Relationships Among MHC Diversity, Parasites and Secondary Sexual Colouration

Sexual selection via honest (costly) signals of fitness can generate patterns in immunogenetic diversity at the major histocompatibility complex (MHC). MHC diversity explains variation in immunocompetence-and therefore fitness-thus impacting the ability to generate honest signals. Differences in guppy (Poecilia reticulata) colour patterns can be driven by opposing selection pressures of predation and female mate choice, but the effect of parasitism has received less attention. By studying 21 wild guppy populations in Trinidad, we sought to quantify covariation among parasitic Gyrodactylus spp. infections, male sexual ornamentation (orange and black colouration) and genetic variation at the major histocompatibility complex (MHC) at both individual and population levels. We found population level support for a correlation between MHC diversity and Gyrodactylus prevalence, including an overall association between MHC supertype composition and prevalence. At the individual level, supertype (functional) diversity again emerged as a more informative measure than allelic richness alone. Predation was associated with Gyrodactylus prevalence and orange, but not black, colouration. Notably, some of these relationships differed between the individual and population levels. Further, much of this variation was explained by processes acting at a broader geographic scale within river drainages, which comprise divergent guppy lineages. These findings suggest that evolutionary history and processes such as genetic drift and gene flow contribute substantially to these relationships, but that ecological variation (i.e., predation) interacts with these processes. Our findings indicate that multi-trait relationships between colour, immunogenetic diversity and infection are best examined within their broader evolutionary and ecological contexts.