Frequency dependence and cooperation: theory and a test with bacteria.

Area of Science:

• Evolutionary biology
• Microbial ecology

Background:

• Hamilton's inclusive fitness theory explains cooperation but generally excludes frequency-dependent selection.
• Microbial systems show that cheaters (non-cooperators) can have higher fitness when rare, indicating frequency dependence.

Purpose of the Study:

• To theoretically and experimentally investigate frequency-dependent selection in microbial cooperation.
• To test if cooperation-dependent population growth drives frequency dependence in Pseudomonas aeruginosa.

Main Methods:

• Theoretical modeling of cooperation under population structuring and growth-dependent selection.
• Experimental competition between siderophore-producing cooperators and non-producing cheaters in Pseudomonas aeruginosa at varying frequencies.
• Manipulating the duration of population growth to assess its impact on frequency dependence.

Main Results:

• Frequency-dependent selection on cooperation was demonstrated in Pseudomonas aeruginosa.
• Cheater fitness and overall culture productivity were negatively related to initial cheater frequency.
• Shortening the population growth period reduced the strength of frequency dependence.

Conclusions:

• Microbial cooperation can exhibit frequency-dependent selection, particularly when linked to population growth.
• Frequency dependence may be more prevalent in microbes than in metazoans due to common factors like strong selection and structured populations.
• The findings challenge traditional inclusive fitness predictions in certain microbial contexts.