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Flatworm Transcriptomes Reveal Widespread Parasitism by Histophagous Ciliates
M Ryan Woodcock1,2,3, Kaleigh Powers1, Kirsten Snead2,4
1Department of Biology, Keene State College, Keene, NH, USA.
Genome Biology and Evolution
|January 19, 2024
Summary
Tetrahymena ciliates are parasites of flatworms, not just free-living bacteria eaters. This study reveals new flatworm hosts and evidence of coevolution, offering insights into parasite-host dynamics.
Area of Science:
- Parasitology
- Bioinformatics
- Evolutionary Biology
Background:
- Unicellular ciliates, such as Tetrahymena, are known as bacterivores but can also be parasites (histophages).
- These histophagous ciliates infect various hosts, including commercially important fish and endangered mussel larvae.
- Platyhelminthes (flatworms) are among the hosts for these parasitic ciliates.
Purpose of the Study:
- To identify new host species for Tetrahymena ciliates within the Platyhelminthes phylum.
- To investigate the coevolutionary relationship between histophagous ciliates and their flatworm hosts.
- To explore the potential role of invasive flatworms in ciliate dispersal.
Main Methods:
- Development of a novel bioinformatics pipeline accounting for the nonstandard ciliate genetic code.
- Screening of over 2.6 million Platyhelminthes expressed sequence tags (ESTs) to identify Ciliophora sequences.
- Culturing and identification of Tetrahymena from various planarian species.
- Co-phylogenetic reconstruction to infer host-parasite evolutionary history.
- Experimental expulsion of Tetrahymena from Schmidtea mediterranea using paromomycin.
Main Results:
- Identification of nearly 6,000 high-confidence ciliate transcripts, confirming parasitism in seven additional flatworm species.
- Isolation and identification of Tetrahymena from nine planarian species, including invasive Bipalium and model organisms like Dugesia japonica and Schmidtea mediterranea.
- Co-phylogenetic analysis strongly supporting the coevolution of histophagous ciliates and Platyhelminthes hosts.
- Demonstration that paromomycin can effectively remove Tetrahymena from S. mediterranea.
Conclusions:
- Platyhelminthes serve as significant hosts for Tetrahymena, expanding the known host range of these ciliates.
- Strong evidence suggests coevolution between histophagous ciliates and their flatworm hosts.
- Invasive flatworms may act as novel dispersal vectors for Tetrahymena parasites.
- The Platyhelminthes phylum is proposed as an excellent model system for studying the ecology and evolution of histophagous ciliates.
