Gut microbiomes of four Australian stink bug species associated with citrus, and fitness effects of a dominant Pantoea-like symbiont in Biprorulus bibax

Gut microbiomes are fundamental to animal biology, evolution and health. True bugs (Hemiptera) maintain heritable microbial gut symbionts and are research models for host-microbe interactions. However, current knowledge is mostly limited to the bacterial symbionts in the Holarctic-Oriental bug fauna, leaving the microbial symbioses of the Australasian diversity unknown. Using high-throughput amplicon sequencing, we characterized the bacterial and fungal communities of four Australian stink bug species associated with citrus: Biprorulus bibax , Poecilometis strigatus (Pentatomidae), Lyramorpha rosea and Musgraveia sulciventris (Tessaratomidae). Across all species, bacterial communities were low in diversity, with each species harbouring a dominant and distinct gammaproteobacterial symbiont within the Pantoea-Erwinia complex. However, L. rosea and P. strigatus contained more diverse assemblages including low-abundance secondary taxa. Furthermore, each host species harboured a differentiated fungal consortium that was diverse across hosts and dominated by taxa including Cladosporium , Eremothecium and Malassezia . Although the dominant bacterial symbionts were host-specific, their phylogeny was incongruent with the host phylogeny, probably indicating host switches and decoupled host-symbiont evolutionary histories. We also found evidence that in B. bibax , the Pantoea -like symbiont was vertically transmitted via egg smearing. Egg surface sterilisation resulted in aposymbiotic offspring with delayed development, reduced longevity and lower fecundity, demonstrating a symbiont contribution to host fitness. Overall, our findings of the first comparative gut microbiome analysis of Australian stink bugs support globally conserved association patterns with Pantoea -like symbionts alongside species-specific microbial community structure and advanced the understanding of host-microbe evolution in these insects.