SYMBIOSIS - Cyanobacteria - page 393

been successful in G . chilensis with N . punctiforme (Reinke, 1873; Jönsson, 1894) or Nostoc symbionts
from G . chilensis or G . arenaria (Johansson and Bergman, 1992). Nostoc symbionts from G . arenaria ,
C . revoluta , Anthoceros spp, P . polydactyla and Nostoc commune reconstituted symbiosis in G . manicata
(Bonnett and Silvester, 1981). Successful establishment of symbiosis was monitored by microscopic
examination, nitrogenase activity and total nitrogen content of the established symbiosis. Symbionts
of Nostoc from G . arenaria (New Zealand), C. revoluta (Green house, New Zealand), Anthoceros sp. (New
Zealand), P. polydactyla and a soil cyanobacterium N. commune (Netherlands) entered into successful
symbiosis with G . manicata . Except the symbiont from P . ploydactyla, the rest of the four associations
performed well with reference to the nitrogenase activity, nitrogen per plant, number of infected
glands per plant and nitrogen content per gland. Strains incapable of producing hormogonia did not
infect the glands. Moreover, Nostoc symbionts from Macrozamia lucida , Anabaena azollae (from Azolla )
and two another Anabaena species ( A . oscillarioides and A . fl os - aquae ) did not establish symbiosis
with G . manicata (Bonnett and Silvester, 1981). This clearly demonstrates that motility is esstential
for infection to take place.
Zimmerman and Bergman (1990) studied the diversity of cyanobionts and correlated it with the
occurrence of Gunnera species and their habitat. For this purpose they had chosen twelve cyanobionts
isolated from Gunnera plants growing in Sweden ( G . manicata 1 and 2 from two sites, G . tinctoria
and Gunnera sp.), New Zealand [ G . arenaria (2), G . chilensis (3) from fi ve different sites] and USA
( G . arenaria , G . kaalensis , G . killipiana and Gunnera sp.) and compared their protein profi les and RFLP
polymorphisms (by hybridizations with heterologous nif H and gln A probes). These results point
towards the identical nature of the cyanobionts from different Gunnera species growing at the same
site in Sweden. However, a different cyanobiont was detected in G . manicata growing at a different site
in Sweden. On the other hand, of the fi ve cyanobionts from two species of Gunnera collected in the
same location in New Zealand three subgroups of cyanobionts have been dectected. The cyanobionts
from three different species of Gunnera grown in different localities were found to be different. These
results emphasize that there is no great critical selective factor required by Gunnera .
In case of heterocystous cyanobacteria a specifi c family of STRR sequences have been described
(Mazel, 1990). The number of copies of such sequences was estimated to be about 100 per genome
in Calothrix species in which these sequences were initially discovered. The long tandemly repeated
repetitive (LTRR) sequences are 37-bp long and have been identifi ed in Anabaena sp. strain PCC 7120.
STRR sequences have been put to use as a valuable tool for the identifi cation and characterization
of cyanobacteria, specially the toxin-producing strains (Rouhiainen et al ., 1995). Thus the conserved
status of these sequences makes them suitable for identifi cation of cyanobacteria. A fi ngerprint
method consisting of STRR and LTRR sequences was developed for the recognition of 35 symbiotic
isolates of Nostoc from Gunnera spp. The results showed a high degree of heterogeneity among
the isolates from the same Gunnera species as well as different species (Rasmussen and Svenning,
1998). Rasmussen and Svenning (2001) have drawn similar conclusions based on RFLP-patterns of
amplifi ed 16S rRNA gene and 16S-23S ITS region as well as DGGE of PCR products of hetR gene.
Similarly, cyanobionts isolated from 11 different geographical areas have been subjected for PCR
fi ngerprinting analysis with STRR sequences as primers. These studies revealed that (i) all the 45
cyanobionts isolated could be divided into ten groups of which fi ve cyanobiont isolates were found
to be unique, (ii) most of the groups are restricted in their distribution to one geographical area and
(iii) a low cyanobiont specifi city has been indicated because many cyanobiont strains have been
detected within and between 11 different Gunnera species. Further, more than one cyanobiont has
been associated with symbiosis within the same plant as well as within the same stem gland (Nilsson
et al ., 2000). The diversity of Nostoc strains occurring in three populations of G . tinctoria from Chile