Fungal infection dynamics in response to temperature in the lepidopteran insect Galleria mellonella

Abstract

This study examines how the dynamics of fungus-insect interactions can be modulated by temperature. The wax moth, Galleria mellonella, is a well-studied and important model insect whose larvae in the wild develop optimally at around 34 °C in beehives. However, surprisingly little research on wax moths has been conducted at relevant temperatures. In this study, the entomopathogenic fungus Metarhizium robertsii inflicted rapid and substantial mortality on wax moth larvae maintained at a constant temperature of 24 °C, but at 34 °C a 10 fold higher dose was required to achieve an equivalent mortality. The cooler temperature favored fungal pathogenicity, with condial adhesion to the cuticle, germination and hemocoel invasion all significantly enhanced at 24 °C, compared with 34 °C. The wax moth larvae immune responses altered with the temperature, and with the infective dose of the fungus. Enzyme-based immune defenses (lysozyme and phenoloxidase) exhibited enhanced activity at the warmer temperature. A dramatic upregulation in the basal expression of galiomicin and gallerimycin was triggered by cooling, and this was augmented in the presence of the fungus. Profiling of the predominant insect epicuticular fatty acids revealed a 4-7 fold increase in palmetic, oleic and linoleic acids in larvae maintained at 24 °C compared with those at 34 °C, but these failed to exert fungistatic effects on topically applied fungus. This study demonstrates the importance of choosing environmental conditions relevant to the habitat of the insect host when determining the dynamics and outcome of insect/fungus interactions, and has particular significance for the application of entomopathogens as biocontrol agents.

Galleria melonella as an experimental in vivo host model for the fish-pathogenic oomycete Saprolegnia parasitica

“…Although melanisation is a key part of the immune system of the insect, no PO activity could be detected in the haemolymph from G. mellonella infected with S. parasitica ( Fig. S2B ) which is in contrast to studies with other fungi and bacteria where PO activity was detected ( Harding et al., 2012 , Grizanova et al., 2014 , Kryukov et al., 2016 ). However, in contrast to infections with human fungi, the immune response to S. parasitica is very local around melanised hyphae and is not systemic.…”

Passive vectoring of entomopathogenic fungus Beauveria bassiana among the wax moth Galleria mellonella larvae by the ectoparasitoid Habrobracon hebetor females

“…We assayed phenoloxidase (PO) activity in the integuments since cuticular PO level is an important parameter of antifungal defense in wax moth larvae (Dubovskiy et al ., ; Kryukov et al ., ). Envenomated and nonenvenomated larvae were infected with 10 6 conidia/mL as described above, but before inoculation, the eggs of H. hebetor were removed from the cuticle using wet cotton wool.…”

HapX, an Indispensable bZIP Transcription Factor for Iron Acquisition, Regulates Infection Initiation by Orchestrating Conidial Oleic Acid Homeostasis and Cytomembrane Functionality in Mycopathogen Beauveria bassiana

“…During interaction of the fungus and the host, BbHapX contributes to its almost complete virulence during topical infection and partial virulence in intrahemocoel infection. The surface of the insect cuticle is deficient in nutrients (22), whereas the host hemolymph is rich in nutrients, including FAs (e.g., oleic acid) (44). These facts readily explain why exogenous oleic acid restores partial fungal virulence during cuticle penetration and complete virulence via direct infection.…”