Epigenetic Allelic States of a Maize Transcriptional Regulatory Locus Exhibit Overdominant Gene Action

Abstract Using alleles of the maize purple plant locus (pl), which encodes a transcriptional regulator of anthocyanin pigment synthesis, we describe a case of single-locus heterosis, or overdominance, where the heterozygote displays a phenotype that is greater than either homozygote. The Pl-Rhoades (Pl-Rh) allele is subject to epigenetic changes in gene expression, resulting in quantitatively distinct expression states. Allelic states with low-expression levels, designated Pl′-mahogany (Pl′-mah), are dominant to the high-expression state of Pl-Rh. Pl′-mah states retain low-expression levels in subsequent generations when homozygous or heterozygous with Pl-Rh. However, Pl′-mah alleles frequently exhibit higher expression levels when heterozygous with other pl alleles; illustrating an overdominant allelic relationship. Higher expression levels are also observed when Pl′-mah is hemizygous. These results suggest that persistent allelic interactions between Pl′-mah and Pl-Rh are required to maintain the low-expression state and that other pl alleles are missing sequences required for this interaction. The Pl-Rh state can be sexually transmitted from Pl′-mah/pl heterozygotes, but not from Pl′-mah hemizygotes, suggesting that fixation of the high-expression state may involve synapsis. The existence of such allele-dependent regulatory mechanisms implicates a novel importance of allele polymorphisms in the genesis and maintenance of genetic variation.