Biology Reference
In-Depth Information
their traditional classifi cation. However, heterocystous cyanobacteria form a monophyletic cluster
or group (Giovannoni
et al
., 1988; Turner, 1997; Wilmotte and Herdman, 2001). This group includes
members of both Sections IV and V defi ned by Rippka
et al
. (1979) corresponding to the orders
Nostocales and Stigonematales, respectively under ICBN.
A number of workers have put forward proposals for the recognition of phylogenetic lineages
or clusters that are not consistent with the classifi cations under ICBN or that of Rippka
et al
. (1979)
or the Bergey's Manual. Wilmotte and Herdman (2001) recognised 14 phylogenetic clusters among
cyanobacteria mainly based on 16S rRNA gene sequencing. This formed a part of Bergey's Manual.
Other workers recognized seven (Honda
et al
., 1999), ten (Turner
et al
., 1999) and fi ve (Seo and
Yokota, 2003; Tomitani
et al
., 2006) phylogenetic clusters among cyanobacteria. They took into
account sequence comparisons of a number of housekeeping genes besides 16S rDNA sequences to
draw phylogenetic relationships. These are presented below.
Honda
et al
. (1999) detected seven major evolutionary lineages in cyanobacteria (including
Prochlorophycean algae) based on 16S rRNA gene sequence analysis which has further been
supported by phylogenetic relationships based on other genes such as
psbA
,
rbcL
,
rnpB
,
rpoC
and
tufA
. Five new
Synechococcus
16S rRNA gene sequences (belonging to
Synechococcus
PCC 7001,
PCC 7003, PCC 7109, PCC 7117 and PCC 7335) have been assigned to Marine Cluster (MC)-B and
MC-C but were separated into three lineages. They suggested that organisms classifi ed in the genus
Synechococcus
have evolved diversely and should be reclassifi ed into several independent taxonomic
units. Further, the
Synechococcus
strains and fi lamentous cyanobacteria make a monophyletic group
refl ecting a convergent evolution of multicellular organization.
Turner
et al
. (1999) recognized 10 monophyletic clusters in cyanobacteria based on 16S rRNA
gene sequence analysis.
Nostoc
(NOST),
Pseudoanabaena
,
Oscillatoria
(OSC) and
Synechococcus
clusters
were common in the studies of Turner
et al
. (1999) and those studied by Tomitani
et al
. (2006). The
Synechocystis
/
Pleurocapsa
/
Microcystis
sequence group identifi ed by Turner
et al
. (1999) does not
derive much support from the work of Tomitani
et al
. (2006), although
Prochloron
and
Pleurocapsalean
cyanobacteria formed a cluster. Turner
et al
. (2001) compared phylogeny, based on 16S rRNA gene
sequencing by using ML method, of strains of nitrogen-fi xing unicellular cyanobacteria. Three
independent lines of descent have been identifi ed without any correlation between aerobic versus
anaerobic nitrogen-fi xing activity.
Cyanothece
PCC 7418 has a sequence similarity of 100% with the
strains assigned to
Dactylococcopsis
and
Euhalothece
. Strains assigned to
Cyanothece
are polyphyletic
with four strains (including
Synechococcus
RF-1 that has been redesignated as
Cyanothece
PCC 8801)
falling into three distinct sequence clusters containing strains assigned to other genera.
Tomitani
et al
. (2006) conducted a detailed phylogenetic analyses of 20 cyanobacterial strains
belonging to 15 diverse representative fi lamentous taxa on the basis of
16S rRNA
,
rbcL
and
hetR
gene sequences in which comparison between ML, NJ and MP methods has been made. Filamentous
cyanobacteria belonging to subsection III appear to be mixed with unicellular species of the
subsections I and II indicating their polyphyletic origin. Further certain of the strains e.g.
Prochloron
and those belonging to subsection II form clusters in all the trees constructed with ML, NJ and
MP methods. Heterocystous cyanobacteria of subsections IV and V form a monophyletic clade in
phylogenetic trees constructed by the ML and NJ methods (with a bootstrap value of 97%) and to a
lesser extent by MP method where the bootstrap values of 66% were found. Further the monophyly
of heterocystous cyanobacteria is supported by
rbcL
phylogeny in all the three phylogenetic trees
constructed by NJ, MP and ML methods. Likewise, the phylogenetic analysis based on
hetR
gene
supports the monophyly of subsection V by NJ and MP methods with bootstrap values of 99% and
