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Background: Long terminal repeat (LTR) retrotransposons are highly abundant in plant genomes and require transcriptional activity for their proliferative mode of replication. These sequences exist in plant genomes as diverse sublineages within the main element superfamilies (i.e., gypsy and copia). While transcriptional activity of these elements is increasingly recognized as a regular attribute of plant transcriptomes, it is currently unknown the extent to which different sublineages of these elements are transcriptionally active both within and across species. In the current report, we utilize next generation sequencing methods to examine genomic copy number abundance of diverse LTR retrotransposon sublineages and their corresponding levels of transcriptional activity in three diploid wild sunflower species, Helianthus agrestis, H. carnosus and H. porteri. Results: The diploid sunflower species under investigation differ in genome size 2.75-fold, with 2C values of 22.93 for H. agrestis, 12.31 for H. carnosus and 8.33 for H. porteri. The same diverse gypsy and copia sublineages of LTR retrotransposons were identified across species, but with gypsy sequences consistently more abundant than copia and with global gypsy sequence abundance positively correlated with nuclear genome size. Transcriptional activity was detected for multiple copia and gypsy sequences, with significantly higher activity levels detected for copia versus gypsy. Interestingly, of 11 elements identified as transcriptionally active, 5 exhibited detectable expression in all three species and 3 exhibited detectable expression in two species. Conclusions: Combined analyses of LTR retrotransposon genomic abundance and transcriptional activity across three sunflower species provides novel insights into genome size evolution and transposable element dynamics in this group. Despite considerable variation in nuclear genome size among species, relatively conserved patterns of LTR retrotransposon transcriptional activity were observed, with a highly overlapping set of copia and gypsy sequences observed to be transcriptionally active across species. A higher proportion of copia versus gypsy elements were found to be transcriptionally active and these sequences also were expressed at higher levels. [ABSTRACT FROM AUTHOR]

Background: Long terminal repeat (LTR) retrotransposons are a class of mobile genetic element capable of autonomous transposition via an RNA intermediate. Their large size and proliferative ability make them important contributors to genome size evolution, especially in plants, where they can reach exceptionally high copy numbers and contribute substantially to variation in genome size even among closely related taxa. Using a phylogenetic approach, we characterize dynamics of proliferation events of Ty3/gypsy-like LTR retrotransposons that led to massive genomic expansion in three Helianthus (sunflower) species of ancient hybrid origin. The three hybrid species are independently derived from the same two parental species, offering a unique opportunity to explore patterns of retrotransposon proliferation in light of reticulate evolutionary events in this species group., Results: We demonstrate that Ty3/gypsy-like retrotransposons exist as multiple well supported sublineages in both the parental and hybrid derivative species and that the same element sublineage served as the source lineage of proliferation in each hybrid species' genome. This inference is based on patterns of species-specific element numerical abundance within different phylogenetic sublineages as well as through signals of proliferation events present in the distributions of element divergence values. Employing methods to date paralogous sequences within a genome, proliferation events in the hybrid species' genomes are estimated to have occurred approximately 0.5 to 1 million years ago., Conclusion: Proliferation of the same retrotransposon major sublineage in each hybrid species indicates that similar dynamics of element derepression and amplification likely occurred in each hybrid taxon during their formation. Temporal estimates of these proliferation events suggest an earlier origin for these hybrid species than previously supposed.