioning preferential survival in the bunching-onion cytoplasm of eggs carrying bunching-onion chromosomes.

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