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

2 CHAPTER 1
Archaeplastida
The monophyly of primary plastids has long been suggested by several features, such as a similar
gene content of plastid genomes, the presence of plastid-specific gene clusters that are distinct from
those found in Cyanobacteria, the conservation of the plastid-protein import machinery and proteintargeting
signals, and phylogenies based on plastid and cyanobacterial gene sequences (Palmer
2003). Nevertheless, several single-gene phylogenies and a few multigene phylogenies have
challenged this hypothesis (e.g., Stiller et al. 2001, Nozaki et al. 2003a, Nozaki et al. 2003b, Stiller and
Harrell 2005). Conclusive evidence for the monophyly of the Glaucophyta, red algae and green plants
was provided only relatively recently by Rodriguez-Ezpeleta et al. (2005) based on: (1) chloroplast
gene phylogenies showing the monophyly of primary plastid and (2) a phylogenomic dataset
containing 143 nuclear genes, ca. 30,000 amino acid positions which show the monophyly of all
organisms with a primary plastid (Fig. 1). The latter study, however, could not reveal the relation
among the three major lineages. Several nuclear genes suggest that red algae are the earliest
diverging Archaeplastida, but such results are inconsistent with many plastid gene trees that identify
glaucophytes as the earliest divergence. Most recent evidence favours the early divergence of
glaucophytes, as demonstrated by Reyes-Prieto et al. (2007) using a concatenated dataset of
conserved nuclear-encoded plastid targeted proteins of cyanobacterial origin. The latter evolutionary
scenario corroborates with two important putatively ancestral characters shared by glaucophyte
plastids and the cyanobacterial endosymbiont that gave rise to this organelle: the presence of
carboxysomes and a peptidoglycan deposition between the two organelle membranes. Both traits
were apparently lost in the common ancestor of red and green algae after the divergence of
glaucophytes.
Figure 2. The green algae exhibit a remarkable cytological diversity ranging from unicellar organisms (coccoid
or flagellates), over multicellular filaments and foliose blades, to coenocytic and siphonous life forms that are
essentially composed of a single giant cell containing countless nuclei (after Coppejans 1998). Arrows indicate
trends in morphological complexity rather than evolutionary hypotheses. For example, green algae are thought
to have evolved from a unicellular flagellate (the Ancestral Green Flagellate, AGF) rather than a coccoid life
form.