Paramutation and Chromosome Organization

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Abstract
Two processes, mutation and paramutation, are postulated to account for the observed heritable changes in''the immediate R region of chromosome 10 in maize. Mutation is sporadic, undirected, and yields stable alleles having distinctive effects on aleurone and plant pigmentation. Paramutation occurs invariably in certain heterozygotes, is directed, and gives unstable alleles conditioning intergrading phenotypes. The paramutant phenotype is R-locus dependent, but paramutagenicity and pigment production are separable functions. Paramutation occurs in somatic cells; and paramutable R alleles are widespread in Zea mays. An R factor in a heterozygote with a strongly paramutagenic allele may itself become weakly paramutagenic. Reciprocal translocations involving chromosome breaks at remote points, proximal or distal to R, and in coupling with the latter, increase pigment-producing action of R, and render R comparatively insensitive to paramutation in Rst heterozygotes. These effects are not immediately detectable following introduction of R into a translocation chromosome but, once established they persist for at least one generation after return of R by crossing over to a structurally normal chromosome. R is relatively insensitive to paramutation when carried by an abnormal chromosome 10 containing a large heterochromatic knob. The meaning of paramutation for chromosome organization is conjectural. The evidence is discussed in relation to a hypothesis in which it is assumed that the chromosome comprises two classes of components, and operates as an information exchange during differentiation and development of the organism. The occurrence of paramutation in other organisms than maize is considered, and it is concluded that possibly paramutation may be a very general phenomenon.