Multiple pathways to the evolution of positive assortment in aggregative multicellularity

Area of Science:

• Evolutionary biology
• Microbiology

Background:

• The transition to multicellularity involves shifting selection from cells to collectives.
• Aggregative organisms face challenges in adapting multicellular traits due to the need for genetic assortment.

Purpose of the Study:

• To investigate the evolution and role of genetic assortment in multicellular adaptation using experimental evolution.
• To understand the mechanisms and adaptive significance of genetic assortment in yeast floccules.

Main Methods:

• Experimental evolution of flocculating *Saccharomyces cerevisiae* under selection for rapid settling.
• Genetic reconstruction experiments and protein structure modeling to identify pathways to assortment.

Main Results:

• 13 of 19 yeast lineages evolved increased positive genetic assortment after 840 generations.
• Assortment did not provide a competitive advantage for settling, suggesting indirect evolution.
• Two pathways to assortment were identified: *FLO1* gene mutations and enhanced general adhesion.

Conclusions:

• Genetic assortment can evolve indirectly through selection on cell-level traits, not necessarily direct selection on collective traits.
• Cell-level adaptations can precede and enable multicellular adaptations.
• Fundamental constraints on aggregative multicellularity may explain the simplicity of such lineages.