LTR retrotransposon landscape in Medicago truncatula: more rapid removal than in rice.

Long terminal repeat retrotransposons (LTR elements) are ubiquitous Eukaryotic TEs that transpose through RNA intermediates. Accounting for significant proportion of many plant genomes, LTR elements have been well established as one of the major forces underlying the evolution of plant genome size, structure and function. The accessibility of more than 40% of genomic sequences of the model legume Medicago truncatula (Mt) has made the comprehensive study of its LTR elements possible. We use a newly developed tool LTR_FINDER to identify LTR retrotransposons in the Mt genome and detect 526 full-length elements as well as a great number of copies related to them. These elements constitute about 9.6% of currently available genomic sequences. They are classified into 85 families of which 64 are reported for the first time. The majority of the LTR retrotransposons belong to either Copia or Gypsy superfamily and the others are categorized as TRIMs or LARDs by their length. We find that the copy-number of Copia-like families is 3 times more than that of Gypsy-like ones but the latter contribute more to the genome. The analysis of PBS and protein-coding domain structure of the LTR families reveals that they tend to use only 4-5 types of tRNAs and many families have quite conservative ORFs besides known TE domains. For several important families, we describe in detail their abundance, conservation, insertion time and structure. We investigate the amplification-deletion pattern of the elements and find that the detectable full-length elements are relatively young and most of them were inserted within the last 0.52 MY. We also estimate that more than ten million bp of the Mt genomic sequences have been removed by the deletion of LTR elements and the removal of the full-length structures in Mt has been more rapid than in rice. This report is the first comprehensive description and analysis of LTR retrotransposons in the Mt genome. Many important novel LTR families were discovered and their evolution is elucidated. Our results may outline the LTR retrotransposon landscape of the model legume.

Wang H ，Liu JS 《BMC GENOMICS》

Evolutionary history of Oryza sativa LTR retrotransposons: a preliminary survey of the rice genome sequences.

LTR Retrotransposons transpose through reverse transcription of an RNA intermediate and are ubiquitous components of all eukaryotic genomes thus far examined. Plant genomes, in particular, have been found to be comprised of a remarkably high number of LTR retrotransposons. There is a significant body of direct and indirect evidence that LTR retrotransposons have contributed to gene and genome evolution in plants. To explore the evolutionary history of long terminal repeat (LTR) retrotransposons and their impact on the genome of Oryza sativa, we have extended an earlier computer-based survey to include all identifiable full-length, fragmented and solo LTR elements in the rice genome database as of April 2002. A total of 1,219 retroelement sequences were identified, including 217 full-length elements, 822 fragmented elements, and 180 solo LTRs. In order to gain insight into the chromosomal distribution of LTR-retrotransposons in the rice genome, a detailed examination of LTR-retrotransposon sequences on Chromosome 10 was carried out. An average of 22.3 LTR-retrotransposons per Mb were detected in Chromosome 10. Gypsy-like elements were found to be >4 x more abundant than copia-like elements. Eleven of the thirty-eight investigated LTR-retrotransposon families displayed significant subfamily structure. We estimate that at least 46.5% of LTR-retrotransposons in the rice genome are older than the age of the species (< 680,000 years). LTR-retrotransposons present in the rice genome range in age from those just recently inserted up to nearly 10 million years old. Approximately 20% of LTR retrotransposon sequences lie within putative genes. The distribution of elements across chromosome 10 is non-random with the highest density (48 elements per Mb) being present in the pericentric region.

Gao L ，McCarthy EM ，Ganko EW ，McDonald JF ... -  《BMC GENOMICS》

Rapid and Recent Evolution of LTR Retrotransposons Drives Rice Genome Evolution During the Speciation of AA-Genome Oryza Species.

The dynamics of long terminal repeat (LTR) retrotransposons and their contribution to genome evolution during plant speciation have remained largely unanswered. Here, we perform a genome-wide comparison of all eight AA-genome species, and identify 3911 intact LTR retrotransposons classified into 790 families. The top 44 most abundant LTR retrotransposon families show patterns of rapid and distinct diversification since the species split over the last ∼4.8 MY (million years). Phylogenetic and read depth analyses of 11 representative retrotransposon families further provide a comprehensive evolutionary landscape of these changes. Compared with Ty1-, independent bursts of Ty3- retrotransposon expansions have occurred with the three largest showing signatures of lineage-specific evolution. The estimated insertion times of 2213 complete retrotransposons from the top 23 most abundant families reveal divergent life histories marked by speedy accumulation, decline, and extinction that differed radically between species. We hypothesize that this rapid evolution of LTR retrotransposons not only divergently shaped the architecture of rice genomes but also contributed to the process of speciation and diversification of rice.

Zhang QJ ，Gao LZ 《G3-Genes Genomes Genetics》

Long terminal repeat retrotransposons of Oryza sativa.

Long terminal repeat (LTR) retrotransposons constitute a major fraction of the genomes of higher plants. For example, retrotransposons comprise more than 50% of the maize genome and more than 90% of the wheat genome. LTR retrotransposons are believed to have contributed significantly to the evolution of genome structure and function. The genome sequencing of selected experimental and agriculturally important species is providing an unprecedented opportunity to view the patterns of variation existing among the entire complement of retrotransposons in complete genomes. Using a new data-mining program, LTR_STRUC, (LTR retrotransposon structure program), we have mined the GenBank rice (Oryza sativa) database as well as the more extensive (259 Mb) Monsanto rice dataset for LTR retrotransposons. Almost two-thirds (37) of the 59 families identified consist of copia-like elements, but gypsy-like elements outnumber copia-like elements by a ratio of approximately 2:1. At least 17% of the rice genome consists of LTR retrotransposons. In addition to the ubiquitous gypsy- and copia-like classes of LTR retrotransposons, the rice genome contains at least two novel families of unusually small, non-coding (non-autonomous) LTR retrotransposons. Each of the major clades of rice LTR retrotransposons is more closely related to elements present in other species than to the other clades of rice elements, suggesting that horizontal transfer may have occurred over the evolutionary history of rice LTR retrotransposons. Like LTR retrotransposons in other species with relatively small genomes, many rice LTR retrotransposons are relatively young, indicating a high rate of turnover.

McCarthy EM ，Liu J ，Lizhi G ，McDonald JF ... -  《GENOME BIOLOGY》

Analyses of LTR-retrotransposon structures reveal recent and rapid genomic DNA loss in rice.

We initially analyzed 11 families of low- and middle-copy-number long terminal repeat (LTR) retrotransposons in rice to determine how their structures have diverged from their predicted ancestral forms. These elements, many highly fragmented, were identified on the basis of sequence homology and structural characteristics. The 11 families, totaling 1000 elements, have copy numbers ranging from 1 to 278. Less than one-quarter of these elements are intact, whereas the remaining are solo LTRs and variously truncated fragments. We also analyzed two highly repetitive families (Osr8 and Osr30) of LTR retrotransposons and observed the same results. Our data indicate that unequal homologous recombination and illegitimate recombination are primarily responsible for LTR-retrotransposon removal. Further analysis suggests that most of the detectable LTR retrotransposons in rice inserted less than 8 million years ago, and have now lost over two-thirds of their encoded sequences. Hence, we predict that the half-life of LTR-retrotransposon sequences in rice is less than 6 million years. Moreover, our data demonstrate that at least 22% (97 Mb) of the current rice genome is comprised of LTR-retrotransposon sequences, and that more than 190 Mb of LTR-retrotransposon sequences have been deleted from the rice genome in the last 8 million years.

Ma J ，Devos KM ，Bennetzen JL 《GENOME RESEARCH》