THE MECHANISM OF MODULATOR TRANSPOSITION IN MAIZE

CCLINTOCK (1956) suggested that a transposable element in maize moves Mfrom the donor site to a different recipient site on the chromosome during the mitotic reduplication process. GREENBLATT and BRINK (1962, 1963) and GREENBLATT (1966) were able to offer experimental support for this idea, and further, developed a working hypothesis of a mechanism of the element's movement. Since the evidence collected by GREENBLATT and BRINK (1962,1963) was based solely on their analysis of red-light variegated twin mutations occurring in medium variegated pericarp, it was not clear at that time whether the model accounted for the way in which all transpositions occur. In fact, it was believed that there might be two or more ways in which an element might transpose. It is the purpose of this report to present a new hypothesis of the mechanism of transposition and show that it is the only mechanism that explains all of the transposition data concerning the autonomous controlling element, Modulator, moving away from the P locus to other chromosomal sites. Since Modulator (Mp) does not produce a direct effect on the maize phenotype, it has been studied by analyzing its interaction with the red pericarp allele on chromosome 1. The listing which follows serves to summarize the pertinent points that are known of this transposable element (Mp)-gene ( P ) interaction. P: This allele is of conventional chromosomal specificity on chromosome 1 conditioning a dominant red pigment in the pericarp, cob, and husk tissues (ANDERSON 1924). Mp: Modulator is a transposable element which has no direct control of any recognizable plant phenotype, but is capable of moving autonomously from one chromosomal site to another during development of the plant. PrMp: When Mp is located at the P locus the PT gene does not come to expression. However, in some cell lineages Mp leaves the P locus during the course of pericarp development, and in these pericarp cells where Mp is absent from PT, the chromosomal gene does come to expression and the red pigment develops. The resultant phenotype is known as medium variegated, and is characterized by a high frequency of red stripes on a colorless background (BRINK and NILAN 1952). P r M p -ttr-Mp: When a transposed Modulator (tr-Mp) is included in a cell lineage which contains the P"Mp complex, the resultant pericarp phenotype ex-