Bacterial gut microbiomes of aculeate brood parasites overlap with their aculeate hosts', but have higher diversity and specialization.

Despite growing interest in gut microbiomes of aculeate Hymenoptera, research so far focused on social bees, wasps, and ants, whereas non-social taxa and their brood parasites have not received much attention. Brood parasitism, however, allows to distinguish between microbiome components horizontally transmitted by spill-over from the host with such inherited through vertical transmission by mothers. Here, we studied the bacterial gut microbiome of adults in seven aculeate species in four brood parasite-host systems: two bee-mutillid (host-parasitoid) systems, one halictid bee-cuckoo bee system, and one wasp-chrysidid cuckoo wasp system. We addressed the following questions: (1) Do closely related species possess a more similar gut microbiome? (2) Do brood parasites share components of the microbiome with their host? (3) Do brood parasites have different diversity and specialization of microbiome communities compared with the hosts? Our results indicate that the bacterial gut microbiome of the studied taxa was species-specific, yet with a limited effect of host phylogenetic relatedness and a major contribution of shared microbes between hosts and parasites. However, contrasting patterns emerged between bee-parasite systems and the wasp-parasite system. We conclude that the gut microbiome in adult brood parasites is largely affected by their host-parasite relationships and the similarity of trophic food sources between hosts and parasites.