Dispersal evolution in a population infected with a male-killing endosymbiont.

Host dispersal influences how frequently a parasite encounters new susceptible hosts, which translates into potential conflict between parasite and host interests regarding dispersal. In vertically transmitted parasites, the impact of host dispersal on parasite spread appears limited. Here, it is less intuitive whether parasites can benefit by manipulating host dispersal and, if so, whether they would evolve to increase or decrease host dispersal. We examine host-parasite conflict over dispersal using the case of the African monarch butterfly (Danaus chrysippus) and the male-killing Spiroplasma as inspiration. Male-killing endosymbionts transmit vertically from host females to their offspring and infected male offspring die. We contrast spread of a male-killing endosymbiont with a hypothetical scenario in which the parasite can additionally manipulate host dispersal. We show that a dispersal-manipulating male killer can switch the system from an overall male-biased dispersal pattern to a female-biased one. Interestingly, while infected females disperse most, the strongest evolutionary response occurs in uninfected females, which evolve towards philopatry (relative to a scenario where parasites cannot manipulate hosts). Our work thus uncovers a novel context where the phenotypic effect of parasitism is strongest in individuals that are not at risk of ever being parasitized.