Abstract
Freshwater planaria (Platyhelminthes, Turbellaria, Tricladida) pose a challenge to current concepts of biological individuality. We review molecular and developmental evidence suggesting that mature intact planaria are not biological individuals but their totipotent stem cells (neoblasts) are individuals. Neoblasts within a single planarian body are, in particular, genetically heterogeneous, migratory, effectively immortal, and effectively autonomous. They cooperate to maintain the planarian body as an obligate environment but compete to make this environment maximally conducive to the survival of their own neoblast lineages. These results suggest that planaria have not fully completed the transition to multicellularity, but instead represent an intermediate form in which a small number of genetically-heterogeneous, reproductively-competent cells effectively “farm” their reproductively-incompetent offspring.
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Acknowledgements
We thank A. Aboobaker for useful discussions. This research was supported by the Allen Discovery Center program through The Paul G. Allen Frontiers Group (12171). In addition, M. L. gratefully acknowledges support of the G. Harold and Leila Y. Mathers Charitable Foundation (TFU141) and the Templeton World Charity Foundation (TWCF0089/AB55 and TWCF0140).
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Fields, C., Levin, M. Are Planaria Individuals? What Regenerative Biology is Telling Us About the Nature of Multicellularity. Evol Biol 45, 237–247 (2018). https://doi.org/10.1007/s11692-018-9448-9
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DOI: https://doi.org/10.1007/s11692-018-9448-9