Systematic and evolutionary investigation of the North American {\it Crepis\/} agamic complex

Three nested phylogenetic analyses of chloroplast DNA restriction site variation are presented for taxa in the subtribe Lactuceae (Asteraceae). The tribal level analysis includes sixty taxa representing forty-two genera in the Lactuceae. Twelve-hundred-sixty mutations are scored and parsimony analysis yields eight equally parsimonious trees. These reveal that many deeper nodes within the Lactuceae are weakly supported, which may indicate rapid diversification of lineages during this phase of the tribe\u27s evolution. Relationships supported by chloroplast DNA data are compared with three existing classifications for the tribe. While there is some support for each of the previous treatments, none is completely congruent with relationships revealed in this analysis. At the second taxonomic level, relationships among species of Crepis and its close relatives are examined. Two-hundred-sixty-six mutations are detected and phylogenetic analysis produced fifty-four equally parsimonious trees. These differ little from one another and all agree that the genus Crepis is not monophyletic. Basal lineages include members of Crepis as well as species of Lapsana and Rhagadiolus. This analysis does however support the monophyly of section Psilochaenia, the only native North American section of the genus. This section was hypothesized to have originated via multiple hybridization and polyploidization events, for which I find no evidence. The third level of analysis is within species and populations of the North American Crepis. This group includes nine species that form an asexual polyploid complex, and a diploid sexual species, C. runcinata. Crepis runcinata occupies the basal position within the North American group. Phylogenetic analysis of variation within and among species and populations in the agamic complex reveals that morphological variation does not correlate well with chloroplast DNA variation, and that multiple morpotypes within populations have different chloroplast DNA haplotyes. Observed patterns suggest that multiple origins of polyploids is the principal mechanism for generating variation within the agamic complex. A fourth chapter reviews the literature on the relationship between gametophytic apomixis and polyploidy in plants and points out that the two phenomena co-occur because they both rely on the formation of unreduced female gametes.