SYMBIOSIS - Cyanobacteria - page 358

1989). Their ability to get excised and get inserted at new locations is based on the presence of ORF
governing the production of DNA endonucleases. At least 30% of the group I introns possess such
ORFs that encode site specifi c DNA endonucleases. Although the location of such ORFs varies in
the conserved secondary RNA structure, it does not interfere with the folding of the catalytic core
(Lambowitz et al ., 1999).
On the contrary, according to Rudi et al . (2002) the evolutionary pattern of tRNA Leu (UAA)
intron is involved with lateral gene transfer (LGT) in cyanobacteria. Evidences adduced in support
of this are a higher sequence similarity with introns in tRNA Ile (CAU) and tRNA Arg (CCU) genes
of α- and β-proteobacteria and sporadic distribution of tRNA Leu (UAA) intron in Nostoc and
Microcystis radiations. Further, the sequences of tRNA gene along with fl anking regions and its
intron have provided suffi cient support for LGT as the means of distribution and evolution of
tRNA Leu (UAA) intron. Intronless strains showed the absence of tRNA gene and fl anking regions
and strains with introns showed the presence of tRNA gene and fl anking regions. It is not due to
intron mobility but rather due to its instability that the sporadic distribution of this intron is seen
in genus Microcystis .
The validity of tRNA Leu (UAA) intron sequences as a molecular marker for the identifi cation and
measuring taxonomic relationships in cyanobacteria has been questioned by Oksanen et al . (2004)
based on the comparison of the molecular phylogeny deduced from 16S rRNA gene sequences. The
two classes of heptanucleotide repeats in the P6b stem-loop described by Costa et al . (2002) have
been found among distant relatives whereas some close relatives harboured different repeat classes
with a high sequence difference. Symbiont specifi city in bipartite lichens P . crocata , P . neglecta and
P . perpetua from Northern and Southern Hemispheres has been investigated based on tRNA Leu
(UAA) intron sequences where the fungal partner was identifi ed on the basis of 5.8S ITS of the
nuclear encoded ribosomal repeat unit and a part of the gene encoding β-tubulin. Both 5.8S ITS
and β-tubulin gene sequence analyses have confi rmed that all the three species of Pseudocyphellaria
examined actually represent morphotypes of the same phylogenetic fungal species. Five cyanobionts
have been identifi ed from a total of 36 specimens of the above three species collected from various
geographical regions of the two Hemispheres. Of these, two strains of Nostoc are represented in a
number of specimens while three have been restricted to one lichen thallus each of P . crocata from
Australia and two specimens from Chile (Summerfi eld and Eaton-Rye, 2006).
The term 'selectivity' has been used instead of 'specifi city' by some investigators to describe
the choice of a partner during symbiosis. According to Galun and Bubrik (1984), selectivity means
'preferential interaction between organisms'. Thus the two terms, specifi city and selectivity have been
used synonymously (Beck et al ., 2002). The foregoing account on the selectivity of a cyanobiont by
an ascomycetous fungus during lichen symbiosis is either based on 16S rRNA gene and/or tRNA Leu
(UAA) intron sequence analysis. According to few workers, the former marker is too conservative
to be relied upon to distinguish between species or strains of Nostoc whereas the latter falls short of
explaining the presence or absence of heptanucleotide repeats in the P6b stem-loop (Fox et al ., 1992;
Stenroos et al ., 2006). Besides, tRNA Leu (UAA) intron sequencing has generated a controversy about
its suitability to be employed for distinguishing between strains of Nostoc (Rudi et al ., 2002; Oksanen
et al ., 2004). Another important aspect that has been over-looked is the lack of correlation of Nostoc
clades with fungal taxa. Stenroos et al . (2006) selected 16S rDNA, tRNA Leu (UAA) intron, partial rbcL
and rbcX genes of the rbcLX gene cluster as molecular markers to investigate selectivity of lichen
mycobionts and cyanobionts ( Nostoc ). Such an analysis of 122 new sequences generated from 45
lichen collections from various geographical regions revealed that lichens Pseudocyphellaria , Sticta ,
Collema , and Leptogium appear to prefer certain Nostoc strains whereas lichen genera Peltigera , Lobaria ,