Though earlier studies had suggested that selection may directly favor the capacity to evolve, they could not definitively rule out that evolvability had arisen for other reasons. In particular, it has been difficult in empirical studies to rule out the possibility that evolvability arises and is maintained as a byproduct of selection on organismal features more directly related to fitness.
In the Lyme disease system, the researchers got around this confounding factor by looking at diversity in the unexpressed cassettes, which would not have been the object of direct selection because they have no known function on their own; they simply exist as a way of increasing the potential diversity of the VlsE protein. Thus diversity in the cassettes would offer a window into past natural selection for a more "evolvable" VlsE.
"Organisms with greater diversity among the cassettes will have a selective advantage as they will be more antigenically evolvable, or better able to repeatedly generate novel antigens, and will thus be more likely to persist within hosts," the researchers wrote.
Using long-established methods in molecular evolution, the researchers evaluated 12 strains of B. burgdorferi for signs that natural selection had acted to increase the diversity of the cassettes.
"The evidence was remarkably strong in favor of evolution for more diversity among cassettes and thus greater evolvability in the expressed protein," Brisson said.
The researchers confirmed that the more genetically diverse the cassettes, the more genetically diverse the expressed protein, VlsE. They also found that mutations in the cassettes that could affect the portion of VlsE that is recognized by the immune
|Contact: Katherine Unger Baillie|
University of Pennsylvania