DIRS retroelements in arthropods: identification of the recently active TcDirs1 element in the red flour beetle Tribolium castaneum.

Members of the DIRS family of retrotransposons differ from most other known retrotransposons in that they encode a tyrosine recombinase (YR), a type of enzyme frequently involved in site-specific recombination. This enzyme is believed to insert the extrachromosomal DNA intermediate of DIRS element retrotransposition into the host genome. DIRS elements have been found in plants, a slime mold, fungi, and a variety of animals including vertebrates, echinoderms and nematodes. They have a somewhat patchy distribution, however, apparently being absent from a number of model organisms such as Saccharomyces cerevisiae, Arabidopsis thaliana and Drosophila melanogaster. In this report we describe the first DIRS retroelement to be identified in an arthropod. This element, TcDirs1, was found in the red flour beetle Tribolium castaneum (Coleoptera). It is generally similar in sequence and structure to several previously described members of the DIRS group: it is bordered by inverted terminal repeats and it has a similar set of protein-coding domains (Gag, reverse transcriptase/ribonuclease H, and the YR), although these are arranged in a novel fashion. TcDirs1 elements exhibit several features indicative of recent activity, such as intact coding regions, a high level of sequence similarity between distinct elements and polymorphic insertion sites. Given their presence in an experimentally tractable host, these potentially active elements might serve as useful models for the study of DIRS element retrotransposition. An element closely related to TcDirs1 was also detected in sequences from a second arthropod, the honey bee Apis mellifera (Hymenoptera), suggesting that these retrotransposons are long-term residents of arthropod genomes.