Fungal symbiont of an ambrosia beetle possesses high nutrient content and suppresses competing fungi with antimicrobial compounds.

Wood-colonizing beetles are associated with a diversity of microbes many of which are thought to act as mutualists with their beetle hosts, but the evidence is usually anecdotal. The ship-timber beetle Elateroides dermestoides, one of the few fungus-farming but nonsocial ambrosia beetles, is described to have a mutualistic relationship with the yeast-like fungus Alloascoidea hylecoeti. Here, we tested the hypothesis that A. hylecoeti has a high nutrient content thus allowing it to function as a valuable food source for the solitary larvae of E. dermestoides, which bore into the wood of dead trees, an extremely nutrient-poor substrate. Our analyses revealed that A. hylecoeti is rich in soluble sugars, free amino acids, ergosterol, phosphorus, and potassium compared to the other fungi measured, and also accumulates high amounts of fatty acids, B vitamins and nitrogen. We also tested whether A. hylecoeti possesses chemical mechanisms to suppress antagonistic microbes. Extracts from A. hylecoeti and chemical compounds produced or accumulated by this fungus were found to significantly inhibit the growth of potentially competing fungi. The active substances include fungal-produced monoterpenes and acetic acid, as well as phenolic compounds accumulated from host tree tissues. Moreover, sufficient acetic acid was released by A. hylecoeti to drop the medium pH to as low as 3.6, which inhibited all tested competitors, whereas the growth of A. hylecoeti was promoted. Taken together, the nutritional properties and competitive ability of A. hylecoeti may make a major contribution to the success of its insect partner, the ship-timber beetle under natural conditions.