Worker-dependent gut symbiosis in an ant

The hallmark of eusocial insects, honeybees, ants, and termites, is division of labor between reproductive and non-reproductive worker castes. In addition, environmental adaption and ecological dominance are also underpinned by symbiotic associations with beneficial microorganisms. Microbial symbionts are generally considered to be maintained in an insect colony in two alternative ways: shared among all colony members or inherited only by a specific caste. Especially in ants, the reproductive caste plays a crucial role in transmission of the symbionts shared among colony members over generations. Here, we report an exceptional case, the worker-dependent microbiota in an ant, Diacamma cf. indicum from Japan. By collecting almost all the individuals from 22 colonies in the field, we revealed that microbiota of workers is characterized by a single dominant bacterium localized at the hindgut. The bacterium belonging to an unclassified member within the phylum Firmicutes, which is scarce or mostly absent in the reproductive castes. Furthermore, we show that the gut symbiont is acquired at the adult stage. Collectively, our findings strongly suggest that the specific symbiont is maintained by only workers, demonstrating a novel pattern of ant-associated bacterial symbiosis, and thus further our understanding of host-microbe interactions in the light of sociobiology.