What lurks in the dark? An innovative framework for studying diverse wild insect microbiota.

Background: Symbiotic microorganisms can profoundly impact insect biology, including their life history traits, population dynamics, and evolutionary trajectories. However, microbiota remain poorly understood in natural insect communities, especially in 'dark taxa'-hyperdiverse yet understudied clades.

Results: Here, we implemented a novel multi-target amplicon sequencing approach to study microbiota in complex, species-rich communities. It combines four methodological innovations: (1) To establish a host taxonomic framework, we sequenced amplicons of the host marker gene (COI) and reconstructed barcodes alongside microbiota characterisation using 16S-V4 rRNA bacterial gene amplicons. (2) To assess microbiota abundance, we incorporated spike-in-based quantification. (3) To improve the phylogenetic resolution for the dominant endosymbiont, Wolbachia, we analysed bycatch data from the COI amplicon sequencing. (4) To investigate the primary drivers of host-microbe associations in massive multi-dimensional datasets, we performed Hierarchical Modelling of Species Communities (HMSC). Applying this approach to 1842 wild-caught scuttle flies (Diptera: Phoridae) from northern Sweden, we organised them into 480 genotypes and 186 species and gained unprecedented insights into their microbiota. We found orders-of-magnitude differences in bacterial abundance and massive within-population variation in microbiota composition. Patterns and drivers differed among microbial functional categories: the distribution and abundance of facultative endosymbionts (Wolbachia, Rickettsia, Spiroplasma) were shaped by host species, genotype, and sex. In contrast, many other bacterial taxa were broadly distributed across species and sites.

Conclusions: This study highlights facultative endosymbionts as key players in insect microbiota and reveals striking variations in distributional patterns of microbial clades. It also demonstrates the power of integrative sequencing approaches in uncovering the ecological complexity and significance of symbiotic microorganisms in multi-species natural communities. Video Abstract.