Abstract
To understand host–parasite interactions, it is necessary to quantify variation and covariation in defence traits. We quantified parasite resistance and fitness tolerance of a polymorphic damselfly (Ischnura elegans), an insect with three discrete female colour morphs but with monomorphic males. We quantified sex and morph differences in parasite resistance (prevalence and intensity of water mite infections) and morph-specific fitness tolerance in the females in natural populations for over a decade. There was no evidence for higher parasite susceptibility in males as a cost of sexual selection, whereas differences in defence mechanisms between female morphs are consistent with correlational selection operating on combinations of parasite resistance and tolerance. We suggest that tolerance differences between female morphs interact with frequency-dependent sexual conflict, which maintains the polymorphism locally. Host–parasite interactions can therefore shape intra- and intersexual phenotypic divergence and interfere with sexual selection and sexual conflict.
Highlights
Phenotypic traits may become negatively associated when they influence the strength of selection on each other by being costly and functionally redundant [1,2]
To estimate morph-specific tolerance, we evaluated the effects of parasites on female fecundity, using a generalized mixed models (GLMM) with Poisson error distribution and fitted by Markov chain Monte Carlo (MCMC) as described above
A- and I-females had similar parasite prevalence and both of these female morphs were more likely to be parasitized by mites than O-females (A versus O pMCMC, 0.001, I versus O pMCMC 1⁄4 0.004)
Summary
Phenotypic traits may become negatively associated when they influence the strength of selection on each other by being costly and functionally redundant [1,2]. This emergent form of phenotypic covariation—i.e. the expression of one advantageous trait or another but not both—is often a result of correlational selection, which favours suites of co-adapted traits resulting in phenotypic and/or genetic correlations between them [3,4,5]. The vast majority of previous research on tolerance variation and the relationship between resistance
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