Biology Reference
In-Depth Information
Phanerozoic age. According to the norms set by the International Commission on Stratigraphy,
seven chronostratigraphic units have been recognized in the Precambrian period. These are
Palaeoproterozoic (2000-1650 My), Lower Mesoproterozoic (1650-1200 My), Upper Mesoproterozoic
(1200-1030 My), Lower Neoproterozoic (1030-850 My), Upper Neoproterozoic (850-630 My), Lower
Edicaran (630-550 My) and Upper Edicaran (550-542 My) units (Sergeev, 2009). The evolution and
distribution of life in the Precambrian era has been reviewed (Sharma and Shukla, 2009). Fossil
microfl ora, dating to Late Precambrian, from Bitter Spring Formation of Central Australia has
been reported (Schopf, 1968). Schopf and Walter (1983) reported simple trichomes of Oscillatorian
cyanobacteria from ~2800 My Fortescne Group, Western Australia. Further evidences on the existence
of microfossils of the early Archaean Apex Chert (Schopf, 1993) and Laser-Raman imagery of the
Earth's earliest fossils (Schopf
et al.
, 2002) have been presented. Some of the fossils of cyanobacteria
have been traced to ~2,100 My from Franceville Group of Gabon formations. These are represented
mostly by coccoid, star-like, fi lament, tubular forms and nostocacean akinetes i.e.
Archaeoellipsoides
(Amard and Bertrand-Sarfati, 1997).
Archaeoellipsoides
is a well preserved representative occurring
abundantly in the ~1500 My Cherts from Billyakh group of Siberia (Golubic
et al
., 1995). This genus
has also been found in the silicifi ed carbamates of the ~1650 My Amelia Dolomite of northern
Australia (Page
et al
., 2000) as well as 1631±5-My Cherts from the Kheinjua Formation in India
(Srivastava, 2005).
Rudi
et al
. (1998) studied the evolutionary rates in a number of strains of cyanobacteria (16 strains
of
Microcystis
, 6 strains of
Tychonema
, 10 strains of
Planktothrix
and 12 strains of
Nostoc
) by comparing
sequence divergence in the neutrally-evolving genes such as
rbcL
and
rbcX
and compared them
with 16S rRNA phylogeny. While there was low sequence divergence within
Microcystis
,
Tychonema
and
Planktothrix
for
rbcLX
and 16S rDNA, strains of
Nostoc
revealed three genetically clustered
lineages for these genes. The 16S rDNA and
rbcLX
phylogenies were not congruent with those in
the clustered groups. The evolutionary model presented by them explains as to how the modern
species has evolved by sequence homogenization involving extensive exchange of genetic material
for neutrally-evolving genes. Further, they emphasized that the macroevolutionary characters or
morphological characters have been stable for long periods if a comparison is made between the
fossils and present day representatives of cyanobacteria. This is likely to be achieved by exchange of
genetic material between phyletically closely related strains of cyanobacteria for neutrally-evolving
genes. This is supported by the existence of greater sequence homogeneity for the closely related
cyanobacteria and the exchange of genetic material probably stabilizes the function and structure
of proteins. According to Rudi
et al
. (1998) their model of gene transfer explains the morphological
similarities between the fossil and the recent species.
Due to selection pressures operative in the populations of pure cultures, the genotype that
dominates is other than the dominant genotype in the natural population at the time of collection. So
a culture thus no longer represents the dominant genotype supposed to represent the type specimen.
Due to this departure, how far is it justifi ed to extrapolate the results from the laboratory to the fi eld
samples? Additionally, if phylogenetic analysis of fi eld samples is done directly (circumventing
the process of culture techniques) by metagenomics, a situation arises where we have a sequence
identifi ed without any proper identifi cation of the organism based on classical morphological or a
polyphasic approach (Ward
et al
., 1994; Amann
et al
., 1995). It means at the time the type species is
proposed, it has to be genetically characterized. Now the situation is that whatever type species for
the representative taxa of cyanobacteria we have now, they lack genetic characterization. Palinska
et al
. (2006) presented evidences to show that historic identifi cations made on the basis of classical
taxonomy could be confi rmed by subjecting herbarium specimens to microscopic (light and electron
