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, 99 (4), 677-701

Nuclear DNA Content Estimates in Green Algal Lineages: Chlorophyta and Streptophyta

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Nuclear DNA Content Estimates in Green Algal Lineages: Chlorophyta and Streptophyta

Donald F Kapraun. Ann Bot.

Abstract

Background and aims: Consensus higher-level molecular phylogenies present a compelling case that an ancient divergence separates eukaryotic green algae into two major monophyletic lineages, Chlorophyta and Streptophyta, and a residuum of green algae, which have been referred to prasinophytes or micromonadophytes. Nuclear DNA content estimates have been published for less than 1% of the described green algal members of Chlorophyta, which includes multicellular green marine algae and freshwater flagellates (e.g. Chlamydomonas and Volvox). The present investigation summarizes the state of our knowledge and adds substantially to our database of C-values, especially for the streptophyte charophycean lineage which is the sister group of the land plants. A recent list of 2C nuclear DNA contents for isolates and species of green algae is expanded by 72 to 157.

Methods: The DNA-localizing fluorochrome DAPI (4',6-diamidino-2-phenylindole) and red blood cell (chicken erythrocytes) standard were used to estimate 2C values with static microspectrophotometry.

Key results: In Chlorophyta, including Chlorophyceae, Prasinophyceae, Trebouxiophyceae and Ulvophyceae, 2C DNA estimates range from 0.01 to 5.8 pg. Nuclear DNA content variation trends are noted and discussed for specific problematic taxon pairs, including Ulotrichales-Ulvales, and Cladophorales-Siphonocladales. For Streptophyta, 2C nuclear DNA contents range from 0.2 to 6.4 pg, excluding the highly polyploid Charales and Desmidiales, which have genome sizes of up to 14.8 and 46.8 pg, respectively. Nuclear DNA content data for Streptophyta superimposed on a contemporary molecular phylogeny indicate that early diverging lineages, including some members of Chlorokybales, Coleochaetales and Klebsormidiales, have genomes as small as 0.1-0.5 pg. It is proposed that the streptophyte ancestral nuclear genome common to both the charophyte and the embryophyte lineages can be characterized as 1C = 0.2 pg and 1n = 6.

Conclusions: These data will help pre-screen candidate species for the on-going construction of bacterial artificial chromosome nuclear genome libraries for land plant ancestors. Data for the prasinophyte Mesostigma are of particular interest as this alga reportedly most closely resembles the 'ancestral green flagellate'. Both mechanistic and ecological processes are discussed that could have produced the observed C-value increase of >100-fold in the charophyte green algae whereas the ancestral genome was conserved in the embryophytes.

Figures

F<sc>ig</sc>. 1.
Fig. 1.
Summary results of combined analysis using morphological, ultrastructural and large- and small-subunit rRNA gene sequences for chlorophyte and streptophyte algae (McCourt, 1995).
F<sc>ig</sc>. 2.
Fig. 2.
Estimated 2C nuclear DNA contents and 2n chromosome complements (Sarma, 1982) superimposed on a phylogenetic tree for Volvocales based on multiple plastid gene sequences (Nozaki et al., 1999, 2000).
F<sc>ig</sc>. 3.
Fig. 3.
Estimated 2C nuclear DNA contents superimposed on a phylogenetic tree for Trebouxiophyceae based on 18S rRNA gene sequence analyses (Friedl, 1995; Krienitz et al., 2004).
F<sc>ig</sc>. 4.
Fig. 4.
Estimated 2C nuclear DNA contents superimposed on a cladogram of Ulvophyceae inferred from small-subunit rRNA gene sequence data (Zechman, 1990).
F<sc>ig</sc>. 5.
Fig. 5.
Phylogenetic tree of Ulotrichales inferred from 18S rRNA and rbcL gene sequence analysis (Hayden and Waaland, 2002).
F<sc>ig</sc>. 6.
Fig. 6.
Phylogenetic tree of the Ulvales inferred from 18S rRNA and rbcL gene sequence analysis (Hayden and Waaland, 2002).
F<sc>ig</sc>. 7.
Fig. 7.
Estimated 2C nuclear DNA content estimates superimposed on a cladogram of Trentepohliales inferred from small-subunit rRNA genes (López-Bautista and Chapman, 2003, 2005).
F<sc>ig</sc>. 8.
Fig. 8.
Estimated 2C nuclear DNA contents superimposed on a phylogenetic tree for Caulerpales based on cladistical analyses (Vroom et al., 1998; Woolcott et al., 2000). Proposed polyploidy events are indicated by [P].
F<sc>ig</sc>. 9.
Fig. 9.
Estimated 2C nuclear DNA contents superimposed on a phylogenetic tree for Dasycladales based on rbcL gene sequence analyses (Zechman, 2003; Berger et al., 2003). Proposed polyploidy events are indicated by [P].
F<sc>ig</sc>. 10.
Fig. 10.
Estimated 2C nuclear DNA contents superimposed on a phylogenetic tree for the Cladophorales/Siphonocladales complex based on 18S rRNA gene sequence analysis (Hanyuda et al., 2002).
F<sc>ig</sc>. 11.
Fig. 11.
Ranges of 2C nuclear DNA contents superimposed on a consensus molecular phylogenetic tree for the streptophyte green algae and bryophytes based on sequence analyses (Bhattacharya et al., 1994; McCourt et al., 2000; Denboh et al., 2001; Karol et al., 2001; Cimino and Delwiche, 2002; Delwiche et al., 2002; Turmel et al., 2002c; Gontcharov et al., 2003). DNA data for streptophyte algae are from Kapraun (2005) and the Appendix, and for the bryophytes from Renzaglia et al. (1995) and Voglmayr (2000).
F<sc>ig</sc>. 12.
Fig. 12.
Comparison of chromosome numbers and nuclear genome sizes in the charophycean and embryophyte lineages. Closed circles: DNA data for streptophyte algae from Kapraun (2005) and the Appendix, and for the bryophytes from Renzaglia et al. (1995) and Voglmayr (2000). Open circles: DNA data from Mandoli (2001).

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