The En mutable system in maize : III. Transposition associated with mutational events.

1. The mutable allele, a 1 (m(pa-pu)) of the En system at the a 1 locus in maize mutates somatically and germinally to pale, colorless, and purple. 2. Colorless and pale germinal deviants arise at a high frequency. The colorless is more frequent than the pale, and each is more frequent than purple. Frequency is correlated with timing of the somatic mutation event - the earlier colorless sectoring is correlated with the higher frequency of colorless deviants. 3. The regulatory element, En, has been identified at the a 1 locus. The origin of colorless and pale deviants is accompanied by the transposition of an En element away from the a 1 site. 4. The transposing event may lead to implantation of En on the same chromosome, on another chromosome, or no implantation occurs. Transposition to a linked site occurs approximately 25% of the time. There is a preference for transposition to sites 6-20 units from a 1. 5. Secondary transpositions of En occur, and in one test, approximately 12% of the time, to an independent position. Secondary transpositions take place to new linked sites. 6. Preliminary data indicate that transpositions can occur to both distal and proximal positions on chromosome 3. 7. Since differences exist in the behaviour of elements in transposition, it is likely that the transposition event probably is dependent on the elements of specific mutable systems and differing elements within a system. 8. Theoretical aspects of diverse types of impairment of normal gene function by inserted elements is discussed.