Cabbage looper (Trichoplusia ni Hübner) labial glands contain unique bacterial flora in contrast with their alimentary canal, mandibular glands, and Malpighian tubules

Parasitoid Calyx Fluid and Venom Affect Bacterial Communities in Their Lepidopteran Host Labial Salivary Glands

The influence of gut and gonad bacterial communities on insect physiology, behaviour, and ecology is increasingly recognised. Parasitism by parasitoid wasps alters many physiological processes in their hosts, including gut bacterial communities. However, it remains unclear whether these changes are restricted to the gut or also occur in other tissues and fluids, and the mechanisms underlying such changes are unknown. We hypothesise that host microbiome changes result from the injection of calyx fluid (that contain symbiotic viruses known as polydnaviruses) and venom during parasitoid oviposition and that these effects vary by host tissue. To test this, we microinjected Pieris brassicae caterpillars with calyx fluid and venom from Cotesia glomerata, using saline solution and natural parasitism by C. glomerata as controls. We analysed changes in the bacterial community composition in the gut, regurgitate, haemolymph, and labial salivary glands of the host insects. Multivariate analysis revealed distinct bacterial communities across tissues and fluids, with high diversity in the salivary glands and haemolymph. Parasitism and injection of calyx fluid and venom significantly altered bacterial communities in the salivary glands. Differential abundance analysis showed that parasitism affected bacterial relative abundance in the haemolymph, and that Wolbachia was only found in the haemolymph of parasitized caterpillars. Altogether, our findings reveal that parasitism influences the host haemolymph microbiome, and both parasitism and injection of calyx fluid and venom drive changes in the bacterial community composition within the host salivary glands. Given that the composition of salivary glands can influence plant response to herbivory, we discuss these results in the broader context of plant-parasitoid interactions.

Comparative gut microbiome research through the lens of ecology: theoretical considerations and best practices

Comparative approaches in animal gut microbiome research have revealed patterns of phylosymbiosis, dietary and physiological convergences, and environment-host interactions. However, most large-scale comparative studies, especially those that are highly cited, have focused on mammals, and efforts to integrate comparative approaches with existing ecological frameworks are lacking. While mammals serve as useful model organisms, developing generalised principles of how animal gut microbiomes are shaped and how these microbiomes interact bidirectionally with host ecology and evolution requires a more complete sampling of the animal kingdom. Here, we provide an overview of what past comparative studies have taught us about the gut microbiome, and how community ecology theory may help resolve certain contradictions in comparative gut microbiome research. We explore whether certain hypotheses are supported across clades, and how the disproportionate focus on mammals has introduced potential bias into gut microbiome theory. We then introduce a methodological solution by which public gut microbiome data of understudied hosts can be compiled and analysed in a comparative context. Our aggregation and analysis of 179 studies shows that generating data sets with rich host diversity is possible with public data and that key gut microbes associated with mammals are widespread across the animal kingdom. We also show the effects that sample size and taxonomic rank have on comparative gut microbiome studies and that results of multivariate analyses can vary significantly with these two parameters. While challenges remain in developing a universal model of the animal gut microbiome, we show that existing ecological frameworks can help bring us one step closer to integrating the gut microbiome into animal ecology and evolution.

Tomato defences modulate not only insect performance but also their gut microbial composition

Plants protect their tissues from insect herbivory with specialized structures and chemicals, such as cuticles, trichomes, and metabolites contained therein. Bacteria inside the insect gut are also exposed to plant defences and can potentially modify the outcome of plant-insect interactions. To disentangle this complex multi-organism system, we used tomato mutants impaired in the production of plant defences (odorless-2 and jasmonic acid-insensitive1) and two cultivars (Ailsa Craig and Castlemart), exposed them to herbivory by the cabbage looper (Trichoplusia ni H.) and collected the insect frass for bacterial community analysis. While the epicuticular wax and terpene profiles were variable, the leaf fatty acid composition remained consistent among genotypes. Moreover, larval weight confirmed the negative association between plant defences and insect performance. The distinctive frass fatty acid profiles indicated that plant genotype also influences the lipid digestive metabolism of insects. Additionally, comparisons of leaf and insect-gut bacterial communities revealed a limited overlap in bacterial species between the two sample types. Insect bacterial community abundance and diversity were notably reduced in insects fed on the mutants, with Enterobacteriaceae being the predominant group, whereas putatively pathogenic taxa were found in wildtype genotypes. Altogether, these results indicate that plant defences can modulate insect-associated bacterial community composition.

