Agronomy and Horticulture Department


Date of this Version

Summer 7-31-2015


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Agronomy, Under the Supervision of Professor Thomas O. Powers. Lincoln, Nebraska: August, 2015


Approximately 200 years ago, North American landscapes were dominated by vast expanses of grassland. The Great Plains, an ecologically complex and diverse ecoregion, extended from Canada to the Gulf of Mexico, and from the Rocky Mountains to the border of the eastern hardwood forest in Indiana. Now the original tallgrass prairie exists in small widely dispersed remnant patches surrounded by agroecosystems. This study is an effort to characterize soil nematode diversity and population structure within those remnant patches. The plant parasitic nematode family Criconematidae serves as our indicator for nematode diversity, due to its global distribution, high abundance in natural areas, broad host range, limited dispersal capabilities, and the availability of a highly resolved COI gene tree. Thirty-one grassland sites representing 13 ecoregions were sampled by extracting soil cores within a 40 x 40m grid. Criconematid nematodes were screened and isolated using soil sieves and centrifugation. Nematodes were individually photographed, measured and amplified with a primer set that, after removing primers, results in 721 nucleotides of the COI mitochondrial gene. Diversity was studied at different levels, from morphospecies to haplotype lineages and haplotype diversity within lineages. Grassland lineages were investigated using phylogenetic methods and species delimitation methods. One hundred and ninety five nematodes conforming morphologically to Mesocriconema curvatum of broad geographic distribution, spanning the central tallgrass ecoregion were analyzed as cryptic species. Where morphological analysis would indicate the presence of a widespread, cosmopolitan species, species delimitation analyses, including the construction of phylogenetic trees, Automatic Barcode Gap Discovery, the Species Delimitation Plugin implemented through Geneious R8, and statistical parsimony networks indicate approximately ten well-supported, genetically distinct clades. Two of these clades contain over seventy specimens each and are characterized by distinct intra-specific population structures, suggesting unique evolutionary forces responsible for their speciation and diversification. While these two clades, or cryptic species possess independent population structures, they are both associated with sister clades of southern sites, indicative of ancestral origins in states such as Arkansas, Oklahoma, and Texas.

Advisor: Thomas O. Powers