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

20 CHAPTER 2
Introduction
The green plant lineage or Viridiplantae represents one of three groups of photosynthetic eukaryotes
that diverged after enslavement of a cyanobacterium to make a primary chloroplast (Rodriguez-
Ezpeleta et al. 2005). Ultrastructural and molecular studies have identified a major split within the
Viridiplantae giving rise to two lineages, the Chlorophyta and the Streptophyta (Pickett-Heaps and
Marchant 1972, Lewis and McCourt 2004). The Streptophyta consists of several lineages of
freshwater green algae from which land plants evolved approximately 470 million years ago 2 (Karol
et al. 2001, McCourt et al. 2004, Hall and Delwiche 2007). Whereas considerable progress has been
made during the past decade in clarifying the relationships among the streptophyte green algae and
land plants (Parkinson et al. 1999, Turmel et al. 2003, Turmel et al. 2006, Lemieux et al. 2007, Moore
et al. 2007, Rodriguez-Ezpeleta et al. 2007, Saarela et al. 2007), the phylogeny and evolutionary
history of the Chlorophyta has been more difficult to elucidate.
Members of the Chlorophyta are common inhabitants of aquatic environments and exhibit a
remarkable morphological and cytological diversity ranging from the world’s smallest free-living
eukaryote Ostreococcus, over multicellular filaments and foliose blades, to highly complex siphonous
life forms. Four classes are recognized within the Chlorophyta: Prasinophyceae, Ulvophyceae,
Trebouxiophyceae and Chlorophyceae. The predominantly marine planktonic Prasinophyceae form a
paraphyletic assemblage of unicellular organisms from which the Ulvophyceae, Trebouxiophyceae
and Chlorophyceae (UTC) are derived (Steinkötter et al. 1994, Fawley et al. 2000, Lopez-Bautista and
Chapman 2003, Guillou et al. 2004). The latter three classes form a monophyletic group termed UTC
clade but the relationships among them form the basis of a longstanding debate (Pröschold and
Leliaert 2007). All possible relationships between the UTC classes have been hypothesized over the
years, depending on which ultrastructural characters were interpreted or which DNA locus or
phylogenetic analysis method was used (Fig. 1). The unstable relationships exhibited among these
three classes are likely due to a combination of their ancient age and the short time span over which
they diverged from one another (O'Kelly 2007). The fossil record indicates the presence of the classes
in the mid-Neoproterozoic (Butterfield et al. 1994) and molecular clock estimates situate the UTC
divergence in the early Neoproterozoic (Douzery et al. 2004, Zimmer et al. 2007, Herron et al. 2009).
The UTC lineages occupy distinct and widely divergent ecological niches. The Chlorophyceae and
Trebouxiophyceae are ubiquitous in freshwater and soil ecosystems. Furthermore, trebouxiophycean
algae are the favored partner in symbiotic relationships with fungi to form lichens. Members of the
Ulvophyceae, on the other hand, are best known as marine macroalgae in coastal ecosystems, but
some of them live in damp subaerial habitats such as humid soil, rocks, tree bark and leaves (Lopez-
Bautista and Chapman 2003, Watanabe and Nakayama 2007). Having evolved from a marine
prasinophyte ancestor, the UTC lineages have experienced one to several transitions to a freshwater
2 This estimate is based on a plant phylogeny derived from 27 plastid protein-coding genes calibrated with
plant fossils (Sanderson et al. 2003). Estimates based on phylogenies of eukaryotes derived from 75 nuclear
protein coding genes calibrated with vertebrate fossil suggest that vascular plant already diverged from mosses
about 700 mya, which suggest that land plants are at least 700 my old (Heckman et al. 2001; Hedges 2004). A
possible explanations for this huge difference might be the use of other calibration points (i.e. plant versus
vertebrate fossils).