Diet and ontogeny drastically alter the larval microbiome of the invertebrate model Galleria mellonella

Larvae of the greater wax moth (Galleria mellonella) are an emerging animal model to study the innate immune response and biodegradation of plastic polymers. Both of these complex biological processes are likely impacted by the plasticity of host-microbe interactions, which remains understudied in lepidopterans. Consequently we carried out 16S rRNA sequencing to explore the effect diet (natural, artificial) has on the bacterial assemblages of G. mellonella in different tissues (gut, fat bodies, silk glands) throughout development (eggs, six instar stages, adults). The microbiome was rich in diversity, with Proteobacteria and Firmicutes being the most represented phyla. Contrary to other lepidopterans, G. mellonella appears to possess a resident microbiome dominated by Ralstonia. As larvae progress through development, the bacterial assemblages become increasingly shaped by the caterpillar's diet. In particular, a number of bacteria genera widely associated with the G. mellonella microbiome (e.g., Enterococcus and Enterbacter) were significantly enriched on an artificial diet. Overall these results indicate that the G. mellonella microbiome is not as simplistic and homogenous as previously described. Rather, its bacterial communities are drastically affected by both diet and ontogeny, which should be taken into consideration in future studies planning to use G. mellonella as model species.

Novel mechanisms of epithelial ion transport: insights from the cryptonephridial system of lepidopteran larvae

Lepidopterans are among the most widespread and easily recognized insects. Whereas adult lepidopterans are known for their beauty and ecological importance as pollinators and sources of food for other animals, larvae are economically important pests of forests and agricultural crops. In the larval body, rapid growth while feeding on plant-based diet is associated with extreme alkalinity (up to pH = 11) of the midgut lumen that helps digest plant proteins. Additionally, the presence of plant secondary metabolites which serve as anti-herbivory agents requires uninterrupted excretory function, accomplished primarily by the Malpighian tubules (MTs). The so-called cryptonephridial condition, along with extreme regional heterogeneity of the MTs, and the ability to rapidly and reversibly alter the direction of epithelial ion transport are features that allow uninterrupted MT functioning and recycling of base equivalents. Studies of MTs in lepidopteran larvae have revealed that rapid adjustments in epithelial ion transport include unexpected roles for voltage-gated, ligand-gated and mechanosensitive ion channels, as well as gap junctions. These molecular components are present in epithelia of a variety of vertebrates and invertebrates and thus are likely to constitute a universal epithelial toolkit for rapid autonomous regulation of epithelial function.

Cabbage looper (Trichoplusia ni Hübner) labial glands contain unique bacterial flora in contrast with their alimentary canal, mandibular glands, and Malpighian tubules

In recent years, several studies have examined the gut microbiome of lepidopteran larvae and how factors such as host plant affect it, and in turn, how gut bacteria affect host plant responses to herbivory. In addition, other studies have detailed how secretions of the labial (salivary) glands can alter host plant defense responses. We examined the gut microbiome of the cabbage looper (Trichoplusia ni) feeding on collards (Brassica oleracea) and separately analyzed the microbiomes of various organs that open directly into the alimentary canal, including the labial glands, mandibular glands, and the Malpighian tubules. In this study, the gut microbiome of T. ni was found to be generally consistent with those of other lepidopteran larvae in prior studies. The greatest diversity of bacteria appeared in the Firmicutes, Actinobacteria, Proteobacteria, and Bacteriodetes. Well‐represented genera included Staphylococcus, Streptococcus, Corynebacterium, Pseudomonas, Diaphorobacter, Methylobacterium, Flavobacterium, and Cloacibacterium. Across all organs, two amplicon sequence variants (ASVs) associated with the genera Diaphorobacter and Cloacibacterium appeared to be most abundant. In terms of the most prevalent ASVs, the alimentary canal, Malpighian tubules, and mandibular glands appeared to have similar complements of bacteria, with relatively few significant differences evident. However, aside from the Diaphorobacter and Cloacibacterium ASVs common to all the organs, the labial glands appeared to possess a distinctive complement of bacteria which was absent or poorly represented in the other organs. Among these were representatives of the Pseudomonas, Flavobacterium, Caulobacterium, Anaerococcus, and Methylobacterium. These results suggest that the labial glands present bacteria with different selective pressures than those occurring in the mandibular gland, Malpighian tubules and the alimentary canal. Given the documented effects that labial gland secretions and the gut microbiome can exert on host plant defenses, the effects exerted by the bacteria inhabiting the labial glands themselves deserve further study.