Evolutionary trade-of between innate and acquired immune defences in birds

Abstract

Background The development, maintenance, and use of immune defences are costly. Therefore, animals face tradeofs in terms of resource allocation within their immune system and between their immune system and other physi‑ ological processes. To maximize ftness, evolution may favour investment in one immunological defence or subsystem over another in a way that matches a species broader life history strategy. Here, we used phylogenetically-informed comparative analyses to test for relationships between two immunological components. Natural antibodies and com‑ plement were used as proxies for the innate branch; structural complexity of the major histocompatibility complex (MHC) region was used for the acquired branch. Results We found a negative association between the levels of natural antibodies (i.e., haemagglutination titre) and the total MHC gene copy number across the avian phylogeny, both at the species and family level. The family-level analysis indicated that this association was apparent for both MHC-I and MHC-II, when copy numbers within these two MHC regions were analysed separately. The association remained signifcant after controlling for basic life history components and for ecological traits commonly linked to pathogen exposure. Conclusion Our results provide the frst phylogenetically robust evidence for an evolutionary trade-of within the avian immune system, with a more developed acquired immune system (i.e., more complex MHC architec‑ ture) in more derived bird lineages (e.g., passerines) being accompanied by an apparent downregulation of the innate immune system.