Phylogeny of the Chlorophyceae With Special Reference to the Sphaeropleales: A Study of 18S and 26S rDNA Data1


Mark A. Buchheim2, Faculty of Biological Science and the Mervin Bovaird Center for Molecular Biology and Biotechnology, The University of Tulsa, 600 South College Avenue, Tulsa, Oklahoma 74104-3189,
Eugenia A. Michalopulos3, Department of Biological Science, The University of Tulsa, 600 South College Avenue, Tulsa, Oklahoma 74104-3189
and
Julie A. Buchheim, Department of Biological Science, The University of Tulsa, 600 South College Avenue, Tulsa, Oklahoma 74104-3189


1received accepted .

2mark-buchheim@utulsa.edu

3Current Address: Hamon Center for Therapeutic Oncology, University of Texas Southwestern Medical Center, Dallas, TX 75390

Running title: Phylogeny of the Chlorophyceae
Abstract
Ultrastructural analyses of the flagellar apparatus suggested that Sphaeroplea, Atractomorpha, the Hydrodictyaceae, and the Neochloridaceae, all of which produce biflagellate motile cells with directly-opposed (DO) basal bodies, are allied in an order Sphaeropleales. Recent studies of 18S rDNA sequence data supported an alliance of the DO group, but no data from Sphaeroplea and its allies were included. This investigation presented a test of the phylogenetic hypothesis suggested by the flagellar apparatus evidence using sequence data from the nuclear-encoded, small-subunit rDNA (18S) and large subunit rDNA (26S) genes, combined with additional taxon sampling. Results from phylogenetic analyses weakly supported monophyly of biflagellate DO taxa and indicated that pyrenoids with cytoplasmic invaginations are present in numerous, distinct lineages. Analysis of both molecular data sets supported a class Chlorophyceae comprised of at least six major groups that generally correspond to currently recognized orders or families: Chaetophorales, Chaetopeltidales, Chlamydomonadales, Sphaeropleales, Sphaeropleaceae and Oedogoniales. In addition, Cylindrocapsa, Elakatothrix, Treubaria and Trochiscia formed a seventh chlorophycean clade that is new to science. This investigation demonstrated that the 26S rDNA gene provides more phylogenetic signal, per unit sequence, than the 18S rDNA gene and that combined analysis yields topologies with more robust support than independent analysis of either data set.

Key Index Words: 18S rDNA, 26S rDNA, Ankyra, Atractomorpha, Chaetopeltidales, Chaetophorales, Chlamydomonadales, Cylindrocapsa, Elakatothrix, flagellar apparatus, Oedogoniales, Ourococcus, phylogeny, Pseudoschroederia, pyrenoid, Schroederia, Sphaeroplea, Sphaeropleales, Treubaria, Trochiscia

Abbreviations: CCW = counter-clockwise, CW = clockwise, DO = directly-opposed, NNI = nearest-neighbor-interchange, ME = minimum evolution, PAUP = phylogenetic analysis using parsimony, TBR = tree-bisection-reconnection.
Introduction
The azoosporic, filamentous, green algal genus Sphaeroplea is distinctive in exhibiting both multinucleate (coenocytic) cells and oogamous sexual reproduction (Fritsch 1929). Traditional classifications based on comparative gross morphology placed Sphaeroplea in one of three orders, the Ulotrichales (Fritsch 1929, Smith 1950, Ramanathan 1964, Christensen 1966), Cladophorales (Oehlkers 1956, Fott 1971), or Sphaeropleales (Pascher 1931, Prescott 1951, Bourrelly 1990, Round 1971, Cáceres and Robinson 1980, Silva 1982, Mattox and Stewart 1984, Bold and Wynne 1985). Ultrastructural investigations of Sphaeroplea (Cáceres and Robinson 1980, 1981) rendered obsolete any classifications linking Sphaeroplea with Ulothrix or Cladophora. The latter two are now firmly regarded as belonging to the class Ulvophyceae (sensu Mattox and Stewart 1984), whereas; Sphaeroplea is placed in the Chlorophyceae (sensu Mattox and Stewart 1984). Thus, the order Sphaeropleales remained as the only reasonable classification scheme. Although a narrowly circumscribed Sphaeropleales was readily defensible, this taxonomic assessment offered little insight into the allies of Sphaeroplea within the Chlorophyceae.
Ultrastructural investigations cited similarities between sphaeropleacean taxa (including Atractomorpha [Hoffman 1983, 1984]) and selected members of the Chlorococcales (Cáceres and Robinson 1980, 1981, Hoffman 1983, 1984a, Buchheim 1988, van den Hoek et al. 1995) as indicative of an alliance. Studies of the flagellar apparatus (Cáceres and Robinson 1981, Hoffman 1984b, Buchheim 1988, Buchheim and Hoffman 1985, 1986, 1987) demonstrated that Atractomorpha and Sphaeroplea possess biflagellate motile cells (male gametes) with basal bodies that are directly opposed (DO). The DO condition of biflagellate motile cells (gametes or zoospores) also was found in members of the Hydrodictyaceae (Wilcox and Floyd 1988), in the Neochloridaceae (Watanabe and Floyd 1989a), in the genus Characiopodium (Floyd et al. 1993), and in the genus Bracteacoccus (Watanabe and Floyd 1992). The DO condition of quadriflagellate zoospores was discovered in both basal body pairs of Chaetopeltis and allies (O'Kelly and Floyd 1984, O'Kelly et al. 1994) and in the upper pair of basal bodies in zoospores of chaetophoralean taxa (Manton 1964, Melkonian 1975, Moestrup 1978, Floyd et al. 1980, Floyd and Hoops 1980, Bakker and Lokhorst 1984, Watanabe and Floyd 1989b).
Deason et al. (1991) proposed an emended order Sphaeropleales that included all biflagellate DO taxa (but excluded taxa that possess DO basal bodies in quadriflagellate zoospores). O'Kelly et al. (1994) have since argued for recognizing another distinct order of DO taxa, the Chaetopeltidales. The Chaetopeltidales (sensu O'Kelly et al. 1994) include all taxa exhibiting quadriflagellate zoospores that possess an exclusively DO architecture (both pairs of basal bodies are DO). In support of a new order, Chaetopeltidales, O'Kelly et al. (1994) cited the presence of body scales on the zoospores of Chaetopeltis, Hormotilopsis, and Planophila as indicative of the distinctiveness of the group.
Phylogenetic studies of 18S rDNA sequence data have been used to test hypotheses derived from the ultrastructural studies. The earliest of these investigations demonstrated a close alliance among Pediastrum, Hydrodictyon, Neochloris, and Characiopodium, all of which possess biflagellate motile cells (gametes or zoospores) exhibiting DO basal bodies (Wilcox et al. 1992 and Lewis et al. 1992). In a more recent study, Lewis (1997) showed that molecular phylogenetic analysis of 18S rDNA data supports an alliance of the biflagellate DO taxon, Bracteacoccus, with the biflagellate DO taxa cited in previous 18S rDNA investigations (Wilcox et al. 1992, Lewis et al. 1992). In the latest investigation of 18S rDNA data, Booton et al. (1998b) found support for an alliance of the Chaetopeltidales (sensu O'Kelly et al. 1994) with the biflagellate DO taxa. Thus, the results from analyses of 18S rDNA data support the hypothesis of DO monophyly (exclusive of the Chaetophorales). As Lewis (1997) pointed out, sequence data from the critical DO taxa, Sphaeroplea and Atractomorpha, are needed to complete the test of the DO hypothesis. Furthermore, the 18S rDNA gene may not provide enough resolution at some of the critical nodes within the Chlorophyceae (Booton et al. 1998a, b) and limited taxon sampling within the Chlorophyceae may be influencing the resolution. Therefore, a new gene to augment the 18S rDNA data and additional sampling of chlorophycean taxa are needed to further our understanding of relationships within the class.
This investigation has three principal goals. The first of these goals is to test the DO monophyly hypothesis (Deason et al. 1991, Wilcox et al. 1992, Lewis et al. 1992, Lewis 1997, Booton et al. 1998a, b) by examining three taxa that exhibit the DO architecture in biflagellate motile cells. Atractomorpha echinata (Hoffman 1984b), Sphaeroplea robusta (Buchheim and Hoffman 1986, Buchheim 1988), and Sphaeroplea soleirolii v. crassisepta have been documented as taxa bearing biflagellate motile cells with DO basal bodies (Buchheim 1988). The second goal of this investigation is to expand data sampling by including new data from the large-subunit of the nuclear-encoded, ribosomal RNA (26S rDNA) gene. A 2.2 kb segment at the 5' end of the 26S rDNA gene was selected for study because early investigations of partial sequences indicated that the 26S rDNA gene was more variable than the 18S gene (Buchheim et al. 1990b, Kantz et al. 1990, Zechman et al. 1990, Larson et al. 1991). More recent investigations of the 26S rDNA gene from selected trebouxiophyte green algae (Friedl and Rokitta 1997) and seed plants (Stefanovic et al. 1998, Kuzoff et al. 1998) indicated that this gene is likely to provide more phylogenetic information than the 18S rDNA gene. The final goal of this investigation is to sample additional chlorophycean taxa that exhibit morphological features which link them to sphaeropleacean taxa or that will provide a more complete taxonomic context in which to interpret sphaeroplealean and chlorophycean diversity. As a consequence of this new data-sampling scheme, the number of sites that can provide new character data for the green algae will be more than doubled. Furthermore, the new taxon-sampling scheme will provide the most comprehensive, molecular phylogenetic study of the Chlorophyceae (sensu Mattox and Stewart 1984) to date.

