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Illuminating the transposon insertion landscape in plants using Cas9-targeted Nanopore sequencing and a novel pipeline
Kirov I., Merkulov P., Gvaramiya S., Komakhin R., Omarov M., Dudnikov M., Kocheshkova A., Soloviev A., Karlov G., Divashuk M.
Transposable elements (TEs) are a major source of genomic novelty, contributing to plant adaptation, speciation and new cultivar production. The often large, complex genomes of plants make identifying TE insertions from short reads challenging, while whole-genome sequencing remains expensive. In addition, rare somatic insertions reflecting mobilome dynamic are difficult to track using short reads. To overcome these challenges, we applied the Cas9-targeted Nanopore sequencing (CANS) approach and a novel pipeline, NanoCasTE, to trace both genetically inherited and somatic TE insertions in plants. We performed CANS of EVD retrotransposon in wild-type Arabidopsis thaliana and obtained up to 40x coverage of the targets after only a few hours of sequencing on a MinION sequencer. To estimate the ability to detect new TE and T-DNA insertions, we exploited the A. thaliana ddm1 mutant, which has elevated TE activity. Using CANS, we detect the hemizygous T-DNA insertion site and genetically inherited EVD insertions. CANS also detected >800 somatic TE insertions allowing monitoring of TE insertions in real time. By this, we built a high-density physical map of EVD insertion preferences in the A. thaliana ddm1 and found that EVD insertions are biased toward pericentromeric regions in ddm1 that differ from wild-type A. thaliana ecotypes suggesting a pivotal role of DNA methylation in the shaping of EVD insertion preferences. Using CANS, we were able to detect transposition events of ONSEN transposon family that occurred in response to heat stress and found hotspots of ONSEN insertions. Our results indicate that CANS and NanoCasTE are effective tools for the detection of TEIs in real time, opening an avenue for obtaining a deeper understanding of mobilome organization in response to stress and in different genetic backgrounds. The ability of CANS to detect genetically inherited TEIs and T-DNA integration sites make it useful for screening insertion mutants and transgenic plants.