18S RDNA AND EVOLUTION IN THE DASYCLADALES (CHLOROPHYTA) - MODERN LIVING FOSSILS

Phylogenetic relationships were inferred from parsimony and distance a nalyses of nuclear small-subunit ribosomal DNA sequences taken from 14 species representing 8 Of the 11 extant genera in the Dasycladales. O f 1733 aligned positions, 412 (23.8%) were variable and 251 (61%) of t hose were phylogenetically informative within the Dasycladales. Second ary structure was analyzed and taken into account during all phases of data analysis. Robustness of the trees was assessed using bootstrap a nalysis and g, statistics of tree-length decay. Strongly supported bra nches were robust to all methods of analysis regardless of weighting s chemes used. The secondary structure of the 18S within the Dasycladale s 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 mo lecular clock was calibrated from the dasyclad fossil record and sugge sts a radiation of the Acetabulariaceae at 120 +/- 3O million years (M a) ago and the Dasycladaceae 215 +/- 40 Ma ago. The split of the two l ineages from a shared ancestor is estimated at 265 +/- 50 Ma ago. With in the Dasycladaceae, Neomeris and Cymopolia are sister taxa, as are B atophora and Chlorocladus. Bornetella groups with the Neomeris and Cym opolia clade in 78% of the bootstrap replicates. Re Relationships amon g the Acetabulariaceae show that Acetabularia and Polyphysa do not for m monophyletic groups as presently circumscribed. No evidence indicate s that Acicularia is the oldest genus. Halicoryne, Chalmasia, and Dasy cladus were not included in the analysis. Molecular data provide afres h background perspective from which to discuss the evolution of one of the most ancient lineages of green plants.