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

A new family of ant-associated fungi in Chaetothyriales

The order Chaetothyriales comprises the black yeasts and relatives, of which numerous species are prevalent as opportunists on human hosts. The present paper introduces a clade of species that live in ant nests inside hollow structures of tropical plants (so-called domatia) and their closest relatives. To clarify the evolutionary trajectory of the domatia-associated clade, molecular, morphological, and physiological data were analysed. The position of the domatia clade within the Chaetothyriales was assessed by phylogenetic analysis of ITS and LSU. Species delimitations were calculated and genealogical concordance performed with a dataset including the gene of the ribosomal operon, β-tubulin (BT2) and RNA polymerase II largest subunit (RBP1). Genome sequencing allowed additional analysis of mating types, mitochondrial genomes, and estimation of a species tree based on the proteins of 770 single copy orthologous genes. A new family with two new genera in Chaetothyriales was introduced to accommodate the taxa from ant-inhabited domatia and a related clade of plant- and rock-colonizing species. The family is monophyletic and has strong statistical support. Although species delimitation criteria suggested the separation of more than 10 species in the domatia-clade, genealogical concordance of ribosomal and housekeeping gene markers indicated genetic exchange. Seven new species were delineated, with species also being characterized by phenotypic features of fungal colony morphology, micromorphology, physiology and ecology. However, intra-specific variability remained exceptionally large and did not always match with ecological and geographic data. It is hypothesized that the high degrees of intra- and interspecific variability of some of the clades acknowledged as separate species might be related to extended periods of molecular evolution. The newly described species seem to have their preferred habitat in tropical ant nests, and they have adapted to this specific environment. Ant-domatia provide a remarkable habitat rich in volatile chemicals, which could be tolerated by the fungi under study. The family is distantly related to the family Herpotrichiellaceae comprising numerous human-opportunistic species, where hydrocarbon tolerance has been hypothesized to play a role in black yeast evolution. Taxonomic novelties: New family: Domatiomycetaceae Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog. New genera: Domatiomyces Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog, Lapsomyces Meizhu Wang, S.A. Ahmed & de Hoog. New species: Domatiomyces globalis Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog, Domatiomyces clavatus Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog, Domatiomyces catenatus Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog, Domatiomyces disarticulatus Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog, Domatiomyces pauciseptatus Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog, Domatiomyces germinans Meizhu Wang, Voglmayr, V.E. Mayer, S.A. Ahmed & de Hoog, Lapsomyces furvus Meizhu Wang, S.A. Ahmed & de Hoog. New combinations: Lapsomyces behniae (Crous) Meizhu Wang, S.A. Ahmed & de Hoog, Lapsomyces hostae (Crous) Meizhu Wang, S.A. Ahmed & de Hoog, Lapsomyces scillae (Deighton) Meizhu Wang, S.A. Ahmed & de Hoog. Citation: Wang MZ, Belmonte-Lopes R, Pan T, Ahmed SA, Rodrigues Lustosa BP, Quan Y, Al-Hatmi AMS, Mayer VE, Voglmayr H, Grisolia ME, de Souza Lima BBJF, Vicente VA, Zhou SQ, Cao Y, Kang YQ, de Hoog GS (2025). A new family of ant-associated fungi in Chaetothyriales. Studies in Mycology 110: 111-143. doi: 10.3114/sim.2024.110.02.

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.

The origin of human pathogenicity and biological interactions in Chaetothyriales

Fungi in the order Chaetothyriales are renowned for their ability to cause human infections. Nevertheless, they are not regarded as primary pathogens, but rather as opportunists with a natural habitat in the environment. Extremotolerance is a major trend in the order, but quite different from black yeasts in Capnodiales which focus on endurance, an important additional parameter is advancing toxin management. In the ancestral ecology of rock colonization, the association with metabolite-producing lichens is significant. Ant-association, dealing with pheromones and repellents, is another mainstay in the order. The phylogenetically derived family, Herpotrichiellaceae, shows dual ecology in monoaromatic hydrocarbon assimilation and the ability to cause disease in humans and cold-blooded vertebrates. In this study, data on ecology, phylogeny, and genomics were collected and analyzed in order to support this hypothesis on the evolutionary route of the species of Chaetothyriales. Comparing the ribosomal tree with that of enzymes involved in toluene degradation, a significant expansion of cytochromes is observed and the toluene catabolism is found to be complete in some of the Herpotrichiellaceae. This might enhance human systemic infection. However, since most species have to be traumatically inoculated in order to cause disease, their invasive potential is categorized as opportunism. Only in chromoblastomycosis, true pathogenicity might be surmised. The criterion would be the possible escape of agents of vertebrate disease from the host, enabling dispersal of adapted genotypes to subsequent generations.

