Frequency-dependent selection on Borrelia burgdorferi is not supported by a mechanistic model and temporal infection data

Background/Question/Methods

Genetic diversity in the bacterium Borrelia burgdorferi, particularly at the outer surface protein C locus (ospC), has been proposed to be maintained by negative frequency-dependent selection. To test this hypothesis, in which host populations mediate selection by acquired immunity, we constructed a mechanistic model to simulate transmission of and selection on ospC genotypes in a community of vertebrate hosts. We also collected temporal field data from B. burgdorferi-endemic host populations to support model assumptions. 

Results/Conclusions

Two necessary conditions that also determine the strength of negative frequency-dependent selection are: the duration of host immunity must last over three years, and these hosts must be responsible for 90% or more of all transmission. Comparisons of these conditions to our empirical observations suggest that frequency-dependence is not a strong selective mechanism in this system.