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.