Context‐dependent symbiosis between black flies (Diptera: Simuliidae) and trichomycete fungi (Harpellales: Legeriomycetaceae)

Abstract

The context‐dependent nature of a symbiotic relationship between a trichomycete fungus (Smittium culisetae) and a larval black fly (Simulium vittatum) is demonstrated in the present study. No significant difference was found between the size of larvae colonized by trichomycetes and those free of trichomycetes, regardless of the trichospore dosage or initial age of the larvae. This trend suggests that the trichomycete has no detectable effect on host fitness, indicating a commensalistic relationship. However, in half of the experiments, stressed (i.e. starved) larvae exposed to trichospores at a dosage of 20 000 spores ml−1 had significantly higher survival than did trichomycete‐free larvae, indicating a mutualistic relationship. Trichomycetes in adult female black flies can replace the ovaries. The symbiotic association between trichomycetes and simuliids, therefore, is dynamic: commensalistic when larvae are well fed, mutualistic when larvae are starved, and parasitic in adults. The trichomycete‐black fly relationship represents a rare case of symbiosis shifting among three states.