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

The mode of transposition of miniature inverted-repeat transposable element s (MITEs) is unknown, but it has been suggested that they are duplicated ra ther than excised at transposition. However, the present investigation demo nstrates that a particular family of MITEs, Stowaway, is excised. Mapped on to 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 sub sequently be reduced in length or deleted. Excision of Stowaway elements le nds strong support to the suggestion that MITEs are DNA transposons and sho uld 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 subst itutions 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 e lements may gradually be reduced.