Microbiome Dynamics and Functional Composition in Coelopidae (Diptera: Sciomyzoidea): Insights into Trophic Specialization of Kelp Flies

Background:Coelopidae (Diptera), known as kelp flies, exhibit an ecological association with beached kelp and other rotting seaweeds. This unique trophic specialization necessitates significant adaptations to overcome the limitations of an algal diet. In this study, we aimed to investigate whether the flies' microbiome could be one of these adaptive mechanisms. Our analysis focused on assessing the composition and diversity of the adult and larval microbiota of the kelp fly Coelopa frigida. The feeding habits of the larvae of this species have been subject of numerous studies, with debates whether they directly consume kelp or primarily feed on associated bacteria.Results:By using a 16S rRNA metabarcoding approach, we found that the larval microbiota displayed considerably less diversity than adults, heavily dominated by only four OTUs. Phylogenetic placement recovered the most dominant OTU of the larval microbiome, which is the source of more than half of all metabarcoding sequence reads, as an undescribed genus of the Orbaceae (Gammaprotebacteria). This taxon has been repeatedly shown to include members which play an important role in the gut microbiome of different insects. Interestingly, this OTU is barely found among the 15 most abundant taxa of the adult microbiome, where it is responsible for less than 2% of the metabardoing sequence reads. The other three OTUs dominating the larval microbiome have been assigned as Psychrobacter (Gammaproteobacteria), Wohlfahrtiimonas (Gammaproteobacteria), and Cetobacterium (Fusobacteriota). Moreover, based on our metabarcoding data we also uncovered a distinct shift in the functional composition between the larval and adult stages.