Paramutation: Directed Genetic Change

Abstract

It was observed about 10 years ago that an R^r gene in maize, which conditions anthocyanin formation in seed and plant, invariably has lowered anthocyanin-forming potential following passage through a heterozygote with the stippled (R^nt) allele. The changed form of R^r was gametically transmissible. It reverted toward the standard type when made homozygous, but only partially. This unusual kind of heritable change was termed paramutation. The R^r allele in question was said to be paramutable, and the stippled factor was described as paramutagenic (1).

The initial studies revealed that paramutation involved chromosome components that do not conform to the ordinary rules of gene stability and integrity in heterozygotes. Furthermore, the induced heritable changes were directed. R paramutation appeared to be comparable in these respects to certain phenomena reported much earlier in Pisum by Bateson and Pellew (2), in Malva by Lilienfeld (3), and in Oenothera by Renner (4) as anomalous exceptions to Mendelian inheritance. The R case in maize offered advantages for experimental analysis which these other systems lacked, and it has been intensively investigated. The present article is limited to discussion of the R studies, for the most part, but the results appear to be meaningful for paramutation in general.

The biological significance of paramutation rests on the fact that the phenomenon involves constraint on gene expression during development of the individual which is exerted by factors located within the chromosome itself. The illuminating advances made in recent decades in characterizing the genetic substance biochemically have disclosed genomic components...