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Algae
As ancient as the primary endosymbiotic origin of plastid was (1.8 billion years ago), it is worth
noting that cases of “recent” cyanobacterium–eukaryote endosymbiosis are known, for example,
in the testate amoeba Paulinella cromatophora and the diatom Rhopalodia gibba. The photosynthetic consortium between Paulinella and an α-cyanobacterium was established 60 million years
ago. This amoeba is surrounded by the cell wall called theca, which is composed of silica scales.
Apart from typical eukaryotic organelles such as nucleus and mitochondria, it harbors two cyanobacterium-derived endosymbionts. The endosymbionts are photosynthetically active, deeply integrated with the host cell, and their genome has been reduced to one-third in comparison to their
cyanobacterial ancestors. The pennate diatom R. gibba harbors endosymbionts closely related to
extant cyanobacteria. Some of the closest free-living relatives of these so-called spheroid bodies
are diazotrophic cyanobacteria of the Cyanothece sp. group. The spheroid bodies encode genes for
nitrogen fixation and have the capacity to fix molecular nitrogen under light conditions only, unlike
all other unicellular nitrogen-fixing cyanobacteria. Although the spheroid bodies are of cyanobacterial origin, they lack the typical photosynthetic pigmentation.
Plantae, that is, Glaucophyta, Chlorophyta, and Rhodopyta, represent just the beginning of the
endosymbiosis story. In fact, while mitochondria originated once and have apparently never been
lost, plastids have spread between eukaryotic lineages several times in events referred to as secondary and tertiary endosymbiosis, that is, the uptake and retention of a primary or secondary algal cell
by another eukaryotic lineage.
Euglenozoa and Cercozoa derived from this primary plastid lineage by two separate secondary
endosymbiosis acquiring the plastid by engulfing a green alga. Two independent lineages of green
algae were captured by two distinct lineages of phagotrophic protists via secondary endosymbiosis;
chloroplast genome analyses suggest that the chlorarachniophyte plastid is derived from a green
alga belonging to the “core chlorophyte” group (Trebouxiophyceae, Ulvophyceae, Chlorophyceae),
while the ancestor of the euglenophyte plastid is related to prasinophyte green algae. There is
no strong similarity between the plastid of euglenids and chlorarachniophytes: euglenids have
plastids bounded by three membranes and store paramylon in the cytosol, whereas chlorarachniophytes have plastids bounded by four membranes with a nucleomorph and store β-1,3-glucan in
the cytosol.
Euglenozoa contains photoautotrophic species and species with different types of feeding strategies, osmotrophic (e.g., Rhabdomonas) or phagotrophic (e.g., Peranema). The plastids have been
subsequently and independently lost in several branches within the phototrophic clade. Current
knowledge on the phylogeny of euglenozoa implies that the secondary endosymbiotic event happened after the split of Peranema, but before the split of Eutreptia and Eutreptiella, which form
the basal lineages of the phototrophic clade. Recently, a number of red algal origin genes have
been identified in the photosynthetic Euglena gracilis as well as in the heterotrophic Peranema.
It is likely that these genes have been acquired via eukaryote-to-eukaryote lateral gene transfer,
giving rise to a complex pattern of genome mosaicism in euglenids. These genes may come from
prey organisms, and the lineage of euglenids might have experienced a cryptic red algal plastid
endosymbiosis before the current green algal plastid was established. Derived genes may have
contributed to the successful integration and functioning of the green algal secondary plastid in
modern-day euglenids.
Multiple red algal-derived Calvin cycle genes have been detected also in Cercozoa
(Chlorarachniophyta) nuclear genomes. One possible explanation is that these genes were transferred from red alga prey organisms via HGT. The prey organism might have been captured by and
retained in an ancestral and probably nonphotosynthetic chlorarachniophyte as an endosymbiont,
which was then replaced by a green algal endosymbiont, giving rise to the extant secondary plastid
in Chlorarachniophyta.
The plastid of cryptophytes, ochrophytes, dinoflagellates, and haptophytes likely arose from a single initial event of secondary endosymbiosis of a red alga with a nonphotosynthetic eukaryotic host.