Materials and Methods
Taxon Selection
Fifty ingroup taxa were included in the phylogenetic analyses (Table 1). New 18S rDNA sequence data for Ankyra judayi, Aphanochaete magna, Atractomorpha echinata, Cylindrocapsa geminella, Elakatothrix viridis, Ourococcus multisporus, Pseudoschroederia antillarum, Schizomeris leibleinii, Schroederia setigera, Sphaeroplea soleirolii v. crassisepta, Sphaeroplea robusta, Treubaria schmidlei, Treubaria setigera, Trochiscia hystrix, and Uronema belkae are presented in this investigation (Table 1). Included are published 18S rDNA sequences (see Table 1) from other members of the Sphaeropleales (sensu Deason et al. 1991), from the DO genera Bracteacoccus, Chaetopeltis, Hormotilopsis and Planophila, from the Chlamydomonadales (including colonial forms), from the Chaetophorales, and from the Oedogoniales. Each of these taxa is generally regarded as a member of the class Chlorophyceae (sensu Mattox and Stewart 1984). New partial 26S rDNA sequence data for all of the 50 ingroup taxa were completed for this project (Table 1). With only a few exceptions (Carteria crucifera, Chlamydomonas reinhardtii, Fritschiella tuberosa, and Oedogonium cardiacum), the same isolates were used to generate sequencing templates for both the 18S and 26S rDNA data.

DNA Extraction and Preparation of Sequencing Templates
Genomic DNA was obtained using extraction protocols described previously (Buchheim and Chapman 1992). Double-stranded DNA sequencing templates were obtained by symmetrically amplifying genomic DNA using the Polymerase Chain Reaction (PCR). Amplification of the 18S gene followed previous work (Buchheim et al. 1997a). The flanking primers used to amplify a portion of the 26S gene, ITS-4rc and 26F, are described by White et al. (1990) and Hamby et al. (1988), respectively (see Table 2). In some cases, LS-18 (Table 2) was substituted for 26F as an amplification primer.

Manual Sequencing
Sequence data for two of the ingroup taxa (Sphaeroplea robusta and Atractomorpha echinata) were obtained by using the protocols and reagents for cycle-sequencing that accompany the AmpliCycle™ cycle sequencing kit (Perkin-Elmer). All manual sequencing reactions used in the present investigation have been described previously (Buchheim et al. 1997a).

Automated Sequencing
New sequence data from the remaining taxa (see Table 1) were obtained with the protocols and reagents for cycle-sequencing that accompany the four-color PRISM™ reagent cycle sequencing kit (Perkin-Elmer) designed for use in ABI automated DNA sequencing systems. All automated sequencing reactions used in the present investigation have been described previously (Buchheim et al. 1997a). Sequencing primers for the 18S rDNA gene have been described previously (Hamby et al. 1988, Buchheim et al. 1997a). Sequencing primers for ca. 2200 base pairs on the 5' end of the 26S rDNA gene are presented in Table 2.

Sequence Alignments
Previous work (Buchheim et al. 1997a) served as the starting point for all 18S rDNA alignments. Alignments from Michalopulos (1998) served as the starting point for all 26S rDNA alignments. The manually aligned sequences were edited using SeqApp (Gilbert 1994). One hundred sites were excluded from the phylogenetic analyses of 18S data and 170 sites were excluded from the phylogenetic analyses of 26S data. The exclusion sets are characterized by blocks of sequence data that are not clearly alignable (i.e., exhibit questionable homology due to base substitution coupled with sequence length heterogeneity) or correspond to a flanking primer region. The data sets and trees have been deposited in TreeBase.

