Mutable R-navajo alleles of cyclic origin in maize

The generation in cyclic fashion of 26 mutable R-Navajo (mR^nj) alleles in maize involved transposition of a non-specific repressor of gene action, Modulator (Mp), first away from, and then back to, the R locus represented by the R-Navajo (R^nj) allele on chromosome 10. The mR^nj alleles reconstituted in this way varied widely, and continuously, in mutability to R^nj—that is, in transposition of Mp away from the R locus, thus derepressing the R^nj gene. They were alike, or nearly so, however, in activating Ds chromosome breakage and in increasing the stability of variegated pericarp, another unstable compound allele comprising Mp conjoined with P^rr on chromosomal 1. These latter two phenomena are based primarily on loci elsewhere in the genome. It is postulated that the 26 reconstituted mR^nj alleles carry a common Mp which, however, is intercalated at a different site within each allele. Nucleotide sequence in the regions adjacent to Mp is assumed to determine the frequency with which a form of micro-nondisjunction occurs whereby Mp is released from a donor site. Transposition to a new site is interpreted in terms of a chromosome model that gives effect to nicking, or single strand breaks, occurring throughout the genome as a prerequisite to unwinding, strand separation, and replication, of the DNA double helix.