Adaptive evolutionary trajectories in complexity: Transitions between unicellularity and facultative differentiated...

Area of Science:

• Evolutionary biology
• Theoretical biology
• Origin of life

Background:

• Multicellularity ranges from simple cell clusters to complex organisms.
• Simple multicellularity evolves readily under selective pressures.
• The evolution of increased multicellular complexity is not well understood.

Purpose of the Study:

• To investigate adaptive trajectories of unicellular organisms under periodic abiotic stress.
• To determine conditions favoring multicellularity, cell differentiation, or both.
• To explore how mutations affect the evolution of complexity.

Main Methods:

• Mathematical modeling of unicellular populations under periodic stress.
• Defining a parameter space of fitness-relevant traits.
• Simulating the effects of beneficial mutations on adaptive trajectories.

Main Results:

• Multicellularity and differentiation offer survival benefits against stress but incur costs in its absence.
• The fitness landscape determines the optimal strategy (unicellularity, multicellularity, or differentiation).
• Mutation order (historical contingency) can influence the permanence of complexity transitions.

Conclusions:

• Continued selective pressure for multicellularity can result in either increased complexity or a reversion to unicellularity.
• Adaptive trajectories can involve cycles of gaining and losing complexity.
• The evolution of complexity is sensitive to mutation order and fitness trade-offs.