Phylogenetic Analyses of Independent Data Sets
Three methods of phylogenetic reconstruction were initially employed for independent analyses of 18S rDNA and 26S rDNA data: character analysis using maximum parsimony (MP) optimality criteria, a distance matrix approach using minimum evolution (ME) optimality criteria, and maximum likelihood. Analyses were conducted using PAUP* (version 4.0b4a, Swofford [2000] or version 4.0b6, Swofford [2001]).
Maximum Parsimony Method. Tree searches for MP analyses were conducted heuristically using the tree-bisection-reconnection (TBR) option. To increase the probability of finding all islands of most parsimonious trees, the order of taxon addition was randomized 50 times. Bootstrap values (Felsenstein 1985) from 1000 resamplings using heuristic, nearest-neighbor-interchange (NNI) searches with simple taxon addition were calculated for each set of data. All characters were regarded as unordered (Fitch 1971). No differential character weighting was employed.
Distance Matrix Method. Modeltest 3.0b (Posada and Crandall 1998) was used to test the goodness-of-fit of DNA substitution models against the various data sets (18S rDNA, 26S rDNA, and combined 18S and 26S rDNA data) to be analyzed. In addition to identifying the model of DNA substitution that best fits a data set, Modeltest 3.0b (Posada and Crandall 1998) exploits PAUP* (Swofford 2000, 2001) to estimate the percent of invariant sites (I) and the shape parameter (G) for calculations assuming a discrete gamma distribution of site-to-site variation (Yang 1994). Based on results from the hierarchical series of likelihood ratio tests for 56 different models of nucleotide substitution (Posada and Crandall 1998), distance matrices for ME analysis were constructed using the general time-reversible (GTR) model (Lanave et al. 1984, Rodríquez et al. 1990) as implemented in PAUP* (Swofford 2000, 2001) and I and G were estimated from the data. Tree searches for ME analyses were conducted heuristically using the TBR option. To increase the probability of finding all islands of optimal trees under the ME criterion, the order of taxon addition was randomized 50 times. Bootstrap values (Felsenstein 1985) from 1000 resamplings using heuristic NNI searches with simple taxon addition were calculated for the data. Starting trees for each replicate were obtained by the neighbor-joining method.
Maximum Likelihood. Parameter estimates (I and G) and rate matrix estimates derived from Modeltest 3.0b (Posada and Crandall 1998) analyses were used for heuristic search strategies using maximum likelihood. Based on results from the likelihood ratio tests (Posada and Crandall 1998), the general time-reversible (GTR) or the Tamura-Nei (TrN) models of nucleotide substitution, as implemented in PAUP* (Swofford 2000, 2001), were used for ML analyses. Tree searches for ML analyses were conducted heuristically using the TBR option. Bootstrap values (Felsenstein 1985) from 100 resamplings using heuristic NNI searches with simple taxon addition were calculated for the data. Starting trees for each replicate were obtained by the neighbor-joining method.
Rooting. The outgroup method was used to root all trees. Sequence data from Fusochloris perforata and Chlorella ellipsoidea (Table 1) were used to root the trees. These trebouxiophycean taxa (sensu Friedl 1995) have been resolved as part of a sister group to the Chlorophyceae (sensu Mattox and Stewart 1984) in previous studies of 18S rDNA data (Friedl 1995, Melkonian and Surek 1995).

Combined Data Analysis
In order to assess the utility of combining data, statistical comparisons (Kishino-Hasegawa [ML and MP], Templeton, and Winning Sites) of trees derived from independent analyses of 18S and 26S rDNA were conducted using PAUP* 4.0b4a (Swofford 2000). In addition, the partition homogeneity test (Swofford 2000, see also Farris et al. 1994 who use the name "incongruence length difference" test) was applied to the 18S and 26S rDNA data. Lastly, the topologies of trees from independent analyses were compared to determine the number of shared nodes.

Tree Comparisons
Pairwise comparison of optimal trees from all analyses were undertaken using results from Kishino-Hasegawa (likelihood and parsimony), Templeton, and Winning Sites tests as recorded using PAUP* 4.0b4a (Swofford 2000). Similar tree comparisons (Kishino-Hasegawa [likelihood and parsimony], Templeton, and Winning Sites tests) were conducted to assess the statistical significance of differences between optimal trees and alternative phylogenetic hypotheses that were input to PAUP as constraint trees.


Results
Phylogenetic Information Content
18S rDNA Data. A total of 1631 aligned sites were included in the phylogenetic analyses of 18S rDNA. Aligned sequence data from all ingroup taxa yield 527 variable sites (excluding gap-only sites) of which 396 are informative for parsimony analysis. A summary of the variability in the 18S rDNA and 26 rDNA is presented in Table 3.
26S rDNA Data. A total of 2053 aligned sites were included in the phylogenetic analyses of 26S rDNA. Aligned sequence data for the ingroup include 940 variable sites (excluding gap-only sites) of which 680 are phylogenetically informative. The partial 26S rDNA sequences used in this investigation include the structural domains I-IV and a portion of domain V (Raué et al. 1988). A summary of the variability in the 26S rDNA (and 18S rDNA) studied in this investigation is presented in Table 3.
Introns. A putative group I intron was revealed by sequence analysis of the 18S rDNA gene from Schroederia setigera (UTEX LB 2454). The intron extends from position 1129 to position 1575 of AF277650. A putative group I intron was also revealed by sequence analysis of the 26S rDNA gene from Neochloris aquatica (UTEX 138). The intron extends from position 883 to position 1258 of AF277653.

Phylogenetic Reconstruction of Independent Data Sets
MP Analyses. The MP analysis of 18S rDNA data yielded four, equally parsimonious resolutions (Fig. 1) and the MP analysis of 26S rDNA data yielded three, equally parsimonious resolutions (Fig. 2). The results from MP analysis of the two data sets differ in topology in a number of general cases. The 18S data and the 26S data differ in the arrangement of taxa in a clade that includes Ascochloris, Chlamypodium, Chlorococcum, Haematococcus, and Protosiphon. In addition, the two data sets differ in the relative position of the Carteria radiosa clade (Car. obtusa, Car. radiosa, Car. sp.), the Chaetopeltidales clade (Chaetopeltis, Hormotilopsis, and Planophila), the Oedogoniales clade (Bulbochaete and Oedogonium), the Cylindrocapsa clade (Cylindrocapsa, Elakatothrix, Treubaria, and Trochiscia), and the Sphaeroplea clade (Ankyra, Atractomorpha, and Sphaeroplea). Lastly, the two data sets differ in the position of Pseudoschroederia relative to Schroederia and the position of Schizomeris relative to Aphanochaete. Overall, the trees from MP analyses of 18S and 26S data have 29 of 47 ingroup nodes in common with one another.
ME and ML Analyses. The results from ME and ML analysis of 18S rDNA data (not shown) differ from the results of ME and ML analysis of 26S rDNA data (not shown) in the same general cases as described for the MP analysis (see above). The trees from ME analysis of 18S and 26S data share 28 of 47 ingroup nodes. The trees from ML analysis of 18S and 26S data share 29 of 47 ingroup nodes.
Comparing Trees From Different Methods. The results from MP, ME and ML analysis of the 18S rDNA data differed principally in the relative position of the Chaetophorales clade, the Cylindrocapsa clade, and the two Carteria clades. The results from MP, ME and ML analysis of 26S rDNA data differ principally in the relative position of the Sphaeroplea clade, the Cylindrocapsa clade, the Carteria crucifera clade, and the Oedogoniales clade. None of the nodes supporting the various alliances from different methods of analysis exhibited robust (>75) bootstrap support.

