Pathogenicity of Escovopsis weberi: The parasite of the attine ant-microbe symbiosis directly consumes the ant-cultivated fungus

Abstract

Fungi in the genus Escovopsis are known only from the fungus gardens of attine ants. Previous work has established that these anamorphic fungi, allied with the Hypocreales, are specialized and potentially virulent parasites of the ancient mutualism between attine ants and their fungal cultivars. It is unclear whether the primary nutrient source for the pathogen is the mutualist fungal cultivar or the vegetative substrate placed on the gardens by the ants. Here, we determine whether Escovopsis weberi is a parasite of the fungal cultivar, a competitor for the leaf substrate, or both. Bioassays reveal that E. weberi exhibits rapid growth on pure cultivar and negligible growth on sterilized leaf fragments. Light microscopy examination of hyphalhyphal interactions between E. weberi and the ant fungal cultivar indicate that E. weberi, unlike invasive necrotrophs that always penetrate host hyphae, can secrete compounds that break down host mycelium before contact occurs. Thus, E. weberi is a necrotrophic parasite of the fungal cultivar of attine ants.

• Acromyrmex
• Atta
• fungus-growing ants
• mutualism
• mycoparasite
• necrotroph

We would like to thank Matías Cafaro, Robert Lichtwardt, Ainslie Little, Keith Seifert, Merlin White and an anonymous reviewer for valuable suggestions on this manuscript and Anne Danielson-François, Emily Davenport, María Le-one, Lacey Loudermilk, Emily Magee, Shauna Price and Alison Stuart for logistical support. This research was supported by grants from NSF (Integrative Research Challenges in Environmental Biology DEB-0110073) and from the University of Kansas Honors Program. We also thank STRI, Autoridad Nacional del Ambiente of the Republic of Panama for facilitating research and granting collecting permits.