Plant transposable elements generate the DNA sequence diversity needed in evolution

Two germinal and 16 somatic reversion events induced by the Enhancer (En) transposable element system at the wx-8::Spm-I8 allele of Zea mays were cloned and studied by sequence analysis. Excision of the Spm-I8 receptor element from the wx gene results in various mutant DNA sequences. This leads to altered gene products, some of which are still capable of restoring the wild-type phenotype. Possible 'foot-print' sequences that may have arisen by the excision of transposable elements were observed when intron sequences of the wild-type (wx+) and mutant (wx-m8) alleles of the wx gene were compared. The sequence divergence generated by visitation of a locus by plant transposable elements is discussed with respect to the molecular evolution of the new gene functions.     

The Spm (En) transposable element controls the excision of a 2-kb DNA insert at the wx allele of Zea mays

The waxy (Wx) locus of Zea mays was cloned from strains carrying the wild-type and wx^m-8 mutant alleles. The receptor component of the Suppressor-Mutator (Spm) controlling element system in the wx^m-8 allele was shown to be a 2 kb long insertion within the transcribed region of the Wx gene. The insertion, termed Spm-I8, is excised during somatic reversion events induced by the autonomous controlling element Enhancer (En), which is an equivalent to Spm. Integration of Spm-I8 into the Wx gene generates a 3-bp target site duplication. Spm-I8 has a 13 bp long inverted repeat at its termini. The ends of the element can be further folded to build a large double-stranded structure consisting of five perfectly matching double-stranded regions of 9–13 bp in length, interrupted by single-stranded loops. A comparison of the wild-type and wx^m-8 alleles revealed two additional insertions 6 (insert-1) and 0.25 (insert-2) kb in length. No En-induced excision of insert-1 and insert-2 could be detected so far. There is remarkable structure and sequence homology between Spm-I8 and the transposable elements Tam1 and Tam2 of Antirrhinum majus at their termini, reflecting a possible evolutionary and/or functional relationship between transposons in different plant species.