A survey of Gypsy and Copia LTR-retrotransposon superfamilies and lineages and their distinct dynamics in the Populus trichocarpa (L.) genome

In this work, we report a comprehensive study of long terminal repeat retrotransposons of Populus trichocarpa. Our research group studied the retrotransposon component of the poplar genome in 2012, isolating 1479 putative full-length elements. However, in that study, it was not possible to identify the superfamily to which the majority of isolated full-length elements belonged. Moreover, during recent years, the genome sequence of P. trichocarpa has been updated, deciphering thek sequences of a number of previously unresolved loci. In this work, we performed a complete scan of the updated version of the genome sequence to isolate full-length retrotransposons based on sequence and structural features. The new dataset showed a reduced number of elements (958), and 21 fulllength elements were discovered for the first time. The majority of retroelements belonged to the Gypsy superfamily (57%), while Copia elements amounted to 41.1% of the dataset. Fulllength elements were dispersed throughout the chromosomes. However, Gypsy and, to a lesser extent, Copia elements accumulated preferentially at putative centromeres. Gypsy elements were more active in retrotransposition than Copia elements, with the exception of during the past million years, in which Copia elements were the most active. Improved annotation procedures also allowed us to determine the specific lineages to which isolated elements belonged. The three Gypsy lineages, Athila, OGRE, and Chromovirus (in the decreasing order), were by far the most abundant. On the other hand, each identified Copia lineage represented less than 1 % of the genome. Significant differences in the insertion age were found among lineages, suggesting specific activation mechanisms. Moreover, different chromosomal regions were affected by retrotransposition in different ages. In all chromosomes, putative pericentromeric regions were filled with elements older than themean insertion age. Overall, results demonstrate structural and functional differences among plant retrotransposon lineages and further support the view of retrotransposons as a community of different organisms in the genome.