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Transposon mediated horizontal transfer of the host-specific virulence protein ToxA between three fungal wheat pathogens

By Megan McDonald, Adam P. Taranto, Erin Hill, Benjamin Schwessinger, Zhaohui Liu, Steven Simpfendorfer, Andrew Milgate, Peter Solomon

Posted 17 Jun 2019
bioRxiv DOI: 10.1101/671446 (published DOI: 10.1128/mBio.01515-19)

Most known examples of horizontal gene transfer (HGT) between eukaryotes are ancient. These events are identified primarily using phylogenetic methods on coding regions alone. Only rarely are there examples of HGT where non-coding DNA is also reported. The gene encoding the wheat virulence protein ToxA and surrounding 14 kb is one of these rare examples. ToxA has been horizontally transferred between three fungal wheat pathogens ( Parastagonospora nodorum, Pyrenophora tritici-repentis and Bipolaris sorokiniana ) as part of a conserved ~14kb element, which contains coding and non-coding regions. Here we use long-read sequencing to define the extent of HGT between these three fungal species. Construction of near-chromosomal level assemblies enabled identification of terminal inverted repeats on either end of the 14kb region, typical of a Type II DNA transposon. This is the first description of ToxA with complete transposon features, which we call ToxhAT. In all three species, ToxhAT resides in a large (140-250 kb) transposon-rich genomic island which is absent in toxA -isolates. We demonstrate that the horizontal transfer of ToxhAT between Pyrenophora tritici-repentis and P. nodorum occurred as part of a large ~80kb HGT which is now undergoing extensive decay. In contrast, in B. sorokiniana ToxhAT and its resident genomic island are mobile within the genome. Together these data provide insight into the non-coding regions that facilitate HGT between eukaryotes and the genomic processes which mask the extent of HGT between these species.

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