Molecular and Morphological Inference of the Phylogeny, Origin, and Evolution of Aesculus L. (Sapindales)
HARRIS, AJ. Molecular and Morphological Inference of the Phylogeny, Origin, and Evolution of Aesculus L. (Sapindales). (Under the direction of Qiu Yun Xiang.)
Aesculus L. (Hippocastanaceae or Sapindaceae) is a Laurasian disjunct genus of 13-19 species in four endemic areas. The genus is a good model for biogoegraphic study due to a small number of species, a rich fossil record, and an intercontinental disjunct distribution of extant species. Therefore, further study of this genus may offer important insights into biogeographic history of modern plant biota in the northern hemisphere. Conflicting biogeographic hypotheses have been proposed for Aesculus in previous studies. The main goal of the study is to evaluate these alternative hypotheses using new and more data. I first reconstructed a more robust phylogeny of Aesculus by increasing taxon and character sampling and integrating fossils, morphology, and molecular data. Using the phylogeny as the basis, I performed biogeographic analysis and divergence time dating to determine the place and time of origin and migration pattern of the genus. This analysis provided insight into the timing of speciation and historical events responsible for modern distribution of the genus. DNA sequences of several molecular regions including rps16, trnHK, and matK from chloroplast genome and the ITS of nuclear ribosomal gene and intron 1 of the LEAFY (LFY) gene from the nuclear genome. These were used for phylogenetic analyses in combination with morphological data. The results largely agree with previous molecular analyses preformed using less inter and intra-specific sampling and molecular data from two gene regions. The major difference was the placement of Sect. Aesculus. The phylogeny reconstructed in this study revealed three major lineages in Aesculus including an Asian clade consisting of all Asian constituents of Sect. Calothyrsus, a clade containing A. californica and Sect. Macrothyrsus, and a clade consisting of Sect. Aesculus, Sect. Pavia, and Sect. Parryana. Section Aesculus was placed as sister to Sect. Parryana + Sect. Pavia, a relationship previously unreported. Relationships between these three major clades remained incompletely resolved, probably due to a rapid radiation of the genus in its early evolutionary history. For reconstruction of biogeography in cases of phylogenetic uncertainty, we developed a probabilistic approach with the method of dispersal -vicariance analysis (DIVA). In the analysis, 100 phylogenetic trees from Bayesian analysis were optimized for ancestral distribution with DIVA. The ancestral area of the root of a given lineage was determined as the most probable area(s) summed over the results from the 100 tree analyses. This new application of DIVA was shown to work with incompletely resolved phylogenies and to provide probability on estimated ancestral areas. Our results from DIVA, using a more traditional approach to this software, and divergence time dating using a Bayesian approach without a constant clock supported an Early Tertiary origin of Aesculus in Northeastern Asia and probably also western North America as an element of the high-latitude part of the boreotropical flora. Migrations to other areas occurred in both eastward and westward directions during the early Tertiary. This biogeographic history is substantially different from all previously proposed hypotheses, highlighting the importance of including fossils, particularly the newly discovered early Tertiary fossils of A. hickeyi. Incongruent positions of A. californica in the chloroplasts and nuclear DNA phylogenies supported the previous hypothesis that an ancestral polymorphism and subsequent lineage sorting of haplotypes had generated the discrepancy in the placements of A. californica.
Advisor:Qiu Yun (Jenny) Xiang; Dennis Werner; Deyu Xie; Candace Haigler
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Date of Publication:08/21/2007