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based on sequences of these genes and the combined amino acid sequences deduced from rpoC1 and
rpoD1 suggested that LGT of rRNA might be suspected in case of Synechocystis sp. strain PCC 6803.
The 24 cyanobacterial sequences have been resolved into fi ve clusters (A-E). Cluster A comprises
sequences from Anabaena cylindrica , Anabaena variabilis , Anabaenopsis circularis , Chlorogloeopsis sp.
PCC 6718, Chroococcidiopsis sp., Fischerella muscicola and Nodularia spumigena . Cluster B consists of
Oscillatoria agardhii, Oscillatoria nigro-viridis , Phormidium sp. and Spirulina platensis while Cluster C
is composed of Microcystis wesenbergii NIES-104, Pleurocapsa sp. PCC 7327, Pleurocapsa sp., Stanieria
cyanosphaera PCC 7437, Synechocystis sp. strain PCC 6803 and Xenococcus sp. Cluster D consists of
Pseudoanabaena sp. PCC 7367, Pseudoanabaena sp. PCC 7403 and Synechococcus sp. PCC 7001. Cluster
E is represented by Leptolyngbya PCC 7375 and Synechococcus leopoliensis IAMM-6.
A number of studies have been devoted to resolve taxonomic disparities and also draw
phylogenetic relationships amongst different groups of cyanobacteria. Of the 14 clusters recognized
on the basis of 16S rRNA gene sequences in cyanobacteria (Wilmotte and Herdman, 2001), such
attempts were made with cyanobacteria belonging to six clusters. The taxonomic resolution of strains
of cluster I comprising of heterocystous members ( Anabaena , Aphanizomenon , Cylindrospermopsis,
planktic Anabaena , Planktothrix ), cluster II of Synechococcus , cluster V of freshwater Leptolyngbya
lineage, cluster VI of Pseudoanabaena , cluster XII of Trichodesmium - Oscillatoria PCC 7515 lineage and
cluster XIV of Geitlerinema lineage has been undertaken during recent years. These are summarized
below.
i) Heterocystous cyanobacterial cluster: (Cluster I as per Wilmotte and Herdman, 2001): Studies
on the phylogeny of the heterocystous cyanobacteria based on 16S rRNA gene sequencing revealed
that these form a monophyletic cluster or group (Giovannoni et al ., 1988; Turner, 1997; Wilmotte
and Herdman, 2001). Within this cluster, phylogenetic relationships of planktonic heterocystous
cyanobacteria based on the 16S rRNA gene sequence comparisons (Lyra et al ., 1997; 2001; Barker et
al ., 1999; Beltran and Neilan, 2000; Lehtimäki et al ., 2000; Li et al. , 2000; Moffi t et al ., 2001; Saker and
Neilan, 2001), DNA-dependent RNA polymerase gene ( rpoC1 ; Fergusson and Saint, 2000; Wilson et al .,
2000), the intergenic spacer region of the phycocyanin operon (PC-IGS; Neilan et al ., 1995; Baker et al .,
1999; 2000; Bolch et al ., 1999) and the IGS between two copies of the gvpA encoding major structural
gas vesicle protein (Barker et al ., 1999) have been drawn.The extent of morphological diversity among
the planktonic heterocystous members has been depicted (Figs. 15, 16 and 17). Iteman et al . (2002)
reported the existence of fi ve monophyletic clusters among heterocystous cyanobacteria. Cluster 1
comprises forms assigned to Anabaena and Aphanizomenon . Cluster II comprises strains of Anabaena .
Cluster III is composed of Anabaena circinalis and A . affi nis (Beltran and Neilan, 2000). Cluster IV
and V possess planktonic and non-planktonic strains of Nodularia and Cylindrospermopsis raciborskii ,
respectively. Although on the basis of morphological characateristics or heterocyst differentiation
patterns the members of the last two clusters can be identifi ed but 16S rRNA gene sequence similarity
is so high that all of them can be assigned to a single species. Alternatively, it can also be stated that
the resolving power of the 16S rRNA is not suffi cient enough to make a distinction of these members.
Keeping this in view, Iteman et al . (2002) compared the 16S rRNA gene sequences and the 16S rRNA-
23S rRNA ITS sequences of 11 planktonic members ( Anabaenopsis fl os - aquae PCC 9302, PCC 9332,
PCC 9349, PCC 7905; Anabaenopsis sp. strains PCC 9215, PCC 9216, PCC 9608, Anabaenopsis elenkinii
PCC 9420, Cyanospira capsulata PCC 9502, Cyanospira rippkae PCC 9501 and Nodularia sp. PCC 9350)
that formed a distinct subclade within the monophyletic clade of heterocystous cyanobacteria with
the exception of strains of Cylindrospermopsis . Five distinct phylogenetic clusters could be identifi ed
with in this subclade. Strains of Anabaena and Aphanizomenon are dispersed through three clusters
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