Effects of entomopathogenic fungi on the mortality and immune system of the leaf‐cutting ant Acromyrmex subterraneus subterraneus

Abstract

Acromyrmex leaf-cutting ants use the antibiotics produced by ectosymbionts (Actinobacteria) to suppress the growth of Escovopsis spp., which are the specialized parasites of attine fungus gardens. However, the spectrum of activity of these bacteria and their interactions with insect immune systems have not been thoroughly studied. The objective of this study was to evaluate whether the presence of symbiotic actinobacteria covering the entire cuticle of Acromyrmex subterraneus subterraneus (Forel) (Hymenoptera: Formicidae) workers provides protection against two fungus species, Metarhizium anisopliae (Metschn.) Sorokin (Hypocreales), a persistent entomopathogenic, and Aspergillus ochraceus G. Wilh. (Eurotiales), an opportunistic insect pathogen. We assessed the mortality rates of three groups of Ac. subterraneus subterraneus workers inoculated with a suspension of one of the two pathogens: external workers (EXT), internal workers with actinomycetes covering the whole body (INB), and internal workers without actinomycetes covering the whole body (INØ). We also assessed the encapsulation response, which is a parameter of innate immunity, to determine whether the pathogens affect immune responses in the ant. All bioassays were conducted on workers from seven laboratory colonies of Ac. subterraneus subterraneus. Mortality tests showed that actinomycetes do protect the workers of group INB against M. anisopliae, but not against A. ochraceus. Regarding the encapsulation response, INB ants treated with A. ochraceus conidia exhibited immunosuppression due to this pathogen. All three groups of workers that received M. anisopliae conidia presented some degree of immunosuppression after pathogen exposure. These results indicate that workers without visible bacteria cover, specially the external workers (EXT), are more resistant to entomopathogenic fungi. Nevertheless, bacterial actinomycetes present in young workers are important in protecting them from ubiquitous entomopathogenic fungi.

Immunity and survival response of Atta cephalotes (Hymenoptera: Myrmicinae) workers to Metarhizium anisopliae infection: Potential role of their associated microbiota

“…In contrast, external workers of Acromyrmex subterraneus , which are naturally devoid of actinobacterial cover, are more resistant to entomopathogenic fungal infection than internal workers that exhibit an actinobacterial cover. This finding may suggest that under natural conditions, cuticular microbiota may exert immunomodulation on this species [ 59 ]. Nevertheless, these authors did not observe differences in the encapsulation rate of external workers covered by bacteria and internal workers without bacteria.…”

“…subterraneus workers where removal of the visible actinobacterial cover did not influence the worker encapsulation rate [ 60 ]. This implies that bacterial coverage is not an exclusive modulator of the encapsulation rate and that it is likely worker age and/or their risk of exposure to pathogens shaped their response [ 59 ].…”

Bacterial microbiota associated with the leaf‐cutting ant Atta cephalotes (Hymenoptera: Myrmicinae): Dynamics during development and potential role in defence

“…In Acromyrmex species, the secretion of antimicrobial compounds is mediated in part by actinobacteria symbionts of the genus Pseudonocardia , which are harboured on the cuticle of workers (Cafaro et al, 2011; Currie et al, 2003; Poulsen et al, 2002). Its ability to inhibit the growth of entomopathogenic fungi has been demonstrated, as well as to protect the symbiotic fungus due to its effect on antagonists (Couceiro et al, 2016; Currie et al, 2003; De Souza et al, 2013; Douglas, 2014; Kost et al, 2007; Marsh et al, 2013; Mattoso et al, 2012; Mueller, 2012; Schultz & Brady, 2008; Sen et al, 2009; Worsley et al, 2021). This association constitutes an important defence component in this genus.…”