1. Department of Plant Biology, University of Georgia, Athens, Georgia 30602, USA
2. Howard Hughes Medical Institute and Department of Genetics, Washington University, St Louis, Missouri 63108, USA
3. These authors contributed equally to this study
4. Present addresses: Department of Horticulture, Michigan State University, East Lansing, Michigan 48824, USA (N.J.); Department of Genome Sciences, University of Washington, Seattle, Washington 98195, USA (Z.B.); Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, California 90095, USA (X.Z.)

Correspondence to: Susan R. Wessler¹ Email: suew@plantbio.uga.edu

Mutator-like transposable elements (MULEs) are found in many eukaryotic genomes and are especially prevalent in higher plants^{1, 2, 3}. In maize, rice and Arabidopsis a few MULEs were shown to carry fragments of cellular genes^{4, 5, 6}. These chimaeric elements are called Pack-MULEs in this study. The abundance of MULEs in rice and the availability of most of the genome sequence permitted a systematic analysis of the prevalence and nature of Pack-MULEs in an entire genome. Here we report that there are over 3,000 Pack-MULEs in rice containing fragments derived from more than 1,000 cellular genes. Pack-MULEs frequently contain fragments from multiple chromosomal loci that are fused to form new open reading frames, some of which are expressed as chimaeric transcripts. About 5% of the Pack-MULEs are represented in collections of complementary DNA. Functional analysis of amino acid sequences and proteomic data indicate that some captured gene fragments might be functional. Comparison of the cellular genes and Pack-MULE counterparts indicates that fragments of genomic DNA have been captured, rearranged and amplified over millions of years. Given the abundance of Pack-MULEs in rice and the widespread occurrence of MULEs in all characterized plant genomes, gene fragment acquisition by Pack-MULEs might represent an important new mechanism for the evolution of genes in higher plants.

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