Frequent and Widespread Horizontal Transfers in Plants

Transposable Elements (TEs) are mobile genomic DNA sequences that are found in almost all living organisms. They so densely populate the genomes of many eukaryotic species that they are often the major components, as in man (more than 50%) or bread wheat (more than 95%). TEs achieve their transposition cycle within their host genome and are thus considered to be lineage-specific as they are inherited vertically from one generation to another, like genes. However, unlike genes, they do not encode any information necessary for their host and their insertion into genes can in some cases have a negative effect on its fitness. This « selfish » and potentially deleterious nature has raised the question of their persistence in eukaryotic lineages, especially after it was shown that TEs are strictly controlled by several silencing pathways and efficiently eliminated from their host genomes through deletions. Based on a comparative genomics analysis of 40 sequenced plant genomes from both monocots and dicots, we have identified 32 recent horizontal transfers, based on both their patchy phylogenetic distributions and high sequence identity, and provide evidence that these TEs have remained active since their transfer. We infer that more than two million HTs have probably occurred among monocots and dicots during the last 3 million years. This suggests that frequent horizontal transfers of LTR-retrotransposons (LTR-RTs) are essential in TE biology and constitute a major mechanism ensuring their long term survival in host genomes.