Nutritional profile and effects on fungal growth of yeast symbionts associated with North American spruce beetle (Dendroctonus rufipennis)

Abstract We characterized yeasts isolated from the integument of North America spruce beetle (Dendroctous rufipennis Kirby) across four populations, using molecular methods to identify 59 representative isolates. Seven yeast species were detected; Wickerhamomyces canadensis (Wickerham) Kurtzman et al. (Sachharomycetales: Saccharomycetaceae) was the most common (74% of isolates) and found in all populations. Isolates of W. canadensis were subsequently tested for in vitro competitive interactions with symbiotic (Leptographium abietinum) and pathogenic (Beauvaria bassiana) fungi, and isolates were nutritionally profiled (N and P content). Three key findings emerged: (1) exposure to yeast headspace emissions had isolate-dependent effects on colony growth of symbiotic and pathogenic fungi; most isolates of W. canadensis slightly inhibited growth rates of symbiotic (L. abietinum) and entomopathogenic (B. bassiana) fungi. (2) A single volatile (2-phenylethanol) was produced by W. canadensis on growth media and was emitted at an average rate of 1.47 µg− 1·mg− 1 yeast·min− 1. Exposure to vapors of synthetic 2-phenylethanol reduced growth rates of both L. abietinum and B. bassiana by 36% on average. (3) W. canadensis cultures were high in both protein (0.8%) and phosphorous (7.2%) in comparison to spruce phloem, but isolates varied in nutritional content with the most nutrient-rich strains isolated from populations in outbreak or post-outbreak population phases. We conclude that W. canadensis is a primary yeast symbiont of D. rufipennis in the southern Rocky Mountains and may serve as a source of volatile emissions that can affect growth of associated microbes. Although W. canadensis has higher nutritional content than spruce tree phloem, the concentration of limiting nutrients (protein and phosphorous) is less than reported for the symbiotic fungus L. abietinum, and variation in nutritional content may be associated with beetle population density.