UNDERSTANDING OF ONION BULB COLOR INHERITANCE AT THE MOLECULAR LEVEL

Sunggil Kim, Kil-Sun Yoo, Leonard M. Pike

A new locus conditioning a pink trait in onions was identified. Unusual pink onions were found in haploid populations induced from a F1 hybrid between yellow and dark red parents and in F3 populations originating from the same cross. Segregation ratios of red to pink in F2, backcross, and F3 populations indicated that this pink trait is determined by a single recessive locus. RT-PCR was carried out to look for any differential expression of anthocyanin synthesis genes between dark red and pink F3 lines. The transcript level of anthocyanidin synthase was significantly reduced in the pink line. To examine co-segregation of ANS gene and pink phenotypes in segregating populations, promoter and coding sequences of the ANS genes from both red and pink alleles were isolated to detect any polymorphism to be used as molecular markers. There was 97% nucleotide sequence identity between the promoter sequences of the two alleles. A 390-bp insertion was identified 632 bp upstream from the putative transcription start site in the pink allele. A pair of primers was designed on the flanking sequences of the inserted region and utilized as a PCR-based marker for allelic selection of the ANS gene. The reliability of the marker was tested using parents, F1 hybrids, and F3 lines whose genotypes had been identified by progeny tests. The marker was also used to evaluate the distribution of the pink allele in white and yellow breeding lines. The results indicated that a majority of the breeding lines tested were homozygous recessive.

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