Combined Analysis
Comparing Data Sets. Statistical comparisons of trees derived from independent analyses of 18S and 26S rDNA data indicated that the topologies from the two are significantly different (P = 0.05) regardless of the method of analysis or the context of the comparison. In addition, partition homogeneity tests (Swofford 2000, Farris et al. 1994) revealed significant incongruence (P = 0.01) between 18S and 26S rDNA data sets suggesting that combination may result in a decrease in resolution when compared to results from independent analysis. However, empirical studies (Cunningham 1997) suggested that congruence tests can over-estimate incongruence when, in fact, combined analysis of the same data yields improved phylogenetic accuracy. Recent studies have corroborated this observation (e.g., Gatesy et al. 1999, Smith 2000).
As noted above, consensus analyses reveal that trees from different data sets share a majority of nodes. Moreover, most of the conflicting nodes in all sets of comparisons exhibit low bootstrap support in one or both data sets. Long branches (e.g., the Sphaeroplea clade, Figs. 1, 2) may be confounding the statistical tests of congruence. When the partition homogeneity test is administered after removal of some taxa that possess long branches in both independent analyses (i.e., the Sphaeroplea clade, the Cylindrocapsa clade, the Oedogoniales clade, and the Carteria radiosa clade), the resulting probability value (P = 0.32) indicates that the two data sets are congruent. Because of these observations, two sets of combined analyses were completed. Combined analysis I was conducted using all ingroup taxa. Model parameters for combined analyses using ME and ML criteria were estimated using Modeltest and PAUP. In addition to MP, ME and ML analyses, the substitution rate calibration (SRC) method of Van de Peer (Van de Peer et al. 1993, 1996), as implemented in TREECON, was used to minimize the impact of long-branch attraction effects of some lineages. Combined analysis II was conducted using the same set of criteria as combined analysis I, but including only a subset of all ingroup taxa in which potential long-branch lineages were removed. A total of 100 bootstrap replicates were completed for the SRC analysis.
Combined Analysis I: All Taxa. Results from MP (Fig. 3), ME (not shown), ML (not shown) and SRC (Fig. 4) analysis of combined 18S and 26S rDNA data collectively differ in (1) the arrangement of taxa in a clade that includes Ascochloris, Chlamypodium, Chlorococcum, Haematococcus, and Protosiphon, (2) the relative position of the Carteria radiosa clade, (3) the relative position of the Cylindrocapsa clade, (4) the relative position of the Oedogoniales clade, and (5) the position of Schizomeris relative to Aphanochaete.
Combined Analysis II: Reduced Data Sets. Results from MP combined analyses, in which all members of the Cylindrocapsa clade, the Sphaeroplea clade, the Oedogoniales, and the Carteria radiosa clade were omitted, are presented in Fig. 5. The tree from MP combined analysis II differs from the ME and ML trees (not shown) only in the relative position of some members within the Sphaeropleales clade and the relative position of some members within the Chlamydomonadales clade (see Fig. 5). Bootstrap values from MP, ME and ML combined analysis II are generally larger than bootstrap values from independent MP, ME and ML analyses of 18S and 26S data (Fig. 5).

Discussion
Comparing Data Sets
The results from a study of variability in the 18S and 26S rDNA data sets (Table 3) demonstrate that the 26S data are providing a higher density of both variable and parsimony-informative sites than the 18S data. Moreover, a greater percentage of informative sites in the 26S data have no detected homoplasy as compared to the 18S rDNA data (Table 3). Although the topologies from independent analyses of 18S (Fig. 1) and 26S (Fig. 2) data are significantly different from one another (P = 0.05), the topologies from combined analysis I generally are not significantly different (P > 0.05 [up to P = 0.8335]) from the 26S topologies. In contrast, most of the 18S topologies (e.g., Fig. 1) are significantly different (P = 0.05) from the topologies generated by combined analysis I (e.g., Fig. 3).
These statistical comparisons indicate that the topologies from combined analysis I have the most support from the 26S data and that the 18S data are, at least in part, being overwhelmed. However, none of the cases of incongruence between the 18S topologies and the topologies from combined analysis I involves branches from the 18S trees that possess robust (>75) support as assessed by the bootstrap. Furthermore, the results from combined analysis II (Fig. 5) reveal substantial topological congruence between 18S and 26S data when putative long-branch clades are removed from both sets of data. The partition homogeneity test (P = 0.32) and a comparison of bootstrap values from independent and combined data analyses (Table 4) confirm this assessment of high congruence between 18S and 26S data sets. The data in Table 4 demonstrate that combining data resulted in an enhancement of phylogenetic signal in comparison to results from analysis of independent data sets.

Broad Taxonomic Perspectives
Six major clades that generally correspond to traditionally-circumscribed orders or families (i.e., Chlamydomonadales, Chaetopeltidales, Chaetophorales, Oedogoniales, Sphaeropleales and Sphaeropleaceae [Sphaeroplea and close allies]) are supported by independent data analysis (Figs. 1, 2) and combined analysis I (Figs. 3, 4). A seventh major clade that has no basis in traditional taxonomy, the Cylindrocapsa clade (incertae sedis, Figs. 1-4), is strongly supported by all analyses. Each of the major clades is examined in more detail in the following sections.

Sphaeroplea and Allies
Phylogenetic analysis of the 18S and 26S rDNA data confirms a close alliance between Atractomorpha and Sphaeroplea, two of the recognized members of the Sphaeropleaceae (sensu Mattox and Stewart 1984). Thus, phylogenetic analysis of the 18S and 26S rDNA genes strongly supports the validity of a family Sphaeropleaceae that includes Atractomorpha and Sphaeroplea. Ankyra judayi, a zoosporic taxon for which no ultrastructural data are available, is resolved as the sister group to the clade that includes Sphaeroplea and Atractomorpha. In contrast to the Ankyra case, neither the 18S rDNA nor the 26S rDNA data support the Mattox and Stewart (1984) proposal that Cylindrocapsa be regarded as a member of the Sphaeropleaceae. Mattox and Stewart (1984) also noted that Sphaeroplea and Cylindrocapsa share a distinctive pyrenoid in which the matrix is penetrated by cytoplasmic invaginations. An evolutionary interpretation of the pyrenoid evidence, which is discussed in more detail below, suggests that the pyrenoids with cytoplasmic invaginations in Sphaeroplea and Cylindrocapsa are either similar by analogy or represent a symplesiomorphy.

