Bacterial diversity in gut compartments of Tomicus brevipilosus: from shoot-boring preconditioning of Pinus yunnanensis to trunk-boring reproduction

Abstract

Gut bacteria are critically important to bark beetles, especially under environmental fluctuations. Tomicus brevipilosus undergoes a niche shift from shoot-boring (which weakens Pinus yunnanensis ) to trunk-boring for reproduction. The relationship between this transition and the gut microbiota remains to be elucidated. Using 16S rRNA sequencing, we characterized gut bacterial communities across adult feeding phase, sexes, and intestinal compartments. The alimentary canal was composed of six morphologically distinguishable regions, to which Malpighian tubules were affixed. At the shoot-boring stage, no clear compartmentalization was observed in either sex, and no particular genus emerged as prominently associated. In trunk-boring adults, females undertake oviposition, whereas males evacuate excreta and food residues from the burrow entrance using their abdominal apex and elytral declivity. During the trunk-boring phase, compartment-specific differentiation was evident: the male rectum exhibited the highest bacterial ASV richness and complexity, harboring the greatest number of unique bacterial taxa at all taxonomic levels (phylum to genus), with significant enrichment of two short-chain fatty acid (SCFA)-producing genera ( Dietzia and Acinetobacter ), suggesting a potential defensive role against gallery pathogens. In contrast, female trunk-boring adults showed no clear gut compartmentalization, although the genera Delftia and Halomonas may contribute to cellulose breakdown and thereby facilitate reproduction. These functional inferences, however, await direct experimental confirmation. Interaction analyses further revealed that gut bacterial communities across intestinal segments varied significantly with both stage and sex. These stage- and sex-specific patterns likely reflect differing physiological and behavioral demands during the niche shift. Our findings demonstrate that this transition drives structural alterations in gut bacterial communities across both compartments and sexes, and provide a basis for future exploration of symbiont-based management strategies for this destructive forest pest.