Innate immunity and the evolution of resistance to an emerging infectious disease in a wild bird

Abstract

Innate immunity is expected to play a primary role in conferring resistance to novel infectious diseases, but few studies have attempted to examine its role in the evolution of resistance to emerging pathogens in wild vertebrate populations. Here, we used experimental infections and cDNA microarrays to examine whether changes in the innate and/or acquired immune responses likely accompanied the emergence of resistance in house finches (Carpodacus mexicanus) in the eastern United States subject to a recent outbreak of conjunctivitis‐causing bacterium (Mycoplasma gallisepticum—MG). Three days following experimental infection with MG, we observed differences in the splenic transcriptional responses between house finches from eastern U.S. populations, with a 12‐year history of MG exposure, versus western U.S. populations, with no history of exposure to MG. In particular, western birds down‐regulated gene expression, while eastern finches showed no expression change relative to controls. Studies involving poultry have shown that MG can manipulate host immunity, and our observations suggest that pathogen manipulation occurred only in finches from the western populations, outside the range of MG. Fourteen days after infection, eastern finches, but not western finches, up‐regulated genes associated with acquired immunity (cell‐mediated immunity) relative to controls. These observations suggest population differences in the temporal course of the response to infection with MG and imply that innate immune processes were targets of selection in response to MG in the eastern U.S. population.

Gene expression differences underlying genotype-by-genotype specificity in a host–parasite system

“…North American house finches are infected by a zoonotic bacterium Mycoplasma galliseptum. Bonneaud et al (26,60) described a distinct difference between birds from a western population that had remained disease-free and an eastern population that had evolved resistance to this parasite over the course of the disease outbreak. Birds that came from the eastern population had a lower infection load and less parasite-induced immune suppression than those from the naive western population, or in samples from the eastern population early in the outbreak.…”

Parallel Patterns of Increased Virulence in a Recently Emerged Wildlife Pathogen

“…Because of these effects, and recent work by Bonneaud and colleagues that detected differences in pathogen load [62] and immune responses [62],[63] between Arizona and Alabama house finches in response to MG infection, we limited our qualitative interpretations about virulence evolution to within rather than between experiments. Although we were able to statistically estimate and control for host effects in response to the reference isolate (VA1994), there may be isolate-by-host interaction effects on virulence that our experimental design was unable to test for.…”

Variation at Innate Immunity Toll-Like Receptor Genes in a Bottlenecked Population of a New Zealand Robin

“…Innate immune responses are expected to play an important role in the evolution of resistance to novel diseases in wild populations, an expectation that was upheld in a recent study of the consequences of a Mycoplasma outbreak in wild house finches [69]. However, little is known about the role of TLR gene diversity in disease susceptibility of threatened species.…”