Escovopsis PARASITES DO NOT SOLELY DEPEND ON THEIR HOSTS FOR NUTRITION

Parasite-host associations are widespread in nature. In such interactions, a parasite usually obtains most (or all) of its nutrients from their hosts. A model system for studying parasite nutrition occurs in colonies of fungus-growing ants (subtribe Attina: hereafter named “the attines”). These insects cultivate basidiomycetous fungi for food, which are threatened by parasites in the genus Escovopsis. This parasite-host interaction takes place in the fungus garden, where ants grow their fungal cultivars and it usually contains high concentrations of simple sugars. Here, we investigate whether these sugars increase the parasite’s infectivity and growth towards its hosts. We carried out dual culture bioassays using six Escovopsis strains isolated from colonies of higher attine ants towards four fungal cultivar strains: two cultivated by Atta sexdens rubropilosa (a leafcutter ant) and two cultivated by Trachymyrmex tucumanus (a non-leafcutter ant). This bioassay was performed on culture medium supplemented with one or two simple sugars as the only carbon sources. Thus, this bioassay had a total of 72 experimental combinations. Our aim was to assess if the addition of a second sugar would increase the parasite’s infectivity and growth. When cultured in the presence of only one simple sugar, we observed that interactions between Escovopsis and its hosts resulted in a lower infectivity by the parasite, when compared with interactions between these fungi on nutrient rich medium (i.e. potato-dextrose agar). In 26.4% of the experimental combinations (19 out of 72) the parasite was not able to inhibit the hosts, even after 14 days in dual culture. Except for one strain, our results show that the presence of a second carbon source in the culture medium does not significantly increase Escovopsis infectivity, when compared with the negative control (culture medium with only one simple sugar available; Two-way ANOVA, P > 0.05). However, the addition of a second sugar increases the mycelial growth of the parasite. Most importantly, in 29.2% of the experimental combinations (21 out of 72), the hosts significantly inhibited Escovopsis growth. These results suggest that nutritional stress negatively influences the parasite, enough to allow fungal cultivar defenses to inhibit Escovopsis. Multiple lines of evidence suggest that, along with the hosts’ mycelia, Escovopsis may use additional nutrient sources for its development.