Predicting interactions of the frass-associated yeast Hyphopichia heimii with Olea europaea subsp. cuspidata and twig-boring bark beetles.

Bark beetles are destructive insect pests known to form symbioses with different fungal taxa, including yeasts. The aim of this study was to (1) determine the prevalence of the rare yeast Hyphopichia heimii in bark beetle frass from wild olive trees in South Africa and to (2) predict the potential interaction of this yeast with trees and bark beetles. Twenty-eight culturable yeast species were isolated from frass in 35 bark beetle galleries, including representatives of H. heimii from nine samples. Physiological characterization of H. heimii isolates revealed that none was able to degrade complex polymers present in hemicellulose; however, all were able to assimilate sucrose and cellobiose, sugars associated with an arboreal habitat. All isolates were able to produce the auxin indole acetic acid, indicative of a potential symbiosis with the tree. Sterol analysis revealed that the isolates possessed ergosterol quantities ranging from 3.644 ± 0.119 to 13.920 ± 1.230 mg/g dry cell weight, which suggested that H. heimii could serve as a source of sterols in bark beetle diets, as is known for other bark beetle-associated fungi. In addition, gas chromatography-mass spectrometry demonstrated that at least one of the isolates, Hyphopichia heimii CAB 1614, was able to convert the insect pheromone cis-verbenol to the anti-aggregation pheromone verbenone. This indicated that H. heimii could potentially influence beetle behaviour. These results support the contention of a tripartite symbiosis between H. heimii, olive trees, and bark beetles.