Dominance of the white bulb color in onions (Allium cepa L.) changes depending on the positions of mutations in the AcMYB1 gene encoding a R2R3-MYB transcription factor

The white bulb color in onions (Allium cepa L.) is known to be controlled by two independent loci, C and I, which are responsible for recessive and dominant white colors, respectively. Previous studies have shown that the AcB2 and AcMYB1 genes are the causal genes for the C and I loci, respectively. To identify additional mutant AcMYB1 alleles, 85 white accessions were analyzed in this study. The results revealed four mutant alleles, designated AcMYB1^Ivana, AcMYB1^CRM, AcMYB1^Reina, and AcMYB1^LINE, which had large-sized insertions in exon3. Long terminal repeat (LTR)-retrotransposons from different families were found in the AcMYB1^Ivana, AcMYB1^CRM, and AcMYB1^Reina alleles, while a non-LTR long interspersed nuclear element (LINE) was identified in the AcMYB1^LINE allele. In addition, four other mutant alleles designated AcMYB1^DEL-A, AcMYB1^DEL-G, AcMYB1^DEL-CG, and AcMYB1^E48K were found to harbor critical single nucleotide polymorphisms (SNPs) in exon1 or exon2. Deletions (one or two base pairs) creating premature stop codons were identified in AcMYB1^DEL-A, AcMYB1^DEL-G, and AcMYB1^DEL-CG, and an SNP resulting in the change of a negatively charged glutamic acid to a positively charged lysine was found in the highly conserved R2 domain of AcMYB1^E48K. Interestingly, yellow bulb colors were observed in heterozygous individuals containing AcMYB1^LINE, AcMYB1^DEL-G, AcMYB1^E48K, or AcMYB1^DEL-A, indicating the recessive inheritance of the white bulb color. On the other hand, dominant inheritance patterns were observed in heterozygotes harboring AcMYB1^CRM or AcMYB1^Reina. Taken together, the positions of mutations in AcMYB1 determined the dominance of the white bulb color in onions.