A Complex Hierarchy of Avoidance Behaviors in a Single-Cell Eukaryote

Area of Science:

• Single-cell biology
• Behavioral ecology
• Evolutionary biology

Background:

• Decision-making and learning are observed in non-neural organisms, including single-celled ciliates.
• Herbert Spencer Jennings documented complex behaviors in the ciliate Stentor roeseli in 1906, but these findings were later disputed.
• Previous claims of non-reproducibility were based on experiments with a different ciliate species, Stentor coeruleus.

Purpose of the Study:

• To re-examine and confirm Herbert Spencer Jennings' observations of complex behaviors in Stentor roeseli.
• To investigate the behavioral hierarchy and decision-making processes in this single-celled organism.
• To explore the evolutionary implications of complex behavior in unicellular eukaryotes.

Main Methods:

• Laboratory culture of Stentor roeseli.
• Micromanipulation techniques to apply stimuli.
• Video microscopy to record and analyze behavioral responses.

Main Results:

• Stentor roeseli exhibits a distinct hierarchy of avoidance behaviors: bending, ciliary alteration, contraction, and detachment.
• The choice between contraction and detachment appears to be a random process, akin to a coin toss.
• Observed behaviors differ from habituation or classical conditioning, suggesting a more complex decision-making mechanism.

Conclusions:

• The study validates Jennings' pioneering insights into ciliate behavior, demonstrating complex responses in a single-celled organism.
• Such behavioral complexity in ciliates may have provided an evolutionary advantage in ancient protist ecosystems.
• The ciliate cortex might have served as a precursor to mechanisms enabling complex behavior before the evolution of multicellularity.