Sphaeropleales Clade
Biflagellate DO Taxa. All of the biflagellate DO taxa included in these analyses (i.e., Characiopodium, Hydrodictyon, Neochloris, and Pediastrum) except Atractomorpha and Sphaeroplea are unambiguously resolved as part of a larger clade that includes a number of green algae that lack demonstrable motile stages (i.e., Scenedesmus, Ankistrodesmus, Ourococcus). In addition, Schroederia and Pseudoschroederia, for which no flagellar apparatus data are currently available, are zoosporic taxa with robust support as members of the sphaeroplealean clade (Figs. 1-4).
The Sphaeropleaceae. Sphaeroplea and other members of the Sphaeropleaceae are resolved as sphaeroplealean taxa in all analyses of 18S rDNA data (see Fig. 1) and in MP (Fig. 3), ME (not shown) and SRC (Fig. 4) global combined analyses. The 26S rDNA data, alone, do not support a monophyletic Sphaeropleales (sensu Deason et al. 1991). Furthermore, neither the 18S rDNA analyses nor the results from combined analysis I demonstrate robust support for a monophyletic Sphaeropleales (sensu Deason et al. 1991). However, results from SRC analysis, which is designed to minimize long-branch problems (Van de Peer et al. 1993, 1996), support a monophyletic Sphaeropleales (sensu Deason et al. 1991) with the Sphaeropleaceae as a basal member of the order (Fig. 4).
Taxon Deletion Experiments. Results from taxon deletion experiments conducted in combined analysis II, in which taxa with putative long branches were excluded (Fig. 5), indicate that the core Sphaeropleales (minus the Sphaeropleaceae) are robustly resolved as the sister group to the Chlamydomonadales regardless of the method of analysis.
MP Tests of DO Monophyly. Tests of four different hypotheses of DO monophyly were undertaken by constraining various groups of DO taxa using MP analysis and statistically comparing the constraint trees against the MP trees using PAUP* (Swofford 2000). The four DO hypotheses differ from one another by inclusion of the Sphaeropleaceae (Ankyra, Atractomorpha, Sphaeroplea), the Chaetopeltidales (Chaetopeltis, Hormotilopsis, Planophila), and/or the Chaetophorales (Aphanochaete, Chaetophora, Fritschiella, Schizomeris, Uronema) with the core Sphaeropleales (Ankistrodesmus, Bracteacoccus, Characiopodium, Hydrodictyon, Neochloris, Ourococcus, Pediastrum, Pseudoschroederia, Scenedesmus, and Schroederia). Results of the constraint tree comparisons indicate that monophyly of the core Sphaeropleales with the Sphaeropleaceae has the strongest support across data sets (values range from P= 0.2817 to P = 1.000). Among all DO taxa, the core Sphaeropleales and the Sphaeropleaceae share the biflagellate condition of zoospores (except Sphaeroplea and other autosporic taxa that lack a motile vegetative stage). Thus, the flagellate condition of vegetative motile cells coupled with the DO architecture may be structural indicators of a close, phylogenetic relationship between the core Sphaeropleales and the Sphaeropleaceae. However, even the combined molecular data are largely ambiguous regarding alternative hypotheses of alliance among DO taxa. Only the constraint that forced monophyly of all DO taxa (core Sphaeropleales, Sphaeropleaceae, Chaetopeltidales, and Chaetophorales) using 26S rDNA data resulted in a uniform rejection of the null hypothesis of no difference between trees (values range from P = 0.005 to P = 0.0146).

Cylindrocapsa and Allies
Cylindrocapsa Clade. One of the most striking findings presented here is the robust grouping of Cylindrocapsa with Treubaria, Trochiscia, and Elakatothrix (Figs. 1-4). Preliminary analyses of rbcL data also support an alliance of at least some members of this distinctive group of green algae (Buchheim, unpubl. observations). An ultrastructural feature that unites Treubaria and Cylindrocapsa is the distinctive pyrenoid with cytoplasmic invaginations (see discussion below). In addition, preliminary ultrastructural analyses indicate that both Trochiscia and Elakatothrix possess pyrenoids with cytoplasmic invaginations (Table 5, Buchheim unpubl. observations). Despite these similarities, an alliance of Cylindrocapsa with Treubaria, Trochiscia, and Elakatothrix cannot easily be interpreted in a more traditional morphological and life history context. The filamentous Cylindrocapsa geminella produces quadriflagellate zoospores (Hoffman and Hofman 1975). In contrast, the unicellular Treubaria, Trochiscia, and Elakatothrix are described as autosporic taxa in general treatments (e.g., Smith 1950, Bourrelly 1990). However, Treubaria may not be exclusively autosporic. Reymond (1979, 1980) reported that Tre. setigera is capable of producing quadriflagellate zoospores. Moreover, non-flagellated spores of Treubaria have been shown to exhibit contractile vacuoles and eyespots, earning the term "hemizoospores" (Fott and Kovácik 1975). While this observation regarding Treubaria is not conclusive, it highlights another feature that could be used to support an alliance with Cylindrocapsa.
Chlamydomonad Alliance. More surprising than an alliance of Elakatothrix, Treubaria, Trochiscia and Cylindrocapsa, is the sister status of the Cylindrocapsa clade and the chlamydomonad flagellate lineage observed in several analyses (Figs. 1, 4). Although an ultrastructural analysis of the flagellar apparatus orientation in zoospores of Cylindrocapsa has not been published, Hoffman (1976) described these motile cells as bearing a fine outer investment that exhibits a pattern similar to "wire gauze." Grazing sections of this outer investment on zoospores of Cylindrocapsa (Hoffman 1976) resemble, in scale and pattern, the lattice of crystalline walls described for many members of the chlamydomonad lineage (Roberts 1974). A crystalline cell wall, or "chlamys," appears to be a diagnostic feature of the chlamydomonad lineage (assuming loss in some taxa, see Buchheim et al. 1996) and forms the basis of a formal diagnosis of the group (Chlamydophyceae, sensu Ettl 1981). Thus, the "wire gauze" observed as an investment surrounding zoospores of Cylindrocapsa might be homologous to the chlamys found in virtually every member of the order Chlamydomonadales. Kouwets (1994) also argued, based on ultrastructural evidence, that Cylindrocapsa has its closest allies among chlamydomonad taxa. A basal alliance of chlamydomonad taxa producing quadriflagellate stages (i.e., Carteria) is consistent with observations of a quadriflagellate motile stage in Treubaria (Reymond 1979, 1980) and Cylindrocapsa. Given that an alliance of the Cylindrocapsa clade and the chlamydomonad clade is only weakly supported by the 18S data and is not supported by the 26S data, the phylogenetic affinities of the Cylindrocapsa lineage must remain an open question.

