image: The divergence time estimation of Ciliophora inferred from the data matrix of 224 taxa (190 ciliates and 34 outgroup taxa) and 718 genes.
Credit: ©Science China Press
Ciliated protists (the phylum Ciliophora) are among the most abundant and diverse single-celled eukaryotes on Earth, playing pivotal roles in microbial food webs and serving as model organisms in a wide range of studies. However, their internal evolutionary relationships have long been contentious due to limited molecular markers and incomplete taxonomic sampling, hindering a deep understanding of how this ancient lineage achieved its spectacular diversity.
A new study published in Science China Life Sciences has now reconstructed the most comprehensive phylogenomic tree for ciliates to date. An international research team sequenced the genomes and/or transcriptomes of 52 ciliate species spanning 10 classes. By combining this new data with publicly available datasets, they assembled a taxon-rich matrix encompassing 190 species, representing 16 of the 17 recognized classes and 49 orders (80% of known orders) within the phylum Ciliophora. Among these, 74 species were analyzed phylogenomically for the first time. Using an advanced analytical pipeline, the team built a robust and highly resolved ciliate "Tree of Life."
The study establishes a new framework for the phylum Ciliophora, defining three main lineages: the distinct class Mesodiniea; the subphylum Postciliodesmatophora (containing Heterotrichea and Karyorelictea); and the subphylum Intramacronucleata, which encompasses two major clades (SLAOMP and CONthreeP) and the separate class Protocruziea. This resolves long-standing controversies regarding groups like Mesodinium and anaerobic ciliates.
Phylogenomic dating indicates that ciliates originated approximately 1.05 billion years ago in the Mesoproterozoic era. Major class-level radiations occurred between 500–700 million years ago, predating the Cambrian Explosion. The study provides a detailed divergence timeline, linking key splits (e.g., Postciliodesmatophora at ~730 Mya, SLAOMP at ~874 Mya) to major Earth history events.
To support further research, the authors curated a set of 200 core gene families with corresponding multiple sequence alignments. This resource, hosted in a public database, offers a standardized toolkit to facilitate robust phylogenomic and comparative genomic analyses of ciliates and other microbial eukaryotes.
This research marks a significant improvement in deciphering the deep evolutionary history of eukaryotic life. This robust phylogenetic framework enables precise tracing of key ciliate evolutionary innovations and provides a stable reference that will improve ecological interpretations and environmental DNA studies. While this study establishes a solid framework, the authors note that future work is needed to include environmentally restricted and poorly sampled lineages to more fully complete the ciliate evolutionary puzzle.