Plumage color and pathogen-induced gene expression in a wild bird

Ornamentation is hypothesized to signal the capacity of an individual to cope with environmental challenges. At the molecular level, organisms respond to their environments largely by altering gene transcription, but the transcriptional responses linking ornamentation and disease resistance are virtually unstudied. In the house finch (Haemorhous mexicanus), carotenoid coloration displayed by males is important in female choice, and plumage redness predicts a male's ability to recover from bacterial infection with Mycoplasma gallisepticum. To investigate the molecular mechanisms underlying links between ornamentation and disease response, we experimentally infected wild finches and used microarrays to identify genes that were differentially expressed in males with redder and yellower plumage coloration. Sixteen candidate genes were then investigated across individuals. Principal components analysis revealed that infection induced a marginally significant decrease in 11 genes, several with primary immune functions. The treatment by plumage color interaction effect was largely driven by greater disease-induced changes in expression of heat shock protein 90 and ubiquitin c (the second principal component) by red birds than by yellow birds. In a direct comparison of disease severity between infected birds, plumage color on its own did not significantly predict disease symptoms. However, in multivariate models, expression of genes loading on the second principal component, plumage color, and their interaction were significant predictors of the severity of disease symptoms. Our observations demonstrate links between ornamentation and expression of genes related to disease resistance; we suggest that male plumage color may reflect functionally associated differences in inducibility or plasticity of gene expression in this species.