Semester

Spring

Date of Graduation

2026

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

Craig F. Barrett

Committee Member

Edward Brzostek

Committee Member

Stephen DiFazio

Committee Member

Daniel Panaccione

Committee Member

Amy Welsh

Abstract

Mycoheterotrophic plants exploit fungi for most or all nutritional needs, providing a compelling system to address the evolutionary consequences of genome reduction, population-level variation, and germination requirements. I am using the widespread Corallorhiza maculata species complex to address questions of genomic variation, fungal host specificity, morphological variation, and germination requirements across the broad geographic range from Mexico to northern North America. This complex contains C. bulbosa, C. macrantha, and C. mertensiana, the latter with three varieties (maculata, occidentalis, and mexicana). These taxa vary in flowering time, floral morphology, and fungal associations. However, the degree of taxonomic and ecological distinctness is not as well characterized. My goals are to: 1) quantify range-wide geographic variation in orchid-fungal associations and 2) directly test the fitness of mycoheterotrophic plants. I begin this dissertation by discussing what is known regarding mycoheterotrophy and the genomic consequences of this lifestyle, how it may dictate fungal host associations, and how it impacts fitness of plants. Then I show the impacts of mycoheterotrophy on plastid genomes (plastomes) by identifying variation across multiple individuals of this complex with varying degrees of degradation and structural variation. I additionally identified unprecedented amounts of structural heteroplasmy in a single individual, conflicting with prior studies regarding the conserved nature of plastomes. Due to the lack of differentiation between two northern North American varieties (maculata and occidentalis), I conducted a population-level study to identify the variation below the species level. I also investigated the putative fungal host specificity across the complex using fungal metabarcoding to target the internal transcribe spacer 2 (ITS2) region to identify the full breadth of potential hosts targeted by these orchids. I leveraged a nuclear SNP, floral morphological, and fungal metabarcoding data to address questions of host specificity and population dynamics across the complex. I found support for the lack of differentiation for the two northern North American varieties, and I also found that putative fungal host associations may be broader than previously thought. This was further supported when taking the complete dataset into consideration, with plant genotypes potentially influencing which fungal hosts were being selected. Finally, I investigated the germination requirements of the complex using C. maculata vars. maculata and occidentalis for both field and in-vitro germination assays. While our field germination assay yielded no recoverable seeds, our in-vitro germination assay yielded asymbiotic germination of a mycoheterotrophic species. To my knowledge, this is the first record of asymbiotic germination of a mycoheterotrophic species in-vitro and greatly expands what can be done for other rare or endangered species. Overall, the results of my study highlight the importance of below species level sampling to capture the nuances of evolution and population-level variation. All aspects of mycoheterotrophy were taken into consideration and yielded a comprehensive study that investigated the plant, plant-fungal, and fitness consequences of mycoheterotrophy.

Download

Share

COinS