Uniparental and transgressive expression of alpha-zeins in maize endosperm of o2 hybrid lines

The alpha -zein gene family encodes the most abundant storage proteins of maize (Zea mays) endosperm. Members of this family are expressed in a parent-of-origin manner. To characterize this phenomenon further, we investigated the expression of a subset of alpha -zein polypeptides in reciprocal crosses between o2 lines that were characterized by a simplified alpha -zein pattern. Maize lines that suppressed the expression of alpha -zeins when used as female parents were identified. The suppression was cross-specific, occurring only when specific genetic backgrounds were combined. Four alpha -zein sequences that were sensitive to uniparental expression were isolated. Molecular characterization of these alpha -zeins confirmed that their expression or suppression depended on the genetic proprieties of the endosperm tissue instead of their parental origin. DNA methylation analysis of both maternally and paternally expressed alpha -zeins revealed no clear correlation between this epigenetic marker and parent-of-origin allelic expression, suggesting that an additional factor(s) is involved in this process. Genetic analyses revealed that the ability of certain lines to suppress alpha -zein expression was unstable after one round of heterozygosity with non-suppressing lines. Interestingly, alpha -zeins also showed a transgressive expression pattern because unexpressed isoforms were reactivated in both F2 and backcross plants. Collectively, our results suggest that parent-of-origin expression of specific alpha -zein alleles depends on a complex interaction between genotypes in a manner that is reminiscent of paramutation-like phenomena.