Phylogenetic Analysis of Pseudochlorodesmis Strains Reveals Cryptic Diversity Above the Family Level in the Siphonous Green Algae (Bryopsidales, Chlorophyta)1
Total Page：16
File Type：pdf, Size：1020Kb
J. Phycol. 45, 726–731 (2009) Ó 2009 Phycological Society of America DOI: 10.1111/j.1529-8817.2009.00690.x PHYLOGENETIC ANALYSIS OF PSEUDOCHLORODESMIS STRAINS REVEALS CRYPTIC DIVERSITY ABOVE THE FAMILY LEVEL IN THE SIPHONOUS GREEN ALGAE (BRYOPSIDALES, CHLOROPHYTA)1 Heroen Verbruggen,2 Caroline Vlaeminck Phycology Research Group and Center for Molecular Phylogenetics and Evolution, Ghent University, Krijgslaan 281, building S8, 9000 Ghent, Belgium Thomas Sauvage, Alison R. Sherwood Botany Department, University of Hawaii, 3190 Maile Way, Honolulu, Hawaii 96822, USA Frederik Leliaert and Olivier De Clerck Phycology Research Group and Center for Molecular Phylogenetics and Evolution, Ghent University, Krijgslaan 281, building S8, 9000 Ghent, Belgium The genus Pseudochlorodesmis (Bryopsidales) is and fuses in various patterns to form a broad composed of diminutive siphons of extreme mor- range of shapes upon which generic boundaries phological simplicity. The discovery of Pseudochlo- are based. The majority of the volume of the tubu- rodesmis-like juveniles in more complex Bryopsidales lar cells, usually referred to as siphons, consists of (e.g., the Halimeda microthallus stage) jeopardized the central vacuole, which is surrounded by a thin the recognition of this genus. Confronted with this layer of cytoplasm and a cell wall (Vroom and uncertainty, taxonomists transferred many simple Smith 2003). Cells are multinucleate, and cytoplas- siphons into a new genus, Siphonogramen. In this mic streaming transports organelles and nutrients study, we used a multimarker approach to clarify throughout the thallus (Littler et al. 1988, Drew the phylogenetic and taxonomic affinities of and Abel 1990). the Pseudochlorodesmis-Siphonogramen (PS) complex Thallus architecture is surprisingly diverse in the within the more morphologically complex bryopsida- Bryopsidales. In structurally simple genera, the lean taxa. Our analyses reveal a new layer of diver- tubular cell can be readily observed as its uniaxial sity largely distinct from the lineages containing the branches determine the main morphological charac- structurally complex genera. The PS complex shows ters. Well-known examples are the genera Bryopsis profound cryptic diversity exceeding the family and Derbesia, both common inhabitants of rocky level. We discuss a potential link between thallus shores worldwide. Less broadly known representa- complexity and the prevalence and profundity of tives featuring simple morphologies are the tropical cryptic diversity. For taxonomic simplicity and as a sand-dwelling genus Boodleopsis, the tropical reef first step toward clarifying the taxonomy of these alga Chlorodesmis, and Dichotomosiphon, the only bry- simple siphons, we propose to maintain Pseudochlo- opsidalean genus to have colonized freshwater habi- rodesmis as a form genus and subsume Siphonogra- tats (Hillis-Colinvaux 1984). Although much more men and Botryodesmis therein. sturdy and thick-walled, the genus Caulerpa has a very similar architecture consisting of branched Key index words: Botryodesmis; Bryopsidales; cryp- siphons that do not form complex tissues. Pseudo- tic diversity; molecular phylogenetics; Pseudochlo- chlorodesmis, the subject of this study, has one of the rodesmis; Siphonogramen; thallus complexity simplest morphologies among bryopsidalean algae. Abbreviations: AIC, Akaike information criterion; Its diminutive siphon, if branched at all, does so PS, Pseudochlorodesmis-Siphonogramen only a few times (e.g., Meinesz 1980b). Thalli grow out of rocky (often calcareous) substrates, under which a network of constricted siphons is embedded Representatives of the marine green algal order (Kraft 2007). Thalli rarely exceed a few millimeters Bryopsidales are characterized by siphonous archi- in length but can occur in extensive populations tecture (Hillis-Colinvaux 1984). Their thallus con- (Meinesz 1980b). sists of a single giant tubular cell that branches Several bryopsidalean lineages have evolved more complex thalli, in which individual siphons adhere or coalesce into more expansive tissues that form 1Received 25 August 2008. Accepted 28 January 2009. thick, multiaxial branches, stipes and blades, or 2Author for correspondence: e-mail [email protected]. 