Putting the waste out: a proposed mechanism for transmission of the mycoparasite Escovopsis between leafcutter ant colonies

Production of Escovopsis conidia and the potential use of this parasitic fungus as a biological control agent of leaf-cutting ant fungus gardens

This study was carried out to investigate the use of different substrates for the production of Escovopsis conidia and verify the virulence of four different isolates cultured on four types of substrates using a novel bioassay. Escovopsis isolates were molecularly identified, based on Internal Transcribed Spacer (ITS) nucleotide sequences. To evaluate conidial production, suspensions (1 × 106 conidia mL-1) of each Escovopsis isolate were inoculated onto four substrates (parboiled rice, white rice, rolled oats, and corn grits). After 14 days, conidial yields were assessed. The virulence of each isolate cultured on the four substrates was tested against Leucoagaricus fungus garden fragments, by directly applying 500 µL of each conidial suspension (1 × 107 conidia mL-1), and the development of the parasite was monitored daily until it completely colonized the fungus garden. It was observed that rolled oats were the best substrate for conidial production, with a yield of 1.7 × 107 to 2.0 × 108 conidia mL-1. Furthermore, isolate AT-01 produced the highest number of conidia when compared with the other isolates. Regardless of the substrate used to produce AT-01 conidia, this isolate completely colonized the fungus garden 6 days post inoculation (dpi), followed by AT-02, AC-01, and AC-2. High levels of both conidial production and virulence against the leaf-cutting ant fungus garden were observed here.

Fungal Cultivars of Higher Attine Ants Promote Escovopsis Chemotropism

In varied environments, microorganisms search for partners or nutritional resources using chemical signals. Microbes are drawn (chemotaxis) or grow directionally (chemotropism) towards the chemical source, enabling them to establish and maintain symbiosis. The hypocrealean fungi Escovopsis enhance their growth towards the basidiomycete fungus Leucoagaricus gongylophorus, which is cultivated by leaf-cutting attine ants for food. Although directional growth is well documented in this symbiosis, it is unclear whether non-volatile or volatile organic compounds participate in the interaction between cultivar and Escovopsis, and which specific chemical compounds might attract and induce chemotropism. In this study, we examined the growth responses of Escovopsis isolates to non-volatile and volatile organic compounds produced by fungal cultivars of higher attine ants. We also isolated and identified molecules released by the ant-cultivar and assessed the chemotropism of Escovopsis towards them. Our results indicate that the growth of Escovopsis is stimulated in the presence of both non-volatile and volatile compounds from fungal cultivars. We also identified three isomeric diketopiperazines molecules from crude extracts of the ant cultivar, suggesting that these might play a role in Escovopsis chemotropism. Our findings provide insights into the complex chemical interactions that govern the association between Escovopsis and fungal cultivars.

Production of Escovopsis weberi (Ascomycota: Hypocreales) Mycelial Pellets and Their Effects on Leaf-Cutting Ant Fungal Gardens

The maintenance of the symbiosis between leaf-cutting ants and their mutualistic fungus Leucoagaricus gongylophorus Singer (Moller) is vital for the survival of both species. The specialist fungal parasite Escovopsis weberi Muchovej & Della Lucia is a threat to this symbiosis, causing severe damage to the fungal garden. Mycelial pellets are resistant fungal structures that can be produced under laboratory conditions. These structures were studied for use in biological pest control, but the production of mycelial pellets has not previously been documented in Escovopsis. One of the aims of this study was to induce Escovopsis weberi to produce mycelial pellets and investigate the potential of these pellets for the control of leaf-cutting ants. We compared the pathogenicity of Escovopsis weberi mycelial pellets and conidia against mini-colonies of Acromyrmex subterraneus subterraneus Forel when applied in the form of baits. Worker ants were able to distinguish mycelial pellets from conidia, as baits with mycelial pellets were more attractive to workers than those with conidia, causing a greater negative impact on colony health. All types of baits containing Escovopsis weberi influenced the foraging activity but only treatments with viable fungal propagules resulted in an increase in the quantity of waste material, with a significant negative impact on the fungal garden biomass. The results provided novel information regarding Escovopsis recognition by worker ants and differences between conidia and mycelial pellet dynamics in leaf-cutting ant colonies, with new perspectives for the biological control of these important pests.

Low Virulence of the Fungi Escovopsis and Escovopsioides to a Leaf-Cutting Ant-Fungus Symbiosis

Eusocial insects interact with a diversity of parasites that can threaten their survival and reproduction. The amount of harm these parasites cause to their hosts (i.e., their virulence) can be influenced by numerous factors, such as the ecological context in which the parasite and its host are inserted. Leaf-cutting ants (genera Atta, Acromyrmex and Amoimyrmex, Attini: Formicidae) are an example of a eusocial insect whose colonies are constantly threatened by parasites. The fungi Escovopsis and Escovopsioides (Ascomycota: Hypocreales) are considered a highly virulent parasite and an antagonist, respectively, to the leaf-cutting ants' fungal cultivar, Leucoagaricus gongylophorus (Basidiomycota: Agaricales). Since Escovopsis and Escovopsioides are common inhabitants of healthy colonies that can live for years, we expect them to have low levels of virulence. However, this virulence could vary depending on ecological context. We therefore tested two hypotheses: (i) Escovopsis and Escovopsioides are of low virulence to colonies; (ii) virulence increases as colony complexity decreases. For this, we used three levels of complexity: queenright colonies (fungus garden with queen and workers), queenless colonies (fungus garden and workers, without queen) and fungus gardens (without any ants). Each was inoculated with extremely high concentrations of conidia of Escovopsis moelleri, Escovopsioides nivea, the mycoparasitic fungus Trichoderma longibrachiatum or a blank control. We found that these fungi were of low virulence to queenright colonies. The survival of queenless colonies was decreased by E. moelleri and fungus gardens were suppressed by all treatments. Moreover, E. nivea and T. longibrachiatum seemed to be less aggressive than E. moelleri, observed both in vivo and in vitro. The results highlight the importance of each element (queen, workers and fungus garden) in the leaf-cutting ant-fungus symbiosis. Most importantly, we showed that Escovopsis may not be virulent to healthy colonies, despite commonly being described as such, with the reported virulence of Escovopsis being due to poor colony conditions in the field or in laboratory experiments.