Oedogonialean Clade
The Oedogoniales are generally resolved as basal (Figs. 1, 3) or near-basal (Figs. 2, 4) members of the Chlorophyceae in the present investigation. These results are consistent with the original studies of 18S rDNA data from oedogonialean taxa (Booton et al. 1998b). However, no single analysis in the present investigation yielded a robust position for the Oedogoniales within the Chlorophyceae. This observation is likely attributable to the long oedogonialean branches associated with both the 18S (Fig. 1) and 26S data (Fig. 2). In such cases, combining data is unlikely to improve resolution (Felsenstein 1978). The combined analyses presented here fail to provide a robust or consistent placement for the Oedogoniales (Figs. 3, 4). Unless future investigations reveal substantial variation within oedogonialean genera (Bulbochaete, Oedocladium, Oedogonium) that can be exploited for interrupting the long oedogonialean branch, the problem will only be resolved by the chance discovery of a cryptic sister group (if one exists) or by analysis using more conserved genes.

Chaetopeltidales and Chaetophorales
Chaetopeltidales. When all taxa are included in phylogenetic analyses the Chaetopeltidales (sensu O'Kelly et al. 1994) are variously supported as sister to the Sphaeroplea clade (Fig. 1), as sister to the Oedogoniales (not shown), as a near-basal member of a chlorophycean grade (Fig. 2) or as sister to the Chaetophorales (Fig.3). Although an alliance of the Chaetopeltidales and the Sphaeropleales is consistent with previous investigations of 18S data (Booton et al. 1998a), none of the various chaetopeltidalean groupings has robust bootstrap support in the current investigation when all taxa are considered.
Chaetophorales. The data presented here confirm the predictions of more traditional taxonomists (e.g., Mattox and Stewart 1984) who included Aphanochaete and Schizomeris in the order Chaetophorales. Both the 18S data (Fig. 1) and the 26S data (Fig. 2) identify Aphanochaete and Schizomeris as basal members of the order. Thus, the addition of these two taxa has expanded the known molecular diversity for the group. The Chaetophorales (sensu Mattox and Stewart 1984) are resolved as a basal or near-basal member of a chlorophycean grade (Figs. 1, 2), as a sister to the Chlamydomonadales (not shown), or as sister to the Chaetopeltidales (Figs. 3-5). A sister relationship between the Chaetophorales and the Chlamydomonadales is consistent with previous 18S rDNA studies (Booton et al. 1998a), but none of the various chaetophoralean alliances is robustly supported in the present investigation.
Test of the Chlamydomonadales/Chaetophorales Alliance. A monophyletic Chlamydomonadales/Chaetophorales was constrained in a MP analysis and the resulting topologies were compared to the optimal MP topologies from analysis of all taxa. Although the values for the statistical comparisons were no higher than P = 0.29, most of the trees constrained to fit the results of the Booton et al. (1998a) investigation were not statistically different from the optimal trees. Thus, global taxon sampling fails to identify a robust resolution for either the Chaetophorales or the Chaetopeltidales.
Taxon Deletion Experiments. When long-branch-taxa are excluded from the combined analyses, an alliance of the Chaetopeltidales and Chaetophorales is supported by MP, ME and ML analysis (Fig. 5) with robust bootstrap values recorded in the MP and ML analyses. In support of this finding, O'Kelly et al. (1994) noted that the Chaetopeltidales and Chaetophorales show similarities in zoospore structure including (1) the quadriflagellate condition of DO or near-DO basal bodies, (2) lack of proximal fibers, (3) proximal sheaths that subtend the basal bodies, (4) similar rhizoplast attachment points, and (5) a microtubule-associated component adjoining with the proximal end of the "d" rootlets. On the other hand, the Chaetopeltidales possess a number of unique features such as body scales on zoospores (O'Kelly et al. 1994). O'Kelly et al. (1994) concluded that the distinctions between the Chaetopeltidales and the Chaetophorales were sufficiently large to preclude a close alliance. However, the Chaetophorales were hypothesized to be derived from a chaetopeltidalean-like ancestor (O'Kelly et al. 1994). This conclusion is consistent with the results from combined analysis of the taxon-limited data set that supports a sister relationship between the Chaetopeltidales and Chaetophorales (Fig. 5). Reconciling chaetopeltidalean scales with the molecular phylogenetic analysis presented here, however, requires invoking one or more extra assumptions about scale evolution. At the present time, scales are only known from ulvophyte, prasinophyte, and streptophyte taxa (O'Kelly et al. 1994).

Chlamydomonadalean Clade
The limited taxon sampling within the chlamydomonadalean clade as compared to previous work (e.g., Buchheim et al. 1996, 1997a, b), does not permit new insights into relationships within the group. However, both the 26S data and the results from combined analysis corroborate earlier assertions of non-monophyly for Carteria (Buchheim and Chapman 1992). Statistical comparisons demonstrate that constraint trees for Carteria monophyly are either significantly different from optimal trees or exhibit low P values (values range from P = 0.003 to P = 0.19). Thus, both data sets suggest that Carteria is not a monophyletic group and that the Car. crucifera lineage is likely basal within the Chlamydomonadales.

Character Evolution
Pyrenoids With Cytoplasmic Invaginations. Mattox and Stewart (1984) suggested that pyrenoids with cytoplasmic invaginations were rare and should be interpreted as an apomorphic feature. However, vegetative cells of a rather large number of green algal taxa have been documented as possessing this distinctive pyrenoid architecture. Taxa known to possess this pyrenoid type include Atractomorpha (Hoffman and Ichimura 1986), Sphaeroplea (Cáceres and Robinson 1980), Ankyra (Swale and Belcher 1971), Bulbochaete (Retallack and Butler 1970), Chaetopeltis (O'Kelly et al. 1994), Characiochloris (Lee 1974), Characiosiphon (Stewart et al. 1978), Cylindrocapsa (Hoffman 1976), Hafniomonas (Ettl and Moestrup 1980), Hormotilopsis (O'Kelly et al. 1994), Lobocharacium (Kugrens et al. 2000), Oedocladium (Markowitz and Hoffman 1974), Oedogonium (Hoffman 1968), Planophila (O'Kelly et al. 1994), and Treubaria (Reymond 1980). In order to formally examine the pyrenoid question, studies of character evolution in the Chlorophyceae using MacClade 3.1.1 (Maddison and Maddison 1992) were undertaken. The following assumptions were regarded as well-supported by most or all analyses: (1) sister status of the Chaetopeltidales and the Chaetophorales, (2) basal status of the Oedogoniales, Chaetopeltidales and Chaetophorales, and (3) monophyly of the Sphaeropleaceae, Sphaeropleales, Chlamydomonadales, and Cylindrocapsa clade. The distribution of known pyrenoid characters (Table 5) on a consensus topology (Fig. 6A) does not support the existence of a single lineage that can be diagnosed by the presence of cytoplasmically invaginated pyrenoids. Moreover, these tests of character evolution indicate that pyrenoids with cytoplasmic invaginations may be an ancient plesiomorphy for the class Chlorophyceae (Fig. 6A).
Flagellar Orientation. The stephanokont motile cells of the Oedogoniales cannot currently be assigned to any group of flagellar orientation types (CCW, CW, or DO). In addition, the Oedogoniales are basal or near-basal in most analyses of the Chlorophyceae (Booton et al. 1998b, present investigation). Consequently, it is difficult to unequivocally map the pattern of evolution of flagellar orientation within the Chlorophyceae to a consensus topology of molecular phylogenetic analyses. However, if the Chaetophorales are regarded as polymorphic (DO+CW) for flagellar orientation, studies of character evolution using MacClade 3.1.1 (Maddison and Maddison 1992) indicate that the DO condition is likely plesiomorphic within the Chlorophyceae (Fig. 6B). Furthermore, this assessment of character evolution suggests that the CW orientation independently arose in the Chaetophorales and the Chlamydomonadales (Fig.6B).

