Dynamics and drivers of fungal communities in a multipartite ant-plant association.

BACKGROUND: Fungi and ants belong to the most important organisms in terrestrial ecosystems on Earth. In nutrient-poor niches of tropical rainforests, they have developed steady ecological relationships as a successful survival strategy. In tropical ant-plant mutualisms worldwide, where resident ants provide the host plants with defense and nutrients in exchange for shelter and food, fungi are regularly found in the ant nesting space, inhabiting ant-made dark-colored piles ("patches"). Unlike the extensively investigated fungus-growing insects, where the fungi serve as the primary food source, the purpose of this ant-fungi association is less clear. To decipher the roles of fungi in these structures within ant nests, it is crucial to first understand the dynamics and drivers that influence fungal patch communities during ant colony development. RESULTS: In this study, we investigated how the ant colony age and the ant-plant species affect the fungal community in the patches. As model we selected one of the most common mutualisms in the Tropics of America, the Azteca-Cecropia complex. By amplicon sequencing of the internal transcribed spacer 2 (ITS2) region, we analyzed the patch fungal communities of 93 Azteca spp. colonies inhabiting Cecropia spp. trees. Our study demonstrates that the fungal diversity in patches increases as the ant colony grows and that a change in the prevalent fungal taxa occurs between initial and established patches. In addition, the ant species significantly influences the composition of the fungal community in established ant colonies, rather than the host plant species. CONCLUSIONS: The fungal patch communities become more complex as the ant colony develops, due to an acquisition of fungi from the environment and a substrate diversification. Our results suggest a successional progression of the fungal communities in the patches during ant colony growth and place the ant colony as the main driver shaping such communities. The findings of this study demonstrate the unexpectedly complex nature of ant-plant mutualisms in tropical regions at a micro scale.

Bacterial diversity in arboreal ant nesting spaces is linked to colony developmental stage.

The omnipresence of ants is commonly attributed to their eusocial organization and division of labor, however, bacteria in their nests may facilitate their success. Like many other arboreal ants living in plant-provided cavities, Azteca ants form dark-colored "patches" in their nesting space inside Cecropia host plants. These patches are inhabited by bacteria, fungi and nematodes and appear to be essential for ant colony development. Yet, detailed knowledge of the microbial community composition and its consistency throughout the life cycle of ant colonies was lacking. Amplicon sequencing of the microbial 16S rRNA genes in patches from established ant colonies reveals a highly diverse, ant species-specific bacterial community and little variation within an individual ant colony, with Burkholderiales, Rhizobiales and Chitinophagales being most abundant. In contrast, bacterial communities of early ant colony stages show low alpha diversity and no ant species-specific community composition. We suggest a substrate-caused bottleneck after vertical transmission of the bacterial patch community from mother to daughter colonies. The subsequent ecological succession is driven by environmental parameters and influenced by ant behavior. Our study provides key information for future investigations determining the functions of these bacteria, which is essential to understand the ubiquity of such patches among arboreal ants.