Cellular packing, mechanical stress and the evolution of multicellularity

Area of Science:

• Evolutionary biology
• Cell biology
• Biophysics

Background:

• Multicellularity enables organismal complexity.
• The evolution of increased size in early multicellular organisms is poorly understood.
• Cellular clusters face mechanical stress limiting growth.

Purpose of the Study:

• Investigate how multicellular clusters overcome mechanical stress to increase size.
• Understand the cellular and physical adaptations during the evolution of larger group size.
• Identify solutions to spatial constraints limiting multicellular evolution.

Main Methods:

• Subjecting snowflake yeast ( *Snowflake yeast* ) clusters to daily selection for large size over seven weeks (~291 generations).
• Measuring cluster radius and analyzing cellular morphology and volume fraction.
• Quantifying internal mechanical stress and cellular elongation.

Main Results:

• Snowflake yeast clusters increased their radius 1.7-fold.
• Clusters evolved reduced internal stress, delaying fracture.
• Cells within clusters became more elongated, decreasing cellular volume fraction and increasing free space.

Conclusions:

• Natural selection favors physical solutions to spatial constraints.
• Reduced crowding and internal stress allow for increased multicellular group size.
• This study elucidates a fundamental mechanism in the evolution of multicellularity.