Summary and Conclusions
Results from SRC analysis (Fig. 4), other combined analyses (Fig. 3) and some analyses of 18S data support the Deason et al. (1991) concept of a monophyletic Sphaeropleales diagnosed by the DO condition of biflagellate motile cells. Although the alliance of the Sphaeropleaceae within the Sphaeropleales is only weakly supported by molecular data, congruence between these data and interpretations of ultrastructural diversity lead to the conclusion that the Sphaeropleales (sensu Deason et al. 1991) should be retained until other data are presented that unambiguously falsify the hypothesis. If the Deason et al. (1991) concept of the Sphaeropleales is retained, the results presented here indicate that Ourococcus, Pseudoschroederia and Schroederia should be regarded as sphaeroplealean taxa. Two quadriflagellate DO groups, the Chaetopeltidales and Chaetophorales, were not formally included in the Sphaeropleales (sensu Deason et al. 1991) and are not supported as sphaeroplealean in combined analyses. Rather, the Chaetopeltidales and Chaetophorales are resolved as sister taxa comprising a lineage separate from the Sphaeropleales. In taxon deletion experiments, the Chaetopeltidales-Chaetophorales clade is resolved as basal to the Sphaeropleales and Chlamydomonadales.
Long branch problems likely confound interpretations of pattern for three major chlorophycean groups. Neither the placement of the Oedogoniales, the Sphaeropleaceae, nor the Cylindrocapsa clade could be determined without ambiguity. Ultrastructural investigations of motile cell architecture in Cylindrocapsa and Treubaria would have a profound impact on any systematic assessment and these data are clearly needed to fill the gap in our understanding of what may be a new chlorophycean order. In the case of the Oedogoniales, detailed ultrastructural studies of development in the flagellar apparatus of the unusual stephanokont gametes or zoospores might provide insights into the phylogeny of this distinctive group. As noted above, identifying sister taxa and studying conserved genes would likely help resolve the long-branch problems. Regardless of the approach taken, a better understanding of the phylogenetic position of the Oedogoniales within the Chlorophyceae is necessary if questions about character evolution are to be answered.
Comparison of the 18S and 26S rDNA genes indicates that the latter is more variable and provides more phylogenetic information than the former (Table 3). The differences in phylogenetic information content between 18S and 26S rDNA indicate that the latter should be examined in other green algal groups for which 18S rDNA data have not yielded globally robust phylogenies due to numerous, short internodal branches (e.g., the Trebouxiophyceae [Friedl 1995] and the basal streptophytes [Melkonian and Surek 1995]). The 18S and 26S data sets exhibit only modest levels of topological congruence when problematic branches (i.e., long branches) are included. When potentially confounding branches are excluded, the data sets exhibit high levels of topological congruence. Moreover, virtually all clades in combined analysis II recorded an enhancement of support, as determined by the bootstrap, over independent data analysis (Table 4). These findings are consistent with other studies that have discovered "hidden" support for groups when data sets are combined (Gatesy et al. 1999, Smith 2000). Similarly, Wenzel and Siddall (1999) concluded that combining data leads to an additive effect for the signal and an averaging of the noise associated with the independent data sets. Thus, despite the questions that remain unanswered, the results from the combined analyses presented here suggest that a fully- and robustly-resolved hypothesis for relationships among chlorophycean algae is an attainable goal. Additional data from other sources including the chloroplast genome (e.g., McCourt et al. 2000) and the mitochondrial genome (e.g., Renzaglia et al. 2000) will almost certainly further advance our understanding of chlorophycean diversity. Moreover, molecular approaches to phylogeny are allowing green algal systematists to interpret structural data in new ways and these data are suggesting new lines of structural investigation.

Acknowledgments
This research was supported by grants from the National Science Foundation (DEB 9220834 and 9726588), the DOE/NSF/USDA Joint Program on Collaborative Research in Plant Biology (USDA grant no. 94-37105-0173 and 97-35105-4678), The University of Tulsa (Faculty Research and Summer Faculty Development grants), and the Mervin Bovaird Center for Molecular Biology and Biotechnology. EPSCoR support of The University of Tulsa DNA Sequencing Facility through the Oklahoma Biotechnology Network helped bring this project to fruition. Larry Hoffman provided the cultures of Atractomorpha and Sphaeroplea. Deborah Guthrie, Sean Larimore, Tonya Leonard, Joanna Michalopulos, Trish Merkel, Whitney Palmer, Matthew Rebstock, and Tracy Tran assisted with DNA extraction and amplification. Karen Draeger performed the automated DNA sequencing.

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_____________________________________________________________________________
Table 1. List of taxa, source of culture material, sequence accession number, and original literature citation.
_____________________________________________________________________________

