Graduate Thesis Or Dissertation
 

Evolutionary Dynamics of Genomes and Endosymbionts in Plant-Parasitic Nematodes

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  • Plant-parasitic nematodes cause more than US $100 billion in annual agriculture loss worldwide. Thorough knowledge of their genetic diversity, and interactions with endosymbionts and environment have the potential to provide valuable insights into the basic biology of these animals, and assist future efforts aimed at management of these plant parasites. In this dissertation, genetics and genomic strategies were utilized to advance our understanding of plant-parasitic nematode evolution and symbiosis. In chapter 2, a molecular-genetic strategy that targeted the 16S rRNA gene was used to assess the prevalence and distribution of two endosymbiotic bacteria, Wolbachia and Cardinium, in plant-parasitic Pratylenchus penetrans field populations collected from varying plant hosts. The study also explored the potential effects of Wolbachia on reproduction in this nematode species. Variable abundances and distributions for both endosymbionts were observed in the P. penetrans populations examined. The complete absence of Wolbachia in some populations and low prevalence in others suggest that this endosymbiont is not an obligate mutualist of P. penetrans, making this system distinct from the obligately mutualist Wolbachia-filarial nematode symbioses. The observed sex ratio bias in Wolbachia-infected nematode populations suggests that Wolbachia acts as a reproductive manipulator, also unlike the well-studied Wolbachia-filarial nematode symbiosis. In chapter 3, a genome skimming strategy was used to investigate Wolbachia and Cardinium occurrence in economically important plant-parasitic nematode populations representing twelve different species, including samples from North and South America. The study further investigated the population level genetic variability among the detected endosymbionts. Limited occurrence of Wolbachia (in P. penetrans) and Cardinium (in P. penetrans and Heterodera glycines) across plant-parasitic nematode species was observed suggesting non-obligatory function of both endosymbionts in these nematodes. Wolbachia genomes were highly similar across populations infecting P. penetrans. Cardinium displayed dissimilarities in their genomes from different nematode species, P. penetrans and H. glycines, suggesting potential to act in a species-specific manner in plant-parasitic nematodes. In chapter 4, population genetic analyses were performed to identify genome-wide patterns of genetic variation of the plant-parasitic nematode Globodera pallida across Idaho field populations to evaluate the origin, and potential evolutionary responses since the time of infestation. Most of the nematode genetic variation was shared among the Idaho fields, indicating that the infestation likely resulted from a single introduction. The significantly differentiated allele frequencies at certain loci among G. pallida populations across time suggested adaptive potential of this parasite invading new environments, and identified candidate genetic loci under positive selection. The dissertation provided important new insights into plant-parasitic nematode biology and evolution through providing information on (i) microbial endosymbionts that have the potential for future development in biological control applications, and (ii) evolutionary adaptive potential of plant-parasitic nematode populations invading new environments.
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  • Pending Publication
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  • 2020-06-12 to 2021-07-13

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