Development of a co-dominant, PCR-based marker for allelic selection of the pink trait in onions (Allium cepa), based on the insertion mutation in the promoter of the anthocyanidin synthase gene.

Bulb color in onions (Allium cepa) is an important trait and is inherited in a complex manner. However, the mechanism of color inheritance is poorly understood at the molecular level. A previous study showed that pink bulb color in onions is inherited as a single recessive trait. This trait is attributable to a significantly reduced transcription of the anthocyanidin synthase (ANS) gene. In this study, we developed a PCR-based marker for an allelic selection of the ANS gene to avoid the laborious progeny tests traditionally employed. To identify polymorphisms between pink and red alleles of the ANS gene, promoter sequences of both alleles were isolated. 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.