Rapid evolution of disease resistance is accompanied by functional changes in gene expression in a wild bird

Abstract

Wild organisms are under increasing pressure to adapt rapidly to environmental changes. Predicting the impact of these changes on natural populations requires an understanding of the speed with which adaptive phenotypes can arise and spread, as well as of the underlying mechanisms. However, our understanding of these parameters is poor in natural populations. Here we use experimental and molecular approaches to investigate the recent emergence of resistance in eastern populations of North American house finches (Carpodacus mexicanus) to Mycoplasma galliseptum (MG), a severe conjunctivitis-causing bacterium. Two weeks following an experimental infection that took place in 2007, finches from eastern US populations with a 12-y history of exposure to MG harbored 33% lower MG loads in their conjunctivae than finches from western US populations with no prior exposure to MG. Using a cDNA microarray, we show that this phenotypic difference in resistance was associated with differences in splenic gene expression, with finches from the exposed populations up-regulating immune genes postinfection and those from the unexposed populations generally down-regulating them. The expression response of western US birds to experimental infection in 2007 was more similar to that of the eastern US birds studied in 2000, 7 y earlier in the epizootic, than to that of eastern birds in 2007. These results support the hypothesis that resistance has evolved by natural selection in the exposed populations over the 12 y of the epizootic. We hypothesize that host resistance arose and spread from standing genetic variation in the eastern US and highlight that natural selection can lead to rapid phenotypic evolution in populations when acting on such variation.

Fig. 1.

Symptoms of M. gallisepticum infection and MG load in the conjunctivae of house finches. (A) Naturally infected (Left) and healthy (Right) wild house finches. (B) Quantification of MG load in the conjunctiva of infected finches from Arizona and Alabama sampled in 2007, 2 wk postinfection. Raw values of MG load are expressed as a ratio of host cell number; horizontal lines indicate mean values of raw data.

Fig. 2.

Comparisons and patterns of splenic gene expression. (A) Schematic of the four analytical comparisons made with gene expression data. (B) Heat map of gene expression patterns in comparisons 1–4 (Fig. 2A). Red and green indicate significantly higher and lower expression levels, respectively, with bright colors reflecting at least a threefold difference in magnitude and values in black indicating no difference. Comparisons in each of the four columns shown for first treatment/population vs. second one are outlined in Fig. 2A. The 52 genes included showed differential expression in at least one comparison (1–4) and were of known identity and function (see Table S1 for full details). Asterisks indicate genes with direct and auxiliary immune functions. (C) Fold difference in expression levels of immune (n = 10), immune-related (n = 6), and stress (n = 1) genes in comparison 4. Genes shown were differentially expressed and known to have direct immune (I1–I10), indirect immune (R1–R3; Si1, P1, C1), or stress (St1) functions (Table S1). Negative values represent lower expression in infected birds from Arizona relative to those from Alabama. Red (I1–I10): immune genes (T-cell Ig and mucin domain containing-4; MHC class II-associated invariant chain I1; lectin galactoside-binding soluble-2-protein; programmed death ligand 1; TCR β-chain; Ig J; neutrophil cytosolic factor-4; Ig superfamily member 4A isoform a; parathymosin; and complement factor-H). Yellow (R1–R3): redox metabolism genes (thioredoxin; spermidine/spermine N1-acetyltransferase variant 1; and squalene epoxidase). Light (Si1), medium (P1), and dark (C1) blue: signal transduction (RhoA GTPase), proteolysis (ubiquitin C), and cytoskeleton (lymphocyte cytosolic protein) genes, respectively. Purple (St1): stress gene (heat-shock protein 90a). The stress gene was included because it was one of the few up-regulated in comparison 4, suggesting that birds from Arizona were more stressed by the infection. Gene I10, complement factor-H, is expected to have increased expression in infected Arizona finches and is not anomalous with the other immune genes (see Discussion).