Elevated phytohaemagglutinin-induced skin-swelling response at an intermediate number of MHC class II alleles in bluethroat nestlings

Abstract

Genes of the major histocompatibility complex (MHC) are crucial for adaptive immunity, and theory predicts that the optimal number of alleles is intermediate rather than maximal. Support for such an intermediate optimum in MHC allele count has mainly been demonstrated in studies on parasite load and reproductive traits. In this study we tested if an intermediate number of MHC class II (MHCII) alleles based on positively selected sites was correlated with an increased skin-swelling response to two consecutive injections of phytohaemagglutinin (PHA) in bluethroat Luscinia svecica nestlings. We found that nestlings that were closer to the population mean in their number of alleles had a higher response to the first injection, supporting an intermediate, optimal number of alleles. After the second PHA injection, we found that the skin swelling was associated with MHCII allele count only at the among-brood level, and with paternity (extra-pair offspring had a larger swelling than within-pair offspring). We therefore conclude that the PHA response is partly correlated with MHCII allele count, but that additional factors must be involved in both the PHA response and in female extra-pair mate choice in this species.