For the insect microbiome, transience doesn’t always equate to irrelevance

Many insects’ gut microbiota derive partly or wholly from environmental sources. These microbes may be transient, passing through in a matter of hours, days, or a host generation. It is often assumed these microbes are commensal and serve no function for their hosts. However, there are major exceptions. In some cases, such as larval mosquitos, the taxonomic identity of a transient microbe is not important, but its functional abilities (vitamin B synthesis) are essential for host survival. In other cases, acquisition of environmental microbes may be uniquely suited to provide functions relevant to local conditions. We present evidence to refute the assumption that transient and/or environmentally-acquired microbiota must be functionally irrelevant to their insect hosts. First, we use Illumina amplicon sequencing and whole-community microbial physiological profiling (Biolog assays) to estimate that an average of 85% of the adult lepidopteran gut microbiome may derive from the diet, but the result is functional matching between diet chemical composition and the microbiome’s digestive abilities. Next, we discuss how environmental acquisition of Caballeronia bacteria with different thermal optima may buffer host leaf-footed bugs (Leptoglossus phyllopus) against changing climate. We conclude with an exploration of the potential for gut bacteria of folivorous grasshoppers and beetles – many of which derive from the environment – to detoxify plant defense chemicals. Ultimately, these results from disparate systems show that the transient, environmentally-derived derived nature of many insects’ microbiota does not preclude, and may in some cases enable, important contributions to host fitness.