LTR retrotransposons reveal recent extensive inter-subspecies nonreciprocal recombination in Asian cultivated rice

Long Terminal Repeats retrotransposons (LTR elements) are ubiquitous Eukaryotic transposable elements (TEs). They are considered to be one of the major forces underlying plant genome evolution. Because of relatively high evolutionary speed, active transposition of LTR elements in the host genomes provides rich information on their short-term history. As more and more genomes, especially those of closely related organisms, have been sequenced, it is possible to perform global comparative study of their LTR retrotransposons to reveal events in the history.

The present research is designed to investigate important evolutionary events in the origin of Asian cultivated rice through the comparison of LTR elements. We have developed LTR_INSERT, a new method for LTR elements discovery in two closely related genomes. Our method has a distinctive feature that it is capable of judging whether an insertion occurs prior or posterior to the divergence of genomes. LTR_INSERT identifies 993 full-length LTR elements, annotates 15916 copies related with them, and discovers at least 16 novel LTR families in the whole-genome comparative map of two cultivated rice subspecies. From the full-length LTR elements, we estimate that a significant proportion of the rice genome has experienced inter-subspecies nonreciprocal recombination (ISNR) in as recent as 53,000 years. Large-scale samplings further support that more than 15% of the rice genome has been involved in such recombination. In addition, LTR elements confirm that the genome of O. sativa ssp. indica and that of japonica diverged about 600,000 years ago.

A new LTR retrotransposon identification method integrating both comparative genomics and ab initio algorithm is introduced and applied to Asian cultivated rice genomes. At whole-genome level, this work confirms that recent ISNR is an important factor that molds modern cultivated rice genome.