Absence of the Parasite Escovopsis in Fungus Garden Pellets Carried by Gynes of Atta sexdens

Abstract

Before the mating flight, the gyne of leaf-cutting ants takes a small pellet of their mutualistic fungus garden to start fungus culture in its new colony by vertically transmitting it. This mutualism is threatened by the specialized microfungal parasite Escovopsis, which is exclusively associated with the ant's fungus gardens. Evidences suggest that Escovopsis transmission between colonies is horizontal, i.e. the parasite is transferred between established nests. However, such studies analyzed a relatively small number of fungal pellets or were restricted to a few ant colonies. Additionally, there is a report of rapid parasite dispersion, compatible with a winged vectored mechanism, suggesting that there is also vertical transmission. Herein, we carried out a complementary study on the possibility of vertical transmission of Escovopsis by sampling a large number of fungus pellets from gynes of Atta sexdens, a species not previously studied from this perspective. Gynes were collected during their mating flights in 2009 and 2010, and were left in moist chambers upon fungus regurgitation. Each pellet was inoculated on potato dextrose agar and incubated at 25 degrees C, resulting in prevalence of the mutualistic cultivar, low proportions of other fungal species, and absence of Escovopsis. Thus, our study consolidates the results of previous reports that Escovopsis vertical transmission does not occur or is negligible, thus enabling the characterization of this parasite transmission as horizontal. Future studies on Escovopsis transmission mechanisms may explain why, although horizontal, it seems to be as fast as the transmission mediated by winged vectors.