Mutation and paramutation at the R locus in maize.

HE R locus in maize exhibits two different kinds of genetic variability, mutation and paramutation (BRINK 1960). Mutation is a rare event, not predictable in an individual gamete, and produces a limited number of visually distinct mutant phenotypes. In contrast, paramutation is a directed change in the pigmenting ability of an R allele which invariably occurs in certain genotypes, and results in a continuous array of genetically different forms. Presumably mutation is concerned with changes in the basic genetic material of the locus, while paramutation affects expression of the basic material during development of the individual. On this basis, the ability of an R' allele to undergo paramutation should not be related to the ability of the allele to mutate. Alternatively, if paramutation affects the basic structure or composition of the locus, the abilities to mutate and to undergo paramutation might be interrelated. The paramutability of an R allele is readily measured by passing the factor through a heterozygote with an overtly paramutagenic allele, and mutation of R to colorless or near-colorless forms provides one measure of mutability. Quantitative data on mutation and paramutation for a series of R alleles provide a basis for determining whether any general relationship exists between the levels of expression of the two processes. Another approach is to examine the sequential effect of one process on the other (i.e., paramutation of mutant R alleles, and mutation of paramutant R alleles). BROWN (1963) has shown that the paramutational capacity of an Rr allele may or may not be changed by mutation, depending on the class of mutant, and BRINK (1958) made a preliminary study on mutation of a paramutant R allele. Further data on the interrelationship between mutation and paramutation are presented in the present paper.