Major histocompatibility complex and mate choice in two passerine birds

Abstract

All organisms need to fight pathogens to avoid infectious diseases. In vertebrates, two classes of polymorphic genes from the major histocompatibility complex (MHC) are crucial for triggering immune responses against specific intracel-lular (MHC class I; MHCI) and extracellular (MHC class II; MHCII) pathogens. Whenever possible, females should choose mates that render an optimal MHC constitution in the offspring. Such genetic effects of mate choice can be elu-cidated by studying species where females engage in extra-pair mating. This thesis explores MHC-based extra-pair mate choice in two songbirds, using new sequencing technology. The hypervariable songbird MHC has been difficult to geno-type, but new methodology offers great improvements. For the bluethroat (Luscinia svecica), evidence for non-random, self-referential female choice of extra-pair males based on an intermediate optimum in MHCII diversity was found. For the willow warbler (Phylloscopus trochilus), females with few MHCI variants were more likely to engage in extra-pair mating. This thesis sheds light on the variation in MHC-based mate choice, and demonstrate an adaptive benefit of fe-male extra-pair mating in songbirds