18S rDNA AND EVOLUTION IN THE DASYCLADALES (CHLOROPHYTA): MODERN LIVING FOSSILS1

Phylogenetic relationships were inferred from parsimony and distance analyses of nuclear small‐subunit ribosomal DNA sequences taken from 14 species representing 8 of the 11 extant genera in the Dasycladales. Of 1733 aligned positions, 412 (23.8%) were variable and 251 (61%) of those were phylogenetically informative within the Dasycladales. Secondary structure was analyzed and taken into account during all phases of data analysis. Robustness of the trees was assessed using bootstrap analysis and g1statistics of tree‐length decay. Strongly supported branches were robust to all methods of analysis regardless of weighting schemes used. The secondary structure of the 18S within the Dasycladales agrees with that of other green algae with the exception of a shared deletion in stemloop E10‐1 (ca. 13 nucleotides long), which provides additional support for the uniqueness of this monophyletic group. A molecular clock was calibrated from the dasyclad fossil record and suggests a radiation of the Acetabulariaceae at 120 ± 30 million years (Ma) ago and the Dasycladaceae 215 ± 40 Ma ago. The split of the two lineages from a shared ancestor is estimated at 265 ± 50 Ma ago. Within the Dasycladaceae,NeomerisandCymopoliaare sister taxa, as areBatophoraandChlorocladus. Bornetellagroups with theNeomerisandCymopoliaclade in 78% of the bootstrap replicates. Relationships among the Acetabulariaceae show thatAcetabulariaandPolyphysado not form monophyletic groups as presently circumscribed. No evidence indicates thatAciculariais the oldest genus.Halicoryne, Chalmasia, andDasycladuswere not included in the analysis. Molecular data provide afresh background perspective from which to discuss the evolution of one of the most ancient lineages of green plants.