CACTA Transposons in Triticeae. A Diverse Family of High-Copy Repetitive Elements

In comparison with retrotransposons, which comprise the majority of the Triticeae genomes, very few class 2 transposons have been described in these genomes. Based on the recent discovery of a local accumulation of CACTA elements at the Glu-A3 loci in the two wheat species Triticum monococcum and Triticum durum, we performed a database search for additional such elements in Triticeae spp. A combination of BLAST search and dot-plot analysis of publicly available Triticeae sequences led to the identification of 41 CACTA elements. Only seven of them encode a protein similar to known transposases, whereas the other 34 are considered to be deletion derivatives. A detailed characterization of the identified elements allowed a further classification into seven subgroups. The major subgroup, designated the "Caspar " family, was shown by hybridization to be present in at least 3,000 copies in the T. monococcum genome. The close association of numerous CACTA elements with genes and the identification of several similar elements in sorghum (Sorghum bicolor) and rice (Oryza sativa) led to the conclusion that CACTA elements contribute significantly to genome size and to organization and evolution of grass genomes.