The action of a chemical mutagen on a mutable system in maize.

The A1 locus in maize controlling anthocyanin pigmentation along with a few other loci, shows an interesting behaviour in that while some alleles like a1, am are highly mutable and mutate to the dominant A1 state in the presence of the non-allelic Dt gene, others like alt are highly stable and would not respond to the action of Dt. The a1-Dt system first studied by Rhoades (1938) and more recently by Neuffer (1961) is rather unique as an example of the control of gene action by other genetic elements inasmuch as the action of the Dt element as a mutator is highly specific being confined to only one locus and to only a few alleles of this locus. This system then appears to provide an opportunity to study several problems of regulation of gene action. For example: What confers specificity to Dt as a mutator and to the alleles at the A1 locus the capacity to respond differentially? Can the action of Dt be altered or copied by genetic elements or nongenetic factors? What is the mechanism underlying stability or conversely high degree of mutability of a genetic element, and particularly to what extent is the molecular organization of a chromosomal segment responsible for such behaviour? Enquiries in these directions would tell us more not only about the regulation of gene action but also the mechanism of gene mutation. Information on any one line of enquiry is likely to offer some clue for the others. To being with, the simplest problem is to explore if the mutability of the Dt-responsive alleles can be altered experimentally and particularly through the action of chemicals with some degree of spe cificity as mutagens. Conversely, it should also be interesting if the stable or non-responsive alleles could be rendered highly mutable through the action of such agents. The most responsive allele, known yet, is a1m. We record below some results obtained with the a1m-Dt system in response to treatment with maleic hydrazide (MH)-a chemical mutagen in a broad sense and strictly speaking a radiomimetic agent, known to act preferentially on heterochromatin (McLeish 1952). A dual action of the chemical is indi cated: (1) induction of point mutation at the A1 locus (apparent mutation of the recessive to the dominant allele) and (2) possible loss of a chromosomal segment carrying the Dt element.