Phase Variation of Regulatory Elements in Maize

HASE variation of regulatory elements in maize refers to changes in functioning of these elements from periods of activity to periods of inactivity or vice versa (PETERSON 1965a). For example, a case of pattern variation representing alternative phenotypes (presence or absence of mutability) was observed recently in aleurone tissue. This paper deals with an analysis of the “flow” and ‘‘cr(yw11’’ phenotypic patterns of mutability associated with the mutable a, allele, which is known to be a part of the two-element regulator-controller system that includes the regulatory element, Enhancer, En (PETERSON 1960,1961). In studies of the Ac-Ds system, MCCLINTOCK (1947, 1951) showed that the expression of alternative phenotypes (representing a mutation event) during growth and development of a tissue reflected the transposition of the inhibitory controlling element Ds away from the affected locus. Such mutational changes associated with the movement of a transposable element have been reported for Mp at the P locus (BRINK and NILAN 1952; BRINK 1954), for Spm (MCCLINTOCK 1956, 1957,1961), En (PETERSON 1965c) and Dt (NUFFER 1961) at the aI locus, and for a factor affecting the Ab allele (NEUFFER 1963). In Salmonella, the movement of an episomic element, comparable to the transposition of controlling elements in maize, is also associated with a mutation event (DAWSON and SMITHKEARY 1963). In the above cited cases, the mutation event is associated with transposition of the element which controls the expression of the affected locus. Phase variation implies a phenotypic change that is not to be ascribed to transposition of the inhibitory element but to the inception or termination of its activity. Further examination of this concept will follow the analysis of the “flow” and “crown” pattern types (PETERSON 1965b).