Impact of transgenerational host switch on gut bacterial assemblage in generalist pest, Spodoptera littoralis (Lepidoptera: Noctuidae)

16S rRNA Sequencing Analysis Uncovers Dose-Dependent Cupric Chloride Effects on Silkworm Gut Microbiome Composition and Diversity

Copper-based pesticides are extensively used in agriculture, yet their impacts on beneficial insects remain poorly understood. Here, we investigate how cupric chloride exposure affects the gut microbiome of Bombyx mori, a model organism crucial for silk production. Using 16S rRNA sequencing, we analyzed the gut bacterial communities of fifth-instar silkworm larvae exposed to different concentrations of cupric chloride (0, 4, and 8 g/kg) in an artificial diet. The high-dose exposure dramatically altered the microbial diversity and community structure, where the Bacteroidota abundance decreased from 50.43% to 23.50%, while Firmicutes increased from 0.93% to 18.92%. A network analysis revealed complex interactions between the bacterial genera, with Proteobacteria and Firmicutes emerging as key players in the community response to copper stress. The functional prediction indicated significant shifts in metabolic pathways and genetic information processing in the high-dose group. Notably, the low-dose treatment induced minimal changes in both the taxonomic composition and predicted functions, suggesting a threshold effect in the microbiome response to copper exposure. Our findings provide novel insights into how agricultural chemicals influence insect gut microbiota and highlight potential implications for silkworm health and silk production. This work contributes to understanding the ecological impacts of copper-based pesticides and may inform evidence-based policies for their use in sericulture regions.

Microbiome analysis of monarch butterflies reveals effects of development and diet

Diet profoundly influences the composition of an animal's microbiome, especially in holometabolous insects, offering a valuable model to explore the impact of diet on gut microbiome dynamics throughout metamorphosis. Here, we use monarch butterflies (Danaus plexippus), specialist herbivores that feed as larvae on many species of chemically well-defined milkweed plants (Asclepias sp.), to investigate the impacts of development and diet on the composition of the gut microbial community. While a few microbial taxa are conserved across life stages of monarchs, the microbiome appears to be highly dynamic throughout the life cycle. Microbial diversity gradually diminishes throughout the larval instars, ultimately reaching its lowest point during the pupal stage and then recovering again in the adult stage. The microbial composition then undergoes a substantial shift upon the transition from pupa to adult, with female adults having significantly different microbial communities than the eggs that they lay, indicating limited evidence for vertical transmission of gut microbiota. While diet did not significantly impact overall microbial composition, our results suggest that fourth instar larvae exhibit higher microbial diversity when consuming milkweed with high concentrations of toxic cardenolide phytochemicals. This study underscores how diet and developmental stage collectively shape the monarch's gut microbiota.

Gene expression plasticity facilitates different host feeding in Ips sexdentatus (Coleoptera: Curculionidae: Scolytinae)

Host shift is ecologically advantageous and a crucial driver for herbivore insect speciation. Insects on the non-native host obtain enemy-free space and confront reduced competition, but they must adapt to survive. Such signatures of adaptations can often be detected at the gene expression level. It is astonishing how bark beetles cope with distinct chemical environments while feeding on various conifers. Hence, we aim to disentangle the six-toothed bark beetle (Ips sexdentatus) response against two different conifer defences upon host shift (Scots pine to Norway spruce). We conducted bioassay and metabolomic analysis followed by RNA-seq experiments to comprehend the beetle's ability to surpass two different terpene-based conifer defence systems. Beetle growth rate and fecundity were increased when reared exclusively on spruce logs (alternative host) compared to pine logs (native host). Comparative gene expression analysis identified differentially expressed genes (DEGs) related to digestion, detoxification, transporter activity, growth, signalling, and stress response in the spruce-feeding beetle gut. Transporter genes were highly abundant during spruce feeding, suggesting they could play a role in pumping a wide variety of endogenous and xenobiotic compounds or allelochemicals out. Trehalose transporter (TRET) is also up-regulated in the spruce-fed beetle gut to maintain homeostasis and stress tolerance. RT-qPCR and enzymatic assays further corroborated some of our findings. Taken together, the transcriptional plasticity of key physiological genes plays a crucial role after the host shift and provides vital clues for the adaptive potential of bark beetles on different conifer hosts.