Independent Evolution Has Led to Distinct Genomic Signatures in Dutch Elm Disease-Causing Fungi and Other Vascular Wilts-Causing Fungal Pathogens

Vascular wilts are important diseases caused by plant pathogenic fungi that result in the rapid death of their plant hosts. This is due to a systemic defense mechanism whereby the plant induces the compartmentalization of the infected vascular system in order to reduce the propagation of the fungus. The ascomycete class Sordariomycetes contains several species that cause vascular wilts in diverse plant hosts, and they can be classified into four taxonomic orders. The genetic mechanisms of pathogenesis have already been investigated in Fusarium and Verticillium species, but they have not yet been compared with other well-known wilt-causing species, especially fungi causing oak wilt or Dutch elm disease (DED). Here we analyzed 20 whole genome assemblies of wilt-causing fungi together with 56 other species using phylogenetic approaches to trace expansions and contractions of orthologous gene families and gene classes related to pathogenicity. We found that the wilt-causing pathogens evolved seven times, experiencing the largest fold changes in different classes of genes almost every time. However, some similarities exist across groups of wilt pathogens, particularly in Microascales and Ophiostomatales, and these include the common gains and losses of genes that make up secondary metabolite clusters (SMC). DED pathogens do not experience large-scale gene expansions, with most of the gene classes, except for some SMC families, reducing in number. We also found that gene family expansions in the most recent common ancestors of wilt pathogen groups are enriched for carbohydrate metabolic processes. Our study shows that wilt-causing species evolve primarily through distinct changes in their repertoires of pathogenicity-related genes and that there is the potential importance of carbohydrate metabolism genes for regulating osmosis in those pathogens that penetrate the plant vascular system.

Lessons From Insect Fungiculture: From Microbial Ecology to Plastics Degradation

Anthropogenic activities have extensively transformed the biosphere by extracting and disposing of resources, crossing boundaries of planetary threat while causing a global crisis of waste overload. Despite fundamental differences regarding structure and recalcitrance, lignocellulose and plastic polymers share physical-chemical properties to some extent, that include carbon skeletons with similar chemical bonds, hydrophobic properties, amorphous and crystalline regions. Microbial strategies for metabolizing recalcitrant polymers have been selected and optimized through evolution, thus understanding natural processes for lignocellulose modification could aid the challenge of dealing with the recalcitrant human-made polymers spread worldwide. We propose to look for inspiration in the charismatic fungal-growing insects to understand multipartite degradation of plant polymers. Independently evolved in diverse insect lineages, fungiculture embraces passive or active fungal cultivation for food, protection, and structural purposes. We consider there is much to learn from these symbioses, in special from the community-level degradation of recalcitrant biomass and defensive metabolites. Microbial plant-degrading systems at the core of insect fungicultures could be promising candidates for degrading synthetic plastics. Here, we first compare the degradation of lignocellulose and plastic polymers, with emphasis in the overlapping microbial players and enzymatic activities between these processes. Second, we review the literature on diverse insect fungiculture systems, focusing on features that, while supporting insects' ecology and evolution, could also be applied in biotechnological processes. Third, taking lessons from these microbial communities, we suggest multidisciplinary strategies to identify microbial degraders, degrading enzymes and pathways, as well as microbial interactions and interdependencies. Spanning from multiomics to spectroscopy, microscopy, stable isotopes probing, enrichment microcosmos, and synthetic communities, these strategies would allow for a systemic understanding of the fungiculture ecology, driving to application possibilities. Detailing how the metabolic landscape is entangled to achieve ecological success could inspire sustainable efforts for mitigating the current environmental crisis.