Parasitic lifestyles have evolved many times in animals, but how such life-history strategies evolved from free-living ancestors remains a great puzzle. Transitional symbiotic strategies, such as facultative parasitism, are hypothesized evolutionary stepping stones towards obligate parasitism. However, to consider this hypothesis, heritable genetic variation in infectious behaviour of transitional symbiotic strategies must exist. In this study, we experimentally evolved infectivity and estimated the additive genetic variation in a facultative parasite. We performed artificial selection experiments in which we selected for either increased or decreased propensity to infect in a facultatively parasitic mite (Macrocheles muscaedomesticae). Here, infectiousness was expressed in terms of mite attachment to a host (Drosophila hydei) and modelled as a threshold trait. Mites responded positively to selection for increased infectivity; realized heritability of infectious behaviour was significantly different from zero and estimated to be 16.6% (±4.4% SE). Further, infection prevalence was monitored for 20 generations post-selection. Selected lines continued to display relatively high levels of infection, demonstrating a degree of genetic stability in infectiousness. Our study is the first to provide an estimate of heritability and additive genetic variation for infectious behaviour in a facultative parasite, which suggests natural selection can act upon facultative strategies with important implications for the evolution of parasitism.
Animal populations exhibit various patterns in ectoparasite distribution across different sexes and age classes, and numerous factors can potentially contribute to ectoparasite abundance and distribution. We examined the influence of host body size and age on the distribution of a chewing louse among brown-headed cowbirds. Differences in louse prevalence (males 62.9 ± 2.8%, females 47.5 ± 4.9%) and intensity (males 15.42 ± 1.51, females 9.04 ± 1.69) were primarily driven by differences in host body mass and not host sex. Larger birds had larger louse infra-populations, which likely translated into a lower risk of local extinction, a possible explanation for higher louse prevalence observed among larger birds. Among males, younger individuals showed higher louse prevalence (70.21 ± 4.72%) compared to older males (59.36 ± 3.59). We speculate that this pattern is likely driven by behavioural difference and not body size, with young males spending relatively more time foraging in large groups, increasing their risk of louse transmission. By examining the mechanisms that underlie the sex- and age-biased infections observed in natural populations, we can better identify the hosts most responsible for parasite transmission.
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A prevailing hypothesis for the evolution of parasitism posits that the fitness benefits gained from parasitic activity results in selection for and fixation of parasitic strategies. Despite the potential fitness advantage of parasitism, facultative parasites continue to exhibit genetic variation in parasitic behaviour in nature. We hypothesized that evolutionary trade-offs associated with parasitic host-attachment behaviour maintain natural variation observed in attachment behaviour. In this study, we used replicate lines of a facultatively parasitic mite, previously selected for increased host-attachment behaviour to test whether increased attachment trades off with mite fecundity and longevity, as well as the phenotypic plasticity of attachment. We also tested for potential correlated changes in mite morphology. To test for context-dependent trade-offs, mite fecundity and longevity were assayed in the presence or absence of a host. Our results show that selected and control mites exhibited similar fecundities, longevities, attachment plasticities and morphologies, which did not provide evidence for life history trade-offs associated with increased attachment. Surprisingly, phenotypic plasticity in attachment was maintained despite directional selection on the trait, which suggests that phenotypic plasticity likely plays an important role in maintaining attachment variation in natural populations of this facultative parasite.
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