Phylogenetic evidence for excision of Stowaway miniature inverted-repeat transposable elements in triticeae (Poaceae)

The mode of transposition of miniature inverted-repeat transposable elements (MITEs) is unknown, but it has been suggested that they are duplicated rather than excised at transposition. However, the present investigation demonstrates that a particular family of MITEs, Stowaway:, is excised. Mapped onto a gene tree based on partial sequences of disrupted meiotic cDNA1 (DMC1) from 30 species of the Triticeae grasses, it is evident that at least two excisions have occurred, leaving short footprints. These footprints may subsequently be reduced in length or deleted. Excision of Stowaway: elements lends strong support to the suggestion that MITEs are DNA transposons and should be classified as class II elements. The evolution of Stowaway: elements can also be traced by scrutiny of the gene tree. It appears that base substitutions are as frequent in the conserved terminal inverted repeats (TIRs) as in the core of the element. Neither substitutions nor deletions lead to compensatory changes; hence, the highly stable secondary structure of the elements may gradually be reduced.