Date of Award
Doctor of Philosophy (PhD)
Elizabeth A. Zimmer
Since the end of the last century, the predominant theories of the early radiation of the angiosperms have been that the earliest flowering plants were most similar to the present-day Magnoliidae (sensu Takhtajan, 1969). This position has been adopted by many, though there are some who suggest that the base of angiosperm radiation lies within the monocots (Burger, 1981) or a combination of monocots and dicots (Burger, 1977; Donoghue and Doyle, 1989a, 1989b). Many different ancestors to the angiosperms themselves have been proposed including, at one time or another, most of the extant gymnosperms, extinct gymnosperms and the extinct seed ferns. Morphologically-, cytologically- and phytochemically-based classifications have not provided unequivocal phylogenies of the angiosperm lineages, although recent cladistic treatments of morphological characters by Crane (1985) and Donoghue and Doyle (1989a) provide a logical framework for testing molecular genealogies. The most fundamental comparison between homologous molecules of different species is a comparison of the primary nucleotide structure. In this dissertation, I report on comparisons of the primary structure of the nuclear-encoded cytoplasmic ribosomal RNAs (rRNAs) to produce phylogenetic hypotheses for the extant angiosperms and other seed plant lineages. Computer-assisted phylogenetic analyses based on the comparisons of 1700 nucleotides from five regions of the nuclear-encoded cytoplasmic 18S rRNA and three regions of the nuclear-encoded cytoplasmic 26S rRNA from 46 angiosperm taxa, 12 gymnosperm taxa and two seedless vascular plants (as outgroups), suggest that: (1) The seed plants (gymnosperms and angiosperms) are a natural (monophyletic) group; (2) The angiosperms arose from within the gymnosperms and are a natural group; (3) The Gnetales are a coherent group with tenuous support as the sister group of the angiosperms; (4) The earliest angiosperm divergences involve the paleoherbs of Donoghue and Doyle (1989a, 1989b), i.e., the Piperales (Piperaceae and Saururaceae), the Nymphaeales (Nymphaeaceae, Cabombaceae, Barclayaceae, but not Ceratophyllaceae or Nelumboaceae) and the monocots; (5) Both the monocots and dicots are paraphyletic groups.
Hamby, Robert Keith, "Ribosomal RNA and the Early Evolution of Flowering Plants." (1990). LSU Historical Dissertations and Theses. 5053.