Growth variation of an ambrosia fungus on different tree species indicates host specialization

Ambrosia beetles rely on mutualistic fungi as a food source for themselves and especially for their offspring, yet the influence of host tree species on fungal growth and specialization is not well understood. In this study, we investigated the growth performance of the ambrosia fungus Dryadomyces montetyi , the primary symbiont of the oak pinhole borer Platypus cylindrus , on semi-artificial media infused with extracts of four tree species: Quercus robur, Fagus sylvatica, Abies alba , and Pseudotsuga menziesii . Fungal growth was quantified over time using logistic growth models, and final biomass was assessed through dry weight measurements. The growth of D. montetyi differed significantly among the different host tree substrates. Growth on F. sylvatica was comparable to that on Q. robur ; however, both conifer-derived media ( A. alba and P. menziesii ) exhibited significantly reduced surface expansion and biomass accumulation. Tissue density measurements further indicated that the mycelium grown on Q. robur was denser than that grown on conifer media, although overall density was highest on a nutrient-rich medium without tree extract. These results demonstrate that D. montetyi performs best on angiosperm hosts, particularly oak, which aligns with the known host preferences of P. cylindrus . Our findings suggest that fungal performance is not solely determined by the typical polyphagy of ambrosia beetles but also reflects the host-related specialization of the fungal symbiont. This specialization certainly determines the host selection of P. cylindrus and the evolution of the tripartite interactions between this beetle, D. montetyi , and oak trees.