Plant-fungus-beetle interactions: Case studies in Hawaiian endemic Xyleborus species and the black twig borer

This dissertation is a contribution towards understanding the evolution of host plant specialization in ambrosia beetles and the role their fungal symbionts play. Two main research questions I addressed are (1) whether diversification of ambrosia beetles is linked to host plant specialization, and (2) whether fungal symbionts enable the beetles to colonize different host plants. Hawaiian endemic Xyleborus species and the black twig borer ( Xylosandrus compactus) were used to investigate these questions.;Molecular phylogenies of Hawaiian endemic Xyleborus ambrosia beetles were used to investigate the association between host plant specialization and their diversification. Mitochondrial (CO1) and nuclear (EF1-alpha) genes were sequenced and used for phylogenetic inference. The results showed that the Hawaiian Xyleborus species are not monophyletic and are composed of at least two separate clades. Among the 13 species sampled, host plant specialization was generally associated with diversification. However, allopatric speciation after dispersal to neighboring islands was observed in the Araliaceae specialist clade and populations.;The Xylosandus compactus-Fusarium solani symbiotic system was used to investigate the role that ambrosia fungus plays in determining host plant use pattern for a generalist ambrosia beetle. Genetic variation and population structure of F. solani in a local population was first investigated using RAPD markers. The results showed a high level of genetic diversity and population structure. However, the cause of population structure cannot be attributed to either the pigmentation phenotypes in the fungus or the host plants.;Two experiments were carried out to test host plant preferences of X. compactus and growth performance of F. solani, in order to understand the observed variation in attack density and colonization success on different host plants. Host plant preference by X. compactus was observed. However, the preference did not correspond to colonization success by the beetle. Measurements of fungal growth rates on nine different host plants indicated than better fungal growth generally translates to higher beetle success. However, exceptions were observed, and twig structure seemed to play a role in determining colonization success of the beetles.