B. burgfdorferi can cause a chronic infection even if its animal host mounts a strong immune response — evading those defences by tweaking the shape and expression of its main surface antigen, VIsE. A series of unexpressed genetic sequences organized into ‘cassettes’ recombine with the VIsE gene, changing the resulting protein such that it escapes detection by the host’s immune system.
The researchers studied the molecular evolution of the cassettes’ genetic sequences in 12 strains of B. burgdorferi. They found that natural selection seemed to favour bacteria with more genetic variability within their cassettes, and hence a greater capacity to generate different versions of the antigen.
“Greater diversity among the cassettes in itself shouldn’t be a selective advantage considering they aren’t expressed and don’t do anything else,” says lead author Dustin Brisson. “But we did find evidence of selection, so the question is what else could it be for besides evolvability?”