Phylogeny and molecular evolution of green algae - Phycology ...

6 CHAPTER 1
Chlorophyta
The current classification of the Chlorophyta, which relies on a combination of morphology,
ultrastructural features of the flagellar root system, and characters relating to the mitotic spindle
during cell division and cytokinesis, has been largely confirmed by phylogenetic analysis (Figs. 4 and
5, Table 1) (Mattox and Stewart 1984, Pröschold and Leliaert 2007). Four major groups, commonly
regarded as classes, are recognized by consensus: “Prasinophyceae”, Chlorophyceae,
Trebouxiophyceae and Ulvophyceae (Fig. 3) (reviewed in Lewis and McCourt 2004). The
prasinophytes form a paraphyletic group of unicellular flagellates or coccoid cells at the base of the
Chlorophyta (Steinkötter et al. 1994, Fawley et al. 2000, Lopez-Bautista and Chapman 2003, Guillou
et al. 2004). The Ulvophyceae, Trebouxiophyceae and Chlorophyceae are resolved as a wellsupported
clade (UTC clade) in most studies (Mishler et al. 1994), but the relationships among these
lineages form the basis of a longstanding debate. Furthermore, the monophyly of the three classes
remains to be demonstrated unequivocally (O'Kelly and Floyd 1984a, Zechman et al. 1990, Krienitz et
al. 2003).
Certain ultrastructural characteristic are shared between the Ulvophyceae, Trebouxiophyceae and
Chlorophyceae. In all representatives the nuclear envelope remains intact until the chromosomes
finally separate (closed mitosis). The flagella are anchored in the cell by means of cruciate flagellar
roots with mostly an X-2-X-2 configuration of the microtubules (Moestrup 1978, Lewis and McCourt
2004). Other ultrastructural observations are useful diagnostic characters to separate the three
classes. The orientation of the basal bodies, short cylindrical arrays of microtubules at the base of a
flagellum, is one of those discriminative characters. The Ulvophyceae and Trebouxiophyceae have a
counter-clockwise orientation of the basal bodies, while the Chlorophyceae have a direct opposite or
clockwise orientation of the basal bodies (Fig. 4) (Lewis and McCourt 2004). The Ulvophyceae have a
persistent mitotic spindle which helps to keep the daughter nuclei separate until cytokinesis has
been accomplished. The Trebouxiophyceae and Chlorophyceae both have a non-persistent mitotic
spindle and a phycoplast composed of a set of microtubules which lie parallel to the plane of
cytokinesis (Fig. 5) (Friedl 1995, Lewis and McCourt 2004). Based on these ultrastructural
observations Mattox and Stewart (1984) suggest that the Ulvophyceae diverged first, followed by the
Trebouxiophyceae and Chlorophyceae (Fig. 6). While Mattox and Stewart (1984) based their
classification on the orientation of the basal bodies in the flagellar apparatus and differences of the
mitotic spindle during cell division and cytokinesis, Sluiman (1989) only used the orientation of basal
bodies in the flagellar apparatus as diagnostic character and merged the Trebouxiophyceae with the
Ulvophyceae based on the counter-clockwise orientation of the basal bodies in the flagellar
apparatus (Fig. 6).
Molecular phylogenetic studies have been highly inconclusive about the relationships between UTC
classes (Fig. 6). The first molecular phylogenies based on small subunit nuclear ribosomal DNA (SSU
or 18S nrDNA) sequences all observed that Ulvophyceae branch first, leaving Trebouxiophyceae and
Chlorophyceae as sisters (Friedl 1995, Bhattacharya et al. 1996, Krienitz et al. 2001, Lopez-Bautista
and Chapman 2003), while more recent SSU nrDNA phylogenitic studies using expanded taxon
sampling and likelihood-based methods with more realistic models of sequence evolution revealed a
sister relation between Chlorophyceae and Ulvophyceae (Friedl and O'Kelly 2002, Lewis and Lewis
2005, Watanabe and Nakayama 2007). Chloroplast gene order data and genomic structural features