The genetic structure of Borrelia afzelii varies with geographic but not ecological sampling scale

The genetic structure of a pathogen is an important determinant of its potential rate of adaptation and can thereby influence the dynamics of host-parasite interactions. We investigated how the genetic structure of Borrelia afzelii varies with geographic and ecological sampling scale. Genetic structure was measured as the degree of linkage disequilibrium (LD) across three loci. To test for the effects of geographic and ecological scale, we calculated LD across or within populations 4-82 km apart and across or within different mammal host species. There was highly significant LD across populations and host species. However, there was also evidence for genome-wide recombination, and the LD largely resulted from epidemic spread of certain haplotypes, rather than lack of recombination. Interestingly, the degree of LD was higher in each population than in the sample as a whole, i.e. LD increased with decreasing geographic scale. In contrast, there was no effect of ecological sampling scale on LD. Strong LD may impede the rate of adaptive evolution. Our results suggest this effect might be particularly strong at a small geographic scale.