Experimental evolution of infectious behaviour in a facultative ectoparasite

Abstract

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.

Phenotypic plasticity more essential to maintaining variation in host-attachment behaviour than evolutionary trade-offs in a facultatively parasitic mite

“…The cultures were maintained at 24 °C, 70% relative humidity and a 12D: 12L photoperiod. Please see Durkin and Luong (2018) for detailed fly and mite culture methods.…”

“…In both experiments A and B, the ‘increased propensity for attachment’ lines responded positively to selection; the mean realized heritability for attachment behaviour was estimated to be 0.166 ± 0.058 s.e. (Durkin and Luong, 2018). We refer to the ‘increased propensity for attachment’ line as the ‘selected’ line from here on.…”

“…Artificial selection was conducted on M. muscaedomesticae in a previous experiment (Durkin and Luong, 2018); here we report on the correlated changes in life history and morphometric traits as well as attachment plasticity. The selection protocol was constrained by logistical challenges associated with the selection regime, limiting the number of concurrent replicate lines we could maintain.…”

“…observation). Their attachment behaviour is plastic and influenced by their own internal state (Jalil and Rodriguez, 1970; Luong et al ., 2017), the state of their potential hosts (Farish and Axtell, 1971; Campbell and Luong, 2016; Luong et al ., 2017) as well as the external environment (Farish and Axtell, 1971; Durkin and Luong, 2018).…”

“…In a previous study, we experimentally evolved M. muscaedomesticae populations to exhibit an increased propensity to attach to fly hosts using artificial selection (Durkin and Luong, 2018). In this study, we determined whether evolutionary trade-offs, phenotypic plasticity or a combination of both might play a role in maintaining attachment variation in M. muscaedomesticae .…”