Biology Reference
In-Depth Information
PCR fi ngerprinting with primers derived from short tandemly repeated repetitive sequences
(STRR) of cyanobionts from the coralloid roots of a number of cycads revealed the existence of
numerous cyanobacteria in a single coralloid root. The diversity of cyanobionts was noted in the
apical, middle and basal regions of a single cluster (Zheng
et al
., 2002). Contrary to the above fi ndings,
Costa
et al
. (2004) concluded that a single
Nostoc
strain is associated in individual coralloid roots
of cycads on the basis of tRNA
Leu
(UAA) intron sequencing. Natural populations of
M
.
riedlei
and
those growing in greenhouses of Perth (Australia) have been selected for the isolation of cyanobionts
from the coralloid roots. Several
Nostoc
strains appeared to be involved in this symbiosis. However,
when single coralloid roots or when all coralloid roots from the same plant were examined only one
Nostoc
strain appeared to be associated with the symbiosis. On the basis of these fi ndings, Costa
et al
. (2004) contested the fi ndings of Zheng
et al
. (2002). Analysis of cyanobiont diversity from 31
species of Australian cycad genus
Macrozamia
based on sequencing of 16S rDNA revealed that there
is negligible host specialization. From among the 56 cyanobionts isolated into cultures 22 have been
identifi ed to belong to
Nostoc
sp. and one to
Calothrix
sp. The former cyanobiont seems to be widely
represented within 14 species of
Macrozamia
(Gehringer
et al
., 2010).
X.
GUNNERA
The genus
Gunnera
with about 40 species belongs to the monogeneric family Gunneraceae of
angiosperms. However, many workers preferred to classify this genus in the family Haloragaceae
(Moore
et al
., 2010). The species of this genus are distributed almost exclusively throughout
the Southern Hemisphere. Shindler's work (1905) has been recognized as the most complete
morphological revision of the genus. He has recognized fi ve subgenera viz.,
Perpensum
,
Pseudogunnera
,
Milligania
,
Misandra
and
Panke
based on their morphology, respective modes of propagation and
their geographic distribution. A new South American species
G
.
herteri
has been discovered in 1933
that has been placed in a new subgenus
Ostenigunnera
(Mattfeld, 1933).
According to Bergman
et al
. (1992a) the subgenera
Pseudogunnera
(
G
.
macrophylla
),
Perpensum
(
G
.
perpensa
) and
Ostenigunnera
(
G
.
herteri
) have one species each whereas subgenus
Misandra
has
two species (
G
.
lobata
and
G
.
magellanica
, the latter is a small, stoloniferous species). In the subgenus
Milligania
six species have been included. By far the largest subgenus
Panke
is represented by the
gigantic forms with thick and fl eshy stems. The largest and most famous species of this subgenus
are
G
.
manicata
(giant ornamental rhubarb attaining a height of 6-8 ft and a native of Brazil; Fig.
14 A) and
G
.
tinctoria
(Chilean rhubarb; Fig. 14 B). These are very attractive ornamental plants.
Many species of
Gunnera
show preference for high altitudes.
G
.
tinctoria
is a fully naturalized exotic
plant of New Zealand but in view of its weedy potential it has been placed in National Pest Plant
Accord List.
Species of
Gunnera
are perennial herbs. There is great variation in the size of the plant. The upright
stem portion attains a height of 2 cm as in subgenus
Milligania
and up to 6 m as in the subgenus
Panke
with intermediate range of height plants found in the subgenus
Misandra
. Species belonging
to the subgenus
Panke
are the largest herbs known on earth. Other important features are slender or
fl eshy stems; leaves possess long petioles (rhubarb-like); fl owers in compound spikes, uni- or bisexual
without petals. Anatomically polystelic nature provides evidence of their aquatic ancestry.
i)
Cyanobiont
:
The cyanobiont in
Gunnera
has initially been classifi ed as
Scytonema
or
Anabaena
but
now it is generally accepted that the cyanobiont is a species of
Nostoc
. Though initially identifi ed as
