Permanent fixation of a transposable element insert in the A2 gene of maize (Zea mays L.)

Transposable elements are considered to be responsible for creating genetic variation that contributes to evolutionary change. The pervasiveness of transposable elements in certain breeding lines of maize suggests that part of the observed genetic variation in those lines might be the result of transposition activity. Stable genetic variation often results from the allelic differences created by footprints generated in the host genes upon element excision; such variation can also result from insertions that are permanently fixed at a particular locus. The 1.3-kb element within the a2-m1 (class II state) allele of maize is one example of a stable insertion. Though the l element never excises from the A2 gene, it interacts with the TNPA (transposase A) product of the En/Spm element. In this study, we tested whether continued interaction of the l element with TNPA would lead to excision of l or other change of this allele. Our screening of 220,000 kernels did not yield any new states of a2-m1 in the presence of an active autonomous En/Spm element, indicating that this l element is highly stable and permanently fixed at this locus. The probable implications of l-element stability are discussed.