First Advisor

Deborah A. Duffield

Term of Graduation

Winter 2022

Date of Publication


Document Type


Degree Name

Doctor of Philosophy (Ph.D.) in Biology






Helminths -- Phylogeny, Helminths -- Oregon -- Hosts, Helminths -- Morphology, Host-parasite relationships, Western pocket gopher, Thomomys, Coevolution



Physical Description

1 online resource (xi, 173 pages)


Although often viewed as inconsequential, parasites have significant ecological and evolutionary importance, and studying them can generate knowledge regarding their environments and their hosts. Helminths (intestinal parasites) infecting Western pocket gophers (Thomomys species) in Oregon had been documented based only on morphology prior to this study. Basing parasite identification solely on morphology is problematic because the characteristics used to identify species can be vague or similar, leading to misidentification or obscuring the true biodiversity present. Using molecular markers to verify species present not only is more reliable but also can be informative in terms of the host-parasite association. To more accurately quantify the diversity of helminths present in these hosts and to better understand their associations with their hosts and the environment, I performed the following three studies.

In the first study (Chapter 2), I documented the parasites found in Thomomys species in Oregon using a molecular approach. Partial nuclear (either the 18S rRNA gene or the ITS1 region) and mitochondrial (COI gene) sequences were used to construct phylogenetic trees for helminth specimens identified morphologically as Trichuris fossor, Heligmosomoides thomomyos, Ransomus rodentorum, and Hymenolepis tualatinensis. The results verified that each of these species represented a distinctive lineage, and that genetic variability was present for each group. Additionally, I documented the presence of what is likely an undescribed species of Heligmosomoides.

In the second study (Chapter 3), I used the COI mitochondrial gene to create phylogenetic trees for Thomomys species. The two subgenera, Megascapheus and Thomomys formed well-supported monophyletic groups in both the maximum likelihood and Bayesian inference analyses. The COI gene was also used to test for coevolution between Thomomys hosts and the Heligmosomoides species parasitizing them. There was no statistical support for a coevolutionary relationship between Thomomys hosts and their Heligmosomoides species.

In the final study (Chapter 4), I investigated the role of intrinsic and extrinsic factors on helminth infections using a series of statistical analyses. A significant difference in prevalence of overall and H. thomomyos infections among host species was detected. The intensity of overall infections, T. fossor infections, and H. thomomyos infections did not vary among host species. The prevalence of T. fossor infections varied marginally among age classes. There was no significant difference in prevalence or intensities of overall infections, T. fossor infections, or H. thomomyos infections between host sexes. Prevalence and infection intensity did not vary by ecoregion for overall, T. fossor, or H. thomomyos infections. Overall infections, T. fossor infections, and H. thomomyos infections did vary significantly among townships (i.e., the closest town or city) and the intensity of overall infections varied among townships as well. Prevalence of overall infections varied between 2018 and 2019, although intensity of overall infections did not vary between the two collection years. The prevalence of overall infections varied marginally by season and the prevalence of T. fossor and H. thomomyos infections varied seasonally as well.

This dissertation provides further insight into the helminth biodiversity present within Thomomys species, factors that affect infections within these hosts, and the evolutionary relationship between Heligmosomoides species and their hosts.


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