Date of Final Oral Examination (Defense)
Type of Culminating Activity
Master of Science in Biology
Merlin M. White, Ph.D.
James F. Smith, Ph.D.
Stephen Novak, Ph.D.
William Bourland, M.D.
The Ichthyosporea (= Mesomycetozoea) is a relatively understudied class of unicellular symbionts that molecular phylogenies have placed at the divergence of animals and fungi. Subsumed in this class are the cosmopolitan families Eccrinidae and Amoebidiidae (referred to as “protist trichos” or “trichos” herein), which are considered obligate commensal endobionts of various arthropods, including marine, freshwater and terrestrial hosts. Once thought to be members of the fungal class Trichomycetes due to their hyphal-like growth form and ecological similarity, molecular evidence has necessitated reclassification. However, evolutionary relationships within and between them are still unclear as the number of taxa sampled and/or the amount of gene data gathered have been factors limiting resolution. These organisms are also taxonomically challenging since informative, homologous morphological characters are difficult to discern using only a light microscope (the method by which members of Amoebidiidae and Eccrinidae have traditionally been described), and only a few have been obtained in axenic culture. Most protist trichos reported thus far lack sufficiently detailed morphological parameters to permit ease and confidence in species identification. As such, relatively little is known about the ecology and biology of most members, some of which were originally classified as fungi or algae. As new members were discovered or reclassified, two orders were established: Dermocystida and Eccrinida. Whereas members of the Dermocystida are almost entirely parasites of various metazoan hosts, only three clades within the Eccrinida contain known parasites, with the remaining members regarded as commensalistic. Interestingly, the putative closest extant relative to both groups is Ichthyophonus, an economically relevant fish parasite, which can invade vital host tissues (e.g. heart and liver) via circulating amoeba-like cells,causing disease and potentially death. The most recent molecular systematic study of the protist trichos was published about a decade ago, and there is as yet but one Paramoebidium (Amoebidiidae) sequence deposited in GenBank. Currently, based on molecular data, the Amoebidiidae are supported as monophyletic (based on one sample from each of its two genera) while the monophyly of the Eccrinidae is indicated, but not supported. Likewise, the relationship of the protist trichos to Ichthyophonus remains unresolved. As such, the first chapter of this thesis addressed the molecular phylogeny of order Eccrinida, with particular emphasis on the protist trichos by first amplifying and sequencing rDNA genes (18S and 28S) for over 100 new samples. Amplification tests were also attempted for several protein-coding genes, including heat shock protein 70. The resulting tree inferences were used in subsequent analyses of ecological and life history traits via ancestral state reconstructions and Bayesian tip-association significance testing (BaTS).
In the second chapter, samples of Paramoebidium spp. were morphologically and molecularly assessed as a case study into the utility of traditionally described morphological characters for taxonomic delimitation among protist trichos. Morphological differentiation of Paramoebidium spp. has been notoriously problematic due to inter- and intraspecific variability. Host specificity within the genus was early suggested, but later questioned, and has not been subjected to thorough evaluation. Therefore, host and hyphal characters were analyzed via three different methods of ancestral state reconstruction, as well as with BaTS on a molecular phylogeny of over 70 Amoebidiidae samples.
Results of these studies indicate: 1) contrary to previous hypotheses, the Amoebidiidae may be paraphyletic, 2) relationships among Eccrinidae and between the protist trichos and Ichthyophonus remain unresolved, 3) several life history and host characters are significantly associated with both the Eccrinida and Amoebidiidae phylogenies, providing platforms for future hypothesis formulation, 4) the protist trichos and the Eccrinida as a whole are likely much more species rich and widespread than what is currently known, 5) species delimitation within Paramoebidium is complicated by cryptic speciation, but there is evidence for possible host specificity, and 6) future studies of the protist trichos will benefit from an integrated approach that shifts away from an emphasis on the morphological species concept but includes both genetic sequence data and traditional morphological approaches.
Reynolds, Nicole K., "Resolving Relationships at the Animal-Fungal Divergence: A Molecular Phylogenetic Approach for Understanding the Ecology and Evolution of the Protist Trichomycetes" (2015). Boise State University Theses and Dissertations. 997.