A domesticated fungal cultivar recycles its cytoplasmic contents as nutritional rewards for its leafcutter ant farmers

Abstract

Leafcutter ants farm a fungal cultivar (Leucoagaricus gongylophorus) that converts inedible vegetation into food that sustains colonies with up to millions of workers. Analogous to edible fruits of crops domesticated by humans, L. gongylophorus has evolved specialized nutritional rewards--swollen hyphal cells called gongylidia that package metabolites ingested by ant farmers. Yet, little is known about how gongylidia form, and thus how fungal physiology and ant provisioning interact to farming performance. We explored the mechanisms governing gongylidium formation using microscopy imaging of ant-cultivated fungus and controlled in vitro experiments with the cultivar grown in isolation from ant farmers. First, L. gongylophorus is polykaryotic (up to 17 haploid nuclei/cell) and our results suggest intracellular nucleus distributions govern gongylidium morphology with their absence in expanding edges arresting apical growth and their presence mediating complex branching patterns. Second, nanoscale imaging (SEM, TEM) shows that the cultivar recycles its own cellular material (e.g. cytosol, mitochondria) through a process called autophagy and stores the resulting metabolites in gongylidia. This autophagic pathway is further supported by gongylidium suppression when isolated fungal cultures are grown on media with autophagy inhibitors, and differential transcript expression (RNA-seq) analyses showing upregulation of multiple autophagy genes in gongylidia. We hypothesize that autophagic nutritional reward production is the ultimate cultivar service and reflects a higher-level organismality adaptation enabled by strict symmetric lifetime commitment between ant farmers and their fungal crop.