	18S rDNA Data		26S rDNA Data
Taxon	Sourcea	Citation & Accession	Sourcea	Citation & Accession
Chlorella ellipsoidea Gerneck	IAM C-87	Aimi et al. 1993 (D13324)	IAM C-87	Aimi et al. 1994 (D17810)
Fusochloris perforata (Lee et Bold) Floyd et al.	UTEX 2104	Lewis et al. 1992 (M62999)	UTEX 2104	Present Investigation (AF183467)
Ankistrodesmus stipitatus (Chod.) Lemm.	SAG 202-5	Huss and Sogin (1990) (X56100)	SAG 202-5	Present Investigation (AF183447)
Ankyra judayi (G. M. Smith) Fott	SAG B 17.84	Present investigation (U73469)	SAG B 17.84	Present Investigation (AF183448)
Aphanochaete magna Godward	UTEX B 1909	Present investigation (AF182816)	UTEX B 1909	Present Investigation (AF183449)
Ascochloris multinucleata Bold et McEntee	UTEX 2013	Lewis 1997 (U63106)	UTEX 2013	Present Investigation (AF277652)
Atractomorpha echinata Hoffman	LRH	Present investigation (U73470)	LRH	Present Investigation (AF183450)
Bracteacoccus giganteus Bischoff et Bold	UTEX 1251	Lewis 1997 (U63099)	UTEX 1251	Present Investigation (AF183451)
Bracteacoccus minor (Chodat) Petrova	UTEX 66	Lewis 1997 (U63097)	UTEX 66	Present Investigation (AF183452)
Bulbochaete hiloensis (Nordst.) Tiffany	UTEX 952	Booton et al. 1998a (U83132)	UTEX 952	Present Investigation (AF183453)
Carteria crucifera Korshikov	NIES 421	Nakayama et al. 1996 (D86501)	UTEX 432	Present Investigation (AF183454)
Carteria eugametos Mitra	UTEX 233	Present Investigation (U70595)	UTEX 233	Present Investigation (AF183455)
Carteria obtusa Dill	SAG 39.84	Present Investigation (AF182818)	SAG 39.84	Present Investigation (AF183456)
Carteria olivieri G. S. West	UTEX LB 1032	Present Investigation (U70596)	UTEX LB 1032	Present Investigation (AF183457)
Carteria radiosa Korshikov	UTEX LB 835	Present Investigation (AF182819)	UTEX LB 835	Present Investigation (AF183458)
Carteria sp.	UTEX 2	Present Investigation (AF182817)	UTEX 2	Present Investigation (AF183459)
Chaetopeltis orbicularis Berthold	UTEX 422	Booton et al. 1998b (83125)	UTEX 422	Present Investigation (AF183465)
Chaetophora incrassata (Huds.) Hazen	UTEX LB 1289	Booton et al. 1998a (U83130)	UTEX LB 1289	Present Investigation (AF183471)
Characiopodium hindakii (Lee et Bold) Floyd et Wat.	UTEX 2098	Lewis et al. 1992 (M63000)	UTEX 2098	Present Investigation (AF183466)
Chlamydomonas moewusii Gerloff	SAG 11-11	Buchheim et al. 1997a (U70786)	SAG 11-11	Present Investigation (AF183461)
Chlamydomonas noctigama Korshikov	SAG 33.72	Buchheim et al. 1997a (U70782)	SAG 33.72	Present Investigation (AF183460)
Chlamydomonas pitschmannii Ettl	SAG 14.73	Buchheim et al. 1997a (U70789)	SAG 14.73	Present Investigation (AF183462)
Chlamydomonas reinhardtii Dangeard	CC-400	Gunderson et al. 1987 (M32703)	CC-1952	Present Investigation (AF183463)
Chlamydopodium vacuolatum (Lee et Bold) Floyd et Wat.	UTEX 2111	Lewis et al. 1992 (M63001)	UTEX 2111	Present Investigation (AF183468)
Chlorococcum echinozygotum Starr	UTEX 118	Buchheim et al. 1996 (U57698)	UTEX 118	Present Investigation (AF183469)
Chlorococcum ellipsoideum Deason et Bold	UTEX 972	Present Investigation (U70586)	UTEX 972	Present Investigation (AF183470)
Cylindrocapsa geminella Wolle	SAG 3.87	Present investigation (U73471)	SAG 3.87	Present investigation (AF183472)
Dunaliella parva Lerche	UTEX LB 1983	Lewis et al. 1992 (M62998)	UTEX LB 1983	Present Investigation (AF183473)
Elakatothrix viridis (Snow) Printz	LC-CH30	Present investigation (AY008844)	LC-CH30	Present Investigation (AY008845)
Fritschiella tuberosa Iyengar	UTEX 1821	Booton et al. 1998a (U83129)	CBS	Present Investigation (AF183474)
Haematococcus zimbabwiensis Pocock	UTEX LB 1758	Buchheim et al. 1997a (U70797)	UTEX LB 1758	Present Investigation (AF183475)
Hormotilopsis tetravacuolaris Trainor et Bold	UTEX 946	Booton et al. 1998b (U83124)	UTEX 946	Present Investigation (AF183476)
Hydrodictyon reticulatum (L.) Lagerh.	CBS	Wilcox et al. 1992 (M74497)	CBS	Present Investigation (AF183477)
Neochloris aquatica Starr	UTEX 138	Lewis et al. 1992 (M62861)	UTEX 138	Present Investigation (AF277653)
Neochloris vigensis Archibald	UTEX 1981	Wilcox et al. 1992 (M74496)	UTEX 1981	Present Investigation (AF277654)
Oedogonium cardiacum Witt.	UTEX 40	Booton et al. 1998a (U83133)	UTEX LB 39	Present Investigation (AF183478)
Ourococcus multisporus Bischoff et Bold	UTEX 1240	Present Investigation (AF277648)	UTEX 1240	Present Investigation (AF277655)
Pediastrum duplex Meyen	UTEX LB 1364	Wilcox et al. 1992 (M62997)	UTEX LB 1364	Present Investigation (AF183479)
Planophila terrestris Groover et Hofstetter	UTEX 1709	Booton et al. 1998b (U83127)	UTEX 1709	Present Investigation (AF183480)
Protosiphon botryoides (Kützing) Klebs	UTEX 99	Nakayama et al. 1996 (U41177)	UTEX 99	Present Investigation (AF183481)
Pseudoschroederia antillarum (Kom.) Hegewald et Schnepf	SAG B 15.86	Present Investigation (AF277649)	SAG B 15.86	Present Investigation (AF277656)
Scenedesmus obliquus (Turp.) Kützing	SAG 276-3a	Huss and Sogin 1990 (X56103)	SAG 276-3a	Present Investigation (AF183482)
Schizomeris leibleinii Kützing	UTEX LB 1228	Present Investigation (AF182820)	UTEX LB 1228	Present Investigation (AF183483)
Schroederia setigera (Schröd.) Lemm.	UTEX LB 2454	Present Investigation (AF277650)	UTEX LB 2454	Present Investigation (AF277657)
Sphaeroplea robusta Buchheim et Hoffman	LRH	Present investigation (U73472)	LRH	Present investigation (AF183484)
Sphaeroplea soleirolii v. crassisepta (Duby) Montag. ex. Kütz.	LRH	Present investigation (U73473)	LRH	Present investigation (AF183485)
Tetraspora sp.	UTEX LB 234	Booton et al. 1998b (U83121)	UTEX LB 234	Present investigation (AF183486)
Treubaria schmidlei (Schröder) Fott et Kovácik	SAG 36.83	Present investigation (U73474)	SAG 36.83	Present investigation (AF183487)
Treubaria setigera (Archer) G. M Smith	SAG 37.83	Present investigation (U73475)	SAG 37.83	Present investigation (AF183488)
Trochiscia hystrix (Reinsch) Hansg.	UTEX LB 606	Present investigation (AF277651)	UTEX LB 606	Present investigation (AF277658)
Uronema belkae Mattox et Bold (O'Kelly et Floyd)	UTEX 1179	Present investigation (AF182821)	UTEX 1179	Present investigation (AF183489)
Volvox carteri Iyengar	UTEX 1885	Rausch et al. 1989 (X53904)	UTEX 1885	Present investigation (AF183490)

Fig. 1

Fig. 2.

Fig. 3

Fig. 4

Fig. 5

Fig. 6