Perhaps
now is a good time to clarify some of the terminology: Symbiogenesis cannot be
replicated in a lab (and not endosymbiosis as previously stated). There is a difference.
I cannot dispute the process of endosymbiosis which can be demonstrated. However
the complete process of symbiogenesis (the evolutionary theory that explains
the origin of eukaryotic cells from prokaryotic cells by symbiosis) cannot as
yet be demonstrated.
What
should also be kept in mind, is the process of adaptation at work, which is
confirmed by Dr. K. W. Jeon.
It is suggested that the presence of a potent P2 in the
X-bacterial gene is an adaptation for the endosymbiotic bacteria to survive
within a potentially hostile intracellular environment. 1
The
following discusses the organisms or possible organisms involved in Dr. Jeon’s
experiment. Important to note that these organisms remain individual and
identifiable as specific species.
The X-bacteria which initiated organismic association with
the D strain of Amoeba proteus in 1966 as parasites have changed to
obligate endosymbionts on which the host depends for survival. Owing to the
difficulty in cultivating the bacteria in vitro, the identity of
X-bacteria has not been determined. The life cycle of X-bacteria is similar to
that of Legionella spp. in soil amoebae. 2
Not
sure why this oversight has occurred, but what is described here is the typical
(or atypical) immune response of the Amoeba
organism, which is not unusual at all. Our immune system has similar response mechanisms
to counter invading bacteria and viruses.
This indicates that phylogenetically and ecologically
diverse bacteria which thrive inside amoebae exploit common mechanisms for
interaction with their hosts, and it provides further evidence for the role of
amoebae as training grounds for bacterial pathogens of humans. 3
Again,
one should not accept as fact that these organisms are a new species. The
warning is sounded by two renowned biologists:
On the basis of the structural and physiological changes brought
about by the endosymbionts of xD amoebae as described above, one could consider
the the symbiont-bearing xD strain a new species of Amoeba. However, until evidence for genetic differences between D and
xD amoebae is obtained, it would be more prudent to treat xD amoebae as belonging
to a variant strain. 4
There
are huge barriers to overcome in the proposed process of symbiogenesis. I
believe the barriers are insurmountable:
Barriers to endosymbiogenesis
The transformation of an independent endosmbiont into an
organelle faces tremendous barriers. The hurdles include the transfer of much
of the endosymbiont’s genetic material to the host’s nucleus, the acquisition
of the proper regulatory and targeting sequences to ensure that the transferred
gene not only is expressed but possesses the correct targeting information to
redirect it into the plastid. This, of course, leads us to the significant challenge
of acquiring the appropriate protein import apparatus to ensure that the targeted
proteins are properly imported and sorted within the organelle. There is also
the issue of integrating and regulating metabolic pathways. 5
We will see that the archaeal
translation machinery is neither bacterial, nor eukaryotic, but customized to
the archaea. Indeed some parts of the archaeal translation machinery and those
of bacteria or eukarya have similar sequence and/or structures since all life
forms share the same task of decoding information carried by mRNA and
translating the message into the amino acid sequences of proteins. However, the
archaeal translation machinery can’t be exchanged with those of bacteria or
eukarya, including ribosomes, tRNAs, and translation factors. Thus, there
exists an evolutionarily unbridgeable gap between archaea and eukarya and
bacteria in translation, just as in DNA replication and transcription.
The above comparisons of a
few molecules involved in the information processing in the three domains of
life reveals several interesting phenomena: 1) Molecular machines are employed
as modules, that is, a process is either bacterial-like or eukaryote-like. 2)
Each machine is a molecular mosaic of modules that is fine-tuned to meet the
unique need of an organism. 3) The machines for DNA replication, transcription,
and translation in bacteria, archaea, and eukarya are unique and specific for
each domain of life, and thus, can’t be exchanged. 4) Functional annotations of
genes based on sequence homology comparisons can be misleading because they
only take into account isolated parts of proteins, not the entire gene. 5)
Organism-specific protein extensions, such as the CTD of eukaryotic Rpb1, can
be the determinant factor of life vs. death for the specific organism.
We will see that the archaeal
translation machinery is neither bacterial, nor eukaryotic, but customized to
the archaea. Indeed some parts of the archaeal translation machinery and those
of bacteria or eukarya have similar sequence and/or structures since all life
forms share the same task of decoding information carried by mRNA and
translating the message into the amino acid sequences of proteins. However, the
archaeal translation machinery can’t be exchanged with those of bacteria or
eukarya, including ribosomes, tRNAs, and translation factors. Thus, there
exists an evolutionarily unbridgeable gap between archaea and eukarya and
bacteria in translation, just as in DNA replication and transcription.
The above comparisons of a
few molecules involved in the information processing in the three domains of
life reveals several interesting phenomena: 1) Molecular machines are employed
as modules, that is, a process is either bacterial-like or eukaryote-like. 2)
Each machine is a molecular mosaic of modules that is fine-tuned to meet the
unique need of an organism. 3) The machines for DNA replication, transcription,
and translation in bacteria, archaea, and eukarya are unique and specific for
each domain of life, and thus, can’t be exchanged. 4) Functional annotations of
genes based on sequence homology comparisons can be misleading because they
only take into account isolated parts of proteins, not the entire gene. 5)
Organism-specific protein extensions, such as the CTD of eukaryotic Rpb1, can
be the determinant factor of life vs. death for the specific organism. 6
At variance with the earlier belief that mitochondrial
genomes are represented by circular DNA molecules, a large number of organisms
have been found to carry linear mitochondrial DNA. Studies of linear
mitochondrial genomes might provide a novel view on the evolutionary history of
organelle genomes and contribute to delineating mechanisms of maintenance and
functioning of telomeres. Because linear mitochondrial DNA is present in a
number of human pathogens, its replication mechanisms might become a target for
drugs that would not interfere with replication of human circular mitochondrial
DNA. 7
1.
Abstract: “A novel strong promoter of the groEx operon of symbiotic
bacteria in Amoeba proteus.” Abstract: Dr K.W. Jeon, Department of
Zoology, University of Tennessee, Knoxville,
TN 37996-0810,
USA.
2.
Abstract: “Phylogenetic characterization of Legionella-like endosymbiotic
X-bacteria in Amoeba proteus: a proposal for ‘Candidatus Legionella
jeonii’ sp. nov.”
3.
Abstract: “The Genome of the Amoeba Symbiont “Candidatus Amoebophilus
asiaticus” Reveals Common Mechanisms for Host Cell Interaction among
Amoeba-Associated Bacteria.”
4.
Symbiosis as a Source of Evolutionary
Innovation: Speciation and Morphogenesis, by Lynn Margulis and René Fester,
p. 125.
5.
Molecular Phylogeny of Microorganisms, by Aharon Oren and R. Thane Papke, p. 198.
6.
“Information Processing Differences Between Archaea and Eukarya—Implications
for Homologs and the Myth of Eukaryogenesis,” by C. L. Tan and J. P. Tomkins.
7. Linear
mitochondrial genomes: 30 years down the line. Josef Nosek, L’Ubomir
Tomaska, Hiroshi Fukuhara, and Ladislav Kovac.
