Bacterial and archaeal symbioses with protists
Most of the genetic, cellular, and biochemical diversity of life rests within single-celled organisms — the prokaryotes (bacteria and archaea) and microbial eukaryotes (protists). Very close interactions, or symbioses, between protists and prokaryotes are ubiquitous, ecologically significant, and date back at least two billion years ago to the origin of mitochondria. However, most of our knowledge about the evolution and functions of eukaryotic symbioses comes from the study of animal hosts, which represent only a small subset of eukaryotic diversity. Here, we take a broad view of bacterial and archaeal symbioses with protist hosts, focusing on their evolution, ecology, and cell biology, and also explore what functions (if any) the symbionts provide to their hosts. With the immense diversity of protist symbioses starting to come into focus, we can now begin to see how these systems will impact symbiosis theory more broadly.
The crucial role of endosymbiosis in the origin of eukaryotic cells and organelles is now accepted beyond any serious doubt, and that debate has turned to focus on how deep was its impact, and how ancient were the associations that gave rise to mitochondria and plastids1, 2, 3, 4. However, it would be a mistake to codify the effects of endosymbiosis based on a few events of extreme antiquity, because bacterial and archaeal symbionts continue to play major roles today in eukaryotic cell biology, molecular biology, ecology, and evolution. The body of evidence for the importance of these ongoing relationships nearly all comes from studies of multicellular hosts, where both apparently parasitic and beneficial relationships have yielded fascinating insights into the mechanisms and outcomes of long-term cohabitation5, 6, 7, 8, 9. From these studies, new basic principles are taking shape; however, the strong focus on multicellular hosts contrasts with the fact that most of the genetic, cellular, and biochemical diversity of eukaryotes rests within single-celled organisms, called protists10,11 (Figure 1; also see Glossary for a list of key terms used).
Husník F., Tashyreva D., Boscaro V., George E.E., Lukeš J., Keeling P.J. 2021: Bacterial and archaeal symbioses with protists. Current Biology 31: R1–R16. [IF=9.601] DOI: 10.1016/j.cub.2021.05.049