Semester

Spring

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Division of Plant and Soil Sciences

Committee Chair

Teiya Kijimoto

Committee Co-Chair

Matthew Kasson

Committee Member

Daniel Panaccione

Committee Member

Rita Rio

Abstract

Evolutionary Developmental Biology seeks to answer fundamental questions about the mechanisms underlying the evolution and innovation towards increasing structural complexity within the body plan. The ambrosia symbiosis, a polyphyletic group of xylem-feeding beetles, provides a wealth of diversity in novel pouch-like structures called mycangia. This diversity could serve as a rich model for understanding how mutualisms may prompt structural innovation and diversification in a host organism. The ambrosia symbiosis has become a fast-growing research subject in entomology and forest pathology but has yet to experience the same attention under the lens of evolutionary developmental genetics.

Development of mycangia was examined in a local ambrosia beetle species, Euwallacea validus, across lifestage and between sexes using micro-computed tomography. Previously considered a sex-specific trait, these observations uncovered a nascent mycangium-like space in the male head. Furthermore, a second set of paired pouches was observed at a lower, lateral position in both sexes. To identify developmental programs that may have been co-opted to form this novelty, expression of three candidate genes (doublesex (dsx), breathless (btl), and trachealess (trh)) were examined across multiple maturity points for female and male beetles, followed by functional analysis via RNA Interference. In line with developmental observations, results suggested insect sex-determination does not regulate the initiation of mycangia. However, btl and trh expression was sustained in the head across development and phenotypic results of RNAi suggested involvement in mycangial morphogenesis.

To establish the contribution of gene sequence to the novel spore shape observed within the beetle-associated Ambrosia Fusarium Clade (AFC), select genes from the conidiogenesis pathway were assessed across the genus for divergence. Topology of constructed phylogenies suggests that gene sequence is not a driving factor of the modified conidial shape, but individual mutations could alter structure and function. Hydrophobins represent a class of secreted, self-assembling proteins that have implications for adhesion, spore dormancy, and even virulence. Hydrophobin sequences were identified across Fusarium and classified by groups to highlight conserved trends across Fusarium and within the AFC. Through this approach, we were able to discern clade-specific hydrophobins that may play specialized roles in Fusarium. Furthermore, unique protein candidates were found in only clavate spore-producing species linking novel protein with novel symbiosis adaptation.

This work has resolved development of preoral mycangia in E. validus and provided the first evidence of co-opted genetic targets in the construction of an evolutionary novelty within ambrosia symbioses. Comparative proteomic analyses identified unique targets that may serve as the first interface between symbiont and host, creating a foundation for investigations surrounding host-symbiont recognition within this beetle-fungal mutualism.

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