182
S.M. Coelho et al.
been made in recent years in understanding how this came about and genomic-scale
analyses have played an important part in this story (Reyes-Prieto et al. 2007). It is
generally thought that all plastids are derived from a single primary endosymbiosis involving the capture of a coccoid cyanobacterium by a heterotrophic eukaryote
about 1.6 billion years ago (Yoon et al. 2006; it should be mentioned here, however, that there is recent evidence for another independent, and more recent, primary
endosymbiotic event in the filose thecamoeba Paulinella chromatophora, Nowack
et al. 2008). The enslavement of the cyanobacterium involved a gradual reduction in
the size of the bacterial genome over evolutionary time, and transfer of the endosymbiont’s genes to the host nucleus. This process necessitated the evolution of a protein
targeting system allowing the proteins encoded by these genes to be transported into
the plastid across the two surrounding membranes.
The ancient primary endosymbiotic event gave rise to three different groups
within the group Plantae: the glaucophyte, the red and the green lineages. All
the algae in the other supergroups (the chromalveolates, the excavates and the
rhizarians) are derived from either secondary or tertiary endosymbioses involving
a eukaryotic cell capturing a photosynthetic eukaryote from either the red or the
green lineages, which then evolved, as in the primary endosymbiosis, to become a
plastid (Keeling 2004, Yoon et al. 2006).
The chromalveolate hypothesis postulates that haptophytes, cryptophytes, heterokonts (or stramenopiles) and alveolates form a supergroup and that the capture
of a red alga occurred in a common ancestor of these lineages leading to the plastids
in present day groups such as diatoms, brown algae, some dinoflagellates, coccolithophores, etc.. The apicoplast of apicomplexans would also have been derived
from such an event. If, as this hypothesis postulates, a secondary plastid was
acquired very early in a common ancestor, then it would have had to have been lost
from groups like the ciliates and the oomycetes which do not possess plastids. There
is some evidence for this, as genes that could potentially have been derived from
such a secondary endosymbiotic have been identified in both ciliates and oomycetes
(Tyler et al. 2006, Reyes-Prieto et al. 2008). It should be noted, however, that there is
still considerable debate about the chromalveolate hypothesis and it is not possible
to rule out alternative hypotheses in which individual lineages of the chromalveolate
group independently acquired their secondary plastids. Moreover, the chromalveolate taxon is not strongly supported by phylogenies based on nuclear genes so it is
also important to bear in mind the possibility that this group is an artefact, for which
the strongest support may actually be based on similar but independent secondary
endosymbiosis events (Parfrey et al. 2006).
Convincing evidence for secondary endosymbiosis can be found in cryptophytes and chlorarachniophytes, which have retained a remnant of the nucleus
of the endosymbiont, called a nucleomorph (Archibald 2007). Their plastids are
surrounded by four membranes (the two outer membranes presumably originally
corresponded to the host vesicle membrane and cell membrane of the endosymbiont) and the nucleomorph is found between these two membranes and the
two inner membranes. The presence of 4 membranes is a common feature of
S.M. Coelho et al.
been made in recent years in understanding how this came about and genomic-scale
analyses have played an important part in this story (Reyes-Prieto et al. 2007). It is
generally thought that all plastids are derived from a single primary endosymbiosis involving the capture of a coccoid cyanobacterium by a heterotrophic eukaryote
about 1.6 billion years ago (Yoon et al. 2006; it should be mentioned here, however, that there is recent evidence for another independent, and more recent, primary
endosymbiotic event in the filose thecamoeba Paulinella chromatophora, Nowack
et al. 2008). The enslavement of the cyanobacterium involved a gradual reduction in
the size of the bacterial genome over evolutionary time, and transfer of the endosymbiont’s genes to the host nucleus. This process necessitated the evolution of a protein
targeting system allowing the proteins encoded by these genes to be transported into
the plastid across the two surrounding membranes.
The ancient primary endosymbiotic event gave rise to three different groups
within the group Plantae: the glaucophyte, the red and the green lineages. All
the algae in the other supergroups (the chromalveolates, the excavates and the
rhizarians) are derived from either secondary or tertiary endosymbioses involving
a eukaryotic cell capturing a photosynthetic eukaryote from either the red or the
green lineages, which then evolved, as in the primary endosymbiosis, to become a
plastid (Keeling 2004, Yoon et al. 2006).
The chromalveolate hypothesis postulates that haptophytes, cryptophytes, heterokonts (or stramenopiles) and alveolates form a supergroup and that the capture
of a red alga occurred in a common ancestor of these lineages leading to the plastids
in present day groups such as diatoms, brown algae, some dinoflagellates, coccolithophores, etc.. The apicoplast of apicomplexans would also have been derived
from such an event. If, as this hypothesis postulates, a secondary plastid was
acquired very early in a common ancestor, then it would have had to have been lost
from groups like the ciliates and the oomycetes which do not possess plastids. There
is some evidence for this, as genes that could potentially have been derived from
such a secondary endosymbiotic have been identified in both ciliates and oomycetes
(Tyler et al. 2006, Reyes-Prieto et al. 2008). It should be noted, however, that there is
still considerable debate about the chromalveolate hypothesis and it is not possible
to rule out alternative hypotheses in which individual lineages of the chromalveolate
group independently acquired their secondary plastids. Moreover, the chromalveolate taxon is not strongly supported by phylogenies based on nuclear genes so it is
also important to bear in mind the possibility that this group is an artefact, for which
the strongest support may actually be based on similar but independent secondary
endosymbiosis events (Parfrey et al. 2006).
Convincing evidence for secondary endosymbiosis can be found in cryptophytes and chlorarachniophytes, which have retained a remnant of the nucleus
of the endosymbiont, called a nucleomorph (Archibald 2007). Their plastids are
surrounded by four membranes (the two outer membranes presumably originally
corresponded to the host vesicle membrane and cell membrane of the endosymbiont) and the nucleomorph is found between these two membranes and the
two inner membranes. The presence of 4 membranes is a common feature of