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microscopy) and molecular methods of analysis (16S rRNA gene amplifi cation and its sequence
comparisons). They obtained materials deposited at the Herbariums of University of Vienna
(HUW) and Swedish Museum of Natural History, Stockholm (SH) by workers other than those who
discovered them in the corresponding years mentioned after the name of the species. They included
representatives of unicellular [
Microcystis
(
Polycystis
)
aeruginosa
Kützing 1885 and
Chroococcus turgidus
Kützing 1850 of Section I], fi lamentous, non-heterocystous [
Microcoleus chthonoplastes
(Mertens)
Zanardini ex Gomont 1895 and
Trichodesmium
erythraeum
Ehrenberg ex Bornet et Flahault 1884 of
Section III ] and fi lamentous, heterocystous forms [
Nostoc muscorum
Agardh ex Bornet et Flahault 1886
and
Nodularia spumigena
Bornet et Flahault 1906 of Section IV of Rippka
et al
. (1979)].
T
.
erythraeum
was obtained from SH and the rest were from HUW. They are of the opinion that though these
specimens were not characterized genetically at the time collection, the preserved type materials
in all these cases had the advantage of permanently retaining the characteristics of the specimens
noted at the time of collection. In this connection, the suggestions of Castenholz and Norris (2005)
are worth recalling who emphasized that the morphological features be given more weightage in
the species defi nition of cyanobacteria.
Zhaxybayeva
et al
. (2006) developed a tool that helps in comparing phylogenetic trees constructed
for each of the orthologous genes detected in four taxa at a time. This method is known as “embedded
quartet decomposition analysis” or “quartet decomposition”. They included 11 completely sequenced
genomes (
Anabaena
sp. strain PCC 7120,
T. erythraeum
IMS101,
Synechocystis
sp. strain PCC 6803,
P. marinus
CCMP1375,
P. marinus
MED4,
P
.
marinus
MIT9313, marine
Synechococcus
WH 8102,
T. elongatus
BP-1,
Gloeobacter violaceus
PCC 7421;
Nostoc punctiforme
ATCC 29133 and
Crocosphaera
watsonii
WH8501) in their study that represent freshwater, marine and hotspring cyanobacteria. A
comparison of 1128 protein-coding gene families from these genomes revealed the existence of 879
homologues in selected prokaryotic genomes and the remaining 249 data sets have been found to
be “cyanobacteria-specifi c”. According to them, these “cyanobacteria-specifi c” homologue gene
sequences are not suitable for interphylum transfers but 700 of the 879 gene data sets are highly
resolved and 80% of these are distributed in cyanobacteria. Of the 700 genes, 540 support cyanobacteria
as a coherent group (accounting to ≈77%) while the remaining 160 gene sequences (≈23%) either have
sequences interspersed among cyanobacteria or some cyanobacterial gene sequences being found
elsewhere. This suggests for a possibility of transfer events to or from cyanobacteria. Zhaxybayeva
et al
. (2009) extended the embedded quartet scatter plots to understand gene fl ow in the intertwined
evolutionary histories of 18
P
.
marinus
and
Synechococcus
genomes. The conclusions are that: (i) the
HL-adapted
P
.
marinus
strains constitute a monphyletic clade; (ii) the LL-adapted
P
.
marinus
strains
form a paraphyletic clade; (iii) frequent transfer of at least 16 gene families took place between HL-
and LL-adapted strains and (iv) high way of gene sharing occurred between
Synechococcus
spp.
and LL-adapted
P
.
marinus
strains to the extent of erosion of genus boundaries, supporting the
obserevations of Beiko
et al.
(2005). According to them, however, these events have not disturbed the
Prochlorococcus
-specifi c ecological adaptations. Other estimates of LGT point out that cyanobacteria
might have acquired between 9.5% and 16.6% genes through LGT (Ochman
et al
., 2000; Nakamura
et
al
., 2004). These estimates are on a lower side when compared to 27 ± 20% of genes acquired through
LGT by cyanobacteria as mentioned earlier (Dagan
et al
., 2008).