The antifeedant action of climacodon septentrionalis and two species of sphaerobolus to hyphal grazing by the fungus-feeding nematode aphelenchoides sp.

Abstract

Fungi are ubiquitous in forest ecosystems and are, collectively, a major force in driving nutrient and organic matter availability and cycling. Some saprotrophic and mycorrhizal fungi are characterized by a perennial vegetative body, delayed sexual reproduction, and a relatively long lifespan. Such fungi are exposed to a barrage of antagonistic forces throughout their existence, one notable stress being invertebrate grazing. It is suggested that fungi with perennial mycelia must have developed means to deter grazing of their hyphal networks by pervasive mycophagist invertebrates.Controlled inoculation studies with a mycophagous nematode, Aphelenchoides sp., and isolates representing 78 fungal species were conducted to investigate the presence of possible antifeedants. These in vitro pairings resulted in the discovery of two novel antifeedant mechanisms in three species of fungi. In the presence of the saprotrophic fungi Sphaerobolus stellatus and S. iowensis, the anterior portion of nematodes became encapsulated in a material of unknown composition. This encapsulation phenomenon effectively prevented further hyphal grazing by obstructing stylet extension, which resulted in the eventual death of the nematode. Nematodes that died as a result of the encapsulation were never colonized or consumed by the fungus. It is hypothesized that the encapsulating material originates from modified hyphal cells, referred to as gloeocystidia, and is liberated when the cells are punctured by the nematode stylet. The wood-decaying fungus Climacodon septentrionalis was found to produce tall, stalked secretory cells in abundance on the aerial mycelia of the colony. Nematodes were enveloped and immobilized by droplets produced at the apices of the secretory cells. Immobilized nematodes were rarely colonized by the fungus and dead individuals persisted for weeks. A media study was employed to investigate the effect of nutrient concentration on the in vitro production of secretory cells. The discovery of novel antifeedants which mitigate damage caused to the mycelial network by grazing invertebrates offers a stimulus for further investigation into the interactions between fungi and their co-inhabiting microfauna. Keywords: Antifeedant, defence mechanism, fungivory, grazing responses, mycology, mycophagy, perennial mycelium.