Immune responses link parasite genetic diversity, prevalence and plumage morphs in common buzzards

Abstract

The evolution and maintenance of conspicuous phenotypic polymorphisms has challenged evolutionary ecologists for centuries. Polymorphisms in pleiotropic cascades or genetic linkage may lead to correlations of life history traits such as immunity, parasite infection levels, pigmentation and lifetime reproductive success. The common buzzard Buteo buteo is a bird of prey occurring in several plumage morphs, which differ in pigmentation and in the prevalence, infection intensity and clone composition of their most common blood parasite, as well as in ectoparasite infestation levels. Buzzard morphs are heritable and exhibit a heterozygote advantage where intermediates have higher lifetime reproductive success (LRS). We explored the hypothesis that the differences in pigmentation also correspond to differences in immunity. We hence compared an inducible adaptive and an innate constitutive immune response between the buzzard plumage morphs. The increase of specific anti-tetanus antibodies after vaccination was explained by the morph of the nestling’s mother and was highest in offspring of intermediate mothers. Additionally, nestlings with higher humoral response were less infected with blood parasites and, if infected, harboured a lower genetic diversity of these parasites. The phytohaemagglutinin-induced skin swelling, a complex of cellular inflammatory responses, was lowest in intermediate nestlings. The higher LRS of intermediate buzzards suggests that the cellular immunity is an inferior fitness determinant compared to humoral immunity. The strength of immune responses was not linear along the melanisation gradient, indicating that there is most likely no simple genetic correlation between immune responses and plumage morphs.