Are bacterial symbionts associated with gall induction in insects?
Citation
Hammer, T.J., De Clerck-Floate, R., Tooker, J.F., Price, P.W., Miller, D.G., Connor, E.F. (2021). Are bacterial symbionts associated with gall induction in insects?, 15(1), http://dx.doi.org/10.1007/s11829-020-09800-6
Plain language summary
Plant galls are tumor-like growths that are produced by specialized organisms that live within their host plants and benefit from the added protection and sustenance provided by the gall structures they induce. The induction of plant galls by a diverse range of organisms (i.e. bacteria, fungi, insects, mites, nematodes, protists) is one of the most striking instances of a biological process evolving multiple, separate times during the history of life on earth. However, exactly how these organisms create galls is only known for a small group of bacteria, but not for more evolutionarily advanced organisms such as insects. To determine if gall induction by insects may be associated with the presence of bacteria that live symbiotically with the insects, we examined using molecular analysis the bacterial communities (i.e. ‘microbiomes’) associated with a variety of gall-inducing and non-gall inducing insects. Overall, we did not find any evidence of bacteria associated gall induction among the insect species we surveyed; i.e. there were no bacterial taxonomic groups known to cause plant galls on their own that were consistently associated with the gall-inducing insect species examined. The bacteria types making up a microbiome also differed strongly among insect species, but not in a predictable way between gall-inducing and non-gall-inducing species. Also, our data for two specific gall-inducing insect species suggested that no symbiotic bacteria present were any more abundant than other bacteria within the microbiomes of these insects. Together, these findings argue that gall induction is not caused by a consistently associated bacterial symbiont or bacterial community within different gall-inducing insect groups, and at least in some insects, may not be associated with symbiotic bacteria at all. While symbiotic bacteria may still contribute to gall induction in specific instances, we suggest that the repeated occurrence of gall induction across a broad range of distantly-related taxonomic groups of plant-feeding organisms is being driven by other biological mechanisms yet to be explored.
Abstract
© 2021, The Author(s), under exclusive licence to Springer Nature B.V. part of Springer Nature.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.