Transposable elements (TE) are natural constituents of plant genomes. However, their presence becomes only apparent if they become dislodged from their resident positions in the genome and transpose into another gene thereby inducing a mutation. Such TE-induced mutations are somatically unstable because they revert to wild type and hence reconstitute the expression of the mutated gene. The frequent somatic excision of the TE results in a variegated phenotype. Since this instability is inherited in a Mendelian manner the variegated phenotype is nuclear determined. By this criterion TE have been shown to occur in more than 30 species belonging to different families and genera (Nevers et al. 1986).
Many questions arise when dealing with TE, concerning, for example, their structure and functions but also concerning the biological significance of the activity of elements in the differentiation of a normal plant or in the evolution of plant genes.
In an attempt to answer these questions we will deal with one of the many TEs of Zea mays, the En/I or Spm systems and will include briefly Tam 1, one of the three systems of Antirrhinum majus.