Parasitic reproductive endosymbionts are emerging as formidable threats to insect biodiversity. Wolbachia are prevalent maternally inherited intra-cellular bacteria found in >50% of arthropod species. These symbiotic bacteria interact with their hosts in diverse ways, most often they alter host reproduction causing four conditions that all selectively favor infected females: feminization, male killing, parthenogenesis, and cytoplasmic incompatibility (CI). Furthermore, depending on strain-type and host genetic background, Wolbachia are known to affect insect behavior, expand or shift host thermal tolerance ranges, and confer anti-viral protection to their hosts. Because Wolbachia both reside in and are transmitted with host cell cytoplasm, mitochondria and other cytoplasmically inherited genetic elements become linked with the bacteria. Thus, by enhancing their own transmission, Wolbachia-induced phenotypes can lead to mitochondrial selective sweeps, which may have profound impacts on vulnerable and small insect populations. Elucidating the extent to which endosymbionts influence biological and ecological functions is pivotal to making management decisions regarding imperiled insect species.
My dissertation investigates biological and ecological impacts of host-endosymbiont interactions by examining Wolbachia infections in three different host systems. First, I used the federally threatened butterfly species Speyeria zerene hippolyta to determine whether the general reproductive success of local populations was affected by the introduction of CI-inducing Wolbachia-infected butterflies through implemented species recovery programs. Next, by characterizing the Wolbachia infections of parasitoids associated with the Eurema butterfly clade, I analyzed whether host-parasitoid interactions provide a path for interspecies horizontal transmission. Finally, I conducted a laboratory experiment using an isogenic Drosophila melanogaster line to determine whether Wolbachia influence host temperature preference. Together, my research examines how the individual level effects of host-endosymbiont interactions can expand into populations, have broader impacts on insect communities, and potentially impede the conservation and management of insects in nature.
In chapter one, I screened S. z. hippolyta samples from three extant populations for Wolbachia infection. To examine the impacts of Wolbachia on small populations, I analyzed and compared infected and uninfected S. z. hippolyta reproductive data and showed that, in a population composed of infected and uninfected S. z. hippolyta, uninfected butterflies had reduced reproductive success (GLMM z = -8.067, P < 0.0001). I then developed a single-population demographic theoretical model using these same reproductive data to simulate and analyze different potential dynamics of small populations resulting from population supplementation with uninfected, CI-Wolbachia infected, or combined uninfected and infected butterflies. Analysis of model simulations revealed that supplementation with CI-inducing butterflies significantly suppressed host-population size (ANOVA F5,593 = 3349, PWolbachia-infected individuals (Tukey's post-hoc test P < 0.0001). In addition, supplementation by multiple releases using a combination of 50 infected and 300 uninfected butterflies has a less severe suppression effect, reducing the population by 75.8%, but the reduction occurs 42.6% faster than with the single release of 50 Wolbachia-infected butterflies (Tukey's post-doc test P < 0.0001).
Parasitoid-host interactions have emerged as probable ecological relationships to facilitate horizontal transmission of Wolbachia. In chapter two, I addressed horizontal transmission using Eurema butterflies and their associated parasitoids. From four locations in Northern Queensland, Australia, I collected a total of 404 Eurema hecabe butterfly larvae. Twenty-three parasitoids emerged from the larvae of which 21 were Diptera and two were Hymenoptera. I amplified COI loci fragments from each parasitoid for BLAST query searches and found that 20 individual Diptera parasitoids matched to the genus Exorista and one to the genus Senometopia. One of the Hymenoptera parasitoids matched to the genus Microoplitis and the other to the genus Cotesia. To characterize Wolbachia infections, I used Wolbachia Multi Locus Sequencing Technique (MLST) and discovered that all 20 Exorista parasitoids were infected with an identical Wolbachia strain (ST-41), which is the same strain infecting their Eurema hecabe butterfly hosts. Although, further experiments are necessary to definitively determine that ST-41 Wolbachia are incorporated into germline cells of the parasitoids, this is the first study to provide ecological evidence for inter-ordinal Wolbachia transmission between Lepidoptera and Diptera. Furthermore, this discovery exposes the risk of population augmentation programs that move insects, potentially facilitating the spread of Wolbachia between species within a community through the accidental introduction of new Wolbachia-infected parasitoids.
Finally, both Wolbachia and their insect hosts are temperature sensitive organisms. Wolbachia's replication behavior in their hosts is positively-temperature dependent, while environmental variation can have profound effects on insect's immune function, fitness, and fecundity. In chapter three, I conducted a laboratory experiment using a thermal gradient choice assay and an isogenic Drosophila melanogaster line with four different Wolbachia infection statuses -- uninfected, wMel, wMelCS, and wMelPop - to assess whether a relationship existed between Wolbachia infection and host temperature preference. Results from my laboratory experiment revealed that Wolbachia-infected flies preferred cooler temperatures compared to uninfected flies. Moreover, D. melanogaster temperature preferences varied depending on the Wolbachia strain variant with which they were infected; flies infected with the wMel strain had temperature preferences 2°C cooler compared to uninfected flies; flies infected with either wMelCS or wMelPop strains had preferred temperatures 8°C cooler compared to uninfected flies. Wolbachia-associated temperature preference variation within a species can lead to conspecifics occupying different microclimates, genetically adapting to different sets of specific environmental conditions, and may eventually result in ecological and reproductive isolation. While, reproduction isolation is recognized as one of the first stages in speciation, in small populations of endangered and threatened species, the inability to reproduce between conspecifics can drive species to extirpation or extinction.
Collectively, the three chapters of my dissertation set precedent for future integration of host-endosymbiont research prior to implementing population supplementation or translocation programs for the conservation of imperiled insects.
| Identifer | oai:union.ndltd.org:pdx.edu/oai:pdxscholar.library.pdx.edu:open_access_etds-4652 |
| Date | 08 June 2017 |
| Creators | Truitt, Amy Michelle |
| Publisher | PDXScholar |
| Source Sets | Portland State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations and Theses |
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