LACK OF HOST PREFERENCE BY THE PARASITE Escovopsis trichodermoides

Symbiosis allowed the conquest of new ecological niches by organisms. An interesting example is the mutualism between fungus-growing "attine" ants and fungi they grow for food. Although ants protect their food from exploiters, parasitic fungi in the genus Escovopsis threaten the fungal cultivar. Recently described from lower attine ant colonies, the host preference of Escovopsis trichodermoides is still unknown. Here, we evaluated the specificity of E. trichodermoides towards its fungal hosts (Leucocoprinus sp.). Six strains of the parasite had their growth patterns evaluated towards the fungal cultivars of Mycocepurus goeldii and Mycetophylax morschi (lower attines). We carried out two experiments: (i) Dual culture assays between E. trichodermoides and Leucocoprinus sp., in which mycelial fragments of the ant fungal mutualists were grown on PDA for 14 days at 25 °C, then E. trichodermoides strains were inoculated 3.0 cm apart from the fungal hosts. This system was incubated for 14 days at 25 °C, and the growth areas of each fungus were measured and compared with the controls (pure cultures of both the parasite and the host grown alone); and (ii) Host-choice assays by E. trichodermoides. In a Petri plate, the PDA medium was cut with a sterile scalpel to create six equidistant tracks. Afterwards, five different host possibilities were placed at the end of each track, and then mycelial fragments of the parasite were grown at the center of the plate. One track was left without any fungi (control). This system was incubated for 28 days at 25 °C, and the growth of E. trichodermoides was measured. In the dual culture assays, the parasite inhibited the mycelium growth of its hosts by at least 43.7% in all host-parasite combinations, when compared to the control. In the host-choice experiments we observed an absence of preference by the parasite towards the multiple host possibilities. In addition, we observed a darkening of the host mycelium at the contact area with the mycelium of E. trichodermoides. Our results of the dual culture assays indicate the antagonism of E. trichodermoides against Leucocoprinus sp. Taken together, the results of the two experiments indicate a generalist behavior of the parasite. This life-style of E. trichodermoides may have allowed host switches among lower attine ant species over the evolutionary time. Interestingly, this lack of host preference is not a common trait in the fungus-growing ant system, which prompts further investigations on Escovopsis evolution.