The properties, origin, and mechanism of conversion-type inheritance at the B locus in maize.

EGULAR, heritable changes in gene function directed by an allele were first demonstrated by BRINK (1956a, b, c) at the R locus in maize. Changes of this type have been designated paramutation (BRINK 1958a), a convenient generic term emphasizing both the heritable nature of the changes and their originanation through heterozygous association of a sensitive (paramutable) allele with an inducing (paramutagenic) allele. Similar behavior (continuing conversiontype inheritance, an extreme form of paramutation) has been observed at the B locus in maize (COE 1958, 1959a), and somewhat similar behavior (somatic conversion) with the sulf alleles in tomato (HAGEMANN 1958a, b) . In each case, an inducing allele ( R S t , Rmb, and certain Rst derivatives; B’ and certain paramutants; su1fPsra and sulfvag) causes heritable alteration in a sensitive allele (R‘ and certain alleles; B and certain alleles; s u l f f ) in the direction of reduced function. Earlier reports (COE 1958, 1959a, b, 1960, 1961a, b) describe the basic B’ phenomenon. B B plants (intense anthocyanin color) give rise occasionally to weakcolored mutants (B’) . Progeny from these B’ mutants, self-pollinated or crossed to B or b, are all weak in color. Repeated backcrosses of B’ to B yield only B’ plants without segregation, even though markers on either side of B segregate and recombine normally. Segregation is found, however, in progeny from B’ b hybrids, demonstrating association of the phenomenon with the B locus: the cross of B‘ b X B B gives half B’ plants (genetically B’ B’ in the succeeding generation) and half B plants (genetically B b ) . The phenomenon has been considered broadly to indicate conversion of B to B’ during a life cycle of exposure to B’ in heterozygous individuals, with b intractable to conversion. A gene-dependent plasmid model has been considered, but does not appear to be involved ( COE 1961 b) . The present paper reports experiments designed to examine in detail the properties and mechanism of the B’ phenomenon. A model is developed that is compatible with the controlling element ( MCCLINTOCK 1951, 1961 ) , parachromatin BRINK 1960) or episome (JACOB and WOLLMAN 1961) concepts. An abstract of the general conclusions has been published ( COE 1963).