Functional immunogenetic variation, rather than local adaptation,
predicts ectoparasite infection intensity in a model fish species
Abstract
Natural host populations differ in their susceptibility to infection by
parasites, and these intra-population 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 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
intra-population 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 likely reflect variation in
parasite selective pressure and/or host evolutionary potential,
underpinned by functional immunogenetic variation.