Single-cell sequencing uncovers three hidden species in a genus linked to sleeping sickness
Scoop a cup of pond water and you are holding a world that genomics has barely touched. Among the millions of single-celled organisms drifting in that sample are protists - eukaryotic microbes that are neither plant, animal, nor fungus - whose diversity dwarfs what laboratory cultures have captured. A study published in Microbial Genomics demonstrates just how much has been missed, revealing three potentially new species of a protist genus sitting on the evolutionary doorstep of one of humanity's most feared parasites.
Bodo: the free-living cousin of a killer
The genus Bodo comprises small, whip-tailed protists common in freshwater, brackish water, and soil. They feed on bacteria, live freely, and attract little medical attention. But Bodo occupies a scientifically important position: it is the closest known free-living relative of Trypanosoma, the parasitic protist responsible for Human African trypanosomiasis - sleeping sickness - as well as Chagas disease and the livestock disease nagana.
Understanding how Trypanosoma evolved its parasitic lifestyle requires comparing its genome to those of free-living relatives. Until now, researchers had genomic data for only a single Bodo species, B. saltans, grown in laboratory culture. That single reference point left an enormous blind spot.
Seven cells, three surprises
Researchers at the Earlham Institute in Norwich, working with the University of Oxford's Department of Biology, took a different approach. Rather than trying to culture Bodo in the lab - a process that selects for species that happen to thrive under artificial conditions - they went straight to the source. From a freshwater environmental sample, the team isolated seven individual Bodo cells, sequenced their genomes one by one, and assembled them.
The results surprised them. Three of the seven cells represented lineages that diverged significantly from B. saltans, the only previously sequenced species. These three lineages are potentially new species, each distinct enough to warrant separate classification.
But the discoveries did not stop at the protists themselves. Each of the three novel lineages carried its own species of Holosporales bacteria living inside it - bacterial endosymbionts that had gone undetected because conventional sequencing methods cannot distinguish which bacteria live inside which host cell when everything is blended together in a bulk sample.
Why bulk sequencing misses the picture
Most microbial genomics studies work with either cultured cells or bulk environmental DNA. Culture-based approaches capture only the fraction of organisms that grow under laboratory conditions - a well-known bias that excludes the vast majority of microbial diversity. Bulk environmental sequencing (metagenomics) captures more diversity but mashes everything together, making it impossible to assign bacterial sequences to specific protist hosts.
Single-cell sequencing threads the needle. By physically isolating individual cells before sequencing, researchers can reconstruct a complete picture: the protist's genome and the genome of whatever bacteria happen to be living inside it. The method reveals symbiotic relationships that are invisible at the bulk level.
The Earlham team adapted existing single-cell genomics protocols - originally developed for other organisms - to work with protists from environmental samples. That adaptation is part of a broader pipeline being built under the institute's Decoding Biodiversity program and the Wellcome-funded Darwin Tree of Life project.
Endosymbionts with unknown functions
The Holosporales bacteria found inside the new Bodo lineages belong to an order known to include intracellular symbionts of other protists, particularly amoebae. Some Holosporales bacteria are parasitic, some are mutualistic, and for many, the nature of the relationship with their host is simply unknown.
In this case, the researchers identified three distinct Holosporales species, each associated with a different Bodo lineage. Whether these bacteria benefit their hosts, harm them, or exist in a neutral relationship remains to be determined. But their consistent presence - each Bodo lineage with its own bacterial partner - suggests the associations are stable and potentially ancient.
Understanding these relationships could inform research on how parasitism evolves. If free-living protists maintain long-term relationships with intracellular bacteria, those relationships might share evolutionary mechanisms with the host-parasite dynamics seen in Trypanosoma infections.
Practical constraints and next steps
Single-cell sequencing of environmental protists is technically demanding. The cells are small, fragile, and surrounded by debris. Genome assemblies from single cells are inevitably less complete than those from cultured organisms grown in abundance. The seven cells sequenced here represent a proof of concept, not a comprehensive census of Bodo diversity.
The study also cannot determine how widespread these new lineages are. They were found in a single freshwater sample from a single location. Whether they exist globally or are restricted to particular environments is unknown.
The Earlham team plans to incorporate this approach into broader soil biodiversity research, exploring symbiotic associations between protists and soil bacteria. As sequencing costs fall and automation improves, the prospect of routinely surveying protist diversity at the single-cell level - cataloguing not just species but their bacterial partners - moves closer to reality.