Biology Reference
In-Depth Information
ecotypes. If some sequence clusters are not falling in the same ecotype as the most closely related
strains under the species such clusters can be considered as different species.
The study of bacterial species in relation to geography is gaining interest during the past few years
and is known as bacterial biogeography (Staley, 1999; Staley and Gosink, 1999; Martiny
et al
., 2006).
Evidences are in favour of the occurrence geovars of bacteria particularly in extreme environments.
Staley (2006) extended the species concept based on ecological information by introducing a combined
Genomic Phylogenetic Species Concept (GPSC) that is based on genomic and MLST approaches.
The basic principle is that every bacterial species grows in a particular niche of its own and no two
species occupy the same niche. In molecular terms, the functional proteins and enzymes produced
by that particular species are unique and a true refl ection of the niche. The core housekeeping
genes of the species will also refl ect the evolutionary pattern. In case there is a change in the niche,
accordingly a genetic drift and adaptation to the new niche is a consequential event. Because GPSC
takes into account sequence-based information (e.g. protein, RNA, DNA), it is more practical, rapid,
readily accessible from anywhere and slowly gaining universality. To elaborate the concept of
GPSC, Staley (2006) cited
Prochlorococcus
as one of the examples where a comparison between HL-
adapted (MED4) and LL-adapted (MIT 9313) strains revealed interesting features. On the basis of
16S rRNA phylogeny both strains seem to have descended from a common LL-adapted ancestor.
There is a great difference in the size of the genomes of the two strains. MED4 has a genome of 1.65
Mb (with a mol % G+C of 30.8) whereas MIT 9313 possesses a genome size of 2.41 Mb (with a mol%
G+C of 50.7). These differences are in fact major in nature that warrants their separation into not
only separate species but also into two different genera. However, their commonality is the existence
of 1350 genes with additional of 364 genes in case of MED4 and 923 genes in case of MIT 9313. It is
suggested that these additional genes either may have been acquired from their common ancestor
(retained and duplicated) or via LGT. Although these two strains differ from each other by 0.8% of
16S rDNA sequence, the median sequence identity of the shared genes is only 78%. These two strains
are considered as distinct ecotypes and GPSC concept can be readily applied to these strains.
Whitaker (2003) investigated the relationship between genetic and geographical distance in
combined analysis of nine genomic loci of archaebacteria collected from globally distant geothermal
regions. There appeared to be a signifi cant distance effect that showed a distinct relationship between
genetic and geographical distance among populations. Whitaker (2006) discussed allopatry as a
speciation mechanism in microorganisms in which both environmental selection and geographical
isolation can lead to differentiation among populations.
III. EVOLUTION OF BACTERIA AND CYANOBACTERIA
Charles Darwin (1859) presented in his classical work on “The Origin of Species” a theory of natural
selection and envisaged that all organisms have evolved from a common ancestor and diverged
from one another during the course of evolution. Accordingly, a universal “Tree of Life” (TOL) can
be constructed representing gradual evolution of organisms. Although the concept of the existence
of a common ancestor was accepted by many of his contemporaries and subsequent followers
the theory of natural selection was not readily accepted. But due to the objections raised by many
workers, he later modifi ed his theory to the extent that the driving force for natural selection is
spontaneous variations. One of his staunch oppents of the theory of natural selection was Thomas
Morgan who strongly advocated a theory of “mutationism” that explains the evolutionary process
as chiefl y due to benefi cial mutations. Simply because a number of mutations are of deleterious
nature, this theory of Morgan was not accepted by a number of leading personalities (Fisher, 1930;
