Geosmithia—widespread and abundant but long ignored bark beetle symbionts

Geosmithia (Ascomycota: Hypocreales) and their associations with bark beetles have long been in the shadow of the more often studied beetle associates: Ophiostomatales and Microascales. The last 20 years of research have demonstrated that Geosmithia species are widespread and abundant in the subcortical habitat and that they show varied degrees of symbiosis with their insect vectors. This article reviews the taxonomic history of the genus, species diversity, host ranges, and biogeography. We also discuss the ecological roles of various Geosmithia species in relation to insect vectors and host trees, phytopathogenicity, and their potential for biotechnology. An extensive review of primary data shows that bark beetle species vector predominantly either Geosmithia or ophiostomatoid fungi, but not both, and that this dichotomy is due to the tree substrate features, especially water content and decay stage, which a given bark beetle vector exploits. In both Geosmithia and the other beetle-associated fungi, coevolution with insect vectors led from phloem inhabiting ancestors to the formation of ambrosia lineages and reciprocal adaptations in the beetles. Lastly, we define knowledge gaps and suggest further research directions.