Date of Award
Doctor of Philosophy (PhD)
Russell L. Chapman
Cladistic analyses of non-molecular and nuclear-encoded rRNA sequence data provided the basis for hypotheses of relationships for the green algal class Ulvophyceae. Non-molecular data rooted with Chara support hypotheses which group the Chlorophyceae and Pleurastrophyceae with ulotrichalean and ulvalean Ulvophyceae. Analyses of rRNA sequence data group the siphonous and siphonocladous Ulvophyceae (i.e. Caulerpales, Siphonocladales, and Dasycladales) with the Chlorophyceae and Pleurastrophyceae. Although hypotheses supported by these independent data sets are incongruent, they suggest that the Ulvophyceae is not monophyletic. Based on rRNA sequences, pleurastrophycean taxa, which, like the Ulvophyceae, possess a counter-clockwise arrangement of flagellar basal bodies, are more closely related to the Chlorophyceae (which possess clockwise basal bodies) than to the Ulvophyceae. Thus, counter-clockwise basal body orientation does not diagnose a monophyletic group. Parsimony analyses to assess the strength of these hypotheses, including bootstrap, decay index, and character distributions suggest that basal divergences exhibit little character support and lead to ambiguous rooting of the phylogeny. Data randomization tests, however, clearly suggest that there is considerable signal in the data. Examination of ordinal relationships within the siphonous and siphonocladous Ulvophyceae revealed that the Dasycladales is the sister group to the Caulerpales with the Siphonocladales representing a basal lineage. Although inconsistent with hypotheses based on ultrastructural features, this hypothesis is consistent with recently reported fossil evidence that extended the minimum age of the siphonocladalean lineage to ca. 700 million years (concurrent with the oldest dasycladalean fossils). Relative rates of evolutionary divergence between sister taxa (inferred by comparing the number of nucleotide changes along internodes leading to terminal taxa) are higher in the Caulerpales and Dasycladales clade than in the Siphonocladales. Congruence of phylogenetic hypotheses with biogeographic distributions were also explored. Two lineages are identified in the Caulerpales; one with genera of strictly tropical distribution and another with more widespread taxa. The sister group, the Dasycladales, is also restricted to the tropics, suggesting that this is the primitive distribution pattern. The Siphonocladales exhibit a similar pattern: derived cosmopolitan clade and basal tropical genera. Thus, these data support the hypothesis that these algae originated in ancient tropical oceans.
Zechman, Frederick William, "Phylogenetic Systematics of the Ulvophyceae (Chlorophyta) Based on Cladistic Analyses of Ribosomal RNA Genes and Morphology." (1992). LSU Historical Dissertations and Theses. 5368.