Lateral Transmission of Yeast Symbionts Among Lucanid Beetle Taxa

Presence of non-symbiotic yeasts in a symbiont-transferring organ of a stag beetle that lacks yeast symbionts found in other stag beetles

Dispersal from wood to wood is essential for wood-inhabiting fungi and wood-inhabiting insects play an important role in the dispersal success of such fungi. However, it is poorly understood whether wood-inhabiting insects which change the habitats from wood to non-wood environments can contribute to the fungal dispersal. Larvae of most stag beetles (Coleoptera: Lucanidae) are wood feeders, while adults are sap feeders. Female adults of lulcanids possess specialized organs (mycetangia) for transportation of fungal symbionts and harbor specific yeasts (e.g., Scheffersomyces spp.) within. Here, we report that the lucanid Aegus subnitidus harbors non-specific yeasts facultatively in mycetangia. We conducted yeast isolation from mycetangia and hindguts of female adults, in a larval gallery in wood-associated materials, and in female-visiting fermented tree sap using culture-dependent methods. Less than half of the females carried a total of 20 yeast species with small amounts using mycetangia and a female harbored up to five species, suggesting the absence of an intimate association with specific yeasts that are found in other lucanids. Yeast species compositions markedly differed between the larval gallery and sap. Most yeasts from the larval galley exhibited xylose-assimilation abilities, while few yeasts from sap did. Mycetangial yeasts comprised a combination from both sources. In hindguts, most yeasts were found in sap (> 70%) with no yeasts in the larval gallery. Sap-associated yeasts in each female mycetangium were also obtained from the female-visiting sap patch, while mycetangial, larval gallery-associated yeasts were absent from the patch, suggesting the survival success of larval gallery-associated yeasts in mycetangia through sap patches. Therefore, wood-inhabiting insects that possess mycetangia can potentially act as vectors of non-symbiotic wood-inhabiting yeasts dispersing from wood to wood via other environments.

Diverse Heat Tolerance of the Yeast Symbionts of Platycerus Stag Beetles in Japan

The genus Platycerus (Coleoptera: Lucanidae) is a small stag beetle group, which is adapted to cool-temperate deciduous broad-leaved forests in East Asia. Ten Platycerus species in Japan form a monophyletic clade endemic to Japan and inhabit species-specific climatic zones. They are reported to have co-evolutionary associations with their yeast symbionts of the genus Sheffersomyces based on host cytochrome oxidase subunit I (COI) and yeast intergenic spacer (IGS) phylogenies. Here we examined the heat tolerances of the yeast colonies isolated from the mycangia of 37 females belonging ten Japanese Platycerus species. The upper limits of growth and survival temperatures of each colony were decided by cultivating it at ten temperature levels between 17.5 and 40°C. Although both temperatures varied during 25.0–31.25°C, the maximum survival temperatures (MSTs) were a little higher than the maximum growth temperatures (MGTs) in 16 colonies. Pearson’s correlations between these temperatures and environmental factors (elevation and 19 bioclimatic variables from Worldclim database) of host beetle collection sites were calculated. These temperatures were significantly correlated with elevation negatively, the maximum temperature of the warmest month (Bio5) positively, and some precipitative variables, especially in the warm season (Bio12, 13, 16, 18) negatively. Sympatric Platycerus kawadai and Platycerus albisomni share the same lineage of yeast symbionts that exhibit the same heat tolerance, but the elevational lower range limit of P. kawadai is higher than that of P. albisomni. Based on the field survey in their sympatric site, the maximum temperature of host wood of P. kawadai larvae is higher about 2–3°C than that of P. albisomni larvae in the summer, which may restrict the elevational range of P. kawadai to higher area. In conclusion, it is suggested that the heat tolerance of yeast symbionts restricts the habitat range of their host Platycerus species or/and that the environmental condition that host Platycerus species prefers affect the heat tolerance of its yeast symbionts.