Masters Theses

Date of Award

12-1998

Degree Type

Thesis

Degree Name

Master of Science

Major

Botany

Major Professor

Edward E. Schilling

Committee Members

B. Eugene Wofford, Karen W. Hughes

Abstract

Eupatorium has received various levels of delimitation and there is still no consensus on its circumscription. It has been considered to be a heterogeneous genus of nearly 1000 species and encompassing half of the mainly Neotropical tribe Eupatorieae. In contrast, a recent circumscription (King and Robinson, 1987) restricts the genus to 42 species which follow an "Arcto-Tertiary" geographical distributional pattern (mostly E. North America, Europe, and E. Asia) that differs from with the Neotropical distribution for the rest of the tribe. Although there is little argument that the species of Eupatorium (sensu King and Robinson, 1987) fall into four easily recognizable groups, only Eupatoriadelphus is recently treated as a formal taxonomic group. A survey of floral micromorphology was conducted to understand better the micromorphological features that are used to distinguish Eupatorium from its former congeners, and to assess their possible infrageneric phylogenetic utility. Floral micromorphological characters considered to be diagnostic for Eupatorium by King and Robinson (1987) were found to be consistent but not helpful in delineating species within the genus. Characters which were observed to be informative in distinguishing Eupatoriadelphus from Eupatorium s. s. included the phyllotaxy, shape of the style base, shape of the pappus bristle barbs, the infrequency of glands on the innermost involucral bracts, and carpopodium type. Neither qualitative nor quantitative floral micromorphology were helpful in illuminating the relationships among species within either genus. Sequences from the internal transcribed spacer (ITS) region of nuclear rDNA were analyzed through maximum parsimony methods to determine the phylogenetic relationships of Eupatorium and the status of the genus Eupatoriadelphus. In the most parsimonious trees, samples of formerly synonymous genera Fleischmannia, Conoclinium, and Ageratina segregated well away from Eupatorium. The Eupatoriadelphus clade was also very distinct from the Eupatorium s. s. clade. These results support segregation of Eupatoriadelphus to produce a narrowly defined Eupatorium. Differences in DNA sequences were used to estimate the times of divergence for various clades, allowing a more systematic assessment of how Eupatorium achieved its "Arcto-Tertiary" geographic distribution. An estimate of 3.5 million years was obtained for the divergence time between Eurasian and North American Eupatorium; this relatively recent divergence date favors long distance dispersal as the origin of the current distribution of the genus, because it is too recent to be a result of fragmentation of an "Arcto-Tertiary" flora.

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