Author ORCID Identifier

https://orcid.org/0009-0006-0612-8381

Semester

Summer

Date of Graduation

2026

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Division of Forestry and Natural Resources

Committee Chair

Matthew T. Kasson

Committee Co-Chair

Teiya Kijimoto

Committee Member

Daniel G. Panaccione

Committee Member

Elizabeth Rowen

Committee Member

Young-Lak Park

Abstract

Ambrosia beetles, a polyphyletic group of weevils, utilize pouch-like structures at various locations on the body to transport for their sole food source, their fungal partner. Ambrosia fungi are known to originate from multiple unrelated fungal lineages with no consensus of traits required to become a fungal cultivar. While the biology and ecology of both partners is in many cases well understood, the genetic underpinnings of this fascinating mutualism remain a mystery. The aim of this dissertation is to add to the understanding of the ambrosia symbiosis by providing insights into its molecular and genetic underpinnings through RNA sequencing data, focusing on two traits that facilitate this unique interaction: mycangia development and host colonization.

Given that molecular protocols for studying ambrosia symbiosis have lagged behind other model systems, a systematic protocol for transcriptomic analysis of both beetle host and fungal mutualist is needed to create consensus within the field. To this end, a novel protocol for RNA extraction, RNA sequencing, and transcriptomic analysis was created. By using inexpensive, easily accessible materials and a free, open-source transcriptomic analysis workflow manager, this protocol aims to be accessible to all ambrosia symbiosis scientists in the hope that studying this mutualism through the lens of evolutionary developmental biology will become more prevalent. This protocol effectively acquired and processed transcriptome datasets of both host beetle and symbiotic fungi from a single sample. Using this protocol, we found genes potentially facilitating mycangia development and host colonization.

Mycangia are present in many insects interacting with fungi, but none have reached advanced agriculture as seen in ambrosia beetles. Within ambrosia beetles, mycangia have no set size, shape, or location. The paired, dual preoral mycangia of Euwallacea validus are thought to have co-opted the genes breathless (btl) and trachealess (trh) for their development. To further uncover genes potentially involved in mycangial development time series analysis using three developmental stages was performed and included early pupae, late pupae, early adult beetles. This analysis revealed several genes potentially co-opted for mycangia development based on expression patterns, known function, and known transcription factor expression patterns.

The mechanisms by which an ambrosia fungus survives within mycangia and becomes the dominant symbiont remain unknown. Mycangia can be colonized by competing fungi and must therefore rely on other mechanisms to ensure vertical transmission of the primary fungal partner over competing fungi. For the first time the transcriptome of an ambrosia fungus actively growing within mycangia was analyzed. High expression of non-constitutive biological processes revealed that Fusarium oligoseptatum likely relies on alcohol and aldehyde dehydrogenase for survival inside the mycangia and may also produce an antifungal peptide, Victoriocin, to compete with non-primary fungi in the immediate environment. Further analysis revealed that Victoriocin production appears to be widespread in Ambrosia Fusarium Clade (AFC) and may account for the lack of fidelity between distinct AFC members and Euwallacea ambrosia beetle counterparts.

In the final chapter, I reflect on our results from chapters 2–3 from the lens of mutualistic evolutionary biology and pest management to help address pressing questions in the field: 1) how does mutualistic evolution differ from other selective forces; and 2) how can ambrosia beetle pests be better controlled. Together, this work represents necessary steps to fully understanding the genes and molecular mechanism driving the ambrosia symbiosis.

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