Sanctuary: AStarshiptransposon facilitating the movement of the virulence factor ToxA in fungal wheat pathogens

Abstract

AbstractThere is increasing evidence that mobile genetic elements can drive the emergence of pathogenic fungal species by moving virulence genes horizontally. The 14 kbpToxhATtransposon has been shown to be moving the necrotrophic effector,ToxA,horizontally between fungal species that infectTriticum aestivum(wheat), namelyParastagonospora nodorum,Pyrenophora tritici-repentis, andBipolaris sorokiniana. All three species utilise the ToxA protein to infect wheat. Previous genomic evidence foundToxhATin distinct chromosomal positions in two isolates ofB. sorokiniana, indicating that the transposon is still active in this species. Here we confirm the movement ofToxhATusing long-read Nanopore MinION sequencing of eight novel and one previously publishedB. sorokinianaisolates. One event of independent transposition ofToxhATwas observed, and target site duplications of “TA” were identified, confirming this was an autonomous movement facilitated by a yet unidentified transposase. Whole genome analysis revealed thatToxhATis a passenger embedded in a much larger, conserved 170–196 kbp mobile genetic element. This element, termedSanctuary, belongs to the newly describedStarshiptransposon superfamily. This classification is based on the presence of short direct repeats, empty insertion sites, a putative tyrosine recombinase gene and other features ofStarshiptransposons. We also show thatToxhAThas been independently captured by two differentStarships,SanctuaryandHorizonwhich share little to no sequenced identity, outside ofToxhAT.This classification makesHorizonandSanctuarypart of a growing number ofStarshipsinvolved in the horizontal gene transfer of adaptive genetic material between fungal species.ImportanceThe work presented here expands our understanding of a novel group of mobile genetic elements calledStarshipsthat facilitate the horizontal exchange of virulence genes in fungal pathogens. Our analysis shows thatSanctuaryandToxhATare likely active and autonomous transposons in theB. sorokinianagenome. We also show that the smallerToxhATtransposon has been independently captured by two differentStarships,viz. SanctuaryinB. sorokinianaandHorizoninP. tritici-repentisandP. nodorum.Outside ofToxhATthese twoStarshipsshare no sequence identity. The capture ofToxhATby two different mobile elements in three different fungal wheat pathogens demonstrates how horizontal transposon transfer is driving the evolution of virulence in these important wheat pathogens.