198
S.M. Coelho et al.
of this initiative because of the single primary endosymbiosis in this group that
gave rise to the plastids of all the algae in the eukaryotic tree. Cryptophytes (or
cryptomonads) are unicellular algae that are found in both freshwater and marine
environments. This group is of particular interest because their plastids, which
are derived from a secondary endosymbosis, retain a remnant of the endosymbiont nucleus, the nucleomorph. A genome sequencing project is underway for one
member of this group, Guillardia theta, an alga that is found in coastal regions
(Table 6.1). Chlorarachniophytes are marine amoeboflagellates belonging to the
supergroup Cercozoa, which, like the cryptophytes, possess a nucleomorph-bearing
plastid derived from a secondary endosymbiosis. A genome sequencing project has
also been initiated for a member of this group, Bigelowiella natans (Table 6.1).
Considerable EST data is already available for Bigelowiella natans and, interestingly, analysis of this data has found evidence for numerous lateral gene transfers
both from bacteria and from other eukaryotic lineages such as streptophytes, heterokonts and red algae (Archibald et al. 2003). As far as we are aware there is no
genome project currently planned for a photosynthetic member of the Euglenozoa,
although EST sequences are available for the freshwater alga Euglena gracilis
(Durnford and Gray 2006).
6.4.5.4 Dinoflagellates
Although only about half of dinoflagellate species contain plastids and are thus capable of photosynthesis, collectively these species are important primary producers in
the marine environment. Most dinoflagellate plastids are thought to have originated
from a secondary endosymbiosis involving a red algal cell. Dinoflagellate plastids
show several unusual features, such as the presence of the accessory pigment peridinin and the presence of three membranes surrounding the plastid (Nassoury et al.
2003, Patron et al. 2005). Moreover, the plastid genome is highly reduced, containing only a small number of genes, each on a separate minicircular chromosome
(Zhang et al. 1999). All other genes necessary for plastid function have been transferred to the nucleus, including a nuclear-encoded proteobacterial Form II RuBisCO
(Morse et al. 1995, Bachvaroff et al. 2004, Hackett et al. 2004). As mentioned
above, some dinoflagellates have lost these red algal derived plastids and appear
to have replaced them with plastids captured from other photosynthetic eukaryotes
such as haptophytes or diatoms via tertiary endosymbioses (Inagaki et al. 2000,
Bhattacharya et al. 2004). The dinoflagellate nucleus also shows some unusual features. It contains extremely large amounts of DNA (3–250 pg, or the equivalent of
3,000–215,000 Mbp per cell) organised into hundreds of chromosomes (for example
there are 143 in Alexandrium tamarense; Hackett et al. 2005). The high concentration of DNA in the nucleus, which exceeds the concentration of basic DNA-binding
proteins by about 10-fold, results in its being condensed in a liquid crystal state,
and attachment to the nuclear envelope leads to the unusual nuclear morphology
known as a “dinokaryon” (Spector 1984, Gautier et al. 1986, LaJeunesse et al.
2005, Hackett et al. 2005). The large size of dinoflagellate genomes has significantly hindered the application of genomic approaches, and no genome sequencing
S.M. Coelho et al.
of this initiative because of the single primary endosymbiosis in this group that
gave rise to the plastids of all the algae in the eukaryotic tree. Cryptophytes (or
cryptomonads) are unicellular algae that are found in both freshwater and marine
environments. This group is of particular interest because their plastids, which
are derived from a secondary endosymbosis, retain a remnant of the endosymbiont nucleus, the nucleomorph. A genome sequencing project is underway for one
member of this group, Guillardia theta, an alga that is found in coastal regions
(Table 6.1). Chlorarachniophytes are marine amoeboflagellates belonging to the
supergroup Cercozoa, which, like the cryptophytes, possess a nucleomorph-bearing
plastid derived from a secondary endosymbiosis. A genome sequencing project has
also been initiated for a member of this group, Bigelowiella natans (Table 6.1).
Considerable EST data is already available for Bigelowiella natans and, interestingly, analysis of this data has found evidence for numerous lateral gene transfers
both from bacteria and from other eukaryotic lineages such as streptophytes, heterokonts and red algae (Archibald et al. 2003). As far as we are aware there is no
genome project currently planned for a photosynthetic member of the Euglenozoa,
although EST sequences are available for the freshwater alga Euglena gracilis
(Durnford and Gray 2006).
6.4.5.4 Dinoflagellates
Although only about half of dinoflagellate species contain plastids and are thus capable of photosynthesis, collectively these species are important primary producers in
the marine environment. Most dinoflagellate plastids are thought to have originated
from a secondary endosymbiosis involving a red algal cell. Dinoflagellate plastids
show several unusual features, such as the presence of the accessory pigment peridinin and the presence of three membranes surrounding the plastid (Nassoury et al.
2003, Patron et al. 2005). Moreover, the plastid genome is highly reduced, containing only a small number of genes, each on a separate minicircular chromosome
(Zhang et al. 1999). All other genes necessary for plastid function have been transferred to the nucleus, including a nuclear-encoded proteobacterial Form II RuBisCO
(Morse et al. 1995, Bachvaroff et al. 2004, Hackett et al. 2004). As mentioned
above, some dinoflagellates have lost these red algal derived plastids and appear
to have replaced them with plastids captured from other photosynthetic eukaryotes
such as haptophytes or diatoms via tertiary endosymbioses (Inagaki et al. 2000,
Bhattacharya et al. 2004). The dinoflagellate nucleus also shows some unusual features. It contains extremely large amounts of DNA (3–250 pg, or the equivalent of
3,000–215,000 Mbp per cell) organised into hundreds of chromosomes (for example
there are 143 in Alexandrium tamarense; Hackett et al. 2005). The high concentration of DNA in the nucleus, which exceeds the concentration of basic DNA-binding
proteins by about 10-fold, results in its being condensed in a liquid crystal state,
and attachment to the nuclear envelope leads to the unusual nuclear morphology
known as a “dinokaryon” (Spector 1984, Gautier et al. 1986, LaJeunesse et al.
2005, Hackett et al. 2005). The large size of dinoflagellate genomes has significantly hindered the application of genomic approaches, and no genome sequencing