Ciliates as models for symbiosis
Using genomic analyses of functionally diverse symbiotic associations with parallel origins to gain insights into basic evolutionary principles of symbiosis
Patrick Keeling, University of British Columbia
Ciliates are powerful models to study the evolution and function of symbiosis at the genomic level, perhaps second only to animals in potential for fundamental insights. This is due to their experimental amenability, and the vast diversity of ciliate symbioses. Ciliates are diverse, but well-defined, with large cells that are relatively easy to identify and culture. Ciliates can be both symbiont and host, forming endo- or ecto-symbiotic associations with bacteria, archaea, animals, and other protists. These associations evolved multiple times independently – some ancient, some recent – and exhibit a spectrum of obligate-to-facultative interactions based on photosynthesis, chemosynthesis, nutritional supplementation, syntrophy, defence, or behaviour.
The ecological and evolutionary significance of ciliates has led to a long history of study, including ciliate model systems for genomics, cell, and molecular biology. But their potential as models for symbiosis has barely been tapped: many symbioses have been observed, but their evolutionary and ecological basis remain unclear, and genomic data to study their function and evolution are virtually absent. With most data and theory for foundational principles of symbiosis currently coming only from animals, we urgently require the integration of data from more diverse systems, and ciliates are the strongest candidate.