Transposon mediated horizontal transfer of the host-specific virulence protein ToxA between three fungal wheat pathogens
Authors: McDonald Megan C. , Taranto Adam P. , Hill Erin , Schwessinger Benjamin , Liu Zhaohui , Simpfendorfer Steven , Milgate Andrew , Solomon Peter S. 
Published: June 2019

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.ToxAhas been horizontally transferred between three fungal wheat pathogens (Parastagonospora nodorum, Pyrenophora tritici-repentisandBipolaris 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 ofToxAwith 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 intoxA-isolates. We demonstrate that the horizontal transfer of ToxhAT betweenPyrenophora tritici-repentisandP. nodorumoccurred as part of a large ∼80kb HGT which is now undergoing extensive decay. In contrast, inB. sorokinianaToxhAT 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.

IMPORTANCE

This work dissects the tripartite horizontal transferof ToxA; a gene that has a direct negative impact on global wheat yields. Defining the extent of horizontally transferred DNA is important because it can provide clues as to the mechanisms that facilitate HGT. Our analysis ofToxAand its surrounding 14kb suggests that this gene was horizontally transferred in two independent events, with one event likely facilitated by a Type II DNA transposon. These horizontal transfer events are now in various processes of decay in each species due to the repeated insertion of new transposons and subsequent rounds of targeted mutation by a fungal genome defense mechanism known as repeat induced point-mutation. This work highlights the role that HGT plays in the evolution of host adaptation in eukaryotic pathogens. It also increases the growing body of evidence that transposons facilitate adaptive HGT events between fungi present in similar environments and hosts.

DATA AVAILABILITY

All raw sequencing data is available under NCBI BioProject PRJNA505097.

TheP. nodorumSN15 Whole Genome Shotgun project has been deposited at DDBJ/ENA/GenBank under the accession SSHU00000000. The version SSHU01000000 is described in this paper. TheP. nodorumSN79-1087 Whole Genome Shotgun project has been deposited under the accessionsCP039668-CP039689.The Whole Genome shotgun project and accession numbers forB. sorokinianaisolates are as follows: CS10; SRZH00000000, CS27; SRZG00000000, WAI2406; SRZF00000000, WAI2411; SRZE00000000. Transposon annotations, CS10 and CS27 gene annotations are available athttps://github.com/megancamilla/Transposon-Mediated-transfer-of-ToxA