Disentangling strictly self-serving mutations from win-win mutations in a mutualistic microbial community

Area of Science:

• Evolutionary biology
• Microbial ecology
• Systems biology

Background:

• Mutualisms, symbiotic relationships benefiting all partners, can be enhanced by pleiotropic mutations.
• These mutations offer direct benefits to the individual (self-serving) and its partner (partner-serving).
• Partner-serving benefits are often quantified by an individual's benefit supply rate to partners.

Purpose of the Study:

• To challenge the conventional quantification of partner-serving phenotypes in mutualisms.
• To propose and test an alternative metric for evaluating mutualistic interactions under genetic variation.
• To investigate the long-term effects of seemingly beneficial mutations on cooperative dynamics.

Main Methods:

• Evolving well-mixed mutualistic communities of two engineered yeast strains.
• Utilizing yeast strains engineered to exchange essential metabolites: lysine and hypoxanthine.
• Analyzing a specific chromosome duplication mutation affecting lysine affinity and hypoxanthine release.

Main Results:

• A chromosome duplication mutation improved yeast's lysine affinity (self-serving benefit).
• The mutation increased hypoxanthine release rate per cell, but also increased cell size and lysine utilization per birth.
• The exchange ratio (total hypoxanthine release per lysine utilization) remained unchanged, indicating no net benefit to the partner.
• The mutation did not increase the partner's steady-state growth rate, classifying it as solely self-serving in the long term.

Conclusions:

• The direct measure of benefit supply rate is insufficient for quantifying partner-serving phenotypes in mutualisms.
• An individual's increased benefit production rate does not automatically imply a net benefit to the partner.
• Reduced benefit production rate by an individual does not necessarily equate to cheating within a mutualistic system.