Duvall, Melvin R.
Ph.D. (Doctor of Philosophy)
Department of Biological Sciences
Evolution; Botany; Molecular biology
Systematics is an important set of tools for the determination of relationships among living organisms. This toolkit is only as good as the information that helps differentiate the taxa in question. Grasses, Poaceae, has always been of great interest due to the important crops in the family. For this reason, the panic grasses (Panicoideae) have been thoroughly researched for their crops; corn, sugarcane and sorghum, but less is understood for their non-crop species. The goals of this dissertation are to 1) better sample within the panicoid grasses to retrieve a more complete phylogeny, 2) determine the divergence phylogeny and divergence date of the Panicoideae, 3) investigate rare genomic changes that occur within species of the panic grasses, and 4) investigate the evolution of multiple traits that occur within the subfamily. First, I looked at previous studies to determine relationships that largely lack resolution and/or robust support for tribal and subtribal groups. I sequenced 35 new Panicoideae plastomes and combined them in a phylogenomic study with 37 other species. This returned a mostly congruent Panicoideae topology compared to other studies at the time, with five recognized subtribes that were non-monophyletic. An unexpected mutation in the Paspalum lineage was discovered, a mitochondrial DNA (mtDNA) to plastid DNA (ptDNA) transfer. This was thought to be a single rare event that unevenly degraded into smaller fragments in the plastome. Second, I investigated the early diverging grass lineages, as these would help set up the framework for the fourth study. I sequenced three species from two different subfamilies, and aligned these with 43 other grass species and the outgroup Joinvillea ascendens. The alignment displayed some large rare genomic changes that were mostly clade defining. The use of a non-grass outgroup caused a decrease in phylogenetic information, but with over 81,000 nucleotide bases a robust phylogeny was obtained. This was then paired with two new fossil calibration dates that had not been used before. Overall the dates retrieved were older than previous estimates, which was most likely influenced by a more complete sampling, both taxonomic and molecular, and the use of these new fossil calibrations. For the third study, I revisited the unusual discovery made in the first study, with the inserts found in the Paspalum lineage, specifically in the trnI - trnL intergenic spacer region. I sequenced nine new plastomes, seven of which contain an insert within the region of interest. These were combined with 52 other species for phylogenomic and divergence date analyses. The Paspalum topology was robust with the genus originating at 10.6 Ma, and the insert arising at 8.7 Ma. The aligned Paspalum inserts revealed 21 different subregions with pairwise homology in 19, suggesting one shared event. An emergent self-organizing maps analysis of tetranucleotide frequencies determined that mitochondrial DNA grouped with the Paspalum insert, suggesting a mitochondrial origin. Thus, it seems the original insert in Paspalum was at least 17685 bp in size. Finally, the Paspalum inserts contained seven intrastrand deletion events, suggesting uneven degregradation and selective pressures to remove these large inserts. Finally, the last study used information from the first and second to put together a large phylogeny and conduct divergence date analyses paired with an ancestral state reconstruction analysis. I assembled another 35 new plastomes for this analysis, and combined it with 317 other Poaceae plastomes. This was used to create a phylogeny that investigated an extremely well-sampled Panicoideae. The divergence date analysis was used in combination with the six character-state traits in an ancestral state reconstruction analyses. A major finding was that the evolution of the NADP-ME subtype for the Paspaleae first occurred at the node uniting Paspaleae with Arundinelleae and Andropogoneae. This occurred sometime around major climate warming events that took place at roughly 55 Ma. Other unexpected trait findings included stamen counts, which are not as static as originally thought. The last finding is that non-open habitats that do not mirror the pseudopetiole trait, while the converse is mostly true.
Burke, Sean Vincent, "Evolution of panic grasses (Panicoideae; Poaceae) : a plastome phylogenomic study" (2018). Graduate Research Theses & Dissertations. 2954.
Northern Illinois University
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