<i>Sanctuary</i> : a <i>Starship</i> transposon facilitating the movement of the virulence factor ToxA in fungal wheat pathogens

There is increasing evidence that mobile genetic elements can drive the emergence of pathogenic fungal species by moving virulence genes horizontally. The 14 kbp <i>ToxhAT</i> transposon was shown to move the necrotrophic effector, <i>ToxA,</i> horizontally between wheat pathogens, namely <i>Parastagonospora nodorum</i>, <i>Pyrenophora tritici-repentis</i>, and <i>Bipolaris sorokiniana</i>. All three species utilize the ToxA protein to infect wheat. Previous work found <i>ToxhAT</i> in distinct chromosomal positions in two <i>B. sorokiniana</i> isolates, indicating that the transposon remains active in this species. Here, we confirm the movement of <i>ToxhAT</i> using long-read sequencing of eight new and one previously published <i>B. sorokiniana</i> isolates. One event of independent transposition of <i>ToxhAT</i> was observed, and target site duplications of "TA" were identified, confirming that this is an active transposon in this species that likely falls into the <i>Tc1/Mariner</i> transposon family. We propose renaming this non-autonomous transposon to <i>ToxTA</i>. Whole genome analysis revealed that <i>ToxTA</i> is a passenger embedded in a much larger, conserved 170-196 kbp mobile genetic element. This element, termed <i>Sanctuary</i>, belongs to the newly described <i>Starship</i> transposon superfamily. This classification is based on the presence of direct repeats, empty insertion sites, a putative tyrosine recombinase gene, and other features of <i>Starship</i> transposons. We also show that <i>ToxTA</i> has been independently acquired by two different <i>Starships</i>, <i>Sanctuary</i> and <i>Horizon,</i> which share little to no sequence identity, outside of <i>ToxTA</i>. This classification makes <i>Horizon</i> and <i>Sanctuary</i> part of a growing number of <i>Starships</i> involved 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 called <i>Starships</i> that facilitate the horizontal exchange of numerous genes between fungal pathogens. Our analysis shows that <i>Sanctuary</i> and <i>ToxTA</i> are both active transposons within the <i>Bipolaris sorokiniana</i> genome. We also show that the smaller <i>ToxTA</i> transposon has been independently acquired by two different <i>Starships</i>, namely <i>Sanctuary</i> in <i>B. sorokiniana</i> and <i>Horizon</i> in <i>Pyrenophora tritici-repentis</i> and <i>Parastagonospora nodorum</i>. Outside of <i>ToxTA,</i> these two <i>Starships</i> share no sequence identity. The acquisition of <i>ToxTA</i> by 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.