Gut-inhabiting Fungi of Aquatic Insects and Environmental Disturbances
Department of Forest Entomology, Forestry and Forest Products Research Institute
Matsunosato, Tsukuba, Ibaraki, 305-8687 Japan
Species in the order Harpellales inhabit mainly in the digestive tracts of aquatic insects (Ephemeroptera, Plecoptera, and Diptera). Infection begins with ingestion of spores in the environment. Germinated spores attach to the lumen of the midgut or the hindgut of the host by a special structure called holdfast. One spore produces one thallus with one holdfast. Namely, one individual. A huge typhoon hit Japan in 2019. Flooding occurred in Nagano Prefecture, and the environment of my research site, Tsuneda 450m alt., the middle reaches of Chikuma River, was changed drastically. However, it was reported that the species Isonichia japonica (Ephemeroptera) population of the river recovered only one year after the major flooding event. A Harpellales species Ephemereromyces sp. inhabits the hindgut of I. japonica nymphs. Fortunately, I had exuviae (molts) specimens of I. japonica collected in 2012. The fungal infection ratio before and after the flood was examined by counting the holdfasts. The hindgut cuticle is removed with the body surface cuticle at the same time of molting with the fungus attached to it. The hindguts were observed. The infection rate in 2012 was 100 % with from 6 to 95 holdfasts as traces of thalli in each specimen. The infection rate in 2023 was 97 % with from 3 to 82 holdfasts, which was considered to be almost the same as that before the flood. Harpellales have evolved through many environmental disturbances and may have an adaptation to increase infection rates in a short period after the disturbance.