Identification of Large Scale Genomic Rearrangements during Wheat Evolution and the Underlying Mechanisms

Following allopolyploidization, nascent polyploid wheat species react with massive genomic rearrangements, including deletion of transposable element-containing sequences. While such massive rearrangements are considered to be a prominent process in wheat genome evolution and speciation, their structure, extent, and underlying mechanisms remain poorly understood. In this study, we retrieved ~3500 insertions of a specific variant of Fatima, one of the most dynamic long-terminal repeat retrotransposons in wheat from the recently available high-quality genome drafts of Triticum aestivum (bread wheat) and Triticum turgidum ssp. dicoccoides or wild emmer, the allotetraploid mother of all modern wheats. The dynamic nature of Fatima facilitated the identification of large (i.e., up to ~1 million bases) Fatima-containing insertions/deletions (InDels) upon comparison of bread wheat and wild emmer genomes. We characterized 11 such InDels using computer-assisted analysis followed by PCR validation, and found that they occurred via unequal intra-strand recombination or double-strand break events. In most cases, InDels breakpoints were located within transposable element sequences. Additionally, we observed one case of introgression of novel DNA fragments from an unknown source into the wheat genome. Our data thus indicate that massive large-scale DNA rearrangements might play a prominent role in wheat speciation.