Are bacterial symbionts associated with gall induction in insects?

A diverse array of organisms induce plants to form galls. This phenomenon is one of the most striking instances of convergent evolution, yet the underlying mechanism is only well understood in gall-inducing microbes. To determine if gall induction by insects is associated with bacterial symbiosis, we used 16S rRNA gene sequencing to examine the microbiome of a variety of gall-inducing and non-gall-inducing insects. Overall, we did not find any bacterial signature of gall induction among the insect species we surveyed. There were no specific bacterial taxa that were consistently associated with gall induction. Microbiome diversity differed significantly among species of host insects, but not between gall- and non-gall-inducing insect species. Bacterial community composition also differed strongly among insect species, but not in a systematic way between gall-inducing and non-gall-inducing species. Furthermore, two gall-inducing species harbored highly variable microbiomes with relatively few bacterial sequences, characteristics that suggest a lack of abundant bacterial symbionts. Together, these findings argue that gall induction is not consistently mediated by a bacterial symbiont or bacterial community and may be symbiont-independent, at least in some insect species. While symbionts may still contribute to gall induction in specific instances, we suggest that the convergent evolution of gall induction is more typically driven by endogenous mechanisms, with potential contributions from horizontal gene transfer.