Molecular Systematics of North American Eupatorium

This study had two primary goals: first, to prepare a thorough taxonomic revision of North American Eupatorium, and second, to conduct initial evaluations using molecular techniques of how diploid and polyploid populations of Eupatorium rotundifolium and E. sessilifolium relate to one another. Nuclear ribosomal internal transcribed spacer (ITS) region and chloroplast DNA (trnC-psbM) sequences were used for molecular analysis, which allowed assessment of relationships of the polyploidy samples based on both a biparentally and a maternally inherited marker. The Inter Simple Sequence Repeat (ISSR) method was used for examining the relationships among diploid and polyploid populations including the genetic variation between the species. The number and variation in band-patterns between diploid and polyploid populations was examined to answer the question of whether natural hybridization was involved. The trees that were generated from the ISSR data gave evidence on the levels of genetic variation within and between populations of both species, as well as allowing evaluation of whether polyploidy has arisen from autopolyploidy or allopolyploidy.

A total of 20 species of Eupatorium occur in North America. The primary species concept that was employed for classification of Eupatorium in North America was a morphological one. Species were recognized that could be consistently distinguished by features of the capitulescence, numbers of florets per head, shape and arrangement of the involucral bracts, and features of the leaves including size, shape, margin, and venation.

This study provides an example of the successful use of ITS, chloroplast DNA sequence data, and ISSR methods to reconstruct reticulate evolution in plants and confirms that molecular data can be highly informative, even when there is a limitation of morphological characters. The result provided the evidence to suggest that polyploidy populations within both Eupatorium rotundifolium and E. sessilifolium are allopolyploids. The evidence of molecular approaches was consistent with the explanation that these two species showed reticulate evolution. Polyploid populations may have arisen many times from multiple origins, and this was particularly clear in the polyploid, apomictic species Eupatorium godfreyanum. Because ITS and chloroplast DNA sequence data provided low levels of variation at the intraspecific level, the analysis was supplemented with data from the ISSR methods to investigate in detail the relationships among populations within each species and to assess more thoroughly the origin of polyploidy in each.