Evolution of bidirectional costly mutualism from byproduct consumption

Area of Science:

• Microbiology
• Evolutionary Biology
• Ecology

Background:

• Mutualisms are vital for ecosystems but their evolutionary origins remain unclear.
• A two-stage model proposes initial byproduct exchange followed by reciprocal costly provisioning.
• Previous work demonstrated the first stage with *Salmonella enterica* and *Escherichia coli*.

Purpose of the Study:

• To investigate the evolution of bidirectional costly mutualism from byproduct consumption.
• To experimentally validate a two-stage model for the origin of mutualisms.
• To understand the maintenance of cooperation in spatially structured microbial communities.

Main Methods:

• Utilized a synthetic consortium of *Salmonella enterica* and *Escherichia coli* on agar plates.
• Evolved microbial communities through successive growth cycles.
• Analyzed evolved strains for changes in resource production and exchange.

Main Results:

• *Escherichia coli* evolved costly secretion of galactose, benefiting *Salmonella enterica*.
• Galactose secretion evolved repeatedly across replicate communities.
• Spatial structure maintained both cooperative and efficient *E. coli* genotypes.

Conclusions:

• Demonstrated de novo evolution of bidirectional costly mutualism from byproduct consumption.
• Validated the plausibility of costly cooperation emerging from initially costless interactions.
• Provides an experimental model for understanding the evolution of essential mutualistic species interactions.