196
S.M. Coelho et al.
size essentially by eliminating duplicate genes and reducing the amount of noncoding (intergenic and intronic) DNA in their genomes (Keeling 2007). Genome
streamlining in parasitic organisms, on the other hand, tends to involve the elimination of many genes that are non-essential because their function is carried out
by the host. One mysterious feature in both Ostreococcus species is the presence
of two chromosomes that have a markedly different composition to the other chromosomes that make up the genome, and contain the majority of the transposable
elements. The origin and the function of these chromosomes are still poorly understood. Interestingly, these two chromosomes exhibit a very low level of synteny
between the two Ostreococcus genomes despite a generally high level of synteny
when the other chromosomes are compared (Palenik et al. 2007). Based on this
observation and the differences in structure compared to the rest of the genome, it
has been suggested that one of these chromosomes, chromosome 2, may be related
to speciation (Palenik et al. 2007).
O. tauri and O. lucimarinus provide an unusual example of cryptic species.
Despite being indistinguishable morphologically when analysed using electron
microscopy and having 99.8% identical 18S rDNA sequences, orthologous genes
from the two species only share about 70% amino-acid identity (Palenik et al.
2007). At the genome level, therefore, they are clearly separate species and could
even be classified as separate genera. A third Ostreococcus strain whose genome
is currently being analysed exhibits the same phenomenon. Hence the diversity of
phytoplankton algae may be considerably greater than that judged simply on the
basis of cellular morphology.
The O. tauri and O. lucimarinus genome sequences have provided many insights
into prasinophyte biology. Genes encoding all of the enzymes required for C4photosynthesis are present in the Ostreococcus genomes indicating that, like the
diatoms, these algae may use this pathway as a CO 2 concentration mechanism. This
may, therefore, be a strategy that has been adapted by planktonic microalgae from
diverse phylogenetic origins. Similarly, the two Ostreococcus genomes encode an
unusually high number of selenoenzymes (Palenik et al. 2007), and this seems to
be a general feature of marine microalgae, including diatoms (Lobanov et al. 2007).
The increased catalytic activity of selenoproteins compared to equivalent enzymes
that lack selenium might provide a selective advantage in the marine environments
where these organisms are found. It may also be that, for some unknown reason,
aquatic habitats favour the use of selenoproteins, compared to terrestrial habitats
(Lobanov et al. 2007).
Two additional prasinophyte genomes, corresponding to two isolates of
Micromonas, have recently been described (Worden et al. 2009). These isolates
are morphologically identical and have been considered to be two members of the
species Micromonas pusilla but genome sequencing has shown that only about 90%
of the genes identified are present in both genomes. The Micromonas genomes are
larger (20.9 and 21.9 Mbp) than those of the Ostreococcus species, and gene families
are in general more extensive. One particularly interesting feature was the discovery in the one of the genomes of abundant intronic repeat sequences (introners) that
extended nearly to the donor and acceptor sites of the introns.
S.M. Coelho et al.
size essentially by eliminating duplicate genes and reducing the amount of noncoding (intergenic and intronic) DNA in their genomes (Keeling 2007). Genome
streamlining in parasitic organisms, on the other hand, tends to involve the elimination of many genes that are non-essential because their function is carried out
by the host. One mysterious feature in both Ostreococcus species is the presence
of two chromosomes that have a markedly different composition to the other chromosomes that make up the genome, and contain the majority of the transposable
elements. The origin and the function of these chromosomes are still poorly understood. Interestingly, these two chromosomes exhibit a very low level of synteny
between the two Ostreococcus genomes despite a generally high level of synteny
when the other chromosomes are compared (Palenik et al. 2007). Based on this
observation and the differences in structure compared to the rest of the genome, it
has been suggested that one of these chromosomes, chromosome 2, may be related
to speciation (Palenik et al. 2007).
O. tauri and O. lucimarinus provide an unusual example of cryptic species.
Despite being indistinguishable morphologically when analysed using electron
microscopy and having 99.8% identical 18S rDNA sequences, orthologous genes
from the two species only share about 70% amino-acid identity (Palenik et al.
2007). At the genome level, therefore, they are clearly separate species and could
even be classified as separate genera. A third Ostreococcus strain whose genome
is currently being analysed exhibits the same phenomenon. Hence the diversity of
phytoplankton algae may be considerably greater than that judged simply on the
basis of cellular morphology.
The O. tauri and O. lucimarinus genome sequences have provided many insights
into prasinophyte biology. Genes encoding all of the enzymes required for C4photosynthesis are present in the Ostreococcus genomes indicating that, like the
diatoms, these algae may use this pathway as a CO 2 concentration mechanism. This
may, therefore, be a strategy that has been adapted by planktonic microalgae from
diverse phylogenetic origins. Similarly, the two Ostreococcus genomes encode an
unusually high number of selenoenzymes (Palenik et al. 2007), and this seems to
be a general feature of marine microalgae, including diatoms (Lobanov et al. 2007).
The increased catalytic activity of selenoproteins compared to equivalent enzymes
that lack selenium might provide a selective advantage in the marine environments
where these organisms are found. It may also be that, for some unknown reason,
aquatic habitats favour the use of selenoproteins, compared to terrestrial habitats
(Lobanov et al. 2007).
Two additional prasinophyte genomes, corresponding to two isolates of
Micromonas, have recently been described (Worden et al. 2009). These isolates
are morphologically identical and have been considered to be two members of the
species Micromonas pusilla but genome sequencing has shown that only about 90%
of the genes identified are present in both genomes. The Micromonas genomes are
larger (20.9 and 21.9 Mbp) than those of the Ostreococcus species, and gene families
are in general more extensive. One particularly interesting feature was the discovery in the one of the genomes of abundant intronic repeat sequences (introners) that
extended nearly to the donor and acceptor sites of the introns.