Virus-like attachment sites and plastic CpG islands:landmarks of diversity in plant Del retrotransposons.

Full-length Del elements from ten angiosperm genomes, 5 monocot and 5 dicot, were retrieved and putative attachment (att) sites were identified. In the 2432 Del elements, two types of U5 att sites and a single conserved type of U3 att site were identified. Retroviral att sites confer specificity to the integration process, different att sites types therefore implies lineage specificity. While some features are common to all Del elements, CpG island patterns within the LTRs were particular to lineage specific clusters. All eudicot copies grouped into one single clade while the monocots harbour a more diverse collection of elements. Furthermore, full-length Del elements and truncated copies were unevenly distributed amongst chromosomes. Elements of Del lineage are organized in plants into three clusters and each cluster is composed of elements with distinct LTR features. Our results suggest that the Del lineage efficiently amplified in the monocots and that one branch is probably a newly emerging sub-lineage. Finally, sequences in all groups are under purifying selection. These results show the LTR region is dynamic and important in the evolution of LTR-retrotransposons, we speculate that it is a trigger for retrotransposon diversification.

Analysis of plant LTR-retrotransposons at the fine-scale family level reveals individual molecular patterns.

BACKGROUND: Sugarcane is an important crop worldwide for sugar production and increasingly, as a renewable energy source. Modern cultivars have polyploid, large complex genomes, with highly unequal contributions from ancestral genomes. Long Terminal Repeat retrotransposons (LTR-RTs) are the single largest components of most plant genomes and can substantially impact the genome in many ways. It is therefore crucial to understand their contribution to the genome and transcriptome, however a detailed study of LTR-RTs in sugarcane has not been previously carried out. RESULTS: Sixty complete LTR-RT elements were classified into 35 families within four Copia and three Gypsy lineages. Structurally, within lineages elements were similar, between lineages there were large size differences. FISH analysis resulted in the expected pattern of Gypsy/heterochromatin, Copia/euchromatin, but in two lineages there was localized clustering on some chromosomes. Analysis of related ESTs and RT-PCR showed transcriptional variation between tissues and families. Four distinct patterns were observed in sRNA mapping, the most unusual of which was that of Ale1, with very large numbers of 24nt sRNAs in the coding region. The results presented support the conclusion that distinct small RNA-regulated pathways in sugarcane target the lineages of LTR-RT elements. CONCLUSIONS: Individual LTR-RT sugarcane families have distinct structures, and transcriptional and regulatory signatures. Our results indicate that in sugarcane individual LTR-RT families have distinct behaviors and can potentially impact the genome in diverse ways. For instance, these transposable elements may affect nearby genes by generating a diverse set of small RNA's that trigger gene silencing mechanisms. There is also some evidence that ancestral genomes contribute significantly different element numbers from particular LTR-RT lineages to the modern sugarcane cultivar genome.

Diversification of hAT transposase paralogues in the sugarcane genome.

Transposons are abundant components of eukaryotic genomes, and play important role in genome evolution. The knowledge about these elements should contribute to the understanding of their impact on the host genomes. The hAT transposon superfamily is one of the best characterized superfamilies in diverse organisms, nevertheless, a detailed study of these elements was never carried in sugarcane. To address this question we analyzed 32 cDNAs similar to that of hAT superfamily of transposons previously identified in the sugarcane transcriptome. Our results revealed that these hAT-like transposases cluster in one highly homogeneous and other more heterogeneous lineage. We present evidences that support the hypothesis that the highly homogeneous group is a domesticated transposase while the remainder of the lineages are composed of transposon units. The first is common to grasses, clusters significantly with domesticated transposases from Arabidopsis, rice and sorghum and is expressed in different tissues of two sugarcane cultivars analyzed. In contrast, the more heterogeneous group represents at least two transposon lineages. We recovered five genomic versions of one lineage, characterizing a novel transposon family with conserved DDE motif, named SChAT. These results indicate the presence of at least three distinct lineages of hAT-like transposase paralogues in sugarcane genome, including a novel transposon family described in Saccharum and a domesticated transposase. Taken together, these findings permit to follow the diversification of some hAT transposase paralogues in sugarcane, aggregating knowledge about the co-evolution of transposons and their host genomes.