Are Planaria Individuals? What Regenerative Biology is Telling Us About the Nature of Multicellularity

Chris Fields and Michael Levin


Link to paper

15 April 2018


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.

Interesting points:

"When embedded in their obligate environment—a planarian body or fragment thereof, even one completely lacking other neoblasts—each neoblast is capable of fully regenerating a complete planarian body, via which it reproduces its neoblast progeny."

"... Despite hundreds of millions of years of accumulating somatic mutations, planarian regenerative anatomy exhibits almost 100% fidelity—each regenerating planarian is a perfect, normal copy of the standard planarian target morphology. How can the anatomy stay constant and robust while the genome diverges?"

"They resemble, in this sense, reproductive queens inhabiting colonies of sterile workers, all of which are their descendants."

More to read:

  • Hamilton's rule of cooperation
  • Zygotic bottlenecks
  • Queller–Strassmann definition of individuals