Abstract
Microbial symbionts can modulate host interactions with biotic and abiotic factors. Such interactions may affect the evolutionary trajectories of both host and symbiont. Wolbachia protects Drosophila melanogaster against several viral infections and the strength of the protection varies between variants of this endosymbiont. Since Wolbachia is maternally transmitted, its fitness depends on the fitness of its host. Therefore, Wolbachia populations may be under selection when Drosophila is subjected to viral infection. Here we show that in D. melanogaster populations selected for increased survival upon infection with Drosophila C virus there is a strong selection coefficient for specific Wolbachia variants, leading to their fixation. Flies carrying these selected Wolbachia variants have higher survival and fertility upon viral infection when compared to flies with the other variants. These findings demonstrate how the interaction of a host with pathogens shapes the genetic composition of symbiont populations. Furthermore, host adaptation can result from the evolution of its symbionts, with host and symbiont functioning as a single evolutionary unit.
Highlights
Animals and plants live in close association with numerous symbiotic bacteria that often cause strong phenotypic changes in their hosts [1]
We show that a population of Drosophila infected with viruses during several generations adapts to this challenge through turnover in Wolbachia composition
111 were significantly different between Control and Virus-Selected populations, but not between Control and Ancestral populations, showing that these changes in the genetic composition of the Wolbachia populations are mostly specific to the response to viral infection
Summary
Animals and plants live in close association with numerous symbiotic bacteria that often cause strong phenotypic changes in their hosts [1]. The frequency of Spiroplasma in a D. neotestacea population increases in the presence of the parasitic nematode during experimental evolution [11] These studies show changes in the prevalence of endosymbiont infection in host populations, but do not address selection at the level of the genetic diversity of the symbiont itself. Some evidence suggests that this could be the case: 1) some defensive symbiont populations display genetic and phenotypic variability [12,13,14,15,16,17,18] and 2) variants or strains of endosymbionts change in frequency in natural populations or during experimental evolution [19,20,21]. We establish a relation between host adaptation to parasites and changes in the genetic composition of endosymbiont populations
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