Lineage-specific amplification and epigenetic regulation of LTR-retrotransposons contribute to the structure, evolution, and function of Fabaceae species

Abstract Background Long terminal repeat (LTR)-retrotransposons (LTR-RTs) are ubiquitous and make up the majority of nearly all sequenced plant genomes, whereas their pivotal roles in genome evolution, gene expression regulation as well as their epigenetic regulation are still not well understood, especially in a large number of closely related species. Results Here, we analyzed the abundance and dynamic evolution of LTR-RTs in 54 species from an economically and agronomically important family, Fabaceae, and also selected two representative species for further analysis in expression of associated genes, transcriptional activity and DNA methylation patterns of LTR-RTs. Annotation results revealed highly varied proportions of LTR-RTs in these genomes (5.1%~68.4%) and their correlation with genome size was highly positive, and they were significantly contributed to the variance in genome size through species-specific unique amplifications. Almost all of the intact LTR-RTs were inserted into the genomes 4 Mya (million years ago), and more than 50% of them were inserted in the last 0.5 million years, suggesting that recent amplifications of LTR-RTs were an important force driving genome evolution. In addition, expression levels of genes with intronic, promoter, and downstream LTR-RT insertions of Glycine max and Vigna radiata, two agronomically important crops in Fabaceae, showed that the LTR-RTs located in promoter or downstream regions suppressed associated gene expression. However, the LTR-RTs within introns promoted gene expression or had no contribution to gene expression. Additionally, shorter and younger LTR-RTs maintained higher mobility and transpositional potential. Compared with the transcriptionally silent LTR-RTs, the active elements showed significantly lower DNA methylation levels in all three contexts. The distributions of transcriptionally active and silent LTR-RT methylation varied across different lineages due to the position of LTR-RTs located or potentially epigenetic regulation. Conclusion Lineage-specific amplification patterns were observed and higher methylation level may repress the activity of LTR-RTs, further influence evolution in Fabaceae species. This study offers valuable clues into the evolution, function, transcriptional activity and epigenetic regulation of LTR-RTs in Fabaceae genomes.