ALLELIC INTERACTIONS HERITABLY ALTER THE ACTIVITY OF A METASTABLE MAIZE PL ALLELE

The maize pl locus encodes a transcriptional activator of anthocyanin biosynthetic genes. The Pl-Rhoades (PI-Rh) allele confers robust purpl e anthocyanin pigment in several tissues. Spontaneous derivatives of P l-Rh, termed Pl'-mahogany (Pl'-mah), arise that confer reduced pigment and are meiotically heritable. These derivatives influence other Pl-R h alleles such that only Pl'-mah alleles are transmitted from a Pl-Rh/ Pl'-mah heterozygote. Genetic crosses establish that chromosomal segre gation distortion does not explain this exclusive transmission and sug gest that Pl=Rh invariably changes to Pl'-mah when exposed to Pl'-mah. Such behavior is a hallmark of peramutation. Cosegregation experiment s demonstrate that this paramutagenic activity is genetically linked t o the pl locus. By visually quantifying pl action through successive c rosses, we find that phenotypic expression is inversely related to par amutagenic strength. In addition, the paramutagenic state is metastabl e; reversion to a nonparamutagenic Pl-Rh state can occur. The behavior of Pl-Rh is unique, yet it shares characteristics with paramutation a t true other maize loci, b and r. Previous analysis of b and r paramut ation revealed extensive differences and led to suggestions of distinc t molecular mechanisms. Consideration of the common features of all th ree systems reinvigorates the interpretation that the mechanistic proc esses of these three allelic interactions are similar.