Importance of innate immune genes during an outbreak of WNV: evidence from a natural experiment

Numerous studies have demonstrated that genetic diversity of the acquired immune system can play an important role in host defense of pathogens. The innate immune system, however, has received relatively little attention even though it serves as the first wave of immune response. Ideally, one would study immune genes and the diseases of birds in an experimental setting, but this can be logistically challenging and time consuming. Instead, we utilized a large-scale natural experiment that was conducted over several decades and is currently in cold storage at Field Museum of Natural History. We DNA sequenced 10 toll-like receptors (TLRs) of 50 American Crows collected before and after the arrival of West Nile Virus in the Midwestern United States to determine if positive natural selection had occurred, as changes in DNA sequences may alter their ability to recognize specific pathogens and thus their effectiveness. Surprisingly, we found essentially no genetic variation across all TLR loci in either time period, and thus no evidence of selection. We then took a phylogenetic approach to determine if the lack of genetic diversity in American Crow TLRs was characteristic of corvoids in general. TLR sequences from 65 species of corvoid like birds revealed that Corvus has substantially lower genetic diversity than other corvoid like genera. This research highlights the importance of understanding the influences of evolution and phylogenic constraints on disease response by wild bird populations.