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Azolla
plants and the cultures of the symbionts from all over the world. They further concluded that
(i) at least two cultures are not true representatives of the symbiosis with
Azolla
or their antigenic
properties might have undergone a change during isolation and cultivation; (ii) the presence of
cross-reactive antigens between
Azolla
leaves and surface of
A.
azollae
. By the application of indirect
immunofl uorescence technique, Ladha and Watanabe (1984) demonstrated agglutination of human
and rabbit erythrocytes by the extracts of
Azolla
-
Anabaena
symbiosis and
Anabaena
-free
Azolla
plants.
Such agglutination was not observed in case of
A
.
azollae
freshly separated from
Azolla
plants or
free-living
A
.
azollae
.
Additional evidences in support of these differences have been put forward by restriction
fragment length polymorphism (RFLP) as well. Frenche and Cohen-Bazire (1985, 1987) demonstrated
that the cultures of the symbiont from
A
.
fi liculoides
did not reveal any common hybridization bands
with freshly isolated cyanobionts from
Azolla
spp. including
A
.
fi liculoides
. Likewise, cultures of
Anabaena
from
A.
caroliniana
and its freshly isolated symbiont exhibited differences in the RFLP
patterns of genes sequences
glnA
,
psbA
,
rbcS
and
nifH
(Nierzwicki-Bauer and Haselkorn, 1986).
Gebhardt
et al
. (1991) identifi ed a common cyanobacterial symbiont in
A
.
mexicana
and
A
.
pinnata
by RFLP analyses with both single copy
glnA
and
rbcS
gene probes and a multicopy
psbA
gene
probe. In these studies
nifD
excision probe and a
xisA
gene probe of
Anabaena
sp. strain PCC 7120
were employed for comparing the sequences in DNA extracted from free-living isolates and freshly
isolated cyanobionts from the two species of
Azolla
. The sequences homologous to these probes
from free-living isolates were found to be homologous to
Anabaena
sp. strain PCC 7120 sequences
whereas the DNA from freshly isolated symbionts did not reveal any homology. From these studies
Gebhardt
et al
. (1991) concluded that (i) the isolates were different from the major cyanobiont that
resides in the leaf cavities of
Azolla
spp., (ii) these isolates are ubiquitously present as a culturable
minor cyanobacterial symbiont in at least three species of
Azolla
, i.e.
A
.
caroliniana
,
A
.
fi liculoides
and
A
.
pinnata
.
Taxonomically the cyanobionts from other symbiotic associations have been identifi ed as species
of
Nostoc
except in case of
Azolla
where the cyanobiont has been accorded the taxonomic assignment
as
Anabaena
(
A
.
azollae
). Few workers preferred to designate the symbiont of
Azolla
as a species of
Nostoc
instead of
Anabaena
on the basis of formation of hormogonia in the former according to the
classifi cation of Rippka
et al
. (1979). Meeks
et al
. (1988) have assigned the status of
Nostoc
sp. to the
cyanobiont of
Azolla
where as Grilli-Caiola
et al
. (1992) have considered the cyanobiont as a species
of
Trichormus
. Ran
et al
. (2010) confi rmed the symbiont of
A
.
fi liculoides
to be a species of
Nostoc
and
designated it as
Nostoc
azollae
0708. The partners of
A
.
fi liculoides
symbiosis are depicted in Fig. 9.
A comparison of the symbionts from different hosts suggested that those from
Azolla
are markedly
very close to each other on the basis of morphological, physiological properties and phycobiliprotein
content and at the same time different from the symbionts of other hosts. Besides, the property of
marked tendency to produce hormogonia has been noted in
Nostoc
species symbiotic with other hosts,
a tendency that is lacking in
A
.
azollae
(Vagnoli
et al
., 1992). Furthermore, strains of the cyanobiont
from different
Azolla
species share a high degree of morphological identity but have been shown
to differ at molecular level (Frenche and Cohen-Bazire, 1987; McCowen
et al
., 1987; Plazinski
et al
.,
1988). Zimmerman
et al
. (1989) conducted a detailed study of the several cyanobionts from
Azolla
.
Out of 10 secondary (minor) cyanobionts isolated from
Azolla
(
A
.
caroliniana
,
A
.
fi liculoides
,
A
.
mexicana
and
A
.
pinnata
), six of them were identifi ed as
Anabaena
and four of them as
Nostoc
on the basis of
morphological features. All six strains of
Anabaena
are unique as they resembled one another and
did not bind the lectins. Of these, fi ve of them (two isolates of
Anabaena
from
A
.
caroliniana
, isolated
by I. Newton, USA and R. Caudales, Rutgers University, USA; two isolates from
A.
pinnata
isolated
