Figure 1.

The Influence of Population Structure and Relatedness on Bacterial Social Traits in a Subpopulation

A cooperative gene coding for a public good is represented by “x” and can be carried either by the chromosome (A and B) or a plasmid (C).

(A) A small bacterial population where all individuals have the cooperative gene on the chromosome.

(B) Only one individual (B) posseses the gene and produces the public good. A, C, and D benefit from B's actions.

(C) The cooperative gene is carried on the plasmid so individuals A and B both produce the trait. A and B will now have a high genetic relatedness (r > 0) because relatedness is defined as the probability that two individuals bear the same gene. Thus relatedness is influenced by the rate at which the plasmid spreads through the subpopulation. The greater the plasmid infection rate, the more “related” the hosts will be because more individuals within the subpopulation will share the same genes, albeit on the plasmid. This illustrates that it is the focal locus that ultimately matters when dealing with social dilemmas such as public good production.