MHC class IIβ diversity as a correlate of neutral- locus similarity and diversity, and a predictor of overwinter return, in song sparrows (Melospiza melodia)

The major histocompatibility complex (MHC) is a family of genes involved in recognizing pathogens and mounting an immune response. Parasite-mediated selection often favours heterozygosity at MHC because MHC-diverse individuals recognize a wider range of pathogens. Because migratory birds encounter many pathogens, I hypothesized that MHC diversity predicts overwinter and juvenile survivorship in song sparrows (Melospiza melodia). I found no correlation between MHC diversity and neutral-locus (microsatellite) heterozygosity, suggesting that measures of neutral and adaptive genetic diversity provide complementary rather than redundant information. However, pairwise similarity at MHC predicted pairwise similarity at microsatellite loci. In contrast to my hypothesis, MHC diversity did not predict overwinter return rates (interpreted as survivorship). However, age cohort comparisons showed that adults were more MHC-diverse than nestlings in one of the two years examined. MHC-diverse individuals in this population may thus be more likely to survive to adulthood in some years, but not in others.