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

Abstract

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 g1 statistics 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, Neomeris and Cymopolia are sister taxa, as are Batophora and Chlorocladus. Bornetella groups with the Neomeris and Cymopolia clade in 78% of the bootstrap replicates. Relationships among the Acetabulariaceae show that Acetabularia and Polyphysa do not form monophyletic groups as presently circumscribed. No evidence indicates that Acicularia is the oldest genus. Halicoryne, Chalmasia, and Dasycladus were 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.

A multi-locus time-calibrated phylogeny of the siphonous green algae

“…One thing worth mentioning is that the close relationship between Batophora and Chlorocladus, which was suggested by previous molecular phylogenetic studies (Olsen et al, 1994;Zechman, 2003), is confirmed by our multi-locus dataset. As reported in the aforementioned papers, this finding contradicts the traditional tribe-level classification by Berger and Kaever (1992 (Table 3).…”

“…Nonetheless, our results indicate that the fossil record of certain aspects of the evolutionary history of the Dasycladaceae may be more incomplete than previously suspected (see Kenrick and Li, 1998). The node age estimates obtained in our relaxed molecular clock analysis are older than those recovered by Olsen et al (1994). Causes for this discrepancy are difficult to pinpoint because differences between the studies are manifold: the molecular clock method, calibration technique, molecular dataset and selection of fossils all differ.…”

“…As a consequence, node age estimates from our analysis with relaxed molecular clock models should match the true divergence times more closely. The calibration method differs in that fossils are used as minimum age estimates for stem groups in our analyses, whereas they were used as point estimates to determine the rate of 18S rDNA evolution in Olsen et al (1994). Finally, we use a more extensive list of fossil calibration points, including some recently described fossils that represent older occurrences of some dasyclad clades.…”

“…The present dasycladalean diversity consists of 37 species belonging to 10 genera and the two families Dasycladaceae and Polyphysaceae (Berger, 2006). Molecular phylogenetic studies have shown that the Polyphysaceae arose from the Dasycladaceae, leaving the latter paraphyletic (Berger et al, 2003;Olsen et al, 1994;Zechman, 2003).…”

“…Furthermore, biogeographic interpretations are difficult without reference to a temporal framework (Verbruggen et al, 2005(Verbruggen et al, , 2007. So far, the fossil record has been used on one occasion to calibrate a dasycladalean molecular phylogeny in geological time (Olsen et al, 1994). In the years that have passed since the publication of this study, however, several important discoveries have been made in dasyclad paleontology (e.g., Kenrick and Li, 1998;LoDuca et al, 2003) and more advanced calibration methods have been developed (reviewed by e.g., Verbruggen and Theriot, 2008).…”