Genomic Assessment of the Contribution of the Wolbachia Endosymbiont of Eurosta solidaginis to Gall Induction.

We explored the genome of the Wolbachia strain, w Esol, symbiotic with the plant-gall-inducing fly Eurosta solidaginis with the goal of determining if w Esol contributes to gall induction by its insect host. Gall induction by insects has been hypothesized to involve the secretion of the phytohormones cytokinin and auxin and/or proteinaceous effectors to stimulate cell division and growth in the host plant. We sequenced the metagenome of E. solidaginis and w Esol and assembled and annotated the genome of w Esol. The w Esol genome has an assembled length of 1.66 Mbp and contains 1878 protein-coding genes. The w Esol genome is replete with proteins encoded by mobile genetic elements and shows evidence of seven different prophages. We also detected evidence of multiple small insertions of w Esol genes into the genome of the host insect. Our characterization of the genome of w Esol indicates that it is compromised in the synthesis of dimethylallyl pyrophosphate (DMAPP) and S-adenosyl L-methionine (SAM), which are precursors required for the synthesis of cytokinins and methylthiolated cytokinins. w Esol is also incapable of synthesizing tryptophan, and its genome contains no enzymes in any of the known pathways for the synthesis of indole-3-acetic acid (IAA) from tryptophan. w Esol must steal DMAPP and L-methionine from its host and therefore is unlikely to provide cytokinin and auxin to its insect host for use in gall induction. Furthermore, in spite of its large repertoire of predicted Type IV secreted effector proteins, these effectors are more likely to contribute to the acquisition of nutrients and the manipulation of the host's cellular environment to contribute to growth and reproduction of w Esol than to aid E. solidaginis in manipulating its host plant. Combined with earlier work that shows that w Esol is absent from the salivary glands of E. solidaginis , our results suggest that w Esol does not contribute to gall induction by its host.