# Novel killer yeasts and toxins from the gardens of fungus-growing ants

## TL;DR

This paper explores killer yeasts in fungus-growing ant gardens, discovering new antifungal toxins and their ecological roles.

## Contribution

Identification of a novel killer toxin, Ksino, from Candida sinolaborantium in ant fungus gardens.

## Key findings

- 10 out of 59 yeast species isolated from ant fungus gardens are killer yeasts.
- Candida sinolaborantium LESF 1467 inhibits 57% of tested yeast strains.
- Ksino toxin homologs are found in diverse fungal genomes.

## Abstract

Killer toxins are proteinaceous antifungal molecules produced by yeasts, with activity against a wide range of human and plant pathogenic fungi. Fungus gardens of attine ants in Brazil were surveyed to determine the presence of killer toxin-producing yeasts and to define their antifungal activities and ecological importance. Our results indicate that 10 out of 59 yeast species isolated from fungal gardens are killer yeasts. Killer yeasts were less likely to inhibit the growth of yeasts isolated from the same environment but more effective at inhibiting yeasts isolated from other environments, supporting a role for killer yeasts in shaping the community composition. All killer yeasts had genome-encoded killer toxins and lacked cytoplasmic toxin-encoding elements (i.e., double-stranded RNA satellites and linear double-stranded DNAs). Of all the killer yeasts associated with attine ants, Candida sinolaborantium (strain LESF 1467) showed a broad spectrum of antifungal activities against 39 out of 69 (57%) of yeast strains tested for toxin susceptibility. The complete genome sequence of C. sinolaborantium LESF 1467 identified a new killer toxin, Ksino, with similarities in the primary sequence and tertiary structure to the Saccharomyces cerevisiae killer toxin named Klus. Surveys of publicly available genome databases identified homologs of Ksino in the genomes of yeast strains of Saccharomycetes and Pichiomycetes, as well as other species of Ascomycota and Basidiomycota filamentous fungi. This demonstrates that killer yeasts can be widespread in attine ant fungus gardens, possibly influencing the fungal community composition and the importance of these complex microbial communities in the discovery of novel antifungal molecules.