Rediscovery of the tardigrade pathogen <i>Sorochytrium milnesiophthora</i> , a blastoclad boasting the longest-known fungal internal transcribed spacer

<i>Sorochytrium milnesiophthora</i> is a necrotrophic pathogen of tardigrades and a member of the poorly sampled zoosporic phylum Blastocladiomycota. First described in 1985, it was previously known only from the type locality, the Appalachian Mountains of North Carolina, USA, and had not been documented in over 35 years. We rediscovered <i>S. milnesiophthora</i> in a lichen sample from Finland, cultured the fungus, and sequenced its genome. Ribosomal DNA and phylogenomic analyses placed <i>S. milnesiophthora</i> in the Blastocladiales as sister to the rest of the order. The Sorochytriaceae clade also included environmental sequences from Brazil, Italy, Poland, and Sweden, indicating a wide distribution and substantial unexplored biodiversity of <i>Sorochytrium</i>-like fungi. To our surprise, <i>S. milnesiophthora</i> was found to have an exceptionally large nuclear rDNA ITS1-5.8S-ITS2 (internal transcribed spacer [ITS]) region of approximately 5500 bp, the largest known in Fungi. This extraordinary ITS length may represent an adaptation to desiccation by slowing ribosome biogenesis. Further studies should examine how <i>S. milnesiophthora</i> infection intersects with tardigrade cryptobiosis and whether the fungus can also survive the extreme conditions for which its host is famous. Deposition of <i>S. milnesiophthora</i> in the Collection of Zoosporic Eufungi at University of Michigan ensures its long-term availability for future investigations.