Algae
As ancient as the primary endosymbiotic origin of plastid was (1.8 billion years ago), it is worth
noting that cases of “recent” cyanobacterium–eukaryote endosymbiosis are known, for example,
in the testate amoeba Paulinella cromatophora and the diatom Rhopalodia gibba. The photosynthetic consortium between Paulinella and an α-cyanobacterium was established 60 million years
ago. This amoeba is surrounded by the cell wall called theca, which is composed of silica scales.
Apart from typical eukaryotic organelles such as nucleus and mitochondria, it harbors two cyanobacterium-derived endosymbionts. The endosymbionts are photosynthetically active, deeply integrated with the host cell, and their genome has been reduced to one-third in comparison to their
cyanobacterial ancestors. The pennate diatom R. gibba harbors endosymbionts closely related to
extant cyanobacteria. Some of the closest free-living relatives of these so-called spheroid bodies
are diazotrophic cyanobacteria of the Cyanothece sp. group. The spheroid bodies encode genes for
nitrogen fixation and have the capacity to fix molecular nitrogen under light conditions only, unlike
all other unicellular nitrogen-fixing cyanobacteria. Although the spheroid bodies are of cyanobacterial origin, they lack the typical photosynthetic pigmentation.
Plantae, that is, Glaucophyta, Chlorophyta, and Rhodopyta, represent just the beginning of the
endosymbiosis story. In fact, while mitochondria originated once and have apparently never been
lost, plastids have spread between eukaryotic lineages several times in events referred to as secondary and tertiary endosymbiosis, that is, the uptake and retention of a primary or secondary algal cell
by another eukaryotic lineage.
Euglenozoa and Cercozoa derived from this primary plastid lineage by two separate secondary
endosymbiosis acquiring the plastid by engulfing a green alga. Two independent lineages of green
algae were captured by two distinct lineages of phagotrophic protists via secondary endosymbiosis;
chloroplast genome analyses suggest that the chlorarachniophyte plastid is derived from a green
alga belonging to the “core chlorophyte” group (Trebouxiophyceae, Ulvophyceae, Chlorophyceae),
while the ancestor of the euglenophyte plastid is related to prasinophyte green algae. There is
no strong similarity between the plastid of euglenids and chlorarachniophytes: euglenids have
plastids bounded by three membranes and store paramylon in the cytosol, whereas chlorarachniophytes have plastids bounded by four membranes with a nucleomorph and store β-1,3-glucan in
the cytosol.
Euglenozoa contains photoautotrophic species and species with different types of feeding strategies, osmotrophic (e.g., Rhabdomonas) or phagotrophic (e.g., Peranema). The plastids have been
subsequently and independently lost in several branches within the phototrophic clade. Current
knowledge on the phylogeny of euglenozoa implies that the secondary endosymbiotic event happened after the split of Peranema, but before the split of Eutreptia and Eutreptiella, which form
the basal lineages of the phototrophic clade. Recently, a number of red algal origin genes have
been identified in the photosynthetic Euglena gracilis as well as in the heterotrophic Peranema.
It is likely that these genes have been acquired via eukaryote-to-eukaryote lateral gene transfer,
giving rise to a complex pattern of genome mosaicism in euglenids. These genes may come from
prey organisms, and the lineage of euglenids might have experienced a cryptic red algal plastid
endosymbiosis before the current green algal plastid was established. Derived genes may have
contributed to the successful integration and functioning of the green algal secondary plastid in
modern-day euglenids.
Multiple red algal-derived Calvin cycle genes have been detected also in Cercozoa
(Chlorarachniophyta) nuclear genomes. One possible explanation is that these genes were transferred from red alga prey organisms via HGT. The prey organism might have been captured by and
retained in an ancestral and probably nonphotosynthetic chlorarachniophyte as an endosymbiont,
which was then replaced by a green algal endosymbiont, giving rise to the extant secondary plastid
in Chlorarachniophyta.
The plastid of cryptophytes, ochrophytes, dinoflagellates, and haptophytes likely arose from a single initial event of secondary endosymbiosis of a red alga with a nonphotosynthetic eukaryotic host.