Assessment of the dunnocks’ introduction to New Zealand using innate immune-gene diversity

Natural selection directing molecular evolution in vertebrate viral sensors

Diseases caused by pathogens contribute to molecular adaptations in host immunity. Variety of viral pathogens challenging animal immunity can drive positive selection diversifying receptors recognising the infections. However, whether distinct virus sensing systems differ across animals in their evolutionary modes remains unclear. Our review provides a comparative overview of natural selection shaping molecular evolution in vertebrate viral-binding pattern recognition receptors (PRRs). Despite prevailing negative selection arising from the functional constraints, multiple lines of evidence now suggest diversifying selection in the Toll-like receptors (TLRs), NOD-like receptors (NLRs), RIG-I-like receptors (RLRs) and oligoadenylate synthetases (OASs). In several cases, location of the positively selected sites in the ligand-binding regions suggests effects on viral detection although experimental support is lacking. Unfortunately, in most other PRR families including the AIM2-like receptor family, C-type lectin receptors (CLRs), and cyclic GMP-AMP synthetase studies characterising their molecular evolution are rare, preventing comparative insight. We indicate shared characteristics of the viral sensor evolution and highlight priorities for future research.

Selection balancing at innate immune genes: adaptive polymorphism maintenance in Toll-like receptors

Balancing selection is a classic mechanism for maintaining variability in immune genes involved in host–pathogen interactions. However, it remains unclear how widespread the mechanism is across immune genes other than the major histocompatibility complex (MHC). Although occasional reports suggest that balancing selection (heterozygote advantage, negative frequency-dependent selection, and fluctuating selection) may act on other immune genes, the current understanding of the phenomenon in non-MHC immune genes is far from solid. In this review, we focus on Toll-like receptors (TLRs), innate immune genes directly involved in pathogen recognition and immune response activation, as there is a growing body of research testing the assumptions of balancing selection in these genes. After reviewing infection- and fitness-based evidence, along with evidence based on population allelic frequencies and heterozygosity levels, we conclude that balancing selection maintains variation in TLRs, though it tends to occur under specific conditions in certain evolutionary lineages rather than being universal and ubiquitous. Our review also identifies key gaps in current knowledge and proposes promising areas for future research. Improving our understanding of host–pathogen interactions and balancing selection in innate immune genes are increasingly important, particularly regarding threats from emerging zoonotic diseases.

Functional immunogenetic variation, rather than local adaptation, predicts ectoparasite infection intensity in a model fish species

Natural host populations differ in their susceptibility to infection by parasites, and these intrapopulation differences are still an incompletely understood component of host‐parasite dynamics. In this study, we used controlled infection experiments with wild‐caught guppies (Poecilia reticulata) and their ectoparasite Gyrodactylus turnbulli to investigate the roles of local adaptation and host genetic composition (immunogenetic and neutral) in explaining differences in susceptibility to infection. We found differences between our four study host populations that were consistent between two parasite source populations, with no indication of local adaptation by either host or parasite at two tested spatial scales. Greater values of host population genetic variability metrics broadly aligned with lower population mean infection intensity, with the best alignments associated with major histocompatibility complex (MHC) “supertypes”. Controlling for intrapopulation differences and potential inbreeding variance, we found a significant negative relationship between individual‐level functional MHC variability and infection: fish carrying more MHC supertypes experienced infections of lower severity, with limited evidence for supertype‐specific effects. We conclude that population‐level differences in host infection susceptibility probably reflect variation in parasite selective pressure and/or host evolutionary potential, underpinned by functional immunogenetic variation.