726 CRYPTIC DIVERSITY ABOVE THE FAMILY LEVEL 727 more amorphous structures. Such structures com- alignment of a broad range of bryopsidalean algae, monly consist of a medulla and a cortex, both including three Pseudochlorodesmis-Siphonogramen formed by branches of the same tubular cell. The strains. predominantly tropical marine genera Avrainvillea and Udotea form stipes and blades. In the cylindrical MATERIALS AND METHODS branches of the genus Codium, cortical siphons are Strains belonging to the Pseudochlorodesmis-Siphonogramen closely adjoined and swollen into utricles. A similar morphological complex (PS complex) were collected in Hawaii anatomy occurs in the tropical genus Halimeda,but and Mediterranean Spain and grown in sterile seawater until in this case, the thallus consists of segments that are they could be harvested for DNA extraction. DNA extraction attached to one another like compressed beads on a followed a CTAB protocol modified from Doyle and Doyle string. Other less well-known genera with complex (1987). The nuclear ribosomal 18S rRNA region and the morphologies include Penicillus, Rhipilia, and Rhipili- plastid genes rbcL and tufA were amplified and sequenced following previously published protocols (Fama` et al. 2002, opsis. Several tropical bryopsidalean genera with Kooistra 2002, Lam and Zechman 2006). complex thallus architectures deposit calcium car- A data set comprising 33 taxa representing all families of the bonate (aragonite) outside of their cell walls. suborder Halimedineae of the order Bryopsidales was com- It is generally believed that complex bryopsida- piled (Table S1 in the supplementary material). Three mem- lean thalli evolved from simpler ancestors, but this bers of the suborder Bryopsidineae were used as outgroups hypothesis has not been tested formally. Further- (Lam and Zechman 2006). The three loci were aligned separately. Alignment of the protein-coding genes rbcL and more, a phylogenetic study showed that representa- tufA was done by eye based on the corresponding amino-acid tives of the morphologically simple genus sequences. The 18S rDNA was aligned with Muscle v3.6 using Chlorodesmis were nested within a clade characterized standard parameters (Edgar 2004). The three alignments were by more complex thallus architectures, suggesting concatenated prior to phylogenetic analysis. Alignments are that at least some simple thalli may have evolved available from TreeBase (http://www.treebase.org) and the through reduction or neoteny (Kooistra 2002). first author’s Web site (http://www.phycoweb.net). Selection of a model of sequence evolution for phylogenetic The life cycles of most genera with simple thallus analysis was based on the Akaike information criterion (AIC) architectures have been thoroughly studied in labo- (Sullivan and Joyce 2005) implemented in the MrAIC.pl ratory culture (e.g., Rietema 1975, Kobara and Chi- program (Nylander 2004). Maximum-likelihood (ML) phylo- hara 1984). Much less is known about the life genetic analysis was carried out with PhyML (Guindon and history of anatomically complex genera. Consider- Gascuel 2003), using the model suggested by the AIC. The tree able amounts of information are available for a few search was started from a BioNJ tree (Gascuel 1997), and genera, but their life cycles have not been com- 1,000 nonparametric bootstrap replicates were carried out to assess statistical branch support (Felsenstein 1985). Bayesian pleted in laboratory culture (Meinesz 1980a). phylogenetic inference was carried out with MrBayes v.3.1.2 Several tropical genera (e.g., Caulerpa, Halimeda, (Ronquist and Huelsenbeck 2003). Two independent runs, Udotea) are known to engage in mass-spawning each consisting of four incrementally heated chains were events during which male and female gametes are carried out. Chains were run for 2,500,000 generations with a released into the water column and the parent thalli sample frequency of 1,000 and using default priors, heat die (holocarpy) (Clifton 1997). Germinated zygotes increments, and other settings. Convergence was assessed visually in Tracer 1.4 (Rambaut and Drummond 2007), and have been shown to form microthalli with simple an appropriate burn-in value was determined with the auto- thallus architecture, but the development of more mated method proposed by Beiko et al. (2006) with a sliding complex thalli from these microthalli has not been window of size 100. A majority-rule consensus tree was observed. generated from the post-burn-in trees using MrBayes’ sumt Since the study of the life cycle of the genus command. Halimeda, which reported a Pseudochlorodesmis-like life The results obtained were compared with the hypothesis that the PS complex forms a monophyletic lineage using likelihood- stage for the Mediterranean species H. tuna (Meinesz based hypothesis testing (Verbruggen and Theriot 2008). To 1972), Pseudochlorodesmis has often been regarded as a this goal, a phylogeny in which the PS complex was forced to be life stage of other, more complex genera. Assuming monophyletic was computed with Bayesian inference (analysis that P. furcellata was a life stage of H. tuna, Abbott settings as above). A Shimodaira-Hasegawa test was carried out and Huisman (2004) transferred all Pseudochlorodesmis to evaluate