ATTINE ANTS have been a theme at my blog recently. They first showed up in a discussion of their evolutionary tree, then in a post examining their relationship with the actinomycete bacteria that help protect their fungal gardens from parasites. In my reading I stumbled across the article that I will write about today, which seems to have discovered another part to the attine ant symbiosis. As of now this is the sole report on the occurrence of black yeasts living apparently parasitically on the attine ants, reliant upon the cuticular crypts that normally house helpful actinomycetes.
To recap, the attine ants cultivate fungal gardens and eat these fungi. A different fungus called Escovopsis finds these gardens delicious, and can destroy a garden if it is unprotected. The ants protect their gardens by constant cleaning of the fungi and by growing actinomycete bacteria upon their cuticles that produce antifungal agents. The ants evolved crypts in their cuticles to house these bactera and exocrine glands to support their growth. According to Currie and Little, it appears that some species of black yeast have found these crypts inviting and moved in as well.
Currie and Little cultured black yeasts from Apterostigma ants (incidentally, paleoattine coral fungus farmers) of various species. Attempts to culture the yeasts from other ant genera were unsuccessful, but PCR amplification showed the presence of black yeast DNA on multiple other species of attine ants as well. The yeasts were monophyletic, suggesting that their ancestor made the jump to growing upon ants’ chests once, and that these yeasts descended from those fungi. They were collected from multiple locations in South America, showing that the yeast has a wide geographical distribution. Since these fungi are monophyletic, widespread, and occur in both paleoattine and neoattine ants, the authors suggest they were present early in attine ant evolution.
On Apterostigma workers the yeasts were localized in those areas bearing crypts that house mutualistic actinomycetes. The authors report that the yeasts inhibit actinomycete effectiveness in suppressing Escovopsis, but unfortunately that paper has not been published yet (at least not that I can find). This would make the black yeasts parasites upon the attine ant/actinomycete symbiosis. Apparently the black yeasts find attine cuticular crypts inviting homes. It is unclear whether these yeasts have any detrimental effects upon the ants besides harming the health of their fungal gardens.
The authors suggest four possible routes for the acquisition of black yeasts:
- Since a closely related yeast genus, Phialophora, lives endophytically in plants, the yeast may have been transported into ant colonies in leaves used as fertilizer for fungus gardens.
- The association between ants and black yeasts may have pre-dated the evolution of fungus farming, similarly to how non-farming ants may carry actinomycete bacteria on their integuments.
- The black yeasts may have jumped hosts, perhaps from bark beetles to ants.
- The black yeasts may have an association with Pseudonocardia bacteria that pre-dates the mutualistic relationship of these bacteria with ants. Maybe the yeasts are a “third wheel” that the ants have not yet successfully ejected.
The black yeasts represent a new avenue for research in attine ant ecology. I look forward to seeing more about these possibilities in future articles.
Little, A.E., Currie, C.R. (2007). Symbiotic complexity: discovery of a fifth symbiont in the attine ant-microbe symbiosis. Biology Letters, 3(5), 501-504. DOI: 10.1098/rsbl.2007.0253