No evidence for a Ganoderma spore dispersal mutualism in an obligate spore-feeding beetle Zearagytodes maculifer

The role of spore dispersal mutualism remains equivocal in many fungus–insect assemblages. We tested experimentally whether an obligate spore-feeding beetle Zearagytodes maculifer has a mutualistic relationship with its host bracket fungus Ganoderma cf. applanatum via spore dispersal. We asked three specific questions: (1) whether or not Ganoderma spore germination rate is increased via beetle digestive activity and (2) is dependent on temperature and sporocarp identity. Spore germination rates were examined in 2 × 3 × 2 factorial experiments (spores consumed by beetles or not × temperature 20, 25, and 30 °C × two independent pairs of sporocarp–beetle populations) replicated five times in an array of 60 experimental cultures. Analysis showed that consumption by beetles significantly reduced germination rate of Ganoderma spores. The effect of temperature was modulated by the effect of individual sporocarp, and was overridden by beetle feeding. Microscopic analysis revealed that spores from beetle faecal pellets exhibited extensive damage to their thin outer walls (pellicles) and thick inner walls, as well as significant loss of cytoplasm, while control spores were intact. The overall evidence argued against our spore dispersal mutualism hypothesis, suggesting that Z. maculifer can potentially exert a negative, if vanishingly small, fitness effect on its host fungus G. cf. applanatum.

► Mutualism theory predicts no interference of spore germination by insect feeding.
► Beetle causes a reduction of spore germination rates of Ganoderma.
► Spores in faecal pellets physically damaged.
► No evidence for insect-mediated spore dispersal.