A free-living ciliate species called Stentor roeseli, which lacks a central nervous system, is capable of decision making, according to a study published in the journal Current Biology.
Stentor roeseli is colorless, trumpet shaped, and visible to the naked eye. Image credit: Protist Image Database.
In 1906, American biologist Herbert Spencer Jennings reported that Stentor roeseli exhibited complex behavior. In response to an irritating stimulus, this microorganism engaged in four distinct behaviors: bending, ciliary alteration, contraction and detachment.
Over a half-century later, the research was debunked by a 1967 experiment that failed to replicate Dr. Jennings’ results. That study was accepted by the science community even though it used a different species of organism.
Now, Dr. Joseph Dexter from the Neukom Institute for Computational Science at Dartmouth College and Harvard Medical School and his colleagues have confirmed the original finding.
“Our results provide strong evidence that Dr. Jennings’ original observations about the behavior of Stentor roeseli were correct, which should help to resolve the long-standing confusion,” Dr. Dexter explained.
“We now have a transparent dataset, and we invite researchers to view the full set of videos to learn more about the complexities of how Stentor roeseli responds to stimulation.”
The researchers developed a platform for manipulating Stentor roeseli that allowed them to target the delivery of an irritant.
They settled on using polystyrene beads to stimulate reactions from the organism in the test. This was a departure from the powder used in the original experiment, but it led to an observable response that is thought to be part of a generalized avoidance strategy in Stentor roeseli.
As the beads were fed through a microinjection needle using a gravity-based system, the study authors worked to keep the microscope image in focus while they observed and recorded the experiment.
In the video, they demonstrate how Stentor roeseli avoids the irritant by bending away or changing the beat of its hair-like cilia to keep from ingesting it.
In response to the irritation, the organism might also contract into a protective ball, or detach from the piece of algae it is anchored to and swim to a new site.
After years of field work, video microscopy, micromanipulation and quantitative analysis, the scientists finally had the evidence that they needed to confirm Dr. Jennings’ finding that the single-cell organism is capable of complex avoidance behavior.
“The results are the culmination of a long, highly-collaborative process,” Dr. Dexter said.
“It was quite satisfying to work on a problem with such an interesting history and to confront some unusual challenges along the way.”
“Our findings show that single cells can be much more sophisticated than we generally give them credit for,” said Dr. Jeremy Gunawardena, also from Harvard Medical School.
“They have to be ‘clever’ at figuring out what to avoid, where to eat and all the other things that organisms have to do to live. I think it’s clear that they can have complex ways of doing so.”
In addition to demonstrating how the organism responds to stimulus, the team also confirmed Dr. Jennings’ finding that Stentor roeseli uses a hierarchy of behaviors.
While the researchers found few instances of the organism following the full hierarchy, they observed many partial instances with varying orders of occurrence, ultimately concluding that the behavior hierarchy exists.
By generating a much larger and richer dataset than the early 1900s experiment, they also demonstrated that the organism’s decision making is distinct from habituation or classical conditioning.
“The choice between contraction and detachment in the organism resembled the same probability of a fair coin toss,” they said.
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Joseph P. Dexter et al. A Complex Hierarchy of Avoidance Behaviors in a Single-Cell Eukaryote. Current Biology, published online December 5, 2019; doi: 10.1016/j.cub.2019.10.059
