THE TIMING OF Mu ACTIVITY IN MAIZE

The timing of mutator activity of Mu in maize was tested in three ways: (1) by allelism tests of phenotypically similar male-transmitted mutants, (2) by studying the clustering of phenotypically similar mutants as demonstrated by ear maps and the subsequent allelism tests of these mutants, and (3) by the induction of somatic sectors in Mu plants heterozygous for plant and endosperm marker genes. Allelism tests of phenotypically similar mutants in out-crosses of Mu plants as males established that 18.8% were allelic and that premeiotic mutants are induced. This conclusion was supported by ear maps of Mu-bearing plants, which revealed sectors of seeds that produced plants bearing phenotypically similar allelic mutants. The smallness of these sectors indicated that the premeiotic activity of Mu that gave rise to them occurred very late. The lack of visible sectors in mature sporophytic, endosperm and aleurone tissue in plants carrying Mu supports the conclusion that the mutator activity of Mu does not occur throughout the ontogeny of the plant and seems to be restricted to a time shortly before and/or during meiosis.