Sexual divergence in bacterial microbiota of Monochamus alternatus (Coleoptera: Cerambycidae) with differential assembly patterns across geographical populations

The sexual dimorphism in terms of morphology, behaviour, and chemical signalling has been elucidated in many insect species, while taxonomic and functional diversification in endosymbiotic microbial community between insect sexes and potential assembly mechanism underlying the sexual variation at latitudinal population level have been less investigated. As a widely spread forest insect pest transmitting an invasive nematode Bursaphelenchus xylophilus , the pine sawyer beetle Monochamus alternatus presents multiple sexually dimorphic traits of its own. Based on amplicon sequencing of the 16S rRNA gene, dramatic sexual variations were found in the community compositions of the beetle reproductive organ and gut bacterial microbiota across different latitudinal populations. Notably, volatile compound indole was more produced in females and dominant bacterial species were relatively more tolerant to this compound than species preferentially enriched in males. Moreover, several species presented strikingly different optimal growth temperatures, contributing to distinct sexual variation patterns of bacterial community composition associated with each beetle population. Functional predictions showed differential metabolic activities of the bacterial microbiota between beetle sexes, exhibiting potential convergence across populations. Our findings suggest that sexually varied chemical environment and external abiotic factors may collectively shape the bacterial microbiota dimorphism between M. alternatus sexes, providing new insights into the mechanism underlying sex disparity of insect herbivores in nutritional utilization, host plant adaptation, and disease resistance driven by symbiotic microbial community.