Evolution in a bark beetle-fungal symbiosis: A phylogenomic and population genetic analysis of the mutualistic symbionts of western pine beetle

Many bark beetles are in tightly linked symbiotic relationships with fungi and get nutritional supplementation by feeding on propagated fungi. As some of the most important insects in forest ecosystems worldwide, there is surprisingly little known about how these insect-fungal symbioses form and if and how they are maintained through evolutionary time. Here, we use population genetics and genomics to investigate the western pine beetle’s mutualistic symbionts, Ceratocystiopsis brevicomi and Entomocorticium sp. B. Using RAD sequencing and assembled and annotated genomes for both fungal species, we characterize their population genetic structure, reproductive modes, and genomic signatures of divergence. We find that one of the fungal symbionts is comprised of three distinct lineages with a complex association with the beetle, while the other symbiont bares signatures of recent invasions into some beetle populations. In addition, the two fungi show marked differences in recombination suggesting fundamental differences in reproduction. Our results provide a fascinating picture of the western pine beetle-fungal symbiosis and highlight the evolutionary complexities that can underlie current insect-fungal associations.