Biology Reference
In-Depth Information
35 distinct myovirus g20 genotypes were identifi ed. Upon phylogenetic analysis, these could be
divided into seven distinct taxonomic units. The interesting feature is that some of these freshwater
cyanophage sequences have been found to be similar to marine cyanophage sequences than to other
freashwater cyanophage sequences.
Short and Suttle (2005) reported the occurrence of
g20
sequences from samples collected from
wide range of freshwaters (Lake Constance, Germany; Chilliwack Lake and Cultus Lake from Canada;
a Catfi sh production pond located at Limco, Ic. in Crowley, La) as well as marine waters (Gulf of
Mexico, the Arctic, an Arctic cyanobacterial mat, Southern, Northeast and Southeast Pacifi c Oceans
and Chuckchi Sea). After DNA extraction and PCR amplifi cation, the separation was done by DGGE
and then the
g20
gene was amplifi ed with CPS-4 (nondegenerate upstream) and G20-2 (degenerate
downstream) primers. The
g20
gene sequences showed nearly 99% identity from environments
that differed greatly in temperature, salinity and location. They concluded that it is very unlikely
that the viruses and their hosts occur in both freshwaters and marine waters. Moreover, the CPS
primers might not be specifi c for cyanophages due to the detection of
g20
sequences from a depth
of 3,246 m in the Chuckchi Sea. A phylogenetic analysis revealed the presence of four new
g20
clusters of which two have been entirely found in freshwaters. The genetic diversity of cyanophages
in the Gulf of Aquaba has been studied by Mühling
et al
. (2005) who simultaneously determined
the genetic diversity of
Synechococcus
sp. and correlated the prevalence of a particular host strain
with a particular phage genotype. The genetic diversity of
Synechococcus
was assayed with RNA
polymerase core subunit gene
rpoc1
and the phage diversity infecting the particular genotype of
Synechococcus
was identifi ed by PCR amplifi cation of
g20
gene sequence and subsequent analysis
by DGGE. The main fi ndings are that (i)
Synechococcus
abundance was represented at a maximum
cell density of 3.4 x 10
4
cells ml
-1
, though the preponderance of
Prochlorococcus
was greater by
12-folds, (ii) the sudden disappearance of
Synechococcus
was attributed to depleted levels of phosphate
concentration, (iii) plaque assay studies, conducted on four host strains of
Synechococcus
(WH7803,
WH8103, RS9906 and RS9911), revealed clear-plaque formation more readily on WH7803 rather
than on other hosts, (iv) PCR amplifi cation of
rpoc1
gene from the samples taken from a depth of 10
m resulted in eight clone libraries. Thirty six clones from each of the clone libraries were analysed
by RFLP to reveal genetic diversity of
Synechococcus
. Season-specifi c prevalence of the genotypes of
Synechococcus
was noted where a number of genotypes disappeared with the onset of summer. During
autumn RFLP types S19 and S12 were predominantly represented and only RFLP type S17 could be
detected during spring. Genotype S19 accounted for 31 of the 36 clones analyzed during July and 18
of the 36 clones analysed during October. Likewise, the cyanophage population was dominated by
only two genotypes (P26 and P27) during the summer months. As it was not possible to cultivate
the dominant
Synechococcus
RFLP types (S19 and S12), the representative phage genotype infecting
the host strains could not be ascertained. They concluded that viral infection plays a major role in
controlling the populations of
Synechococcus
in the Gulf of Aquaba where grazing is insignifi cant.
The interaction with the co-occurring phages results in a qualitative and quantitative change in
the genetic diversity of
Synechococcus
strains. They did not agree with the suggestion of Waterbury
and Valois (1993) that cyanophage-resistant mutants are dominantly represented in the oligotropic
regions of oceans. Their observations also derive support from the fi ndings of Suttle and Chan
(1994) who demonstrated lack of such resistance in the host cells in offshore environments. Of the
35 marine cyanophages isolated and characterized from the Gulf of Mexico, the
g20
gene sequence
could be amplifi ed from the 20 of the phages suggesting that only 63% of them are myoviruses
(McDaniel
et al
., 2006). PFGE of viral community structure of Norwegian coastal waters revealed
the presence of 29 different viral populations with a genome size variation ranging from 26 to 500
