Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/3/2007
Publication Date: 1/1/2008
Citation: Khar, A., Jakse, J., Havey, M.J. 2008. Segregations for Onion-Bulb Colors Reveal that Red is Controlled by at Least Three Loci. Journal of the American Society for Horticultural Science. 133:42–47. Interpretive Summary: Colors of the seed coats and bulbs of onion are economically important traits. Occasionally undesirable light-red bulb colors occur in hybrids produced by crossing two yellow inbreds, complicating the production of acceptable yellow hybrid onions. This light-red bulb color (L-R-) was proposed to occur when the yellow parental inbreds possess the genotypes CCLLrr and llRR. Anthocyanidin synthase (ANS) and dihydroflavonol 4-reductase (DFR) have been proposed as the candidate genes for the L and R loci of onion, respectively. We developed families segregating for bulb and seed colors, as well as simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs) in the structural genes synthase (ANS) and dihydroflavonol 4-reductase (DFR) in order to assess linkages among these important morphological and molecular markers. The B and C loci were mapped to chromosomes 1 and 6, respectively. The R locus cosegregated with DFR; however the second locus conditioning light-red bulb color did not cosegregate with ANS, but showed linkage to ANS at 6.3 cM. This new locus was named L2 and reveals that light-red bulb color is conditioned by the interaction of DFR and at least two other loci (L and L2) on chromosome 4. These results will be of interest to onion breeders and will help to predict which yellow inbreds will produce undesirable light-red hybrids.
Technical Abstract: Onion seed-coat color is controlled by one locus (B) and bulb color by at least five major loci. White bulbs are conditioned by a dominant allele at the I locus or recessive alleles at the C locus. Colored bulbs (pink, red, yellow, or chartreuse) are homozygous recessive at the I locus and carry a dominant allele at the C locus. Red bulbs are produced when both the L and R loci carry dominant alleles. If either the L or R locus is homozygous recessive, yellow bulbs are produced. We developed families segregating for bulb colors, simple sequence repeats (SSRs), and single nucleotide polymorphisms (SNPs) in genomic amplicons of dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS), putatively the R and L loci respectively. The B and C loci were linked to SSRs on chromosomes 1 and 6, respectively. For three families SNPs in DFR co-segregated with R locus. In the family from B2246 x B11159, red versus yellow bulbs was controlled by DFR and a locus (L2) linked at 6.3 cM to ANS. We propose that yellow-bulb onions have been independently selected numerous times and that yellow populations carry independent mutations in structural or regulatory genes controlling the production of red bulb color in onion.