TEnest: Automated Chronological Annotation and Visualization of Nested Plant Transposable Elements

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Abstract
Organisms with a high density of transposable elements (TEs) exhibit nesting, with subsequent repeats found inside previously inserted elements. Nesting splits the sequence structure of TEs and makes annotation of repetitive areas challenging. We present TEnest, a repeat identification and display tool made specifically for highly repetitive genomes. TEnest identifies repetitive sequences and reconstructs separated sections to provide full-length repeats and, for long-terminal repeat (LTR) retrotransposons, calculates age since insertion based on LTR divergence. TEnest provides a chronological insertion display to give an accurate visual representation of TE integration history showing timeline, location, and families of each TE identified, thus creating a framework from which evolutionary comparisons can be made among various regions of the genome. A database of repeats has been developed for maize (Zea mays), rice (Oryza sativa), wheat (Triticum aestivum), and barley (Hordeum vulgare) to illustrate the potential of TEnest software. All currently finished maize bacterial artificial chromosomes totaling 29.3 Mb were analyzed with TEnest to provide a characterization of the repeat insertions. Sixty-seven percent of the maize genome was found to be made up of TEs; of these, 95% are LTR retrotransposons. The rate of solo LTR formation is shown to be dissimilar across retrotransposon families. Phylogenetic analysis of TE families reveals specific events of extreme TE proliferation, which may explain the high quantities of certain TE families found throughout the maize genome. The TEnest software package is available for use on PlantGDB under the tools section (http://www.plantgdb.org/prj/TE_nest/TE_nest.html); the source code is available from (http://wiselab.org).