Consistent patterns of fungal communities within ant-plants across a large geographic range strongly suggest a multipartite mutualism

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.

Fungi as mutualistic partners in ant-plant interactions

Associations between fungi and ants living in mutualistic relationship with plants ("plant-ants") have been known for a long time. However, only in recent years has the mutualistic nature, frequency, and geographical extent of associations between tropical arboreal ants with fungi of the ascomycete order Chaetothyriales and Capnodiales (belonging to the so-called "Black Fungi") become clear. Two groups of arboreal ants displaying different nesting strategies are associated with ascomycete fungi: carton-building ants that construct nest walls and galleries on stems, branches or below leaves which are overgrown by fungal hyphae, and plant-ants that make their nests inside living plants (myrmecophytes) in plant provided cavities (domatia) where ants cultivate fungi in small delimited "patches". In this review we summarize the current knowledge about these unsuspected plant-ant-fungus interactions. The data suggest, that at least some of these ant-associated fungi seem to have coevolved with ants over a long period of time and have developed specific adaptations to this lifestyle.

Effectiveness of Cashew Nut Shell Liquid (CNSL) with Bait Formulation Against Iridomyrmex cordatus as a Vector of Cocoa Pod Rot Disease Caused by Phytophthora palmivora

<b>Background and Objective:</b> Cashew Nut Shell Liquid (CNSL) is a biopesticide that is environment-friendly in controlling ants <i>Iridomyrmex cordatus</i> is a vector pot rot disease caused by <i>Phytophthora palmivora</i>. The research aimed to obtain the best concentration of cashew nut shell liquid formulated as smart food bait to tackle the ant population of <i>Iridomyrmex cordatus</i> due mainly to a vector of <i>Phytophthora </i>pod rot. <b>Materials and Methods:</b> The research was carried out in cocoa areas in South Sulawesi with a randomized block design consisting of six treatments. Feeding trials consisted of food bait formulation that was added with cashew husk, respectively 1, 10, 20% and 17.5 g carbaryl (recommended dose) and control (without cashew husk or carbaryl). Each trial was replicated five times and set near to petiole and ant tunnel nest. The research focused on the number of ant colonies established in the trees until food bait was given. <b>Results:</b> The trial with CNSL concentration had a positive effect on the ant population the higher concentration given, the lower the ant population was obtained. Meanwhile, the population of <i>I. cordatus</i> had a positive contribution to the increase of disease incidence of <i>Phytophthora</i> pod rot disease<i>.</i> Feeding trials with 1 and 5% CNSL concentrations were less effective to limit ant population and disease incidence according to the efficacy test. In contrast, the trials of 10 and 20% were able to control population density and disease incidence due to over 51.39% efficacy test. <b>Conclusion:</b> There was a positive trend of CNSL concentration to reduce the ant population of <i>I. cordatus</i>. There was the greatest association between the increase of ant population density and the increase in disease incidence of <i>Phytophthora </i>pod rot.

Volatile Organic Compounds in the Azteca/Cecropia Ant-Plant Symbiosis and the Role of Black Fungi

Black fungi of the order Chaetothyriales are grown by many tropical plant-mutualistic ants as small so-called “patches” in their nests, which are located inside hollow structures provided by the host plant (“domatia”). These fungi are introduced and fostered by the ants, indicating that they are important for the colony. As several species of Chaetothyriales tolerate, adsorb, and metabolize toxic volatiles, we investigated the composition of volatile organic compounds (VOCs) of selected domatia in the Azteca/Cecropia ant-plant mutualism. Concentrations of VOCs in ant-inhabited domatia, empty domatia, and background air were compared. In total, 211 compounds belonging to 19 chemical families were identified. Ant-inhabited domatia were dominated by ketones with 2-heptanone, a well-known ant alarm semiochemical, as the most abundant volatile. Empty domatia were characterized by relatively high concentrations of the monoterpenes d-limonene, p-cymene and β-phellandrene, as well as the heterocyclic sulphur-containing compound, benzothiazole. These compounds have biocidal properties and are primarily biosynthesized by plants as a defense mechanism. Interestingly, most of the latter compounds were present at lower concentrations in ant inhabited domatia than in non-colonized ones. We suggest that Chaetothyriales may play a role in reducing the VOCs, underlining that the mutualistic nature of these fungi as VOCs accumulation might be detrimental for the ants, especially the larvae.