Host Susceptibility Modulates Escovopsis Pathogenic Potential in the Fungiculture of Higher Attine Ants

Health and disease emerge from intricate interactions between genotypes, phenotypes, and environmental features. The outcomes of such interactions are context-dependent, existing as a dynamic continuum ranging from benefits to damage. In host-microbial interactions, both the host and environmental conditions modulate the pathogenic potential of a microorganism. Microbial interactions are the core of the agricultural systems of ants in the subtribe Attina, which cultivate basidiomycete fungi for food. The fungiculture environment harbors a diverse microbial community, including fungi in the genus Escovopsis that has been studied as damage-causing agent. Here, we consider the ant colony as a host and investigate to what extent its health impacts the dynamics and outcomes of host-Escovopsis interactions. We found that different ant fungal cultivars vary in susceptibility to the same Escovopsis strains in plate-assays interactions. In subcolony-Escovopsis interactions, while healthy subcolonies gradually recover from infection with different concentrations of Escovopsis conidia, insecticide-treated subcolonies evidenced traits of infection and died within 7 days. The opportunistic nature of Escovopsis infections indicates that diseases in attine fungiculture are a consequence of host susceptibility, rather than the effect of a single microbial agent. By addressing the host susceptibility as a major modulator of Escovopsis pathogenesis, our findings expand the understanding of disease dynamics within attine colonies.

More pieces to a huge puzzle: Two new Escovopsis species from fungus gardens of attine ants

Escovopsis (Ascomycota: Hypocreales, Hypocreaceae) is the only known parasite of the mutualistic fungi cultivated by fungus-growing ants (Formicidae: Myrmicinae: Attini: Attina, the "attines"). Despite its ecological role, the taxonomy and systematics of Escovopsis have been poorly addressed. Here, based on morphological and phylogenetic analyses with three molecular markers (internal transcribed spacer, large subunit ribosomal RNA and the translation elongation factor 1-alpha), we describe Escovopsisclavatus and E.multiformis as new species isolated from fungus gardens of Apterostigma ant species. Our analysis shows that E.clavatus and E.multiformis belong to the most derived Escovopsis clade, whose main character is the presence of conidiophores with vesicles. Nevertheless, the most outstanding feature of both new species is the presence of a swollen region in the central hypha of the conidiophore named swollen cell, which is absent in all previously described Escovopsis species. The less derived Escovopsis clades lack vesicles and their phylogenetic position within the Hypocreaceae still remains unclear. Considering the high genetic diversity in Escovopsis, the description of these new species adds barely two pieces to a huge taxonomic puzzle; however, this discovery is an important piece for building the systematics of this group of fungi.

Transmission of mutualistic bacteria in social and gregarious insects

Symbiotic microbes can confer a range of benefits to social, sub-social, and gregarious insects that include contributions to nutrition, digestion, and defense. Transmission of beneficial symbionts to the next generation in these insects sometimes occurs transovarially as in many solitary insects, but primarily through social contact such as coprophagy in gregarious taxa, and trophallaxis in eusocial insects. While these behaviors benefit reliable transmission of multi-microbial assemblages, they may also come at the cost of inviting the spread of parasites and pathogens. Nonetheless, the overall benefit of social symbiont transmission may be one of several important factors that reinforce the evolution of social behaviors and insect eusociality.

Putting the waste out: a proposed mechanism for transmission of the mycoparasite Escovopsis between leafcutter ant colonies

The attine ant system is a remarkable example of symbiosis. An antagonistic partner within this system is the fungal parasite Escovopsis, a genus specific to the fungal gardens of the Attini. Escovopsis parasitizes the Leucoagaricus symbiont that leaf-cutting ants (Acromyrmex, Atta) have been farming over the past 8-12 Myr. However, it has been a puzzle how Escovopsis reaches its host. During a seasonal survey of nests of Acromyrmex subterraneus subterraneus in Atlantic rainforest in Brazil, Escovopsis was detected in all the sampled fungal garden waste tips or middens (n = 111). Middens were built strategically; always below the nest entrances. Here, we report the first evidence of a putative mechanism for horizontal transmission of Escovopsis between attine colonies. It is posited that leaf-cutting ants pick up the spores from soil and litter during foraging and vector the mycoparasite between attine colonies. Field and laboratory experiments, using At. laevigata and Ac. subterraneus subterraneus, confirm that Escovopsis spores are phoretic, and have an inbuilt dormancy, broken by the presence of their Leucoagaricus host. However, in the coevolutionary arms race, Atta ants may lose out-despite most species in the genus investing in a more advanced waste disposal system-due to the insanitary habits of their Acromyrmex neighbours.