Should Symbionts Be Nice or Selfish? Antiviral Effects of Wolbachia Are Costly but Reproductive Parasitism Is Not.

Symbionts can have mutualistic effects that increase their host's fitness and/or parasitic effects that reduce it. Which of these strategies evolves depends in part on the balance of their costs and benefits to the symbiont. We have examined these questions in Wolbachia, a vertically transmitted endosymbiont of insects that can provide protection against viral infection and/or parasitically manipulate its hosts' reproduction. Across multiple symbiont strains we find that the parasitic phenotype of cytoplasmic incompatibility and antiviral protection are uncorrelated. Strong antiviral protection is associated with substantial reductions in other fitness-related traits, whereas no such trade-off was detected for cytoplasmic incompatibility. The reason for this difference is likely that antiviral protection requires high symbiont densities but cytoplasmic incompatibility does not. These results are important for the use of Wolbachia to block dengue virus transmission by mosquitoes, as natural selection to reduce these costs may lead to reduced symbiont density and the loss of antiviral protection.This study was funded by the Wellcome Trust grant WT094664MA (http://www.wellcome.ac.uk/). FMJ is supported by a Royal Society Research Fellowship.This is the final published version. It first appeared at http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1005021

FHV titer qPCR data in experiment 1 (Original hosts)

Column headers correspond to the Wolbachia infection status (Status), the Wolbachia strain (W_strain), the Ct values for FHV and the control fly gene Rpl32 (2 technical replicates for each)

Wolbachia density qPCR data in experiment 2 (Original hosts and STCP line)

Column headers correspond to the type of host, the fly species, the Wolbachia strain (W_strain), the Ct values for the Wolbachia gene atpD and the control fly gene Rpl32 (2 technical replicates for each)

Survival data in experiment 1 (Original hosts)

Column headers correspond to the Fly species, the Wolbachia strain (W_strain), the Wolbachia infection status (Status), the virus infection treatment (Treatment), the vial number, the total number of flies in a replicate vial (Sum), the number of flies lost during the experiment (Lost) and the number of days post-viral infection. Values in the latter columns indicate the cumulative number of dead flies after virus infection

FHV titer qPCR data in experiment 2 (Original hosts and STCP line)

Column headers correspond to the type of host, the fly species, the Wolbachia strain (W_strain), the Wolbachia infection status (Status), the Ct values for FHV and the control fly gene Rpl32 (2 technical replicates for each)

Survival data in experiment 2 (Original hosts and STCP line)

Column headers correspond to the batch of infection, the type of host, the Fly species, the Wolbachia strain (W_strain), the Wolbachia infection status (Status), the virus infection treatment (Treatment), the vial number, the total number of flies in a replicate vial (Sum), the number of flies lost during the experiment (Lost) and the number of days post-viral infection. Values in the latter columns indicate the cumulative number of dead flies after virus infection

Data from: Symbiont strain is the main determinant of variation in Wolbachia-mediated protection against viruses across Drosophila species

Wolbachia is a common heritable bacterial symbiont in insects. Its evolutionary success lies in the diverse phenotypic effects it has on its hosts coupled to its propensity to move between host species over evolutionary timescales. In a survey of natural host–symbiont associations in a range of Drosophila species, we found that 10 of 16 Wolbachia strains protected their hosts against viral infection. By moving Wolbachia strains between host species, we found that the symbiont genome had a much greater influence on the level of antiviral protection than the host genome. The reason for this was that the level of protection depended on the density of the symbiont in host tissues, and Wolbachia rather than the host-controlled density. The finding that virus resistance and symbiont density are largely under the control of symbiont genes in this system has important implications both for the evolution of these traits and for public health programmes using Wolbachia to prevent mosquitoes from transmitting disease

<i>Wolbachia</i> tissue tropism.

<p>Mean <i>Wolbachia</i> density in (A) head and thorax of females, (B) testes and (C) freshly laid eggs. Error bars are standard errors. Letters indicate significant differences based on a Tukey’s honest significance test on ln-transformed data. All tissues were analyzed in a single linear model to test for difference in tissue tropism: strain effect: F_15,427 = 131. 1; <i>P</i> < 0.0001; tissue effect: F_2,427 = 4448. 8; <i>P</i> < 0.0001; strain × tissue effect: F_30,427 = 11.5; <i>P</i> < 0.0001.</p

Phylogenetic distribution of CI levels and <i>Wolbachia</i> effects on egg hatch rates, fecundity and lifespan.

<p>(A) The phylogeny based on the MLST genes <i>16S rRNA</i>, <i>aspC</i>, <i>atpD</i>, <i>ftsZ</i>, <i>sucB</i>, <i>groEL</i>, <i>coxA</i> and <i>fbpA</i> was inferred using ClonalFrame v1.2 [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005021#ppat.1005021.ref043" target="_blank">43</a>] as in [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005021#ppat.1005021.ref014" target="_blank">14</a>]. Strains in bold conferred significant antiviral protection [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005021#ppat.1005021.ref014" target="_blank">14</a>]. Branch labels represent posterior support values. Nodes with less than 50% support were collapsed. Branch lengths indicate relative time. (B) CI measured as egg hatch rates in crosses between uninfected females and <i>Wolbachia</i>-infected males. (C) Egg hatch rates in crosses between <i>Wolbachia</i>-infected females and <i>Wolbachia</i>-infected males (blue bars) or uninfected males (grey bars). (D) Fecundity of <i>Wolbachia</i>-infected females. (E) Lifespan of <i>Wolbachia</i>-infected females. Error bars are standard errors. *: significance relative to the <i>Wolbachia</i>-free line (Dunnett’s test; *: <i>P</i> < 0.05; **: <i>P</i> < 0.01; ***: <i>P</i> < 0.001). The dotted line indicates for each trait the mean value in the <i>Wolbachia</i>-free controls. (F) Original host species of the <i>Wolbachia</i> strains.</p

Correlations between <i>Wolbachia</i> density in somatic tissues and antiviral protection, CI or other host life-history traits.

<p>The relative <i>Wolbachia</i> density in head and thorax of females is correlated with survival [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005021#ppat.1005021.ref014" target="_blank">14</a>] upon infection with (A) DCV or (B) FHV (0 and positive values mean no difference and increase in survival compared to <i>Wolbachia</i>-free control respectively), (C) the level of CI, (D) the egg hatch rate in crosses with <i>Wolbachia</i>-free males, (E) the decrease in male fertility and (F) the egg number. Means and standard errors are shown. Solid lines show predicted values from linear regressions. <i>r</i> is the Pearson’s correlation coefficient between traits.</p