SYMBIOSIS - Cyanobacteria - page 357

strains associated with lichen symbiosis. All Nostoc strains formed a monophyletic group as revealed
by 16S rDNA sequencing. Further, Nostoc strains were divided into two sub-groups, cyanobionts
of epiphytic lichens designated as Nephroma guild and the rest as Peltigera guild, irrespective of
geographical origin or generic identity of the lichen species. According to them, the cyanolichens
show specifi city to a cyanobiont on a community scale, in other words suggesting that lichens of a
particular habitat exhibit specifi city for a group of cyanobacterial strains. Low cyanobiont selectivity
was reported in lichen specimens from Antarctica. Free-living cyanobacteria and cyanobionts
from fi ve lichen species ( Massalongia carnosa , Leptogium puberulum , Psoroma cinnamomeum , Placopsis
parellina and Placopsis contortuplicata ; the fi rst two being bipartite and the rest tripartite) collected
from Livington Island (maritime Antarctica) were analyzed for tRNA Leu (UAA) intron as a genetic
marker for the identifi cation of cyanobacterial strains. All the lichen species examined shared the
same Nostoc strain with an additional Nostoc strain in two of the lichens. There was no difference
in the Nostoc strains in between bi- and tripartite lichens. This has been explained as a selection
pressure in the harsh environment and that the mycobionts in order to survive in the extreme climatic
condition have no greater choice for the selection of their cyanobionts (Wirtz et al ., 2003).
The identity and specifi city of major photobionts of Pseudocyphellaria have been examined by
comparing 16S rRNA gene sequences. On this basis, cyanobacterial and green algal isolates could
easily be distinguished one from the other. With the help of both 16S rRNA gene and tRNA Leu (UAA)
intron sequences of isolates it was possible to identify the individual photobionts. The genetic
diversity of cyanobiont and mycobionts was investigated using tRNA Leu (UAA) intron sequences and
ITS sequences (of 5.8S gene), respectively. Two Nostoc strains have been identifi ed as species-specifi c.
On the other hand, 5.8S ITS sequences did not show much variation in the mycobionts of P . crocata and
P . neglecta . Further, the two symbionts have been shown to be specifi c for all samples (Summerfi eld
et al ., 2002). The tRNA Leu (UAA) intron sequences of a number of symbiotic strains (54 of them) of
Nostoc that are derived from lichens [species of Peltigera (18), Nephroma (7)], bryophytes [ Blasia (6),
Anthoceros fusiformis (4)] and gymnosperms ( Cycas circinalis , C . rumphii , Encephalartos lebomboensis
and Zamia pumila ) have been compared with the sequences of diverse free-living cyanobacteria
belonging to all fi ve taxonomic subsections and the evolutionary patterns deduced. The tRNA Leu
(UAA) intron sequences in various strains of Nostoc exhibited high similarity and shared a highly
conserved intron sequence with few variable positions. These differences have been found in one
stem-loop (P6b) of the tRNA Leu (UAA) intron. Degenerate heptanucleotide repeats are characteristic
of this region that fold into a hairpin structure. All Nostoc strains exhibited differences in the number
of heptanucleotide repeats thus causing size variations and also by the presence of other sequences
not having the heptanucleotide repeats (Costa et al ., 2002). Further, the regions that fl ank these
sequences contained the same or similar heptanucleotide repeats. The different groups of degenerate
heptanucleotide repeats could be distinguished into two classes from the P6b stem-loop of the intron,
i.e. N. punctiforme PCC 73102 (two groups of repeats one with a consensus sequence 5'-TDNGATT-3'
and the other its pairing repeat with 3'-AATYHAA-5') and Nostoc commune (two groups of repeats
with a consensus sequence 5'-NNTGAGT-3' and its base pairing repeat 3'-AACTCHN-5'). The cause
of variations in the introns has been attributed due to slipped strand mispairing during replication
and homologous recombination among different loci in the genome.
An important aspect of group I introns pertains to their mobility. Group I introns of tRNA Leu
(UAA) gene are once considered to be immobile and are of ancient origin. It was presumed that
these introns are older than the divergence of cyanobacteria and chloroplasts (Kuhsel et al ., 1990;
Xu et al ., 1990; Delwiche and Palmer, 1997). Group I introns have now been shown to be mobile.
They can insert themselves into intronless genes. This process has been termed as homing (Dujon,