PT  - JOURNAL ARTICLE
AU  - M. Ryan Woodcock
AU  - Kaleigh Powers
AU  - Kirsten Snead
AU  - Jason Pellettieri
TI  - Flatworm transcriptomes reveal widespread parasitism by histophagous ciliates
AID  - 10.1101/2023.09.17.558123
DP  - 2023 Jan 01
TA  - bioRxiv
PG  - 2023.09.17.558123
4099  - http://biorxiv.org/content/early/2023/09/17/2023.09.17.558123.short
4100  - http://biorxiv.org/content/early/2023/09/17/2023.09.17.558123.full
AB  - Unicellular ciliates like Tetrahymena are best known as free-living bacteriovores, but many species are facultative or obligate parasites. These ‘histophages’ feed on the tissues of hosts ranging from planarian flatworms to commercially important fish and the larvae of imperiled freshwater mussels. Here, we developed a novel bioinformatics pipeline incorporating the nonstandard ciliate genetic code and used it to search for Ciliophora sequences in 34 publicly available Platyhelminthes EST libraries. From 2,615,036 screened ESTs, we identified nearly 6,000 high-confidence ciliate transcripts, supporting parasitism of seven additional flatworm species. We also cultured and identified Tetrahymena from nine terrestrial and freshwater planarians, including invasive earthworm predators from the genus Bipalium and the widely studied regeneration models Dugesia japonica and Schmidtea mediterranea. A cophylogenetic reconstruction provides strong evidence for coevolution of histophagous Ciliophora with their Platyhelminthes hosts. We further report the antiprotozoal aminoglycoside paromomycin expels Tetrahymena from S. mediterranea, providing new opportunities to investigate the effects of this relationship on planarian biology. Together, our findings raise the possibility that invasive flatworms constitute a novel dispersal mechanism for Tetrahymena parasites and position the Platyhelminthes as an ideal model phylum for studying the ecology and evolution of histophagous ciliates.Competing Interest StatementThe authors have declared no competing interest.