Year of Award
Doctor of Philosophy (PhD)
Forest and Conservation Science
Department or School/College
College of Forestry and Conservation
Jeffrey Good, John McCutcheon, Michael Schwartz, Ylva Lekberg
University of Montana
Bark beetles in the genus Dendroctonus are some of the most important insects in forest ecosystems worldwide and are known to be involved in symbiotic relationships with fungi. However, we have a poor understanding of beetle-fungal coevolution and if the specificity we see in beetle-fungal relationships may translate to cospeciation over longer time frames. In this dissertation, I attempt to answer these questions by investigating the western pine beetle (Dendroctonus brevicomis)-fungal symbiosis, a beetle that consists of two putative cryptic species in the early stages of divergence on two susbspecies of ponderosa pine (Pinus ponderosa). In the first chapter, I describe research aimed at determining if the western pine beetle harbors the same species of mycangial fungi across its entire range. I found widespread fungal fidelity and that the beetle carries two species, Entomocorticium sp. B, and Ceratocystiopsis brevicomi. In the second chapter, I describe research aimed at determining if the western pine beetle is indeed in an obligate mutualism and is adapted to fungal isolates with a shared evolutionary history. I found that E. sp. B was crucial for the successful development of western pine beetles and found no significant difference in the effects of the natal (shared history) and non-natal (no shared history) isolate on beetle fitness parameters. However, brood adult beetles failed to incorporate the non-natal fungus into their mycangium indicating adaption by the beetle to particular genotypes of symbiotic fungi. In the third chapter I describe research exploring if the beetle, its two fungal symbionts, and the host tree are genetically structured in a similar fashion indicative of shared evolutionary history and cospeciation. I sequenced the genomes of the beetle and two fungal symbionts and conducted population genetic and genomic analyses for the host tree, beetle, and for the two fungal symbionts. I found congruent patterns of population genetic structure and phylogenetic relationships between multiple species. Taken together, my dissertation research suggests that the western pine beetle is in a tightly linked and evolving obligate mutualism with fungi and that the entire tree-beetle-fungi system is diverging and cospeciating in concert.
Bracewell, Ryan Russell, "Coevolution and cospeciation in a bark-beetle fungal symbiosis" (2015). Graduate Student Theses, Dissertations, & Professional Papers. 10784.
© Copyright 2015 Ryan Russell Bracewell